EFFECT OF FLUORINE AND HYDROGEN RADICAL SPECIES ON MODIFIED

OXIDIZED Ni(Pt)Si

Sneha Sen Gaddam

Thesis Prepared for the Degree of

MASTER OF SCIENCE

UNIVERSITY OF NORTH TEXAS

May 2010

APPROVED: Jeffry Kelber, Major Professor Steve Cooke, Committee Member Bill Acree, Chair of the Chemistry Department Michael Monticino, Dean of the Robert B. Toulouse School of Graduate Studies

  

Gaddam, Sneha Sen. Effect of fluorine and hydrogen radical species on

modified oxidized Ni(Pt)Si. Master of Science (Chemistry), May 2010, 30 pp., 14

illustrations, 22 references,.

NiSi is an attractive material in the production of CMOS devices. The

problem with the utilization of NiSi, is that there is no proper method of cleaning the

oxide on the surface. Sputtering is the most common method used for the cleaning,

but it has its own complications. Dry cleaning methods include the reactions with

radicals and these processes are not well understood and are the focus of the project.

Dissociated NF3 and NH3 were used as an alternative and XPS is the technique to

analyze the reactions of atomic fluorine and nitrogen with the oxide on the surface. A

thermal cracker was used to dissociate the NF3 and NH3 into NFx+F and NHx+H.

There was a formation of a NiF2 layer on top of the oxide and there was no

evidence of nitrogen on the surface indicating that the fluorine and hydrogen are the

reacting species. XPS spectra, however, indicate that the substrate SiO2 layer is not

removed by the dissociated NF3 and NiF2 growth process. The NiF2 over layer can be

reduced to metallic Ni by reacting with dissociated NH3 at room temperature. The

atomic hydrogen from dissociated ammonia reduces the NiF2 but it was determined

that the atomic hydrogen from the ammonia does not react with SiO2.

Copyright 2010

by

Sneha Sen Gaddam

 ii

ACKNOWLEDGEMENTS

First and foremost, I would like to express my gratitude to my advisor Dr.

Jeffry Kelber for his encouraging guidance and sincere support during the course of

my graduate study. I greatly appreciate his patience in teaching me about the

important aspects of research. His sincerity, acumen, fortitude and diligence are

qualities which I hope to mirror in my professional life. Special thanks to Ms. Sudha

Manandhar whose contributions are very significant to my thesis. She has been a great

mentor and I have learnt a lot under her guidance and acknowledge her patience in

teaching me the aspects and operation of XPS. I also thank my past and present group

members, Mr. Cameron Bjelkevig, Mr. Justin Wilks, Mr.Brian Copp, Ms.Sundari

Pokharel and Ms. Mi Zhou for their constant motivation and discussions. I would like

to extend my gratitude to my committee member Dr. Steve Cooke for his help in

finishing up my thesis. I would like to express my admiration towards his earnestness

and spirited approach in professional life. A very special thanks to Ms. Susan

Brockington for helping me finish out all the necessary requirements for my

graduation. I also thank my family members for their never ending love and

encouragement and my friends for being my support system and being my family

away from home.

 iii

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ..........................................................................................iii

LIST OF FIGURES ......................................................................................................vi

CHAPTERS

1. INTRODUCTION ................................................................................1

1.1 NiSi and Ni(Pt)Si: Source/Drain Contacts for Ultra –Small

Transistors ..................................................................................1

1.2 Cleaning of NiSi and Pt-NiSi Surfaces .....................................3

1.3 Previous Work ..........................................................................4

2. EXPERIMENTAL METHODS ............................................................5

2.1 Characterization by X-ray Photoelectron Spectroscopy (XPS) .5

2.2 Description of the UHV Chamber ............................................9

2.3 Description of the Thermal Cracker .......................................10

2.4 Sample Preparation .................................................................11

3. EXPERIMENTAL RESULTS ............................................................13

3.1 Formation of a Clean Mono Silicide Surface .........................13

3.2 Oxidation of the Monosilicide Surface ...................................16

3.3 Reaction of Dissociated NF3 on SiO2/ Ni(Pt)Si ......................17

3.4 Reaction of Dissociated NH3 with NiFx Overlayer ................23

 iv

 v

4. SUMMARY AND CONCLUSIONS .................................................26

4.1 Summary and Conclusions .....................................................26

4.2 Future Work ...........................................................................28

REFERENCES ............................................................................................................29

LIST OF FIGURES

Page

1.1 TEM image of As-doped 18nm Ni with two steps Ni silicidation ....................1

2.1 Pictorial representation of theory of XPS ..........................................................6

2.2 System for in situ O2 , NH3 and NF3 dosing (molecular and dissociated

species) and XPS studies of the surface modification on Ni(Pt)Si .................10

2.3. Mechanism and schematic representation of a thermal cracker ......................11

3.1(a) XPS spectra of Ni(Pt)Si (a): As received spectra for Ni(2p), Si(2p) and

corresponding surface ......................................................................................14

3.1(b) XPS spectrum for carbon and oxygen. As received spectra and the spectra for

the clean sample ..............................................................................................15

3.2 Graph of the intensity of oxygen vs exposure in langmuir .............................16

3.3 XPS spectra of Ni(2p), Pt(4f), Si(2p) clean surface (top trace) and the XPS

spectra of Ni(2p), Pt(4f), Si(2p) after the oxidation with molecular oxygen at

5x10-6 torr , 8x106 L ........................................................................................18

3.4 Evolution of the F(2p) XPS spectrum upon increasing exposure to dissociated

NF3 : 600L (bottom trace), after 1200L (middle trace) and after 1800L (top

trace) ................................................................................................................19

 vi

3.5 XPS of Ni spectra after oxidation with molecular oxygen of 79200L (a), after

atomic NF3 dose of 600L (b), after atomic NF3 dose for 1200L (c), after

atomic NF3 dose for 1800L (d) ........................................................................20

3.6 Graph of the total Si(2p) and O(1s) intensity vs exposure to fluorine in

langmuir oxygen(■) and silicon(▲) ...............................................................21

3.7 Graph of the total intensity vs exposure to fluorine in langmuir. nickel (■) and

platinum (▲) ...................................................................................................22

3.8 XPS spectra of F(1s), Ni(2p), O(1s) and Si(2p) after the reaction with NF3

with dissociated ammonia ...............................................................................25

4.1 Illustration of the reaction of fluorine and NH3 on oxidized Ni(Pt)Si. ...........27

 vii

CHAPTER 1

INTRODUCTION

This thesis is a compilation of the work done in the Surface Science Laboratory

at the University of North Texas under the guidance of Dr. Jeffry Kelber. This thesis

focuses on the interactions of free radical species with oxidized NiSi and Pt-doped

NiSi (Ni (Pt) Si) surfaces under controlled ultra-high vacuum (UHV) conditions. The

first chapter explains the scientific background for the research; the second chapter

contains an explanation of the experimental methods involved. The third chapter

contains results for the reaction of molecular vs. dissociated NF3 and NH3 with

oxidized Ni(Pt)Si. The fourth chapter contains a discussion of the results and possible

future avenues of research.

1.1. NiSi and Ni(Pt)Si: Source/Drain Contacts for Ultra–Small Transistors

NiSi has become an attractive material for source, drain and gate contacts in

advanced complementary metal-oxide semiconductor (CMOS) devices (Fig 1.1).

Fig 1.1: TEM image of As-doped 18nm Ni with two steps Ni silicidation 1.

 1

The reasons for this are that this silicide material has the lowest achievable

specific resistivity among silicides of interest (e.g., CoSi2, TiSi2) 2 and also consumes

less silicon than disilicides during formation, hence reduces the risk of junction

spiking due to substrate Si depletion. NiSi formation is a one step annealing process

and thus proves to be a simple silicidation process3. Additionally, there is no adverse

line-width dependence of sheet resistance, which is a big problem in TiS2 4. This is

due to the fact that the temperature for the Ni mono silicide formation is lower than

for TiSi2 and is controlled by the rapid diffusion of Ni, rather than Si 5. This diffusion

-controlled formation leads to smoother interfaces and surfaces, and helps in reducing

the voiding observed in Co silicided narrow poly-Si lines 6. NiSi is also known to be

compatible with Ge 6, thus making it a suitable contact material for Si-Ge transistors.

The mechanical stress of NiSi on silicon is also very small.

The major drawback of NiSi is its poor thermal stability, with formation of

disilicides and reduced conductivity occurring for temperatures above 750oC 4. The

introduction of the Pt metal into the NiSi (typically, ~ 5 wt %) improves its thermal

stability and reduces the nucleation of other silicide phases 4. In the course of device

fabrication, NiSi and Pt-NiSi can undergo oxidation due to various processes, like

plasma etching and oxygen exposure. Previous studies in our group8 have shown that

exposure of Pt-NiSi (or NiSi) to molecular oxygen or atmosphere yields a formation

of a thin (6-15 Å) SiO2 layer followed by passivation. In contrast, exposure to atomic

oxygen yields the kinetically-driven formation of a SiO2/metal silicate overlayer

without passivation. The presence of Pt retards but does not halt this process8.

 2

1.2. Cleaning of NiSi and Pt-NiSi Surfaces

The removal of oxide or silicate over layers prior to metallization of the Pt-

NiSi surface is important to ensure good interfacial nucleation and adhesion, as well

as low contact resistance.

Sputtering, thermal treatment and reactive cleaning are widely used techniques

to clean the surfaces of bulk crystalline samples. While a combination of Ar ion

sputtering and heating at elevated temperatures is proven to be an effective method in

the preparation of the surfaces, it has been observed that with multi-component

systems, a complication is introduced. Sputtering and heating cause the surface

composition to change with respect to the bulk composition. There is considerable

roughening of the surface and annealing in vacuum (not always practical in industrial

situations) is necessary to restore the surface structure and desired chemical

composition 9.

Wet cleaning used to clean ultra-large scale integrated circuits (USLI) has

faced difficulties: introduction of cleaning agents and de-ionized water in to fine gaps

and holes. Dry cleaning (e.g., plasma or vapor chemical cleaning) is also expected to

be applicable from the view point of chemical integration and other chemical vapor

deposition techniques. Dry cleaning methods like the HF vapor process have been

studied in detail for the removal of Si native oxides 10. Other dry etching methods like

NF3/H2O exhibit a low oxide etching rate of substrate and excessive etching of Si 10.

Partially dissociated NF3 and heated NH3 gases achieved a high etching rate, as it was

found that the etchant was directly generated by the thermally excited species 10. Such

dry cleaning methods involve the reaction of radical species with surfaces. Such

processes are not well understood, and are a focus of research in this project.

 3

1.3. Previous Work

Previous work in Dr. Kelber’s group8, 11 has characterized the evolution of the

NiSi and Ni(Pt)Si surface structure on exposure to molecular oxygen(O2) and to a

mixed flux of atomic and molecular oxygen (O+O2). It was observed that the

exposure to O2 resulted in a growth of a thin (6 Å-15 Å, depending on O2 pressure)

SiO2 layer that grows slowly and passivates the surface toward further oxidation.

Under high vacuum conditions at 300 K (PO2 ~ 10-6 torr), the limiting oxide average

thickness is 6-7 Å 8. In contrast the exposures to dissociated oxygen resulted in greatly

increased oxidation rate. The high rate of oxidation, combined with the ability of

atomic oxygen to diffuse through SiO2 12, resulting in the kinetically-driven oxidation

of the metal (Ni, Pt) The presence of Pt in the surface region retards, but does not stop

the oxidation process, for which passivation is not observed 8. Since plasma treatment

(and therefore the presence of atomic O) is common in processing of these materials,

the formation of thick metal silicate layers on NiSi or Ni(Pt)Si must be considered in

designing appropriate surface cleaning methods.

X-ray electron spectroscopy was also used to study the effect of atomic oxygen

and dissociated ammonia on ruthenium and RuO2 respectively. It was found that the

exposure to atomic oxygen results in the formation of a disordered multi layer

ruthenium oxide. The oxide formation proceeded until an average thickness of 68Å.

There was no evidence of higher oxides. The oxide was then exposed to dissociated

ammonia which resulted in the removal of lattice oxide and no nitrogen adsorption

was observed 11.

 4

CHAPTER 2

EXPERIMENTAL METHODS

2.1. Characterization by X-ray Photoelectron Spectroscopy (XPS)

2.1.1. Background

XPS (X-Ray photoelectron spectrometry) is a surface analytical technique used

to characterize the chemical composition and electronic structure of the surface region

of materials (typically, the outermost ~ 50 Å). The method involves the detection of

photoelectrons from (usually) the surface region of a material due to the absorption of

photons in the UV or x-ray region of the electromagnetic spectrum. XPS has its

origins in the investigations of the photoelectric effect discovered by Hertz. Innes in

1907 conducted experiments involving a Rontgen tube with a platinum cathode. In

1914, Rutherford stated the first equation of XPS which was later modified13. After

the World War II, Steinhardt and Serfass revived the idea of utilizing XPS as an

analytical tool 13. In 1954, the first X-ray electron spectrum of cleaved sodium

chloride was obtained. During the late 60’s and 70’s, the Siegbahn group (Uppsala)

went on to develop the field of electron spectroscopy for chemical analysis of

molecules and surfaces. Commercial instruments started to appear in 1969-70. Today,

XPS and related methods are commonly used to address a broad variety of issues in

academic and industrial research involving the electronic and chemical structure of

the surface regions of materials. This derives from the inherent surface sensitivity of

the technique under typical laboratory conditions, where the inelastic mean free paths

of photoelectrons are ~ 10 Å – 100 Å.

 5

2.1.2. Theory

The primary process of XPS is the ionization of an atom, typically from a core

level, and is illustrated schematically in Fig. 2.1.

If the energy of the excitation photon is known, the electron binding energy

(BE) of each of the emitted electrons can be determined by using the following

equation

------------------------------------ (1)

Where Ebinding is the binding energy of the electron, Ephoton is the energy of the

X-ray photons being used, Ekinetic is the kinetic energy of the electron as measured by

the instrument and φ is the work function of the spectrometer (not the

material).

KE = hν ‐ BE

Fig 2.1: Pictorial representation of theory of XPS.

 6

http://en.wikipedia.org/wiki/Electron_binding_energyhttp://en.wikipedia.org/wiki/Work_function

Note that addition of the work function term (eqn. 1) due to the photoelectron

entering the spectrometer detector is not shown here.

Since binding energies for core levels of different elements are typically well-

separated in energy, Ebinding serves as a “fingerprint” for elemental identification.

Typical values for Ebinding in XPS are between 20 eV and 1100 eV. Additionally,

Ebinding is sensitive to the oxidation state of the atom and other factors in the chemical

environment. Such “chemical shifts” (ΔEbinding) are typically ~ 0.1 eV – 10 eV,

depending on the element involved and other factors. Detailed discussions and

tabulations of chemical shifts for elements in specific compounds, and their

relationship to electronic structure, can be found in standard text books. The ability to

determine chemical shifts with high precision using modern electron energy analyzers

allows XPS to serve as a powerful tool for characterizing both surface elemental

composition and chemical bonding/electronic structure.

The surface sensitivity of XPS derives from the limited inelastic mean free

paths (IMFP) for photoelectrons in XPS. The IMFP is the average distance an electron

can travel in a solid before undergoing an inelastic collision and therefore losing any

chemical information (eqn. 1). IMFP values very somewhat with the density of the

material through which the photoelectron passes, but are in general a function of the

photoelectron kinetic energy. An approximate universal (element-independent)

relationship is given by:

 7

)]()()ln([ 22

ED

ECEE

E

p +−=

γβλ

MNE vp

ρ8.28= = Free‐Electron Plasmon Energy (eV)

Nv = Number of valence electrons per atom or moleculeρ = Materials density (g cm‐3)M = Atomic or Molecular Weight

‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ (2)

XPS spectra for the experiments discussed here were acquired with non-

monochromatic Mg Kα radiation (hv=1253 eV), and with the analyzer in constant

pass energy mode (22 eV). The electrons escape the surface at about 1000V, which is

very high for the detector to capture and analyze them. Pass energy is the retardation

voltage applied in an electron to slow it down and be captured by the detector. The

high retard ratios Ekin / Epass for XPS lines at high energies principally reduce the

intensities in XPS (and AES) spectroscopy. The pass energy should be as high as

possible, taking in to account the needed minimum energy resolution. Analysis of

XPS spectra was performed according to established methods 14, with commercially

available software (ESCATOOLS) and shirley background subtraction 13. The

analyzer energy scale was calibrated against sputter-cleaned polycrystalline Cu foil.

Sample peak intensities were taken as proportional to the peak area (after background

subtraction) and were determined by fitting to Gaussian–Lorentzian components. The

relative Ni/Si atomic ratio (XNi/XSi) was estimated using the intensities of the

Ni(2p3/2) and Si(2p) intensity at 100 eV according to:

XNi/XSi = FNiSi (INi/SNi)/ (ISi/SSi ) ------------------------------------- (3)

 8

where I and S represent, respectively, the corresponding intensities and atomic

sensitivity factors appropriate to this analyzer. The matrix correction factor, FNiSi is

given by

FNiSi ═ (λNidNi)/(λSidSi ) ----------------------------------------- (4)

Where λ represents the electron inelastic mean free path and d represents the atomic

number density.

2.2. Description of the UHV Chamber

All the experiments were carried out in a system consisting of an ultra-high

vacuum (UHV) analysis chamber for surface characterization, and a processing

chamber permitting the exposure of samples to reactive environments (for, e.g.

chemical vapor deposition) without compromising the UHV integrity or

instrumentation in the analytical chamber. This system is shown schematically in Fig.

2.2. The analytical chamber (turbo molecularly pumped, base pressure 1 x 10-10 torr)

consists of a 100 mm mean radius hemispherical analyzer (Vacuum Generators) and

non- monochromatic X-ray source (Physical Electronics)for acquisition of XPS

spectra, LEED (low energy electron diffraction) as well as an ion gun (Physical

Electronics) for sample bombardment/cleaning. The processing chamber (turbo

molecularly pumped, base pressure 5 x 10-8 torr) consists of a precursor dosing

apparatus, and a commercially available (Oxford Applied Research) thermal catalytic

cracker for the generation of free radical species compatible with UHV operating

conditions. Sample temperature in the main chamber is controllable between 150 and

1200 K by a combination of liquid nitrogen cooling and resistive heating. Sample

transport between chambers occurs via magnetically-coupled feed-thru under UHV

 9

conditions, with chamber separation accomplished by a differentially pumped Teflon

seal when the sample is in the main chamber. Sample temperature is monitored with a

type-k thermocouple attached to the sample holder. The pressure in each chamber is

monitored by nude ion gauges.

LEED

Turbo 1P = 10‐8 Torr

Turbo 2P ~ 5 x 10‐10 Torr

Thermal Cracker

Hemispherical Analyzer (XPS)

View port

Fig 2.2: System for in situ O2 , NH3 and NF3 dosing (molecular and dissociated

species) and XPS studies of the surface modification on Ni(Pt)Si.

2.3. Description of the Thermal Cracker

The TC 50 (Fig. 2.3) is a thermal gas cracker which is designed for use in

UHV. It has the capability to dissociate molecular gases like hydrogen, ammonia and

generating a stream of highly reactive atomic species. The cracker, from Oxford

Applied research employs an iridium tube which is heated up to 1273o K by electron

beam induced heating. The cracker tube is directly connected to the gas lines, thus

 10

reducing the gas load and providing a large surface area and a long path length. The

head is directly water cooled to keep the temperature at ambient and to minimize out

gassing even at maximum power. The gases are sent through the feed and the heated

tube and are then converted to atoms from molecules by catalytic dissociation at the Ir

surface and subsequent desorption as radical species. Dissociation efficiencies vary

with species and pressure, but typically were in the range of 40%-60% for conditions

typical of these experiments.

Gas in

Mixed atomic and molecular flux

H.

Water

Capillary Tube

Filament

H2

Fig 2.3: Mechanism and schematic representation of a thermal cracker.

2.4. Sample Preparation

The sample, 1 cm x 1 cm, was scored from a Ni(Pt)Si wafer (approximately

200Å thick, 5% at. wt. Pt deposited on a Si(100) film) provided by

STMicroelectronics-Crolles. Resistivity measurements indicated that the Si substrate

was As-doped with a density of 5x 1015 cm3. The sample as received was mounted on

 11

a Ta foil which was in turn spot-welded to Cu leads for resistive heating/liquid

nitrogen cooling. The sample was introduced into UHV through the intro chamber

and then transferred in to the main chamber.

The sample was then cleaned in the main chamber to remove any oxide or

carbon over layer present due to the exposure to atmosphere. Sample cleaning and

preparation were accomplished by a combination of Ar+ bombardment (1 keV) and

annealing to about 600 K. XPS was taken to make sure that there is minimum amount

of carbon and SiO2 on the surface. Suitable ion bombardment and annealing in UHV

results in an oxide and carbon-free sample, with a Pt-enriched surface region 8.

 12

CHAPTER 3

EXPERIMENT RESULTS

3.1. Formation of a Clean Monosilicide Surface

The Ni(Pt)Si sample was first mounted in to the introduction chamber and then

transferred in to the main chamber. The sample was cleaned in cycles of Ar+

sputtering and annealing up to 800K for about 15 minutes. Ni (2p), Si (2p) and Pt (4f)

XPS spectra are displayed in Fig. 3.1(a) for the as-received surface and the samples

after Ar+ sputtering and annealing (clean sample). Corresponding C (1s) and O(1s)

spectra are displayed in Fig. 3.1(b). The analysis of the XPS spectra acquired for the

as-received sample indicates the presence of an oxide thickness of 15(2) Å. The Ni

(2p) spectrum after the sputter/anneal cycle shifts to a lower binding energy

corresponding to the Ni in Pt-NiSi binding energy.8. The Si(2p) spectrum for the as-

received sample displays a feature at 103eV and another near 100eV. XPS studies of

Pt-free NiSi report the Si (2p) binding energy at 98.85 and 98.75 for Ni2Si and NiSi

respectively15 and binding energies ~100.2 for Pt silicides.16. We therefore attribute

the Si (2p) peak at around 100eV to Si due to both Pt and Ni silicides.8. After the

sputter anneal, there is a reduction on the peak intensities of carbon and oxygen along

with decrease in the Si (2p) peak at 103eV. This accounts for the removal of the oxide

as well as the carbon on the surface to form a clean surface on the sample. The

persistence of the carbon and oxygen indicates that the Ni(Pt)Si sample is reactive in

the UHV conditions.

 13

880 872 864 856 848-1000

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80 76 72 68

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Clean

As recieved

880 872 864 856 848

-200

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100

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80 76 72 68-50

050

100150200250300

Ni(Pt)Si (854.0) Pt and NiSi (103.1)

SiO2 (103.1)

PtSi( 73.2)

Binding energy(eV)

Inte

nsity

Figure 3.1 XPS spectra of Ni(Pt)Si (a): As received spectra for Ni(2p), Si(2p), Pt(4f)

(bottom trace) and corresponding spectra for the clean surface (after sputter and

anneal) (top trace).

 14

-292 -288 -284 -280-40-20

020406080

-537 -534 -531 -528-50

050

100150200

-292 -288 -284 -280-500

50100150200250300

-537 -534 -531 -528-2000

200400600800

10001200

As received

Clean

C(1s) O(1s)

Fig 3.1(b): XPS spectrum for carbon and oxygen. As received spectra (bottom trace)

and the spectra for the clean sample (top trace).

After annealing, XPS indicated a surface stoichiometry of 1:1 for Si relative to

total (Ni+ Pt). The Ni(2p) peak maximum for the clean surface (Fig. 2.1a, top left)

was 854.0 eV, consistent with a Ni/Si ratio of 1:1 4.

 15

3.2. Oxidation of the Monosilicide Surface

The clean surface was exposed to molecular oxygen at 5x10-6 torr for varying

lengths of time to form a layer of SiO2 on the surface. Increase in O(1s) intensity

(normalized by total Ni(2p) intensity) is shown in Fig. 3.2 The O(1s) intensity

increases monotonically up to a total oxygen exposure of 1x10-6 Langmuir (L) (1 L =

10-6 Torr-sec). The absence of further surface oxidation beyond this point indicates

that a continuous SiO2 film inhibits further O2 chemisorptions, dissociation and

surface oxidation.

0 2 4 6 8

600

900

1200

1500

1800

2100

2400

Inte

nsity

Exposure x105(L)

Molecular Oxygen Uptake

O(1s)

Fig 3.2: Graph of the intensity of oxygen vs exposure in langmuir; P (O2) = 5x10-6

torr T = 298K.

At this point the average SiO2 thickness, calculated from relative Si and O

intensities is about 6-8 Å.

 16

During the oxidation process, the Ni(2p), , and Pt(4f) peaks display a gradual

decrease in the intensity with a negligible change in the binding energies with

increasing exposure to O2 consistent with passivation, no further decrease in Ni(2p) or

Pt(4f) intensity is observed for O2 exposure > 8x106L. The decrease in core level

intensities is due to SiO2 attenuation of photoelectron signals from the substrate, while

the absence of any binding energy shifts indicates that the amount of Si transported

into the oxide overlayer during the oxidation process is too small to significantly alter

the electronic state of Ni or Pt.

XPS of Pt(4f), Ni(2p) and Si(2p) intensities before oxidation (clean) and after

exposure to 8x106 L O2 at room temperature are shown in Fig. 3.3. A feature

corresponding to SiO2 is clearly visible after oxidation, consistent with an O(1s) peak

(not shown) characteristic of SiO2. In summary, figure 3.2 indicates an increase in the

O(1s) intensity with complete passivation at 8x106 L. The SiO2 is formed as a

continuous layer on the top of the sample. As mentioned before, Manandhar et al, has

observed that the oxidation on Ni (Pt)Si with atomic oxygen is much more aggressive

when compared to molecular oxygen.

3.3. Reaction of Dissociated NF3 on SiO2/ Ni (Pt)Si:

The sample was first exposed to molecular NF3 at 1x10-6 torr for 15 min. It was

observed that the molecular NF3 did not have any effect on the sample. The oxidized

Ni(Pt)Si was then exposed to dissociated NF3 (F + NFx; x = 1,2) at 1x10-6 torr for an

exposure of 1800 L. The dissociation efficiency of NF3 was assumed to be around

48%. The exposure is calculated in langmuir. A langmuir is determined by

multiplying the pressure of the gas with the time of exposure. One langmuir is equal

 17

to the gas pressure of 1x10-6 torr for 1 second. The dissociation energy was

previously measured for ammonia. It was also noticed from the nitrogen spectrum that

there was no uptake of nitrogen and thus indicates that the fluorine is the major

species that reacts with the sample.

880 872 864 856 848

0

1000

2000

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84 80 76 72

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0500

10001500200025003000

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050

100150200250300350400450

112 108 104 100 96-100

0100200300400500600700

Binding Energy

Inte

ns

ity(

CP

S)

Clean

After Oxidation

Ni(Pt)Si (854.0)PtSi (73.2) Ni(Pt)Si (100.1)

SiO2 (103.2)

Ni(2p)

Ni(2p)

Pt(4f)

Pt(4f)

Si(2p)

Si(2p)

Fig 3.3 XPS spectra of Ni(2p), Pt(4f), Si(2p) clean surface (top trace) and the XPS

spectra of Ni(2p), Pt(4f), Si(2p) after the oxidation with molecular oxygen at 5x10-6

torr , 8x105 L.

 18

The corresponding evolution of the XPS spectrum indicates that the fluorine

first reacts with the silicon forming a fluoride species (fig 3.4). The peak at 687eV on

the fluorine spectrum indicates the formation of fluorine bound to silicon 17along with

fluorine bound to Si and oxygen at 687.3 18. Further reaction of the sample proceeds to

drive the nickel in the bulk of the sample to the surface to form NiFx species on the

top of all the other layers. The XPS spectrum indicates the formation of the peak at

685 eV which corresponds to NiF2 formation19.

696 694 692 690 688 686 684 682 680

0400800

12001600696 694 692 690 688 686 684 682 680

0400800

12001600696 694 692 690 688 686 684 682 680

0500

1000150020002500

Increase in the Intensity of this side of the peak at about 685eV. NiF2 formation

After Atomic NF3 dose for 600L

After Atomic NF3 dose for 1200L

After Atomic NF3 dose for 1800L

Binding Energy

Inte

nsity

(CP

S)

Fig 3.4: Evolution of the F(2p) XPS spectrum upon increasing exposure to dissociated

NF3 : 600L (bottom trace), after 1200L (middle trace) and after 1800L (top trace).

 19

890 885 880 875 870 865 860 855 850 845-700

0700

140021002800

After oxidation with Mol Oxygen for 79200L890 885 880 875 870 865 860 855 850 845

-4000

400800

12001600 After atomic NF3 dose of 600L

890 885 880 875 870 865 860 855 850 845-400

0400800

1200

After 1800 L Atomic NF3 dose

After 1200 L Atomic NF3 dose890 885 880 875 870 865 860 855 850 845

-4000

400800

1200

Ni rich Si(853.1, 852.8)

NiF2(864.3)

NiF2(858.3)

PtNiSi(854.0)

Ni 2p3/2(857.6)

Binding Energy

Inte

nsity

(CP

S)

Fig 3.5: XPS of Ni spectra after oxidation with molecular oxygen of 79200L (a), after

atomic NF3 dose of 600L (b), after atomic NF3 dose for 1200L (c), after atomic NF3

dose for 1800L (d).

It is also evident from the corresponding XPS spectra for Ni that there is a

reaction of Ni with the fluorine atoms. The peak at 858.2(1) eV (Fig. 3.5) proves that

there is a formation of NiF3 species 20. It is observed from the spectra that there is a

 20

shift in the binding energy which makes it evident that NiSi is first transformed in to a

nickel rich silicide before the reaction with the fluorine species occurs.

Variation in Si(2p) and O(1s) intensities as a function of exposure to NFx +F

are shown in Fig. 3.5. Corresponding changes in Ni(2p) and Pt(4f) intensities are

shown in Fig. 3.6.

-200 0 200 400 600 800 1000 1200 1400 1600 1800 2000500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

Oxygen Silicon

Inte

nsity

exposure to Fluorine (L)

Fig 3.6: Graph of the total Si(2p) and O(1s) intensity vs exposure to fluorine in

langmuir oxygen (■) and silicon (▲).

 21

The intensity of the silicon and oxygen decreases with increasing exposure to F

(fig 2.5). This might be due to the fact that the fluorine reacts with silicon oxide first,

consuming all of the silicon. In contrast, (fig 3.6) the Ni(2p) intensity decreases up to

a total F exposure of 1200 L. Subsequent F exposure results in increase in Ni(2p)

intensity (Fig. 2.6) corresponding to formation of NiF2 . During this process, there is

only minor change, if any, in the Pt(4f).

-200 0 200 400 600 800 1000 1200 1400 1600 1800 2000

0

200

400

600

800

1000

1200

1400

1600

1800

Ni Pt

Inte

nsity

Exposure to Fluorine (L)

NiF2 over layer formationF attack on SiO2

Ni(2p)

Pt(4f)

Fig 3.7 Graph of the total intensity vs exposure to fluorine in langmuir. nickel (■) and

platinum (▲)

 22

The data in Figures 3.6 and 3.7 indicate that the fluorine reacts first with the

silicon dioxide and then eventually reacts with the nickel. It is been observed that the

Ni intensity increases at 1200L. This might be due to the fact the fluorine draws the

Ni up in to the surface after the consumption of silicon, which leads to a decrease in

the intensity of all the other elements. The Ni is drawn up to the surface through the

Cabrera-Mott mechanism. The hypothesis is that a strong field is set up on the oxide

film due to a contact potential difference between the metal and the oxygen atoms

which enables the metal ions to diffuse through the oxide layer without much help

from the change in the temperature 21.

3.4. Reaction of dissociated NH3 with NiFx overlayer

The sample was first treated with molecular NH3 for any reduction of the

fluoride. It was observed that there was no change in the spectra after the dose with

molecular ammonia. The sample was treated with dissociated ammonia at 5x10-6 torr

after the sample was reacted with NF3. The dose of the gases is measured in langmuir

(L): 1 L corresponds to an exposure of 1x10-6 torr for 1 second. Dissociation

efficiency of ammonia is measured to be 48%. The efficiency was previously

measured using a mass spectrometer. Fig 2.7 indicates that hydrogen from dissociated

ammonia preferentially reduces the Ni fluoride species (no N(1s) signal is observed in

the XPS) and forms nickel in its metallic form. After the initial decrease in the NiFx

species, the hydrogen then reduces the SiFx and there was no change in the total

silicon intensity silicon intensity. It was also observed that there was no uptake of

nitrogen in the spectra and this indicates that hydrogen is the main active ingredient in

the reduction of fluoride.

 23

As shown in Fig 3.8 it is observed that the hydrogen mainly reacts with the

nickel and fluorine while the other elements remain largely unaffected. This process

might be due to the fact that the nickel fluoride species is the overlayer covering all

the other elements. The hydrogen is only able to react with the NiFx species. The

dissociated ammonia first has to reduce the NiFx over layer to reach the other

elements. The presence of a (now reduced) Ni overlayer will prevent further atomic H

interaction with SiO2. In any case, atomic H reduction of SiO2 does not become

spontaneous for T < ~ 1020 K due to free energy considerations in forming the SiO

intermediate 22. The silicon oxide requires higher temperatures to be reduced which

might contribute to the selectivity of the hydrogen reaction.

 24

-693 -690 -687 -684 -681-500

0500

10001500200025003000350040004500

-890 -880 -870 -860 -850-500

0500

100015002000250030003500

-538 -536 -534 -532 -530 -528 -526-100

-500

50100150200250300350400

-108 -106 -104 -102 -100 -98 -96-50

0

50

100

150

200

After N F3 dose for 10 m in After N H 3 dose for 40 m in(tot) After N H 3 dose for 70 m in(tot) After N H 3 dose for 130 m in(to t) After N H 3 dose for 190 m in(to t)

F(1s)Ni(2p)

O(1s) Si(2p)

Intensity(CP

S)

Binding Energy(eV)

NiF2 decreases

Ni0, Not NiSi

Atomic H preferentially reduces NiF2

No increase in total or individual Si intensity

SiFxcomponent: ~ contstant after initial decrease

Fig 3.8: XPS spectra of F(1s), Ni(2p), O(1s) and Si(2p) after the reaction with NF3

with dissociated ammonia.

 25

CHAPTER 4

SUMMARY AND CONCLUSIONS

4.1. Summary and Conclusions

Ni(Pt)Si is a material that is been recently used in the advanced

complementary metal oxide devices. It is a better material because of its reduced risk

of junction spiking, less consumption of Si, better compatibility with Ge, smoother

interfaces and less voiding. Newer methods of cleaning the surfaces are required for

better adhesion and nucleation of metal overlayers on the sample. XPS was used to

study the interactions of molecular and dissociated NF3 and NH3 with SiO2/Ni(Pt)Si.

Dissociation of NF3 and NH3 in the thermal cracker source results in F +NFx and H +

NHx (x = 1,2) respectively. No evidence of reaction was observed upon exposure of

the SiO2/Ni(Pt)Si surface to molecular NF3 or NH3. Further, no evidence of N surface

incorporation was observed in the presence of dissociated gases, indicating that

atomic F and H are the reactive species. Exposure of SiO2/Ni(Pt)Si to F results in

initial SiFx species, with eventual formation of a Ni difluoride overlayer on the

surface. It was also observed that Pt does not play any role in the process. The NiF2

layer is formed on the top of all the other metals as the intensity of all the other

elements was reduced. XPS spectra, however, indicate that the substrate SiO2 layer is

not removed by the dissociated NF3 and NiF2 growth process. The NiF2 over layer

can be reduced to metallic Ni by reacting with dissociated NH3 at room temperature.

The atomic hydrogen from dissociated ammonia reduces the NiF2 but it was

determined that the atomic hydrogen from the ammonia does not react with SiO2.

 26

This reaction might be due to the fact that the NiF2 species is the top most

layer and is the first one to react with hydrogen which is summarized in figure 4.1.

The failure of atomic H to reduce SiO2 at room temperature is explained on

thermodynamic grounds. The chemical reaction for the removal of SiO2 is assumed to

be as follows:

SiO2(s) + 2H*(g) SiO(g) + H2O.

The Gibbs’ free energy at room temperature (298 K) is 94KJ, which means that it is

an endergonic or a non-spontaneous reaction. The estimation at which the reaction

becomes spontaneous is 1023 K or 750oC, and this is in good agreement with

experiment 22.

Fig 4.1: Illustration of the reaction of fluorine and NH3 on oxidized Ni(Pt)Si.

 27

4.2. Future Work

Other methods to clean the oxide on the Ni(Pt)Si can be explored. We need to

know whether the oxide or the silicate on the surface is relevant. We also need to

explore the possibility of metallization of Ni after the reduction of NiF2. The surface

composition and the surface thermal compatibility need to be explored.

 28

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