The Pennsylvania State University

The Graduate School

College of Engineering

PROCESSING OF SAPPHIRE SURFACES FOR

SEMICONDUCTOR DEVICE APPLICTIONS

A Thesis in

Electrical Engineering

by

Kevin W Kirby

copy 2008 Kevin W Kirby

Submitted in Partial Fulfillment of the Requirements

for the Degree of

Master of Science

May 2008

ii

The thesis of Kevin W Kirby was reviewed and approved by the following Jerzy Ruzyllo Professor of Electrical Engineering Thesis Adviser John D Mitchell Professor of Electrical Engineering W Kenneth Jenkins Professor of Electrical Engineering Head of the Department of Electrical Engineering

Signatures are on file in the Graduate School

iii

ABSTRACT

This thesis explores the preparation of sapphire surfaces for use in semiconductor

device applications Sapphire has shown promise in a few niche applications as a device

substrate due to its insulating nature and extremely stable behavior These properties

make sapphire a suitable alternative for applications where standard silicon substrates

provide inadequate performance As the technology behind the applications involving

sapphire substrates has improved sapphire substrate surface preparation has become

more of a concern

In an effort to further the development of sapphire surface processing wet chemical

cleaning treatments were explored during this study Chemistries common to the industry

were chosen including Standard Clean 1 (SC1) Standard Clean 2 (SC2) hydrofluoric

acid (HF) and a 31 mixture of sulfuric acid (H2SO4) and phosphoric acid (H3PO4)

Chemical treatments were analyzed by means of wetting angle measurements AFM

analyses and XPS surveys Several alkali metals and other contaminants were shown to

be present on the surface of the samples tested along with a significant amount of carbon

implying organic contamination Several treatments caused a change in surface

morphology but many treatments did not affect surface composition Most noticeably

SC1 appeared to be an effective treatment for organic contamination on sapphire

surfaces Ultimately these treatments could prove to be an integral part of an effective

sapphire surface cleaning sequence and these treatments should be explored in more

detail

iv

TABLE OF CONTENTS

LIST OF TABLES vi

LIST OF FIGURES vii

ACKNOWLEDGEMENTS ix

1 INTRODUCTION 1

2 BACKGROUND 3

21 Sapphire Material Properties 3

211 Sapphire Crystal Structure 3

212 Notable Mechanical and Electrical Properties 4

213 Diffusion Behavior of Various Elements in Sapphire 6

22 Applications for Sapphire in Semiconductor Electronics 9

221 Silicon on Sapphire (SOS) Technology 9

222 Fabrication of SOS wafers 15

223 RF Applications for SOS Technology 20

224 Other Applications for SOS Technology 21

23 Applications for Sapphire in Semiconductor Photonics 23

231 Epitaxial Deposition of GaN on a Sapphire Substrate 23

232 GaN Based Optoelctronic Devices 28

233 ZnO Based Optoelectronic Devices 29

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing 31

241 Wet Chemical Treatments 31

242 Dry Chemical Treatments 35

243 Mechanical Polishing 37

3 OBJECTIVES OF THIS STUDY 40

4 EXPERIMENTAL PROCEDURES 41

41 Chemical Treatments 41

v

42 Measurement Techniques 42

5 EXPERIMENTAL RESULTS AND DISCUSSION 45

51 Wetting Angle Measurements 45

52 Surface Roughness Measurements 48

53 Atomic Composition Analysis 54

6 SUMMARY 61

7 REFERENCES 63

vi

LIST OF TABLES

Table 1 A selection of important electrical and mechanical properties of sapphire 7

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count 55

Table 3 Atomic percentage of various elements on the sapphire surface following treatment 57

vii

LIST OF FIGURES

Figure 1 A graphical representation of the sapphire crystal orientations most commonly

used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml 5

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt Ag Ga Y Cu Co Fe and Cr 10

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20 12

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006 14

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20 17

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20 19

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45 25

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47 27

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57 30

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63 33

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77 38

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time 47

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time 49

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4 50

viii

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4 52

Figure 16 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric phosphoric clean and SC2 clean 60

ix

ACKNOWLEDGEMENTS

We would like to acknowledge RSA le Rubis in Grenoble France for supplying the

sapphire wafers for these experiments Additionally we would like to thank Vince Bojan

and his group at the Penn State Materials Characterization Lab for their help with some

of the testing and analysis presented in this report The author would also like to thank his

thesis advisor Dr Jerzy Ruzyllo for his support and guidance as well as his colleague

Karthikeyan Shanmugasundaram for his assistance throughout the course of this research

project

1

1 INTRODUCTION

Sapphire is an important material used in semiconductor device technology It is a

common choice as a device substrate in a few niche applications where the traditional

workhorse of the industry silicon is not appropriate Sapphire substrates offer several

benefits over their silicon counterparts and are ideal for these particular applications in

part due to their insulating nature and highly stable behavior Sapphire is also naturally

radiation-hardened making it an excellent choice for space and military applications

Sapphire wafer technology is not as developed as silicon technology but sapphire

substrates provide for enough of a performance benefit that they have found some

commercial success in the areas of RF technology and optoelectronic applications

Despite the importance and number of uses for sapphire substrates the chemical

cleaning and surface preparation of sapphire substrates has not been explored in the same

level of detail or in the same context that silicon wafer cleaning has in the past Silicon

wafer cleaning is an industry unto itself and well-documented effective cleaning

techniques exist for a variety of situations Several technical limitations dealing with the

compatibility of sapphire substrates and other materials used during processing have

made the quality and purity of sapphire surfaces less of a concern However as

technology improves and these limitations are surpassed it will become increasingly

important to create sapphire wafers that are both atomically smooth and free of all

particle contamination and other impurities

2

This project intends to explore the cleaning of sapphire substrates in the same frame of

reference as silicon substrate cleaning particularly in the area of wet chemical cleaning

where the least amount of work is found Chapter 2 details all relevant background

information including notable material properties that distinguish sapphire from other

materials applications for sapphire substrates in the electronics device industry

applications for sapphire substrates in the photonics device industry and a review of the

state of the art in sapphire surface cleaning Chapter 3 further defines the scope and

objectives of this study Chapter 4 gives detail of all experimental procedures used to

obtain the data presented in Chapter 5 of this report Chapter 5 also discusses the

relevance of the data collected and any conclusions that can be drawn from the

experiments conducted for this report A summary of this thesis can be found in Chapter

6

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

ii

The thesis of Kevin W Kirby was reviewed and approved by the following Jerzy Ruzyllo Professor of Electrical Engineering Thesis Adviser John D Mitchell Professor of Electrical Engineering W Kenneth Jenkins Professor of Electrical Engineering Head of the Department of Electrical Engineering

Signatures are on file in the Graduate School

iii

ABSTRACT

This thesis explores the preparation of sapphire surfaces for use in semiconductor

device applications Sapphire has shown promise in a few niche applications as a device

substrate due to its insulating nature and extremely stable behavior These properties

make sapphire a suitable alternative for applications where standard silicon substrates

provide inadequate performance As the technology behind the applications involving

sapphire substrates has improved sapphire substrate surface preparation has become

more of a concern

In an effort to further the development of sapphire surface processing wet chemical

cleaning treatments were explored during this study Chemistries common to the industry

were chosen including Standard Clean 1 (SC1) Standard Clean 2 (SC2) hydrofluoric

acid (HF) and a 31 mixture of sulfuric acid (H2SO4) and phosphoric acid (H3PO4)

Chemical treatments were analyzed by means of wetting angle measurements AFM

analyses and XPS surveys Several alkali metals and other contaminants were shown to

be present on the surface of the samples tested along with a significant amount of carbon

implying organic contamination Several treatments caused a change in surface

morphology but many treatments did not affect surface composition Most noticeably

SC1 appeared to be an effective treatment for organic contamination on sapphire

surfaces Ultimately these treatments could prove to be an integral part of an effective

sapphire surface cleaning sequence and these treatments should be explored in more

detail

iv

TABLE OF CONTENTS

LIST OF TABLES vi

LIST OF FIGURES vii

ACKNOWLEDGEMENTS ix

1 INTRODUCTION 1

2 BACKGROUND 3

21 Sapphire Material Properties 3

211 Sapphire Crystal Structure 3

212 Notable Mechanical and Electrical Properties 4

213 Diffusion Behavior of Various Elements in Sapphire 6

22 Applications for Sapphire in Semiconductor Electronics 9

221 Silicon on Sapphire (SOS) Technology 9

222 Fabrication of SOS wafers 15

223 RF Applications for SOS Technology 20

224 Other Applications for SOS Technology 21

23 Applications for Sapphire in Semiconductor Photonics 23

231 Epitaxial Deposition of GaN on a Sapphire Substrate 23

232 GaN Based Optoelctronic Devices 28

233 ZnO Based Optoelectronic Devices 29

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing 31

241 Wet Chemical Treatments 31

242 Dry Chemical Treatments 35

243 Mechanical Polishing 37

3 OBJECTIVES OF THIS STUDY 40

4 EXPERIMENTAL PROCEDURES 41

41 Chemical Treatments 41

v

42 Measurement Techniques 42

5 EXPERIMENTAL RESULTS AND DISCUSSION 45

51 Wetting Angle Measurements 45

52 Surface Roughness Measurements 48

53 Atomic Composition Analysis 54

6 SUMMARY 61

7 REFERENCES 63

vi

LIST OF TABLES

Table 1 A selection of important electrical and mechanical properties of sapphire 7

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count 55

Table 3 Atomic percentage of various elements on the sapphire surface following treatment 57

vii

LIST OF FIGURES

Figure 1 A graphical representation of the sapphire crystal orientations most commonly

used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml 5

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt Ag Ga Y Cu Co Fe and Cr 10

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20 12

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006 14

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20 17

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20 19

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45 25

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47 27

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57 30

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63 33

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77 38

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time 47

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time 49

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4 50

viii

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4 52

Figure 16 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric phosphoric clean and SC2 clean 60

ix

ACKNOWLEDGEMENTS

We would like to acknowledge RSA le Rubis in Grenoble France for supplying the

sapphire wafers for these experiments Additionally we would like to thank Vince Bojan

and his group at the Penn State Materials Characterization Lab for their help with some

of the testing and analysis presented in this report The author would also like to thank his

thesis advisor Dr Jerzy Ruzyllo for his support and guidance as well as his colleague

Karthikeyan Shanmugasundaram for his assistance throughout the course of this research

project

1

1 INTRODUCTION

Sapphire is an important material used in semiconductor device technology It is a

common choice as a device substrate in a few niche applications where the traditional

workhorse of the industry silicon is not appropriate Sapphire substrates offer several

benefits over their silicon counterparts and are ideal for these particular applications in

part due to their insulating nature and highly stable behavior Sapphire is also naturally

radiation-hardened making it an excellent choice for space and military applications

Sapphire wafer technology is not as developed as silicon technology but sapphire

substrates provide for enough of a performance benefit that they have found some

commercial success in the areas of RF technology and optoelectronic applications

Despite the importance and number of uses for sapphire substrates the chemical

cleaning and surface preparation of sapphire substrates has not been explored in the same

level of detail or in the same context that silicon wafer cleaning has in the past Silicon

wafer cleaning is an industry unto itself and well-documented effective cleaning

techniques exist for a variety of situations Several technical limitations dealing with the

compatibility of sapphire substrates and other materials used during processing have

made the quality and purity of sapphire surfaces less of a concern However as

technology improves and these limitations are surpassed it will become increasingly

important to create sapphire wafers that are both atomically smooth and free of all

particle contamination and other impurities

2

This project intends to explore the cleaning of sapphire substrates in the same frame of

reference as silicon substrate cleaning particularly in the area of wet chemical cleaning

where the least amount of work is found Chapter 2 details all relevant background

information including notable material properties that distinguish sapphire from other

materials applications for sapphire substrates in the electronics device industry

applications for sapphire substrates in the photonics device industry and a review of the

state of the art in sapphire surface cleaning Chapter 3 further defines the scope and

objectives of this study Chapter 4 gives detail of all experimental procedures used to

obtain the data presented in Chapter 5 of this report Chapter 5 also discusses the

relevance of the data collected and any conclusions that can be drawn from the

experiments conducted for this report A summary of this thesis can be found in Chapter

6

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

iii

ABSTRACT

This thesis explores the preparation of sapphire surfaces for use in semiconductor

device applications Sapphire has shown promise in a few niche applications as a device

substrate due to its insulating nature and extremely stable behavior These properties

make sapphire a suitable alternative for applications where standard silicon substrates

provide inadequate performance As the technology behind the applications involving

sapphire substrates has improved sapphire substrate surface preparation has become

more of a concern

In an effort to further the development of sapphire surface processing wet chemical

cleaning treatments were explored during this study Chemistries common to the industry

were chosen including Standard Clean 1 (SC1) Standard Clean 2 (SC2) hydrofluoric

acid (HF) and a 31 mixture of sulfuric acid (H2SO4) and phosphoric acid (H3PO4)

Chemical treatments were analyzed by means of wetting angle measurements AFM

analyses and XPS surveys Several alkali metals and other contaminants were shown to

be present on the surface of the samples tested along with a significant amount of carbon

implying organic contamination Several treatments caused a change in surface

morphology but many treatments did not affect surface composition Most noticeably

SC1 appeared to be an effective treatment for organic contamination on sapphire

surfaces Ultimately these treatments could prove to be an integral part of an effective

sapphire surface cleaning sequence and these treatments should be explored in more

detail

iv

TABLE OF CONTENTS

LIST OF TABLES vi

LIST OF FIGURES vii

ACKNOWLEDGEMENTS ix

1 INTRODUCTION 1

2 BACKGROUND 3

21 Sapphire Material Properties 3

211 Sapphire Crystal Structure 3

212 Notable Mechanical and Electrical Properties 4

213 Diffusion Behavior of Various Elements in Sapphire 6

22 Applications for Sapphire in Semiconductor Electronics 9

221 Silicon on Sapphire (SOS) Technology 9

222 Fabrication of SOS wafers 15

223 RF Applications for SOS Technology 20

224 Other Applications for SOS Technology 21

23 Applications for Sapphire in Semiconductor Photonics 23

231 Epitaxial Deposition of GaN on a Sapphire Substrate 23

232 GaN Based Optoelctronic Devices 28

233 ZnO Based Optoelectronic Devices 29

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing 31

241 Wet Chemical Treatments 31

242 Dry Chemical Treatments 35

243 Mechanical Polishing 37

3 OBJECTIVES OF THIS STUDY 40

4 EXPERIMENTAL PROCEDURES 41

41 Chemical Treatments 41

v

42 Measurement Techniques 42

5 EXPERIMENTAL RESULTS AND DISCUSSION 45

51 Wetting Angle Measurements 45

52 Surface Roughness Measurements 48

53 Atomic Composition Analysis 54

6 SUMMARY 61

7 REFERENCES 63

vi

LIST OF TABLES

Table 1 A selection of important electrical and mechanical properties of sapphire 7

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count 55

Table 3 Atomic percentage of various elements on the sapphire surface following treatment 57

vii

LIST OF FIGURES

Figure 1 A graphical representation of the sapphire crystal orientations most commonly

used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml 5

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt Ag Ga Y Cu Co Fe and Cr 10

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20 12

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006 14

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20 17

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20 19

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45 25

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47 27

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57 30

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63 33

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77 38

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time 47

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time 49

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4 50

viii

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4 52

Figure 16 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric phosphoric clean and SC2 clean 60

ix

ACKNOWLEDGEMENTS

We would like to acknowledge RSA le Rubis in Grenoble France for supplying the

sapphire wafers for these experiments Additionally we would like to thank Vince Bojan

and his group at the Penn State Materials Characterization Lab for their help with some

of the testing and analysis presented in this report The author would also like to thank his

thesis advisor Dr Jerzy Ruzyllo for his support and guidance as well as his colleague

Karthikeyan Shanmugasundaram for his assistance throughout the course of this research

project

1

1 INTRODUCTION

Sapphire is an important material used in semiconductor device technology It is a

common choice as a device substrate in a few niche applications where the traditional

workhorse of the industry silicon is not appropriate Sapphire substrates offer several

benefits over their silicon counterparts and are ideal for these particular applications in

part due to their insulating nature and highly stable behavior Sapphire is also naturally

radiation-hardened making it an excellent choice for space and military applications

Sapphire wafer technology is not as developed as silicon technology but sapphire

substrates provide for enough of a performance benefit that they have found some

commercial success in the areas of RF technology and optoelectronic applications

Despite the importance and number of uses for sapphire substrates the chemical

cleaning and surface preparation of sapphire substrates has not been explored in the same

level of detail or in the same context that silicon wafer cleaning has in the past Silicon

wafer cleaning is an industry unto itself and well-documented effective cleaning

techniques exist for a variety of situations Several technical limitations dealing with the

compatibility of sapphire substrates and other materials used during processing have

made the quality and purity of sapphire surfaces less of a concern However as

technology improves and these limitations are surpassed it will become increasingly

important to create sapphire wafers that are both atomically smooth and free of all

particle contamination and other impurities

2

This project intends to explore the cleaning of sapphire substrates in the same frame of

reference as silicon substrate cleaning particularly in the area of wet chemical cleaning

where the least amount of work is found Chapter 2 details all relevant background

information including notable material properties that distinguish sapphire from other

materials applications for sapphire substrates in the electronics device industry

applications for sapphire substrates in the photonics device industry and a review of the

state of the art in sapphire surface cleaning Chapter 3 further defines the scope and

objectives of this study Chapter 4 gives detail of all experimental procedures used to

obtain the data presented in Chapter 5 of this report Chapter 5 also discusses the

relevance of the data collected and any conclusions that can be drawn from the

experiments conducted for this report A summary of this thesis can be found in Chapter

6

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

iv

TABLE OF CONTENTS

LIST OF TABLES vi

LIST OF FIGURES vii

ACKNOWLEDGEMENTS ix

1 INTRODUCTION 1

2 BACKGROUND 3

21 Sapphire Material Properties 3

211 Sapphire Crystal Structure 3

212 Notable Mechanical and Electrical Properties 4

213 Diffusion Behavior of Various Elements in Sapphire 6

22 Applications for Sapphire in Semiconductor Electronics 9

221 Silicon on Sapphire (SOS) Technology 9

222 Fabrication of SOS wafers 15

223 RF Applications for SOS Technology 20

224 Other Applications for SOS Technology 21

23 Applications for Sapphire in Semiconductor Photonics 23

231 Epitaxial Deposition of GaN on a Sapphire Substrate 23

232 GaN Based Optoelctronic Devices 28

233 ZnO Based Optoelectronic Devices 29

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing 31

241 Wet Chemical Treatments 31

242 Dry Chemical Treatments 35

243 Mechanical Polishing 37

3 OBJECTIVES OF THIS STUDY 40

4 EXPERIMENTAL PROCEDURES 41

41 Chemical Treatments 41

v

42 Measurement Techniques 42

5 EXPERIMENTAL RESULTS AND DISCUSSION 45

51 Wetting Angle Measurements 45

52 Surface Roughness Measurements 48

53 Atomic Composition Analysis 54

6 SUMMARY 61

7 REFERENCES 63

vi

LIST OF TABLES

Table 1 A selection of important electrical and mechanical properties of sapphire 7

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count 55

Table 3 Atomic percentage of various elements on the sapphire surface following treatment 57

vii

LIST OF FIGURES

Figure 1 A graphical representation of the sapphire crystal orientations most commonly

used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml 5

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt Ag Ga Y Cu Co Fe and Cr 10

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20 12

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006 14

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20 17

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20 19

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45 25

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47 27

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57 30

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63 33

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77 38

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time 47

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time 49

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4 50

viii

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4 52

Figure 16 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric phosphoric clean and SC2 clean 60

ix

ACKNOWLEDGEMENTS

We would like to acknowledge RSA le Rubis in Grenoble France for supplying the

sapphire wafers for these experiments Additionally we would like to thank Vince Bojan

and his group at the Penn State Materials Characterization Lab for their help with some

of the testing and analysis presented in this report The author would also like to thank his

thesis advisor Dr Jerzy Ruzyllo for his support and guidance as well as his colleague

Karthikeyan Shanmugasundaram for his assistance throughout the course of this research

project

1

1 INTRODUCTION

Sapphire is an important material used in semiconductor device technology It is a

common choice as a device substrate in a few niche applications where the traditional

workhorse of the industry silicon is not appropriate Sapphire substrates offer several

benefits over their silicon counterparts and are ideal for these particular applications in

part due to their insulating nature and highly stable behavior Sapphire is also naturally

radiation-hardened making it an excellent choice for space and military applications

Sapphire wafer technology is not as developed as silicon technology but sapphire

substrates provide for enough of a performance benefit that they have found some

commercial success in the areas of RF technology and optoelectronic applications

Despite the importance and number of uses for sapphire substrates the chemical

cleaning and surface preparation of sapphire substrates has not been explored in the same

level of detail or in the same context that silicon wafer cleaning has in the past Silicon

wafer cleaning is an industry unto itself and well-documented effective cleaning

techniques exist for a variety of situations Several technical limitations dealing with the

compatibility of sapphire substrates and other materials used during processing have

made the quality and purity of sapphire surfaces less of a concern However as

technology improves and these limitations are surpassed it will become increasingly

important to create sapphire wafers that are both atomically smooth and free of all

particle contamination and other impurities

2

This project intends to explore the cleaning of sapphire substrates in the same frame of

reference as silicon substrate cleaning particularly in the area of wet chemical cleaning

where the least amount of work is found Chapter 2 details all relevant background

information including notable material properties that distinguish sapphire from other

materials applications for sapphire substrates in the electronics device industry

applications for sapphire substrates in the photonics device industry and a review of the

state of the art in sapphire surface cleaning Chapter 3 further defines the scope and

objectives of this study Chapter 4 gives detail of all experimental procedures used to

obtain the data presented in Chapter 5 of this report Chapter 5 also discusses the

relevance of the data collected and any conclusions that can be drawn from the

experiments conducted for this report A summary of this thesis can be found in Chapter

6

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

v

42 Measurement Techniques 42

5 EXPERIMENTAL RESULTS AND DISCUSSION 45

51 Wetting Angle Measurements 45

52 Surface Roughness Measurements 48

53 Atomic Composition Analysis 54

6 SUMMARY 61

7 REFERENCES 63

vi

LIST OF TABLES

Table 1 A selection of important electrical and mechanical properties of sapphire 7

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count 55

Table 3 Atomic percentage of various elements on the sapphire surface following treatment 57

vii

LIST OF FIGURES

Figure 1 A graphical representation of the sapphire crystal orientations most commonly

used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml 5

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt Ag Ga Y Cu Co Fe and Cr 10

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20 12

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006 14

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20 17

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20 19

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45 25

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47 27

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57 30

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63 33

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77 38

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time 47

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time 49

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4 50

viii

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4 52

Figure 16 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric phosphoric clean and SC2 clean 60

ix

ACKNOWLEDGEMENTS

We would like to acknowledge RSA le Rubis in Grenoble France for supplying the

sapphire wafers for these experiments Additionally we would like to thank Vince Bojan

and his group at the Penn State Materials Characterization Lab for their help with some

of the testing and analysis presented in this report The author would also like to thank his

thesis advisor Dr Jerzy Ruzyllo for his support and guidance as well as his colleague

Karthikeyan Shanmugasundaram for his assistance throughout the course of this research

project

1

1 INTRODUCTION

Sapphire is an important material used in semiconductor device technology It is a

common choice as a device substrate in a few niche applications where the traditional

workhorse of the industry silicon is not appropriate Sapphire substrates offer several

benefits over their silicon counterparts and are ideal for these particular applications in

part due to their insulating nature and highly stable behavior Sapphire is also naturally

radiation-hardened making it an excellent choice for space and military applications

Sapphire wafer technology is not as developed as silicon technology but sapphire

substrates provide for enough of a performance benefit that they have found some

commercial success in the areas of RF technology and optoelectronic applications

Despite the importance and number of uses for sapphire substrates the chemical

cleaning and surface preparation of sapphire substrates has not been explored in the same

level of detail or in the same context that silicon wafer cleaning has in the past Silicon

wafer cleaning is an industry unto itself and well-documented effective cleaning

techniques exist for a variety of situations Several technical limitations dealing with the

compatibility of sapphire substrates and other materials used during processing have

made the quality and purity of sapphire surfaces less of a concern However as

technology improves and these limitations are surpassed it will become increasingly

important to create sapphire wafers that are both atomically smooth and free of all

particle contamination and other impurities

2

This project intends to explore the cleaning of sapphire substrates in the same frame of

reference as silicon substrate cleaning particularly in the area of wet chemical cleaning

where the least amount of work is found Chapter 2 details all relevant background

information including notable material properties that distinguish sapphire from other

materials applications for sapphire substrates in the electronics device industry

applications for sapphire substrates in the photonics device industry and a review of the

state of the art in sapphire surface cleaning Chapter 3 further defines the scope and

objectives of this study Chapter 4 gives detail of all experimental procedures used to

obtain the data presented in Chapter 5 of this report Chapter 5 also discusses the

relevance of the data collected and any conclusions that can be drawn from the

experiments conducted for this report A summary of this thesis can be found in Chapter

6

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

vi

LIST OF TABLES

Table 1 A selection of important electrical and mechanical properties of sapphire 7

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count 55

Table 3 Atomic percentage of various elements on the sapphire surface following treatment 57

vii

LIST OF FIGURES

Figure 1 A graphical representation of the sapphire crystal orientations most commonly

used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml 5

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt Ag Ga Y Cu Co Fe and Cr 10

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20 12

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006 14

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20 17

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20 19

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45 25

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47 27

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57 30

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63 33

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77 38

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time 47

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time 49

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4 50

viii

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4 52

Figure 16 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric phosphoric clean and SC2 clean 60

ix

ACKNOWLEDGEMENTS

We would like to acknowledge RSA le Rubis in Grenoble France for supplying the

sapphire wafers for these experiments Additionally we would like to thank Vince Bojan

and his group at the Penn State Materials Characterization Lab for their help with some

of the testing and analysis presented in this report The author would also like to thank his

thesis advisor Dr Jerzy Ruzyllo for his support and guidance as well as his colleague

Karthikeyan Shanmugasundaram for his assistance throughout the course of this research

project

1

1 INTRODUCTION

Sapphire is an important material used in semiconductor device technology It is a

common choice as a device substrate in a few niche applications where the traditional

workhorse of the industry silicon is not appropriate Sapphire substrates offer several

benefits over their silicon counterparts and are ideal for these particular applications in

part due to their insulating nature and highly stable behavior Sapphire is also naturally

radiation-hardened making it an excellent choice for space and military applications

Sapphire wafer technology is not as developed as silicon technology but sapphire

substrates provide for enough of a performance benefit that they have found some

commercial success in the areas of RF technology and optoelectronic applications

Despite the importance and number of uses for sapphire substrates the chemical

cleaning and surface preparation of sapphire substrates has not been explored in the same

level of detail or in the same context that silicon wafer cleaning has in the past Silicon

wafer cleaning is an industry unto itself and well-documented effective cleaning

techniques exist for a variety of situations Several technical limitations dealing with the

compatibility of sapphire substrates and other materials used during processing have

made the quality and purity of sapphire surfaces less of a concern However as

technology improves and these limitations are surpassed it will become increasingly

important to create sapphire wafers that are both atomically smooth and free of all

particle contamination and other impurities

2

This project intends to explore the cleaning of sapphire substrates in the same frame of

reference as silicon substrate cleaning particularly in the area of wet chemical cleaning

where the least amount of work is found Chapter 2 details all relevant background

information including notable material properties that distinguish sapphire from other

materials applications for sapphire substrates in the electronics device industry

applications for sapphire substrates in the photonics device industry and a review of the

state of the art in sapphire surface cleaning Chapter 3 further defines the scope and

objectives of this study Chapter 4 gives detail of all experimental procedures used to

obtain the data presented in Chapter 5 of this report Chapter 5 also discusses the

relevance of the data collected and any conclusions that can be drawn from the

experiments conducted for this report A summary of this thesis can be found in Chapter

6

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

vii

LIST OF FIGURES

Figure 1 A graphical representation of the sapphire crystal orientations most commonly

used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml 5

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt Ag Ga Y Cu Co Fe and Cr 10

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20 12

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006 14

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20 17

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20 19

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45 25

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47 27

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57 30

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63 33

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77 38

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time 47

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time 49

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4 50

viii

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4 52

Figure 16 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric phosphoric clean and SC2 clean 60

ix

ACKNOWLEDGEMENTS

We would like to acknowledge RSA le Rubis in Grenoble France for supplying the

sapphire wafers for these experiments Additionally we would like to thank Vince Bojan

and his group at the Penn State Materials Characterization Lab for their help with some

of the testing and analysis presented in this report The author would also like to thank his

thesis advisor Dr Jerzy Ruzyllo for his support and guidance as well as his colleague

Karthikeyan Shanmugasundaram for his assistance throughout the course of this research

project

1

1 INTRODUCTION

Sapphire is an important material used in semiconductor device technology It is a

common choice as a device substrate in a few niche applications where the traditional

workhorse of the industry silicon is not appropriate Sapphire substrates offer several

benefits over their silicon counterparts and are ideal for these particular applications in

part due to their insulating nature and highly stable behavior Sapphire is also naturally

radiation-hardened making it an excellent choice for space and military applications

Sapphire wafer technology is not as developed as silicon technology but sapphire

substrates provide for enough of a performance benefit that they have found some

commercial success in the areas of RF technology and optoelectronic applications

Despite the importance and number of uses for sapphire substrates the chemical

cleaning and surface preparation of sapphire substrates has not been explored in the same

level of detail or in the same context that silicon wafer cleaning has in the past Silicon

wafer cleaning is an industry unto itself and well-documented effective cleaning

techniques exist for a variety of situations Several technical limitations dealing with the

compatibility of sapphire substrates and other materials used during processing have

made the quality and purity of sapphire surfaces less of a concern However as

technology improves and these limitations are surpassed it will become increasingly

important to create sapphire wafers that are both atomically smooth and free of all

particle contamination and other impurities

2

This project intends to explore the cleaning of sapphire substrates in the same frame of

reference as silicon substrate cleaning particularly in the area of wet chemical cleaning

where the least amount of work is found Chapter 2 details all relevant background

information including notable material properties that distinguish sapphire from other

materials applications for sapphire substrates in the electronics device industry

applications for sapphire substrates in the photonics device industry and a review of the

state of the art in sapphire surface cleaning Chapter 3 further defines the scope and

objectives of this study Chapter 4 gives detail of all experimental procedures used to

obtain the data presented in Chapter 5 of this report Chapter 5 also discusses the

relevance of the data collected and any conclusions that can be drawn from the

experiments conducted for this report A summary of this thesis can be found in Chapter

6

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

viii

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4 52

Figure 16 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric phosphoric clean and SC2 clean 60

ix

ACKNOWLEDGEMENTS

We would like to acknowledge RSA le Rubis in Grenoble France for supplying the

sapphire wafers for these experiments Additionally we would like to thank Vince Bojan

and his group at the Penn State Materials Characterization Lab for their help with some

of the testing and analysis presented in this report The author would also like to thank his

thesis advisor Dr Jerzy Ruzyllo for his support and guidance as well as his colleague

Karthikeyan Shanmugasundaram for his assistance throughout the course of this research

project

1

1 INTRODUCTION

Sapphire is an important material used in semiconductor device technology It is a

common choice as a device substrate in a few niche applications where the traditional

workhorse of the industry silicon is not appropriate Sapphire substrates offer several

benefits over their silicon counterparts and are ideal for these particular applications in

part due to their insulating nature and highly stable behavior Sapphire is also naturally

radiation-hardened making it an excellent choice for space and military applications

Sapphire wafer technology is not as developed as silicon technology but sapphire

substrates provide for enough of a performance benefit that they have found some

commercial success in the areas of RF technology and optoelectronic applications

Despite the importance and number of uses for sapphire substrates the chemical

cleaning and surface preparation of sapphire substrates has not been explored in the same

level of detail or in the same context that silicon wafer cleaning has in the past Silicon

wafer cleaning is an industry unto itself and well-documented effective cleaning

techniques exist for a variety of situations Several technical limitations dealing with the

compatibility of sapphire substrates and other materials used during processing have

made the quality and purity of sapphire surfaces less of a concern However as

technology improves and these limitations are surpassed it will become increasingly

important to create sapphire wafers that are both atomically smooth and free of all

particle contamination and other impurities

2

This project intends to explore the cleaning of sapphire substrates in the same frame of

reference as silicon substrate cleaning particularly in the area of wet chemical cleaning

where the least amount of work is found Chapter 2 details all relevant background

information including notable material properties that distinguish sapphire from other

materials applications for sapphire substrates in the electronics device industry

applications for sapphire substrates in the photonics device industry and a review of the

state of the art in sapphire surface cleaning Chapter 3 further defines the scope and

objectives of this study Chapter 4 gives detail of all experimental procedures used to

obtain the data presented in Chapter 5 of this report Chapter 5 also discusses the

relevance of the data collected and any conclusions that can be drawn from the

experiments conducted for this report A summary of this thesis can be found in Chapter

6

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

ix

ACKNOWLEDGEMENTS

We would like to acknowledge RSA le Rubis in Grenoble France for supplying the

sapphire wafers for these experiments Additionally we would like to thank Vince Bojan

and his group at the Penn State Materials Characterization Lab for their help with some

of the testing and analysis presented in this report The author would also like to thank his

thesis advisor Dr Jerzy Ruzyllo for his support and guidance as well as his colleague

Karthikeyan Shanmugasundaram for his assistance throughout the course of this research

project

1

1 INTRODUCTION

Sapphire is an important material used in semiconductor device technology It is a

common choice as a device substrate in a few niche applications where the traditional

workhorse of the industry silicon is not appropriate Sapphire substrates offer several

benefits over their silicon counterparts and are ideal for these particular applications in

part due to their insulating nature and highly stable behavior Sapphire is also naturally

radiation-hardened making it an excellent choice for space and military applications

Sapphire wafer technology is not as developed as silicon technology but sapphire

substrates provide for enough of a performance benefit that they have found some

commercial success in the areas of RF technology and optoelectronic applications

Despite the importance and number of uses for sapphire substrates the chemical

cleaning and surface preparation of sapphire substrates has not been explored in the same

level of detail or in the same context that silicon wafer cleaning has in the past Silicon

wafer cleaning is an industry unto itself and well-documented effective cleaning

techniques exist for a variety of situations Several technical limitations dealing with the

compatibility of sapphire substrates and other materials used during processing have

made the quality and purity of sapphire surfaces less of a concern However as

technology improves and these limitations are surpassed it will become increasingly

important to create sapphire wafers that are both atomically smooth and free of all

particle contamination and other impurities

2

This project intends to explore the cleaning of sapphire substrates in the same frame of

reference as silicon substrate cleaning particularly in the area of wet chemical cleaning

where the least amount of work is found Chapter 2 details all relevant background

information including notable material properties that distinguish sapphire from other

materials applications for sapphire substrates in the electronics device industry

applications for sapphire substrates in the photonics device industry and a review of the

state of the art in sapphire surface cleaning Chapter 3 further defines the scope and

objectives of this study Chapter 4 gives detail of all experimental procedures used to

obtain the data presented in Chapter 5 of this report Chapter 5 also discusses the

relevance of the data collected and any conclusions that can be drawn from the

experiments conducted for this report A summary of this thesis can be found in Chapter

6

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

1

1 INTRODUCTION

Sapphire is an important material used in semiconductor device technology It is a

common choice as a device substrate in a few niche applications where the traditional

workhorse of the industry silicon is not appropriate Sapphire substrates offer several

benefits over their silicon counterparts and are ideal for these particular applications in

part due to their insulating nature and highly stable behavior Sapphire is also naturally

radiation-hardened making it an excellent choice for space and military applications

Sapphire wafer technology is not as developed as silicon technology but sapphire

substrates provide for enough of a performance benefit that they have found some

commercial success in the areas of RF technology and optoelectronic applications

Despite the importance and number of uses for sapphire substrates the chemical

cleaning and surface preparation of sapphire substrates has not been explored in the same

level of detail or in the same context that silicon wafer cleaning has in the past Silicon

wafer cleaning is an industry unto itself and well-documented effective cleaning

techniques exist for a variety of situations Several technical limitations dealing with the

compatibility of sapphire substrates and other materials used during processing have

made the quality and purity of sapphire surfaces less of a concern However as

technology improves and these limitations are surpassed it will become increasingly

important to create sapphire wafers that are both atomically smooth and free of all

particle contamination and other impurities

2

This project intends to explore the cleaning of sapphire substrates in the same frame of

reference as silicon substrate cleaning particularly in the area of wet chemical cleaning

where the least amount of work is found Chapter 2 details all relevant background

information including notable material properties that distinguish sapphire from other

materials applications for sapphire substrates in the electronics device industry

applications for sapphire substrates in the photonics device industry and a review of the

state of the art in sapphire surface cleaning Chapter 3 further defines the scope and

objectives of this study Chapter 4 gives detail of all experimental procedures used to

obtain the data presented in Chapter 5 of this report Chapter 5 also discusses the

relevance of the data collected and any conclusions that can be drawn from the

experiments conducted for this report A summary of this thesis can be found in Chapter

6

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

2

This project intends to explore the cleaning of sapphire substrates in the same frame of

reference as silicon substrate cleaning particularly in the area of wet chemical cleaning

where the least amount of work is found Chapter 2 details all relevant background

information including notable material properties that distinguish sapphire from other

materials applications for sapphire substrates in the electronics device industry

applications for sapphire substrates in the photonics device industry and a review of the

state of the art in sapphire surface cleaning Chapter 3 further defines the scope and

objectives of this study Chapter 4 gives detail of all experimental procedures used to

obtain the data presented in Chapter 5 of this report Chapter 5 also discusses the

relevance of the data collected and any conclusions that can be drawn from the

experiments conducted for this report A summary of this thesis can be found in Chapter

6

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

3

2 BACKGROUND

Sapphire in both its natural and synthetic forms has been a well-known and heavily

used material for a long time It can be described as a very resilient material both

chemically and mechanically which makes it useful for a variety of applications In the

semiconductor community sapphire has been explored as an electronic device substrate

and a few important niche applications for sapphire substrates have become

commercially viable As interest in sapphire as a commercially viable electronic device

substrate has increased so to has interest in sapphire surface preparation While some

work in this area has been preformed there is still room for more understanding

particularly in the area of wet chemical cleaning

21 Sapphire Material Properties

As previously mentioned sapphire is a very resilient material More specifically it

is an excellent electrical insulator that is both physically and chemically stable even

under harsh conditions Material properties of particular note are sapphirersquos crystal

structure measures of its mechanical and electrical capabilities as well as the relative

resistance to the diffusion of impurities through its crystal lattice

211 Sapphire Crystal Structure

Sapphire is one of the varieties of the mineral corundum a crystalline form of alumina

(aluminum oxide - Al2O3) In nature sapphire occurs as the result of iron and titanium

substitutions in the crystal lattice usually resulting in a bluish color The other common

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

4

variety of corundum is ruby which results from chromium impurities in the crystal

lattice Its natural color is red Pure forms of corundum are transparent and colorless

The sapphire crystal has trigonal symmetry and in particular belongs to the crystal

point group

euro

3 m The three-fold axis (0001) is designated as the c-axis and many of the

sapphire substrates used in the semiconductor community are oriented along this

direction The two-fold symmetry axes both perpendicular to the c-axis are designated

the a-axis and m-axis1 The most common crystallographic orientations for

semiconductor applications include the a-plane c-plane and r-plane A representation of

these planes is included in Figure 1

Sapphire crystals can be created synthetically using one of several crystal growth

techniques including the Verneuil process the Czochralski method Edge-defined film-

fed growth the heat exchanger method and gradient solidification An in-depth review of

the history of synthetic sapphire crystal growth along with a description of each of the

previously mentioned growth techniques is provided in reference 1 Synthetic sapphire

production was commercially developed extensively over the course of the 20th century

and is currently a large industry

212 Notable Mechanical and Electrical Properties

Sapphire is notable as a very stable material both mechanically and thermally

Sapphire has a hardness of 9 on the Mohs scale making it second only to diamond in that

regard It is also chemically stable up to its melting point of 2050degC1 The thermal

expansion coefficient of sapphire varies with crystal orientation from 54-66 10-6K2

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

5

Figure 1 A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications Source httpamericaskyoceracomkiccindustrialcrystalhtml

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

6

Sapphire is one of only few materials to exhibit characteristics this strong while being

visually transparent

Sapphire is also an excellent electrical insulator making it ideal for various electrical

applications The dielectric constant of sapphire is 939 and its resistivity is 1014 Ω-cm

making it a superior insulator to SiO23 Sapphirersquos band gap energy is 87 eV a value that

is also much higher than the band gap of SiO24 Additionally sapphire exhibits excellent

radiation hardness that allows for the creation of electrical devices with minimal

performance degradation during radiation exposure All notable sapphire material

properties are summarized in Table 1

213 Diffusion Behavior of Various Elements in Sapphire

One of the important characteristics of sapphire as it relates to semiconductor

applications is the behavior of sapphire as it interacts with contaminants in the

processing environment One of the simplest measurements of a contaminantrsquos potential

to cause problems during a process is its ability to diffuse into the materials used during

that process As such the diffusion behavior of various elements in alumina is of chief

concern In addition it is important to contrast this behavior with the behavior of other

materials that may be subsequently added to the sapphire base

It is difficult to find comprehensive information regarding the diffusion behavior of

elements in alumina Experimental diffusion coefficients defined by the classical

Arrhenius relationship for some elements have been reported but the data is limited and

scattered An in-depth review of the available data has been published by Doremus5

Explanations for the various types of diffusions mechanisms in sapphire are similarly

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

7

Table 1 A selection of important electrical and mechanical properties of sapphire

Material Property Value

Mechanical Hardness 9 Mohs

Melting Point 2050 degC

Dielectric Constant 939

Resistivity 1014 Ω-cm

Thermal Expansion Coefficient 54-66 10-6K

Band Gap Energy 87 eV

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

8

scattered and a universally accepted theory for diffusion in alumina is not currently

known

The diffusion coefficients for silver and platinum were determined in the same study

using the Secondary Ion Mass Spectroscopy (SIMS) technique6 The data was collected

over the range of 800ndash1150degC for silver and 900ndash1200degC for platinum All data pertains

to bulk diffusion in alumina samples oriented in the c-plane direction In both cases the

data implies a combination of interstitial and substitutional mechanisms

The bulk diffusion coefficient for gallium in alumina has also been reported7 The data

was again determined using the SIMS technique in the range between 1400 degC and 1600

degC While the data is limited to only a few samples the trend indicates a relatively severe

dependence on temperature No diffusion mechanism has been proposed

Yttrium has been studied as a dopant of alumina8 and as such the diffusion coefficient

for yttrium in alumina has been explored as well9 Data has been experimentally

determined by the SIMS technique for temperatures between 1150 degC and 1500 degC

Yttrium diffusion properties are very similar to that of chromium and it has been

proposed that they share the same diffusion mechanism

A SIMS study of copper diffusion in alumina has also been preformed10 Data is

available over the range of temperatures between 800 degC and 1100 degC An interstitial-

substitutional interchange mechanism has been proposed A similar study has been

conducted for cobalt over the temperature range from 1000 degC to 1600 degC11 No diffusion

mechanism has been proposed at this time

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

9

Iron and chromium diffusion in alumina are also well studied Diffusion coefficients

for these elements have been determined by the radiotracer technique in the range

between 1200 degC to 1700 degC12 As mentioned earlier chromium and yttrium share similar

diffusion properties

All available diffusion coefficients are summarized in Figure 2 In all comparable

cases the diffusion coefficient for an element in alumina is several orders of magnitude

lower than the value for that same element in silicon13 The difference is most severe for

elements that are known to diffuse in silicon through an interstitial mechanism such as

iron or copper In these cases the difference in diffusion coefficient can be up to 10

orders of magnitude depending on temperature

22 Applications for Sapphire in Semiconductor Electronics

In the semiconductor industry synthetic alumina is often used as a substrate for

integrated circuits and other semiconductor devices These substrates are often referred to

as sapphire despite the fact that they are generally free of the impurities that distinguish

natural sapphire from other forms of the mineral corundum Sapphire substrates provide

several benefits over the most common semiconductor device substrate silicon For this

reason it has found a few niche applications particularly in the area of RF devices

221 Silicon on Sapphire (SOS) Technology

Sapphire is an electrical insulator and thus it is not a suitable material for use in the

production of active devices It is however possible to deposit semiconductor materials

on top of a sapphire surface while taking advantage of its insulating nature In the most

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

10

Figure 2 Arrhenius plots for bulk diffusion in sapphire for Pt6 Ag6 Ga7 Y9 Cu10 Co11 Fe12 and Cr12

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

11

common application silicon is deposited on sapphire in order to create a Silicon on

Insulator (SOI) device structure This particular structure is referred to as Silicon on

Sapphire (SOS)

In CMOS applications SOI technology offers several advantages over traditional bulk

silicon devices Individual devices can be physically separated on an insulating substrate

drastically improving device isolation while eliminating the need for extraneous doped

well regions or isolation trenches The vertical isolation provided by the insulating base

also eliminates the possibility of latch-up between adjacent devices14 This results in a

simplified circuit structure with a higher packing density These differences are

schematically compared in Figure 3

SOI technology can also greatly improve device performance The source and drain

regions of a MOSFET will extend to the insulating substrate greatly reducing the

junction surface area and in turn reducing junction leakage current and junction

capacitance This will improve device speed lower power dissipation and reduce the

impact of short channel effects Ultimately SOI CMOS shows a 30 performance

increase over bulk CMOS of the same technological generation when operating at similar

voltages and shows a 300 performance increase when operating at similar low-power

conditions14 In general it is expected that a previous generation SOI device will compare

favorably to a current generation bulk silicon device

SOS technology provides all the benefits of SOI technology in addition to a few other

significant advantages Sapphire is inherently radiation-hardened making SOS CMOS

great for space or military applications Sapphire has a relatively high thermal

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

12

Figure 3 A schematic comparison of (a) a SOS device and (b) a bulk silicon device The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices Source Reference 20

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108

13

conductivity of 046 Wcm-K This is much higher than the thermal conductivity of

silicon dioxide (0014 Wcm-K) the other common insulator used in SOI applications3

This helps to reduce self-heating effects and leads to a cooler operating temperature

Sapphire is also a better electrical insulator than silicon dioxide with a higher

resistivity and band gap as previously mentioned SOI wafers with silicon dioxide tend to

have a thin layer of buried oxide acting as the insulating barrier By contrast the entire

bulk of an SOS wafer is an insulator giving an SOS wafer a comparatively infinite

dimension This eliminates the possibility of any non-ideal leakage through the oxide

In SOI devices the body of a CMOS transistor is floating At high drain voltages

impact ionization can occur leading to an increase in the substrate potential This

effectively reduces the threshold voltage of the device and leaves a ldquokinkrdquo in the current-

voltage response when the drain voltage reaches a certain threshold SOS schemes reduce

this effect leading to a reduction in the magnitude of this kink effect and an increase in

the drain voltage that will trigger this effect15 In short a more ideal response can be

achieved over a wider range of operating conditions A graphical representation of this

kink effect is shown in Figure 4

SOS technology is not without its limitations Fabrication of SOS wafers is more

complex and more costly than fabrication of silicon wafers The heteroepitaxial

deposition of silicon on sapphire can result in silicon layers with a high number of

crystalline defects The number and severity of these defects has been shown to directly

impact device performance16

14

Figure 4 A graphical depiction of the ldquokinkrdquo effect observed in SOI devices Source D Neamen ldquoAn Introduction to Semiconductor Devicesrdquo New York McGraw-Hill Higher Education 2006

15

However as technology improves it is possible to create epitaxial layers of silicon that

are both thinner and contain fewer crystalline defects This provides additional benefits

for devices created in these layers If the silicon layer is thin enough devices will

experience significant strain which results in an increase of the devicersquos hole mobility

and a decrease in electron mobility17 Matching hole mobility to electron mobility is

important when creating complementary MOS device structures Thinner silicon layers

also exhibit a higher source-drain breakdown voltage and reduced parasitic bipolar gain

as a result of a lower minority carrier lifetime around 1 ns3

In general SOS technology offers significant performance improvements over

standard bulk silicon technology but the process of creating SOS wafers is more costly

and as a result less developed than bulk silicon manufacturing One can expect to be

using SOS wafers that are smaller than state of the art silicon substrates as well as using

device geometry specifications that are older than the most current generation provides

However the performance of these SOS devices should be comparable to the state of the

art bulk silicon devices and in applications like RF technology the benefits afforded by

the sapphire substrate can actually make it a preferable approach

222 Fabrication of SOS wafers

Due to the limitations and complexity of SOS wafers mentioned above SOS wafer

fabrication has been well studied and well documented In many ways the fabrication of

SOS device substrates requires the same amount of attention as the many steps that would

follow for creation of SOS based devices

16

Interest in SOS technology began in the 1960rsquos as it was discovered that crystalline

silicon could be epitaxially deposited on sapphire substrates18 Initially a hydrogen gas

reduction of SiC14 at high temperatures was used to deposit silicon on the surface of a

sapphire substrate It was immediately clear that an epitaxial relationship existed between

the dimension of a sapphire substrate and the deposited silicon

The most common method for depositing epitaxial silicon on sapphire is currently

some form of Chemical Vapor Deposition (CVD) Various techniques can be employed

to deposit a layer with a minimal amount of defects For example it has been

demonstrated that a hydrogenation process can improve the quality of the subsequently

deposited silicon layer19 Despite these efforts it is difficult to directly deposit silicon on

sapphire and obtain a silicon surface suitable for the most demanding applications

without further treatments

The crystalline structure of R-plane oriented sapphire closely matches that of lt100gt

oriented silicon but there is still a lattice mismatch of up to 125 along a single

dimension20 This mismatch is pictured with the r-plane of sapphire in Figure 5

In part because of the lattice mismatch between silicon and sapphire and in part

because of high thermal stresses associated with cooling the SOS structure the sapphire-

silicon interface contains several crystalline dislocations21 As the thickness of the

epitaxial layer increases the defect density tends to decrease This in turn requires

epitaxial layers of silicon with a thickness in excess of 500 nm to minimize crystalline

defects and ensure reliable device performance22

17

Figure 5 A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and lt100gt silicon Source Reference 20

18

Thinner silicon layers are preferred as CMOS device dimensions shrink As device

dimensions shrink relative to the epitaxial layer thickness the benefits gained from

insulating substrates are reduced and the deposited layer tends to behave more like a bulk

device In order to accommodate the increasingly restrictive device dimensions imposed

by improved CMOS technology it became necessary to decrease the thickness of the

epitaxially deposited layers below 500 nm

It has been shown that the number of crystalline defects can be significantly reduced

through a process known as solid-phase epitaxial regrowth (SPER)23 In this process an

amorphous layer of silicon is created near the sapphire-silicon interface through a series

of silicon implants that immediately follow the epitaxial deposition The structure is then

subject to a series of anneals at a temperature less then 600 degC The remaining crystal

layer acts as a template allowing the buried amorphous layer to recrystallize form the top

down At this point a fairly uniform crystal layer is present with a minimal amount of

crystal dislocations The silicon layer can then be thinned to the desired dimension The

process is summarized in Figure 6 It has been shown that this process can result in a

fourfold decrease in the strain of the deposited silicon layer creating an epitaxial layer

with a much smaller defect density24

Further study has shown that applications of the SPER process involving a rapid

thermal anneal will maintain important device characteristics In particular the resistivity

(104 ndash 105 Ω-cm) of the initially deposited epitaxial layers is preserved when a rapid

thermal treatment is used in lieu of a traditional furnace anneal25 The resistivity is

maintained even if the rapid thermal anneal is followed by other prolonged furnace

treatments

19

Figure 6 A process flow diagram for Solid Phase Epitaxial Regrowth (SPER) Source Reference 20

20

It has been demonstrated that various implementations of the SPER process improve

electrical device performance26 It has also been shown that a second set of implants and

anneals near the epitaxial layer surface can further improve performance and crystal

quality while minimizing aluminum outdiffusion27

The fabrication of SOS wafers is well understood To this date most work has focused

on improving the quality of epitaxially deposited silicon layers after the deposition has

occurred It is logical to expect that as methods continue to improve the initial quality of

the sapphire surface will play a more important role in the SOS fabrication process

223 RF Applications for SOS Technology

SOS technology is widely used in RF applications because it provides several benefits

for analog RF circuits in addition to the benefits already discussed for general digital

CMOS circuits As previously mentioned SOS technology is preferred over other SOI

techniques in RF applications because it offers a continuous insulating substrate

Substrate losses are drastically reduced which allows for the creation of higher Q factor

inductors Improvements to the kink effect discussed previously drastically reduce the

parasitic bipolar gain and increase the breakdown voltage15 In short efficiency improves

as well as reliability

Laterally diffused MOSFETrsquos have been created on SOI substrates for use as power

amplifiers in RF designs They can provide high cutoff frequencies and high breakdown

voltages along with the potential of improved gain higher efficiency larger bandwidth

and the possibility of integrating analog components alongside digital components on a

21

single chip28 It has been shown that stacking CMOS structures can further improve

power amplifier performance on SOS substrates29

Impedance matching networks have also been demonstrated using SOS technology30

Circuits were capable of matching highly mismatched loads due to the high quality of

inductors inherent to an SOS device Distributed amplifiers with coplanar waveguides

have also exhibited excellent matching characteristics while operating at 10 GHz31 Low

loss characteristics allow SOS technology to compare favorably to III-IV alternatives in

this application

The implementation of SiGe heterostructures on high quality SOS substrates has

demonstrated great potential for the wireless industry32 Implementations of MODFET

structures with extremely high transconductance properties have shown promise in the

area of high-speed analog communications MOSFET transmit receive switches have also

been demonstrated on SOS technology33 This type of design compares well with other

alternatives while providing the unique opportunity to be implemented alongside other

digital CMOS circuitry on the same substrate

Sapphire substrate based integrated circuit technologies have found a particular

niche in the RF industry They offer the potential for integrating high performance analog

devices alongside digital CMOS circuitry on the same substrate all the while maintaining

equivalent or even improved performance over other alternative technologies

224 Other Applications for SOS Technology

SOS technology has been explored as an alternative to bulk silicon technology in areas

other then RF device design Experimentally it has shown similar potential to be an

22

efficient and effective alternative to more traditional technologies in these areas but as of

yet it has not found comparable commercial success

Optoelectronic switches based on SOS CMOS technology have been explored for

computing applications They are being looked at as an alternative for chip-to-chip

communications where traditional approaches are limited by the inherent speed of

interconnect lines34

SOS CMOS technology using thin silicon films is being explored as an improvement

to bulk CMOS as well as traditional SOS in general high performance computing

applications22 In particular thin film SOS transistors show equal or better performance

in all measured categories when compared to bulk Si transistors and show greatly

improved performance when compared to traditional as-grown SOS devices

Fabrication of a galvanic isolation buffer has been achieved on SOS substrates35

Circuit performance was adequate at operation speeds up to 30 MHz and the package

exhibited a breakdown voltage of 800 V Proper device isolation was verified

experimentally A similarly successful study demonstrated the fabrication of an isolation

charge pump on a SOS substrate36 The circuit demonstrated a 23 efficiency compared

to only a 9 efficiency for standard bulk CMOS

A low-power analog to digital converter has been demonstrated on a sapphire

substrate37 Using an innovative 2C-1C capacitor chain allowed the circuit to achieve

competitive performance while operating at a power dissipation as low as 900 nW with a

11V power supply

23

SOS technology can be a viable alterative for high performance CMOS based

technologies that require the unique benefits provided by a sapphire substrate This

performance has been demonstrated experimentally for several applications

23 Applications for Sapphire in Semiconductor Photonics

Sapphire has become an important substrate for optoelectronic applications

particularly in the case of Gallium Nitiride (GaN) and Zinc Oxide (ZnO) based photonic

devices Both of these materials are wurtzite type crystals that feature a wide and direct

bandgap characteristic and as such have proven to be valuable materials for creating

short wavelength light emitting diodes and lasers Unfortunately both GaN and ZnO are

extremely difficult materials to grow and cost-effective large device substrates have not

been produced The preferred approach has often been to use some other available

substrate and epitaxially deposit GaN or ZnO layers on the wafer with one of a variety of

available techniques Sapphire or Silicon Carbide (SiC) is typically chosen for these

applications

231 Epitaxial Deposition of GaN on a Sapphire Substrate

Sapphire has been the most common choice of substrate for GaN epitaxial deposition

A sapphire substrate provides a base for an epitaxial active layer in much the same

manner that a sapphire substrate is used in SOS applications However in this application

the reason behind the choice of substrate is rather different The sapphire substrate

provides few if any benefits to the operation of the final device In fact there is a slight

lattice mismatch between GaN and c-plane sapphire (the most common choice for this

24

application) that causes deposited GaN layers to be significantly strained and contain a

high number of crystalline defects38 Despite this limitation sapphire was chosen because

of its ability to promote epitaxial growth of GaN its relative commercial viability its low

cost and its wide band gap39 The epitaxial relationship between GaN and sapphire is

illustrated in Figure 7

To improve the quality of epitaxial GaN deposition on sapphire nitride buffer layers

are grown on the sapphire surface at low temperatures Several approaches exist that

either convert the sapphire surface to an aluminum nitride (AlN) film or deposit a low

temperature layer of GaN on the surface Following growth of the buffer layer with a

high temperature deposition of GaN will create GaN layers with a smaller number of

crystalline defects

Initially it was shown that GaN layers deposited on AlN coated sapphire wafers

showed improved electrical and optical performance over GaN films deposited directly

on sapphire surfaces40 Various vapor phase epitaxy techniques can be employed to

generate the AlN film on the sapphire surface prior to GaN deposition41 The AlN layers

can also be fabricated by the nitridation of the sapphire surface in a nitrogen rich plasma

environment42 or an ammonia based gas environment43 It has been shown that the

uniformity and surface morphology of the deposited GaN layers greatly improves with

the addition of a buffer layer44 As a result the optical and electrical properties of the

GaN films are improved

It is also possible to create GaN buffer layers at low temperatures on sapphire and

subsequently deposit improved GaN active layers The molecular beam epitaxy of GaN

25

Figure 7 An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations Source Reference 45

26

buffer layers in ammonia rich environments has been demonstrated and is know to

improve the formation of crystalline GaN films45 These layers are necessary even on A-

plane sapphire where the lattice mismatch of GaN and sapphire is only about 2 rather

than around 14

Both the process of forming these buffer layers often referred to as nucleation layers

and the process of growing high quality GaN on top of these layers is well understood A

comparative study of the AlN and GaN nucleation layers demonstrates the formation of

GaN islands on the nucleation layers as the catalyst for the growth of continuous single-

crystal GaN layers46 Evidence of these islands is given in Figure 8 A low temperature

buffer layer has a complex grain structure that contains several misoriented islands47 As

the buffer layer is annealed at high temperatures during the growth of the GaN film these

islands coalesce and a continuous single crystal film forms as the islands continue to

grow vertically and laterally A thermodynamic model for this process has been

proposed48

More recently self-supporting GaN substrates have been fabricated using sapphire

substrates as an initial base49 Using a similar buffer layer approach a low temperature

seed layer of GaN is grown on the sapphire substrate and then followed by a high

temperature GaN deposition The GaN film will then naturally separate as the highly

defective buffer layer cools down A similar approach using an AlN seed layer failed to

separate naturally

The epitaxial deposition of GaN on sapphire is a complex process GaN layers

deposited directly on sapphire exhibit a high number of crystalline defects in a manner

27

Figure 8 Visual evidence of single crystal GaN island formation on GaN nucleation layers Source Reference 47

28

that is similar to the epitaxial deposition of silicon on sapphire To improve the quality of

deposited GaN nitride buffer layers are grown on the sapphire substrate that promote the

growth of single crystal GaN with a minimal amount of crystalline defects This process

is well understood and general models for the formation of GaN epitaxial layers have

been reported

232 GaN Based Optoelctronic Devices

GaN is a direct bandgap semiconductor with a bandgap energy of 34 eV This makes

GaN ideal for short wavelength optical devices particularly in the blue-green optical

spectrums50 Additionally it is highly tolerant of high temperatures and harsh operating

environments making it a highly desirable alternative for space and military applications

Gallium can also make nitride alloys with aluminum andor indium This alters the band

gap of the material anywhere between 19 eV to 63 eV allowing for the creation of

optical devices across a wide portion of the spectrum from blue-green all the way to ultra

violet51

GaN based LEDrsquos have been demonstrated and are currently commercially available

They satisfy a previously unfulfilled need for blue-green LEDrsquos with a peak wavelength

between 450 nm and 500 nm52 It is possible to make high brightness LEDrsquos suitable for

commercial applications53 A double-heterostructure approach is used combining InGaN

and AlGaN alloys

Gallium based laser diodes have been fabricated as well Quantum well laser

structures are possible in the blue-green and ultraviolet spectrums54 InGaN based

quantum well lasers have been demonstrated in the spectral range between 363 nm ndash 380

29

nm Using AlGaN alloys it is possible to create quantum well lasers with a wavelength as

small as 200 nm It is possible to improve the quality and lifetime of laser diodes using

lateral overgrowth techniques55

233 ZnO Based Optoelectronic Devices

ZnO is also a popular material choice for optoelectronic devices It shares the direct

bandgap properties of GaN that make it a good choice for optical applications

Additionally ZnO features a high exitonic binding energy of 60 meV that allows for

efficient optical output even at high temperatures56

ZnO also features a wurtzite type crystal structure (Figure 9) that allows for an

epitaxial relationship with sapphire substrates The lattice mismatch between ZnO and the

c-plane of sapphire is approximately 18 allowing for fairly good quality epitaxial

deposition of ZnO on sapphire57 In contrast to GaN small ZnO substrates are

commercially available and do provide benefits over sapphire substrates58 However

heteroepitaxial deposition of ZnO on sapphire has still been extensively explored and

ZnO devices on sapphire have been produced using several different methods including

molecular beam epitaxy (MBE) methods and vapor phase deposition techniques These

methods have been known to produce relatively high quality layers of ZnO59

Several optical devices have been demonstrated using ZnO layers deposited on

sapphire substrates For example ultra violet detectors have been produced with

noticeably fast response times60 MgZnO alloys have also shown promise for optical

applications61 Light emitting diodes using MgZnO heterostructures have been

demonstrated56

30

Figure 9 Representation of the lt0001gt sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO Source Reference 57

31

ZnO is another popular optical device material that can take advantage of sapphire

substrates ZnO on sapphire can be used to create devices in the short wavelength

spectrum that compare favorably to other alternatives and exceed the performance of

those alternatives at high temperatures

24 Current Methods for Sapphire Wafer Cleaning and Surface Processing

Sapphire surface preparation is an important issue in the case of epitaxial

deposition on a sapphire surface The initial quality of the sapphire surface will directly

impact the final quality of the epitaxial deposited layer There is some experimental work

that focuses on the area of sapphire surface preparation and cleaning but particularly in

the area of wet chemical cleaning there is comparatively little information available

241 Wet Chemical Treatments

The removal of particle contamination and other chemical impurities on sapphire

substrates has not been directly explored through wet chemical processes in great detail

However there has been significant work done in the area of wet chemical etching and

polishing of sapphire surfaces It is possible to uniformly etch a sapphire surface with wet

chemical treatments and achieve an atomically smooth surface through several different

methods Various acidic mixtures as well as molten solutions have been explored with

varying degrees of success

Etching sapphire substrates with sulfuric andor phosphoric acids is popular Pure

H2SO4 has been shown to be a fast and fairly uniform etchant of sapphire but also has

been shown to produce insoluble reaction products when the temperature was too high or

32

the etch time was too long62 63 On the other hand pure H3PO4 is a highly non-uniform

etchant of sapphire It will smooth sapphire surfaces oriented in the lt0001gt crystal

orientation however along any other orientation it will preferentially attack crystal

dislocations64 In fact H3PO4 has been commonly used to study the dislocations and

crystalline defects in sapphire samples65

The most routinely employed sapphire etchant is a 31 mixture of H2SO4H3PO4 The

inclusion of phosphoric acid in the mixture appears to buffer the effect of the sulfuric

acid slowing the reaction rate of the mixture compared to that of pure sulfuric acid but

preventing the formation of any unwanted reaction products Unfortunately the addition

of phosphoric acid can also cause surface pitting63

The etch rate for sapphire in these various acid mixtures has been reported over the

temperature range from 200degC ndash 400degC with conflicting results but the reaction rate is

relatively slow on the order of 1 micromhr or less One example of achievable experimental

etch rates is presented in Figure 10

The activation energies for these reactions are reported on the order of ~ 10 ndash 30

kcalmol but differ significantly between studies62 63 The activation energy for the

reactions involving sulfuric acid is higher than that for reactions involving phosphoric

acid The activation energy for reactions with the sulfuric-phosphoric mixture falls

between the other two

It is possible to create patterned substrates using some variation of these etchants in

combination with silicon dioxide masks patterned via standard photolithography

techniques In one case grooved substrates were created in preparation for laterally

33

Figure 10 The etch rate vs temperature for various chemical etchants of sapphire Source Reference 63

34

overgrown GaN deposits Etching in both pure H2SO4 and a 31 mixture of H2SO4H3PO4

was preformed62 Here the sulfuric-phosphoric mixture achieved the most appropriate

results due to issues with formation of reaction products as mentioned above In another

study pyramidal shaped pits were etched in preparation for the deposition of InGaN-

based Light Emitting Diodes66 They achieved similarly effective results with a sulfuric-

phosphoric mixture

Earlier research focused on surface smoothing via treatments in molten liquid

solutions These treatments were capable of significant etching of a sapphire surface but

were cumbersome to implement and often left residual contaminants on the waferrsquos

surface In some cases these residuals where difficult to remove

A treatment in molten Vanadium Pentoxide was preformed at 800degC In general it was

successful in removing surface scratches and other defects however in some cases

deposits were left on the surface The deposits were somewhat chemical resistant and it

was not clear if they could be completely removed via thermal treatments67

Molten Na2B4O7 (Borax) has also been explored as a means of chemical polishing

Reaction temperatures are in excess of 800degC and can achieve atomically smooth results

on most crystal orientations except for the lt0001gt orientation where it is more

appropriate to use phosphoric acid64

While well understood wet chemical treatments with the specific focus of improving

sapphire surface quality do not exist there has been some significant experimental work

done in the area of wet chemical etching of sapphire surfaces The process is well

35

documented and the most common method involving sulfuric and phosphoric acids can

achieve predictable etch rates with minimal surface damage

242 Dry Chemical Treatments

Several dry chemical treatments have been studied as methods for improving the

surface of sapphire substrates These methods include thermal anneals gas phase etching

and plasma etching Each approach offers several benefits depending on the application

and the methods compare favorably to wet chemical treatments

It has long been known that high temperature thermal treatments will improve the

color and clarity of natural sapphire gems so it is not a stretch to think that similar

treatments may improve the quality of a synthetic sapphire surface Not surprisingly it

has been shown that thermal annealing can produce atomically smooth sapphire

substrates68 Annealing at temperatures in excess of 1000degC was required for an hour or

more with longer annealing times or higher temperatures needed for misoriented

surfaces These types of treatments created a step and terrace like structure rather than a

perfectly uniform surface These changes in morphology due to high temperature

treatments are now well understood69 70

Gas phase etching was also explored as an alternative to liquid etching techniques

Gas chemistries comprised of sulfur fluorides were proven to be effective etchants of

sapphire but displayed a high selectivity for various orientations of sapphire71 Etching

with fluorinated hydrocarbons displayed similar potential72

More recently there has been a wealth of research in the area of plasma etching of

sapphire surfaces These types of treatments have focused on exposing sapphire wafers to

36

plasmas at modest temperatures In some cases the process was followed by a nitridation

to render the surface inert before the deposition of a III-IV compound often some form

of GaN

Plasma etching and cleaning can offer several benefits if the environment is

appropriate Plasma processing is more dependent on the inductive power and bias

voltage of the plasma environment than temperature As a result plasma cleaning can be

carried out at a temperature that is much lower than thermal furnace treatments and

anneals

Exposure to hydrogen atoms through low temperature plasma treatments proves to be

effective at cleaning the surface of sapphire wafers73 These plasma treatments are

preferable to high temperature treatments in air that tend to lead to surface damage and

promote hydrogen diffusion into the sapphire substrate Hydrogen based plasma

treatments at 300degC have been shown to completely remove carbon contamination from a

sapphire wafer74

It has been shown that O2 as well as H2Ar plasmas are effective at removing

hydrocarbon and fluorine contamination form a sapphire surface75 Samples initially

containing a uniform contamination layer of approximately 7 ndash 10 Aring before treatment

were left with a uniform Al layer following treatment with one of the plasmas and a

furnace anneal at 900degC for 30 minutes

Chlorine based plasmas have also been explored extensively as an etchant of

sapphire76 They are most effective with an inclusion of 80 BCl3 or more Inclusions of

inert gases nominally effect etch rate and etch selectivity The etch rate of sapphire in an

37

inductively coupled 81BCl39HBr10Ar plasma in particular exhibits relatively fast

etch rates on the order of 05 micrommin77 The etch rate is highly dependent on bias voltage

and inductive power The etch chemistry is selective with a selectivity of sapphire over

mask material (photoresist) of approximately 087 An example of experimental etch

times and etch selectivities for this particular chemistry is illustrated in Figure 11

Several dry chemical methods exist for sapphire surface preparation and etching The

methods feature comparably fast etch rates while providing a relatively smooth surface If

the production environment is conducive to the types of processing equipment required

for these dry methods a dry surface preparation approach is very effective

243 Mechanical Polishing

Mechanical polishing is often preformed on commercially available sapphire wafers

but not to a degree that would leave the surface suitable for immediate device fabrication

It is often assumed that wet chemical or dry chemical surface preparation would be more

appropriate for more exacting applications however some mechanical methods have

been explored with the intent of creating an atomically smooth surface for device

fabrication

Colloidal sapphire polishing with SiO2 has been explored in a manner that is similar to

that of silicon The results were positive indicating removal rates of 25 micromhour under the

right conditions78 It is believed that a chemical reaction between the silicon dioxide and

sapphire surface form alumina silicates that promote fine polishing

Chemical Mechanical Polishing (CMP) techniques have been tested on sapphire

substrates79 Removal rates of 180 nmmin can be achieved while maintaining a surface

38

Figure 11 The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V Source Reference 77

39

roughness around 2 nm Further treatments in a plasma environment reduced the surface

roughness to less than 1 nm It has also been shown that the CMP process displays high

anisotropy in the case of sapphire with the removal rate varying greatly with the surface

crystal orientation80

Vibratory polishing has also been explored as a method for removing the damage done

to sapphire wafers after slicing81 The process was successful at creating a smooth surface

free of scratches and pits but the results are somewhat dated and more current techniques

surpass the effectiveness of this approach

Mechanical polishing of sapphire surfaces has been explored with the intention of

creating surfaces ready for device fabrication Some results indicate that polishing may

be an effective technique that offers a relatively speedy etch rate However in general

other methods appear to be preferable in the case of sapphire surface processing

40

3 OBJECTIVES OF THIS STUDY

It has been established that sapphire is an important material used in semiconductor

technology There are currently a few important electronic and photonic devices that use

sapphire as an insulating substrate Despite these available applications for sapphire

substrates there has been comparatively little analysis of sapphire surface cleaning prior

to its use in these applications particularly in the area of wet chemical cleaning As

current technologies improve the initial surface conditions of sapphire substrates will

become more critical The surface preparation and cleaning of sapphire substrates must

be explored in more detail then the current state of the art provides

This study intended to use the most common cleaning methods for silicon substrates

as a starting point for exploring appropriate sapphire substrate cleaning methods There is

evidence that combining chemical solvents with a known etchant of sapphire can create

an effective cleaning process82 A similar process involving Standard Clean (SC)

solutions and hydrofluoric acid already exists for silicon It was intended to use these

preexisting methods as a basis for obtaining a better understanding of sapphire surface

processing in a manner that meets the rigid standards of semiconductor device

manufacturing Determining the most common contaminates of a sapphire substrate and

initiating the process of developing the methods to remove those contaminants were the

chief goals of this study

41

4 EXPERIMENTAL PROCEDURES

These experiments involved the testing and analysis of several sapphire substrate

samples subject to various wet chemical treatments The substrate samples were received

from RSA Le Rubis in Grenoble France The as-received wafers used for these

experiments were 4 inches in diameter oriented in the c-plane (0001) direction and

subject to a thermal anneal prior to delivery Individual wafers were scored and separated

into smaller irregular pieces in order to allow for a more direct comparison where

possible Several wafers were required to perform all of the experiments thus a direct

comparison of treatments is not always appropriate as a significant difference was

observed between the initial surface properties of each wafer

Treatments employed during this study include Standard Clean 1 (SC1) Standard

Clean 2 (SC2) a dilute HF solution and a 31 mixture of H2SO4H3PO4 These samples

were analyzed by means of wetting angle measurements AFM measurements and XPS

analyses A more detailed description of the treatment procedures and methods of

analysis are included below

41 Chemical Treatments

Several chemical treatments were employed during this experiment Combinations of

the following chemical solutions were used Dilute hydrofluoric acid (HF) phosphoric

acid (H3PO4) sulfuric acid (H2SO4) Standard Clean 1 (SC1) and Standard Clean 2

(SC2) In all cases the solutions were prepared in simple chemical beakers and either

42

heated on hot pates or left at room temperature Unless otherwise noted all treatments

lasted 10 minutes Following treatment samples were rinsed in deionized water and

stored in polycarbonate containers until the necessary measurements were taken

The HF solution was prepared by diluting 49 HF in deionized water (DIH2O) in the

ratio 1100 (HFDIH2O) and treatments were performed at room temperature A sulfuric-

phosphoric mixture was also used that is known to etch sapphire (see Chapter 2 for more

details) The mixture was combined in a ratio of 31 sulfuric acid to phosphoric acid and

treatments were preformed at approximately 100 degC SC1 was prepared in a ratio of

6151 (DIH2OH2O2NH4OH) Standard clean 2 was prepared in a ratio of 7151

(DIH2OH2O2HCL) Both Standard Clean treatments were preformed at approximately

80 degC In many cases a series of two or more of these chemicals were used on the same

sample In these situations an intermittent short rinse in deionized water was used

between treatments

In an effort to further explore the effect of these treatments some samples were

intentionally contaminated with a ldquospikedrdquo SC1 solution The spiking mixture contained

iron in 5 HNO3 in a weight ratio of 1 mgmL and 100 microL of this iron mixture was

added to 850 mL of SC1 Samples subject to this treatment were also treated for 10

minutes and followed by a short deionized water rinse

42 Measurement Techniques

Three primary measurement techniques were employed during this experiment

Wetting angle measurements were taken as an initial analysis of the substrate surface

43

energy These measurements were followed by XPS measurements as well as AFM

analyses to determine surface contamination and surface roughness respectively

Wetting angle or contact angle measurements were taken as a preliminary inspection

of a samplersquos surface following several of the treatments A Tantec testing apparatus was

used that included a suspended syringe over a movable stage A small quantity of water

was dropped on the surface of a treated sample and the drop was monitored with a high-

resolution camera The wetting angle can be measured by taking a line that intersects

both the apex of the water droplet and the edge of the dropletrsquos base The wetting angle is

the angle between this line and the surface of the sapphire sample

Depending on the surface energy of a sample a water droplet placed on the surface

would either disperse widening the radius of the base of the droplet or would further

coalesce shortening the radius of the base A decrease in wetting angle implies an

increase in surface energy whereas an increase in wetting angle implies a decrease in

surface energy A change in surface energy can result form a change in surface

morphology or surface composition but wetting angle measurements alone are not

conclusive enough to make a judgment about the exact nature of any surface change

Atomic Force Microscopy (AFM) measurements were taken following several of the

samples as well A Digital Instruments Nanoscope 3000 AFM was used in trapping mode

to take 2 microm by 2 microm scans of several different areas of the same sample These

measurements provided a visual representation of the surface of the treated samples as

well as a measurement of the RMS roughness of the surface

44

Xray-Photoelectron Spectroscopy was used to measure the surface contamination A

Kratos Axis Ultra was used in hybrid mode to take 1 mm by 15 mm scans of the

samples Monochromatic Al-k alpha x rays were used Samples were placed normal to

the analyzer entrance and the unscanned area was covered with aluminum foil A 5mm

by 5mm window was cut in the foil for analysis Measurements were taken at low

pressure (~8-10 torr)

The results from each of these tests are presented in the following chapter More detail

regarding the number of sample treated and the exact combination of treatments used can

be found in that section as well A discussion of the relevance of these results is provided

alongside the summarized data

45

5 EXPERIMENTAL RESULTS AND DISCUSSION

Several sapphire samples were treated with various combinations of SC1 SC2 dilute

HF and a 31 mixture of H2SO4H3PO4 The effect of these treatments was analyzed by

means of wetting angle measurements an AFM analysis and an XPS analysis In many

cases sapphire was proven to be a resilient material as the treatments had minimal effect

on the surface composition of the sapphire samples Yet in some other cases these

treatments caused enough of a change to warrant further study particularly in the

treatment of organic contamination

51 Wetting Angle Measurements

Wetting angle measurements were taken as a preliminary means for analyzing the

effect of chemical treatments on the sapphire surface Four samples were treated with

various combinations of cleaning solutions As received the wetting angle for a drop of

water on the sapphire surface was over 50deg indicating a relatively hydrophobic surface at

the start In all cases the wetting angle of the surface decreased from the no treatment

case implying an increase in surface energy as a result of the treatment

Following a 10 minute treatment with HF the wetting angle decreased to

approximately 15deg falling just short of the value one would expect for a completely

hydrophilic surface Another 10 minute treatment with SC2 produced similar results

decreasing the wetting angle to about 20deg In contrast a treatment for 10 minutes with

SC1 produced an almost immeasurable wetting angle around 7deg indicating fully

46

hydrophilic behavior Finally treatment with SC1 then HF then SC2 was preformed

with each treatment again lasting for 10 minutes This closely replicates a standard

cleaning sequence used for preparing silicon wafers for device fabrication In this case

the wetting angle results were similar to the SC1 treatment however the surface did not

appear to be as strongly hydrophilic It was clear that the sapphire surface responded to

each of these treatments but at this stage it was not possible to determine the cause of the

change without further experimentation

A further test monitored the wetting angle of the surface after treatment with HF for 1

minute 5 minutes and then finally 10 minutes The change in wetting angle was not

linear indicating a similarly nonlinear change in surface energy All wetting angle

measurements are summarized in Figure 12 Figure 12 (a) compares the wetting angle of

the sapphire surface following each of the 10 minute treatments and Figure 12 (b)

illustrates the change in wetting angle over time following several HF treatments

In the case of silicon wafers it is known that surface wetting properties are primarily

controlled by organic contamination83 SC1 is known to be a strong oxidizing agent and

is effective at removing organic contamination from the surface of silicon During these

experiments the response of the sapphire surface to treatments with SC1 produced results

that would similarly indicate organic contamination to be the primary controller of the

surface wetting properties of sapphire

In an effort to further verify the effect of organic contamination on the wetting

properties of sapphire some of the treated samples were stored in a polycarbonate storage

container and exposed to ambient air over a period of several days The wetting angle of

47

(a)

(b)

Figure 12 Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time

48

the sapphire surface was periodically monitored during this time The results of these

measurements are presented in Figure 13 for two samples One sample was etched twice

with HF for a total of 10 minutes and then stored The second sample was etched with HF

a total of three times for 30 minutes The samples were rinsed in deionized water between

each etch The wetting angle increased quickly from a low of 15deg to a high around 35deg

over the first five days before leveling out It should be noted that due to lack of substrate

samples the same sample was measured each time and then rinsed and returned to the

storage container This may explain why the wetting angle never returned to the initial

wetting angle value over 50deg

Based on the change in wetting angle following each treatment and the subsequent

reversal of wetting angle behavior over time it is likely that organic contamination plays

an important role in determining the wetting properties of a sapphire surface

Furthermore it is possible that SC1 a treatment known to reduce organic contamination

on silicon surfaces can be an effective treatment for organic contamination on sapphire

surfaces

52 Surface Roughness Measurements

In an effort to better understand the impact of various chemical treatments on the

sapphire surface an AFM analysis was conducted on several samples The RMS

roughness of each sample was determined and an AFM image of the surface was

obtained The RMS roughness for one set of samples is summarized in Figure 14 (a)

which contains data for samples treated with SC1 SC2 HF and the combination of all

49

Figure 13 Wetting angle over time for samples treated with HF for varying lengths of time

50

(a)

(b)

Figure 14 RMS roughness of wafer samples treated with various combinations of SC1 SC2 and (a) HF or (b) 31 H2SO4H3PO4

51

three A second set of samples is summarized in Figure 14 (b) This set of data contains

samples using a similar series of treatments that replaces HF with a 31 mixture of

sulfuric acid and phosphoric acid This solution is a known etchant of sapphire It should

be noted that the two sets of samples were taken from different wafers and the initial

roughness of each wafer differed significantly This may make direct comparisons of the

two sets of data inappropriate

From the available results it appears that SC1 and SC2 both tended to smooth the

surface of the sapphire samples where as both HF and the sulfuric-phosphoric mixture

tended to roughen the surface The sulfuric-phosphoric mixture is a known etchant of

sapphire but its etch rate is known to be relatively slow It is possible that due to the

relatively short treatment time used for these experiments an uneven surface etch was

achieved accounting for the increase in surface roughness HF on the other hand is not

known to etch sapphire and for all intensive purposes sapphire is considered to be

resistant to HF treatments In these experiments HF roughened the surface significantly

The surface roughness results combined with the wetting angle results seem to indicate a

change in surface morphology due to a change in surface energy

A select few AFM images are presented in Figure 15 Part (a) showcases an as

received wafer with clear surface scratches Part (b) illustrates a wafer treated with HF

where the initial surface features appear to be magnified by the treatment A comparison

of these two images may suggest a selective interaction of the sapphire surface with the

HF treatment as the previously discussed presence of organic contamination is blocking

portions of the sapphire surface from contact with the HF treatment It is also possible

that the crystallographic surface orientation is influencing the effect of the HF treatment

52

(a) (b)

(c) (d)

Figure 15 AFM Images of the sapphire surface after (a) no treatment (b) treatment with HF (c) treatment with SC1 (d) treatment with H2SO4H3PO4

53

Part (c) and part (d) of Figure 15 illustrate samples treated with SC1 and the sulfuric-

phosphoric mixture respectively A smooth surface was seen visually following treatment

with SC1 and this result was confirmed with surface roughness measurements Similar

results were obtained with SC2 Interestingly in the case of a HF treatment followed by a

SC2 treatment the surface roughness was clearly determined by the final SC2 treatment

On the other hand a sapphire surface etched with a sulfuric-phosphoric solution and then

subject to a SC2 treatment was further roughened This appears to further confirm the

fact that treatments of HF modify the surface morphology through a redistribution of

surface energy where as the sulfuric-phosphoric mixture clearly removes portions of the

surface The spotted image seen in part (d) appears to confirm the slow uneven etching

of the sapphire surface

In any case it is clear that HF treatments modify the surface morphology as a result of

a change in surface energy whereas the sulfuric-phosphoric mixture is an effective

etchant of sapphire The wetting angle data in combination with the AFM results indicate

that a cleaning series involving SC1 and the sulfuric-phosphoric etching solution shows

potential as an effective cleaning method However more work is need and in particular

longer etch times need to be explored as it appears the relatively short etch time used in

this experiment may have done more harm then good to the sapphire samples It is also

possible that a simple treatment with SC1 may prove to be the most effective surface

treatment for sapphire substrates SC1 treatments caused the largest change in surface

energy and created the smoothest substrate surfaces

54

53 Atomic Composition Analysis

In an effort to better understand the response of sapphire surfaces to the chemical

treatments used during these experiments an analysis of the atomic composition of

several sapphire samples was conducted The wafer supplier had two independent Total

Reflection X-ray Florescence (TXRF) analyses conducted on separate samples These

results included in Table 2 show significant calcium contamination as well as some

contamination from various alkali metals To a lesser degree other metallic

contamination like iron and zinc appear to be an issue

In an attempt to verify the suppliers results XPS survey scans were taken of several

samples treated with some combination of SC1 SC2 HF andor H2SO4H3PO4 (31) An

example of one of these scans is presented in Figure 16 It was hoped that these scans

could reveal more about the ability of certain treatments to remove particle contamination

from the sapphire surfaces tested In many cases trace amounts of elements were

observed near the detection limits of the available XPS process and as a result it is hard

to make definitive conclusions about the ability of certain chemistries to remove

contamination However some general trends were apparent that warrant further

investigation All XPS results are presented in Table 3 in terms of the atomic percentage

of the elements present on the surface

In general the surface treatments employed did very little to change the surface

composition Oxygen and Aluminum dominated the surface chemistry as expected There

was significant carbon contamination in all cases and all of the treatments tended to

reduce that contamination to some degree This seems to confirm the fact that organic

contamination may play an important role in determining surface properties but it should

55

Table 2 TXRF results from two separate sources (a) and (b) Particle concentrations are given in terms of the atomscm2 count

Element Atomscm2

Ca 11 x 1014

Zn 32 x 1013

Na 14 x 10 13

Fe 38 x 1012

K 000

Ni 000

Cu 000

Cr 000

(a)

Element Atomscm2

Ca 240 x 1012

Zn 96 x 1010

Na 197 x 1012

Fe 13 x 1011

K 34 x 1011

Cr 6 x 109

Ni 12 x 1010

Cu 16 x 1010

(b)

56

Figure 16 Counts per second vs binding energy for an entire XPS survey scan of an as-received wafer sample

57

Table 3 Atomic percentage of various elements on the sapphire surface following treatment

Sample Al O C Si Fe P Ca Na N S No Treatment 3208 5236 1317 093 002 0 027 021 020 061

HF 3438 5677 774 104 004 0 002 0 0 0 SC1 3384 5673 820 077 005 0 008 0 0 0 SC2 3409 5685 783 114 004 0 0 0 0 0

SC1+HF+SC2 3546 5980 379 073 017 0 0 0 0 0 H2SO4H3PO4 (31) 2656 6200 282 071 0 264 0 032 203 287 SC1+H2SO4H3PO4 3199 6107 354 092 0 080 003 0 054 106

SC1+H2SO4H3PO4+SC2 3269 6049 376 112 0 090 0 0 023 075

58

be noted that there was some storage time in between treatment and analysis that could

have allowed carbon to return to the surface Generally it appears that organic

contamination may be a concern in the area of sapphire surface cleaning

A relatively small amount of metallic contamination was present on the surface most

notably some alkali metals and small amounts of iron The presence of elements like

sodium calcium and iron seem to validate some of the findings from the TXRF studies

Unfortunately the concentrations of these metals are so near the detection limits of the

XPS process used it makes a direct comparison of the two sets of data difficult It is

probably safe to assume that various metallic contaminants will exist in small amounts on

sapphire surfaces if the surface is exposed to such contamination A more exacting study

must be used to determine the most problematic particle contamination for these cases

After comparing some trends from the XPS analysis it may appear that treatments

involving the sulfuric-phosphoric etching solutions removed some of the metallic

contamination from the sapphire surface However most of the elements are present in

quantities that are so near the detection limits of the XPS process employed that it is

difficult to make this conclusion confidently It should also be noted that the sulfuric-

phosphoric etching solution appeared to leave trace amounts of sulfur and phosphorus on

the surface following those treatments

In an effort to take a closer look at the behavior of some of these treatments a few

samples were intentionally contaminated with iron during an SC1 treatment and then

subsequently treated with the sulfuric-phosphoric etching solution andor SC2 XPS

survey scans were again completed in an effort to examine the effectiveness of these

59

treatments in dealing with iron contamination Unfortunately this ldquospikingrdquo process

appeared to have no effect on the sapphire surface as a focus on the binding energy range

for iron revels no significant change to the iron levels following contamination

Additionally all treatments following the contamination process showed no signs of

decreasing the initial iron concentration on the surface The XPS counts per second

versus binding energy scans are illustrated in Figure 17 for the range relevant to iron It is

clear that a relatively small amount of iron is present initially but it is also clear that this

concentration does not change significantly after any of the treatments It is also possible

that the detection limits of the XPS process played a role in the accuracy of these results

In general metallic and carbon contamination both appear to be an issue for sapphire

surface contamination The presence of calcium sodium and other alkali metals was

apparent throughout the atomic composition analysis To a lesser extent the presence of

other metals like iron may be an issue of concern The XPS process verified the presence

of carbon which in turn implies the presence of organic contamination on the surface

The presence of particle contamination on sapphire surfaces needs to be explored in

further detail with a more exacting process in order to determine the best course of action

for removing this type of contamination

60

(a) (b)

(c) (d)

(e)

Figure 17 Counts per second versus binding energy for XPS scans of samples treated with (a) no treatment (b) spiked SC1 (c) spiked SC1 and a sulfuricphosphoric clean (d) Spiked SC1 and a SC2 clean (e) Spiked SC1 a sulfuric-phosphoric clean and SC2 clean

61

6 SUMMARY

Sapphire is an important material for a few niche applications in the semiconductor

device industry In the past technical limitations with various fabrication techniques have

made the initial quality of sapphire substrates less of a concern during processing There

has been substantial work done to surpass many of these limitations and as new

processing techniques become more refined the initial quality of sapphire substrates will

become more of a concern As such surface cleaning and processing will need to be

explored in further detail

This thesis has begun the process of exploring wet sapphire surface processing in

more detail The process began with the same approaches used for cleaning devices based

on the traditional workhorse of the industry silicon The chief focus of the study was to

explore surface contamination while monitoring changes in surface properties resulting

from treatment with industry standard cleaning chemistries including dilute HF SC1

SC2 and a sulfuric-phosphoric etching mixture Analysis of treated samples was

preformed by means of wetting angle measurements AFM analyses and XPS

measurements

Treatments involving HF provided no benefit to the surface preparation as it is not

known to etch sapphire in the same manner it etches silicon In fact HF appeared to

modify the surface morphology of the sapphire samples in a negative manner Replacing

HF with a 31 mixture of H2SO4H3PO4 does in fact etch the surface and may prove more

beneficial However in this study relatively short etch times lead to an increase in surface

62

roughness and longer etch times should be explored Standard Clean treatments proved

useful particularly in the case of SC1rsquos treatment of organic contamination SC2

treatments must be explored further to see if it provides a unique benefit to the cleaning

sequences tested Ultimately a simple treatment with SC1 may be the most effective

approach for preparing sapphire substrates for device fabrication

In the cases tested the possible presence of organic contamination emerged as the

largest concern The presence and removal of organic contamination appeared to be the

cause of some of the largest changes in surface properties observed during this study On

the other hand metallic particle contamination proved more difficult to observe as small

trace amounts of these contaminants remained on the sapphire surface both before and

after treatment This stands to be a concern for sapphire surface cleaning because any

surface contamination remaining after treatment is likely to diffuse into material layers

deposited on the sapphire substrates during fabrication

The current state of the art in sapphire surface cleaning needs to be improved if

sapphire based semiconductor device technology is to keep improving at its current pace

There is room for more understanding in the area of wet chemical cleaning in particular

Treatments combining well known semiconductor cleaning agents (SC1 SC2) and a

known etchant of sapphire (31 H2SO4H3PO4) have shown promise as effective cleaning

agents and exploration of the effect of these chemistries on sapphire surface preparation

should continue

63

7 REFERENCES

1 D C Harris ldquoA peek into the history of sapphire crystal growthrdquo Proceedings of SPIE - The International Society for Optical Engineering 5078 (2003) pp 1-11 2 R E Newnham Properties of Materials Oxford Oxford University Press 2005 3 R A Johnson P R de la Houssaye C E Chang P Chen M E Wood G A Garcia I Lagnado and P M Asbeck ldquoAdvanced Thin-Film Silicon-on-Sapphire Technology Microwave Circuit Applicationsrdquo IEEE Transactions On Electron Devices 45 (1998) pp 1047-154 4 J Shan F Wang E Knoesel M Bonn M Wolf and T F Heinz ldquoTransient conductivity in wide band-gap solids probed by THz pump-probe measurementsrdquo Quantum Electronics and Laser Science Conference (2002) pp 187-188 5 R H Doremus ldquoDiffusion in Aluminardquo Applied Physics Reviews 100 (2006) pp 101301 1 ndash 16 6 EG Gontier-Moya J Bernardini and F Moya ldquoSilver and Platinum Diffusion in Alumina Single Crystalsrdquo Acta Materialia 49 (2001) pp 637-644 7 V Apostolopoulos LMB Hickey DA Sager JS Wilkinson ldquoDiffusion of Gallium in Sapphirerdquo Journal of the European Ceramic Soc 26 (2006) pp 2695-2698 8 J D Cawley and J W Halloran ldquoDopant Distribution in Nominally Yttrium-Doped Sapphirerdquo Journal American Ceramic Soc 69 (1986) pp C-195 ndash C-196 9 E G Moya F Moya B Lesage M K Loudjan and C Grattepain ldquoYttrium Diffusion in α-Alumina Single Crystalrdquo Journal of the European Ceramic Soc 18 (1998) pp 591-594 10 F Moya EG Moya D Juvegrave D Tregraveheux C Grattepain and M Aucouturier ldquoSIMS Study of Copper Diffusion into Bulk Aluminardquo Scripta Metallurgica 28 (1993) pp 343 ndash 348 11 E G Gontier-Moya G Erdegravelyi F Moya and K Freitag ldquoSolubility and Diffusion of Cobalt in Aluminardquo Philosophical Magazine A 81 11 (2001) pp 2665-2673 12 B Lesage and AM Huntz ldquoDiffusion of Chromium and Iron in Mono-Crystalline Alpha-Aluminardquo Scripta Metallurgica 14 (1980) pp 1143-1146 13 R C Jaeger Introduction to Microelectronic Fabrication Upper Saddle River Prentice Hall 2002 14 S Cristoloueanu ldquoSOI A Metamorphosis of Siliconrdquo IEE Circuits and Devices Magazine 15 1 (1999) pp 26-32

64

15 J Roig D Flores S Hidalgo J Rebollo and J Millan ldquoThin-film silicon-on-sapphire LDMOS structures for RF power amplifier applicationsrdquo Microelectronics Journal 35 (2004) pp 291-297 16 J L Peel M D Barry and L G Green ldquoEffects of Defects and Impurities in Starting Material on radiation hardness of CMOSSOS devicesrdquo IEE Transactions on Nuclear Science NS23 6 (1976) pp 1594-1597 17 M Roser S R Clayton P R de la Houssaye and G A Garcia ldquoHigh-mobility fully-depleted thin film SOS MOSFETrsquosrdquo presented at 50th Device Res Conf Cambridge MA June 1992 18 H M Manasevit and WI Simpson ldquoSingle-Crystal Silicon on a Sapphire Substraterdquo Journal Applied Physics 35 4 (1964) pp 1349-1351 19 H Y Cho C J Park ldquoHydrogenation Effect on silicon on sapphire grown by rapid thermal chemical vapor depositionrdquo Physica E 16 (2003) pp 489-494 20 T Nakamura H Matsuhashi and Y Nagatomo ldquoSilicon on Sapphire (SOS) Device Technologyrdquo Oki Tech Review 71 4 (2004) pp 66-69 21 M S Abrahams C J Buiocchi J F Corboy and G W Cullen ldquoMisfit Dislocations in heteroepitaxial Si on Sapphirerdquo Applied Physics Letters 28 5 (1976) pp 275-277 22 G A Garcia R E Reedy and M L Burgener ldquoHigh-Quality CMOS in Thin (100 nm) Silicon on Sapphirerdquo IEE Electron Device Letters 9 1 (1988) pp 32-24 23S S Lau S Matteson JW Mayer P Revesz J Gyulai J Roth T W Sigmon and T Cass ldquoImprovement of Crystalline Quality of Epitaxial Si Layers by Ion-implantation Techniquesrdquo Applied Physics Letters 34 1 (1979) pp 76-78 24 I Golecki H L Glass and G Kinoshita ldquoReduction in Crystallographic Surface-defects and Strain in 02-microm Thick Silicon-on-Sapphire Films by Repetitive Implantation and Solid-Phase Epitaxyrdquo Applied Physics Letters 40 8 (1982) pp 670-672 25 Q Wang Y Zan J Wang Y Yu ldquoComparison of properties of solid phase epitaxial silicon on sapphire films recrystallized by rapid thermal annealing and furnace annealingrdquo Materials Science and Engineering B29 (1995) pp 43-46 26 J Pretet S Cristoloveanu N Hefyene M Matsui Y Moriyasu and Y Kawakami ldquoElectrical evaluation of innovating processes for improving SOS materialsrdquo Microelectronic Engineering 59 ( 2001) pp 443-448 27 R E Reedy T W Sigmon and L A Christel ldquoSuppression of Al Outdiffusion in Implantation Amorphized and Recrystallized Silicon on Sapphire Filmsrdquo Applied Physics Letters 42 8 (1983) 707-709 28 J G Fiorenza D A Antoniadis and J A del Alamo ldquoRF Power LDMOSFET on SOIrdquo IEE Electron Device Letters 22 3 (2001) pp 139-141

65

29 J Jeong S Pornpromlikit P M Asbeck and D Kelly ldquoA 20 dBm Linear RF Power Amplifier Using Stacked Silicon-on-Sapphire MOSFETsrdquo IEEE Microwave and Wireless Components Letters 16 12 (2006) pp 684-686 30 A Chamseddine J W Haslett and M Okoniewski1 ldquoCMOS Silicon-on-Sapphire RF Tunable Matching Networksrdquo EURASIP Journal on Wireless Communications and Networking 2006 (2006) pp 1-11 31 PF Chen RA Johnson M Wetzel PR de la Houssaye GA Garcia PM Asbeck and I Lagnado ldquoSilicon-on-Sapphire MOSFET Distributed Amplifier with Coplanar Waveguide Matchingrdquo 1998 IEEE Radio Frequency Integrated Circuits Symposium (1998) pp 161-164 32 I Lagnado PR de la Houssaye and WB Dubbelday ldquoRF Systems Based on Silicon-on-Sapphire Technologyrdquo 2000 IEEE International SOI Conference (2000) pp 32-33 33 RA Johnson PR de la Houssaye ME Wood GA Garcia CE Chang PM Asbeck and I Lagnado ldquoSilicon-on-sapphire MOSFET Transmit Receive Switch for L and S Band Transceiver Applicationsrdquo Electronics Letters 33 15 (1997) pp 1324-1326 34 A B Apsel and A G Andreou ldquoA Low-Power Silicon on Sapphire CMOS Optoelectronic Receiver Using Low- and High-Threshold Devicesrdquo IEEE Trans On Circuits and Systems ndash I Regular Papers 52 2 (2005) pp 253-261 35 E Culurciello PO Pouliquen AG Andreou K Strohbehn and S Jaskulek ldquoMonolithic digital galvanic isolation buffer fabricated in silicon on sapphire CMOSrdquo Electronics Letters 41 9 (2005) 36 E Culurciello PO Pouliquen and AG Andreou ldquoIsolation charge pump fabricated in silicon on CMOS technologyrdquo Electronics Letters 41 10 (2005) 37 Z Fu P Weerakoon and E Culurciello ldquoNano-Watt silicon-on-sapphire ADC using 2C-1C capacitor chainrdquo Electronics Letters 42 6 (2006) 38 SF Yoon XB Li and MY Kong ldquoSome properties of gallium nitride films grown on (0 0 0 1) oriented sapphire substrates by gas source molecular beam epitaxyrdquo Journal of Crystal Growth 180 (1997) pp 27-33 39 R D Vispute V Talyansky Z Trajanovic S Choopun M Downes R P Sharma T Venkatesanc M C Woods K A Jones and A A Iliadis ldquoHigh quality crystalline ZnO buffer layers on sapphire (001) by pulsed laser deposition for III ndash V nitridesrdquo Applied Physics Letters 70 20 (1997) pp 2735-2737 40 S Yoshida S Misawa and S Gonda ldquoImprovement of the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN films by using AlN-coated Sapphire Substratesrdquo Applied Physics Letters 42 5 (1983) pp 427-429

66

41 H Amano H Sawaki I Akasaki and Y Toyoda ldquoMetalorganic Vapor Phase Epitaxial Growth of a High Quality GaN film using an AlN Buffer Layerrdquo Applied Physics Letters 48 5 (1985) pp 353-355 42 C Heinlein J Grepstad T Berge H Riechert ldquoPreconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridationrdquo Applied Physics Letters 71 3 (1997) 341-343 43 K Uchida A Watanabe F Yano M Kouguchi T Tanaka and S Minagawa ldquoNitridation process of sapphire substrate surface and its effect on the growth of GaNrdquo Journal of Applied Physics 79 7 (1996) pp 3487-3491 44 I Akasaki H Amano Y Koide K Hiramatsu and N Sawaki ldquoEffects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga1-xAlxN (0lt x 04) Films Grown on Sapphire Substrate by MOVPErdquo Journal of Crystal Growth 98 (1989) pp 209-219 45 D Doppalapudi E Iliopoulos S N Basu and T D Moustakas ldquoEpitaxial growth of gallium nitride thin films on A-plane sapphire by molecular beam epitaxyrdquo Journal of Applied Physics 85 7 (1999) pp 3582-3589 46 K Lorenz M Gonsalves W Kim V Narayanan and S Mahajan ldquoComparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor depositionrdquo Applied Physics Letters 77 21 3391-3393 47 V Narayanan K Lorenz W Kim and S Mahajan ldquoGallium nitride epitaxy on (0001) sapphirerdquo Philosophical Magazine A 82 5 (2002) pp 885-912 48 E N Vigdorovich A A Arendarenko R V Kharlamov and Y N Sveshnikov ldquoOn the nucleation model for gallium nitride films grown on sapphirerdquo Physica Status Solidi 2 4 (2005) pp 1280ndash1283 49 A D William and TD Moustakas ldquoFormation of large-area freestanding gallium nitride substrates by natural stress-induced separation of GaN and sapphirerdquo Journal of Crystal Growth 300 (2007) pp 37ndash41 50 SN Mohammad A A Salvador and H Morkoc ldquoEmerging Gallium Nitride Based Devicesrdquo Proceedings of the IEEE 83 10 (1995) pp 1306-1355 51 S J Chang Y C Lin Y K Su C S Chang T C Wen S C Shei J C Ke C W Kuo S C Chen C H Liu ldquoNitride-based LEDs fabricated on patterned sapphire substratesrdquo Solid-State Electronics 47 (2003) pp 1539-1542 52 S Nakamura T Mukai and M Senoh ldquoHigh-brightness InGaNAIGaN double-heterostructure blue-green-light-emitting diodesrdquo Journal Applied Physics 76 12 (1994) pp 8189-8191

67

53 S Nakamura T Mukai and M Senoh ldquoCandela-class high-brightness InGaNAIGaN double-heterostructure blue-light-emitting diodesrdquo Applied Physics Letters 64 13 (1994) pp 1687-1689 54 M Kneissl D W Treat M Teepe N Miyashita and N M Johnson ldquoUltraviolet InAlGaN multiple-quantum-well laser diodesrdquo Physica Status Solidi 200 1 (2003) pp 118-121 55 S Nakamura M Senoh S Nagahama N Iwasa T Yamada T Matsushita H Kiyoku Y Sugimoto T Kozaki H Umemoto M Sano and K Chocho ldquoInGaNGaNAlGaN-based laser diodes with cleaved facets grown on GaN substratesrdquo Applied Physics Letters 73 6 (1998) pp 832-834 56 A Osinsky J W Dong M Z Kauser B Hertog A M Dabiran P P Chow S J Pearton O Lopatiuk and L Chernyak ldquoMgZnO AlGaN heterostructure light-emitting diodesrdquo Applied Physics Letters 85 19 (2004) pp 4272-4274 57 K Fujiwara A Ishii T Ebisuzaki T Abe and K Ando ldquoTheoretical investigation on the structural properties of ZnO grown on sapphirerdquo e-Journal of Surface Science and Nanotechnology 4 (2006) pp 544-547 58 DC Look ldquoRecent advances in ZnO materials and devicesrdquo Materials Science and Engineering B80 (2001) pp 383-387 59 Y Chen D M Bagnall H Koh K Park K Hiraga Z Zhu and T Yao ldquoPlasma assisted molecular beam epitaxy of ZnO on c -plane sapphire Growth and characterizationrdquo Journal of Applied Physics 84 7 (1998) pp 3912-3918 60 Y Liu C R Gorla S Liang N Emanetoglu Y Lu H Shen and M Wraback ldquoUltraviolet Detectors Based on Epitaxial ZnO Films Grown by MOCVDrdquo Journal of Electronic Materials 29 1 (2000) pp 69-74 61 A Ohtomo M Kawasaki T Koida K Masubuchi H Koinuma Y Sakurai Y Yoshida T Yasuda and Y Segawa ldquoMgxZn1-xO as a II ndash VI widegap semiconductor alloyrdquo Applied Physics Letters 72 19 (1998) pp 2466-2468 62 J Wang L W Guo H Q Jia Y Wang Z G Xing W Li H Chen and J M Zhou ldquoFabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNrdquo Journal of The Electrochemical Society 153 3 (2006) pp C182-C1851113088 63 F Dwikusuma D Saulys and T F Kuecha ldquoStudy on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatmentsrdquo Journal of The Electrochemical Society 149 11 (2002) pp G603-G608 64 R G Vardiman ldquoThe Chemical Polishing and Etch Pitting of Sapphirerdquo Journal of the Electrochemical Society Solid State Science 118 11 (1971) pp 1804-1809

68

65 W J Alford and D L Stephens ldquoChemical Polishing and Etching Techniques for Al2O3 Single Crystalsrdquo Journal of The American Ceramic Society 46 4 (1963) 193-194 66 D S Wuu W K Wang K S Wen S C Huang S H Lin R H Horng Y S Yu and M H Pand ldquoFabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodesrdquo Journal of The Electrochemical Society 153 8 (2006) pp G765-G770 67 M M Faktor D G Fiddyment and G R Newns ldquoPreliminary Study of the Chemical Polishing of α-Corundum Surfaces with Vanadium Pentoxiderdquo Journal of the Electrochemical Society Electrochemical Science 114 4 (1967) pp 356-359 68 M Yoshimoto T Maeda T Ohnishi and H Koinuma ldquoAtomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabricationrdquo Applied Physics Letters 67 18 (1995) pp 2615-2617 69 LVan J Cousty C Lubin ldquoStep heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHVrdquo Surface Science 549 (2004) pp 157-164 70 P R Ribič G Bratina ldquoBehavior of the (0 0 0 1) surface of sapphire upon high-temperature annealingrdquo Surface Science 601 (2007) pp 44ndash49 71 H M Manasevit and F L Morritz ldquoGas Phase Etching of Sapphire with Sulfur Fluoridesrdquo J Electrochem Soc Solid State Science 114 2 (1967) pp 204-207 72 H M Manasevit ldquoGas Phase Etching of Sapphire II Fluorinated Hydrocarbons rdquo Journal of the Electrochemical Society Solid State Science 115 4 (1968) pp 434-437 73 M Losurdo P Capezzuto and G Bruno ldquoPlasma cleaning and nitridation of sapphire (α-Al2O3) surfaces New evidence from in situ real time ellipsometryrdquo Journal of Applied Physics 88 4 (2000) pp 2138-2145 74 C Heinlein J Grepstad H Riechert R Averbeck ldquoPlasma preconditioning of sapphire substrate for GaN epitaxyrdquo Materials Science and Engineering B43 (1997) pp 253-257 75 M Seelmann-Eggebert H Zimmermann H Obloh and R Niebuhr ldquoPlasma cleaning and nitridation of sapphire substrates for AlxGa1-xN epitaxy as studied by x-ray photoelectron diffractionrdquo Journal of Vacuum Science and Technology A 16 4 (1998) pp 2008-2015 76 CH Jeong DW Kim JW Bae YJ Sung JS Kwak YJ Park GY Yeom ldquoDry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmasrdquo Materials Science and Engineering B93 (2002) 60-63 77 CH Jeong DW Kim HY Lee HS Kim YJ Sung GY Yeom ldquoSapphire etching with BCl3HBrAr plasmardquo Surface and Coatings Technology 171 (2003) pp 280ndash284

69

78 H W Gutsche and J W Moody ldquoPolishing of Sapphire with Colloidal Silicardquo J Electrochem Soc Solid-state Science and Technology 125 1 (1978) pp 136-138 79 L Wang K Zhang Z Song and S Feng ldquoChemical Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Waferrdquo Journal of The Electrochemical Society 154 3 (2007) pp H166-H169 80 H Zhua L A Tessarotob R Sabiac V A Greenhuta M Smith D E Niesza ldquoChemical mechanical polishing (CMP) anisotropy in sapphirerdquo Applied Surface Science 236 (2004) pp 120ndash130 81 M F Ehman D Medellin and F F Forrest ldquoMechanical Preparation of Sapphire Single-Crystal Surfaces by Vibratory Techniquesrdquo Metallography 9 (1976) pp 333-339 82 J Cui A Sun M Reshichkov F Yun A Baski and H Morkoccedil ldquoPreparation of Sapphire for High Quality III-Nitride Growthrdquo MRS Internet Journal of Nitride Semiconductor Research 5 7 (2000) 1-6 83 K Shanmugasundaram et al in Proceedings of the Eighth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing J Ruzyllo et al Editors PV 2003-26 2004 p 108