Laser Oxidized SnfIn Films for Microlithography Applications

by

Jun Perig

THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF APPLIED SCIENCE

In the School of

Engineering Science

O Jun Peng 2006

SIMON FRASER UNIVERSITY

Spring 2006

All rights reserved. This work may not be reproduced in whole or in part, by photocopy

or other means, without permission of the author

Approval

Name:

Degree:

Title of Thesis:

Jun Peng

Master of Applied Science

Laser Oxidized SnIIn Films for Microlithography Applications

Examining Committee:

Chair: Richard F. Hobson Professor of the School of Engineering Science

Glenn H. Chapman Senior Supervisor Professor of the School of Engineering Science

Karen L. Kavanagh Supervisor Professor of Physics Department

-

Ash M. Parameswaran Internal Examiner Professor of the School of Engineering Science

Date DefendedJApproved: A x . I%/&

SIMON FRASER u N l w R s A i brary

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Abstract

This thesis reports a new kind of laser oxidized inorganic film, Snlh film, which has

unique properties and could be used as a direct-write mask, greyscale mask, anisotropic etch

mask and patterned IT0 film in microelectronic manufacture. Upon exposure to laser light the

film's Optical Density (OD) drops from 3 OD to 0.24 OD, and the OD decreases almost linearly

with laser power. Profilometry, XRD, TEM and EI>X results show that the OD change is caused

by an oxidation process. Binary and greyscale masks were successfully created by using these

films. The film has a similar structure to IT0 film and can be used to replace deposited IT0 film.

Since the exposed film has a much lower etch rate compared to Si, it can also be used in

anisotropic masks. Patterns and trenches on Si(100) wafer were successfully created.

Acknowledgements

The author would like to thank Professor Glenn Chapman of Simon Fraser University for

his encouragement and support during this project. Special thanks are extended to Dr. Yuqiang

Tu, Dr. Chinheng Choo, Jozsef Dudasa and David Poon given many help and directions in the

whole research time. Thanks to Bill Woods, whose help in the lab made valuable contributions to

the research presented in this document. Also I would like to thank Dr. Karen Kavanagh and Dr.

Ash M. Parameswaran for their helpful suggestions. This work was supported in pilrt by CREO

Products Inc. and the Science Council of British Columbia.

Table of Contents

.. Approval ......................................................................................................................................... 11

... Abstract ......................................................................................................................................... III

Acknowledgements ........................................................................................................................ iv

Table of Contents ............................................................................................................................ v . . List of Figures ............................................................................................................................. vn

List of Tables .................................................................................................................................... x

List of Abbreviations and Acronyms .............................................................................................

Chapter 1: Introduction ................................................................................................................. 1 ................................................................................................................................... 1 . 1 General 1

1.2 Introduction to Microlithography ........................................................................................... 3 1.2.1 Photoresists ...................................................................................................................... 5

....................................................................................................................... 1.2.2 Photomask 7 1.3 Thesis Objective ................................................................................................................... 14

.............................................................................................................................. 1.4 Summary 1 5

Chapter 2: Introduction to Laser Oxidized Bimetallic Thermal Resists ................................ 17 .......................................................................................................................... 2.1 Introduction 17

................................................................................... 2.2 Studies of Bimetallic Thermal Resist 17 2.2.1 Requirements for the Inorganic Resists .......................................................................... 17 2.2.2 Material of the Bimetallic Resist .................................................................................... 18 2.2.3 Structure of the Bimetallic Resists ................................................................................. 20

.................................................................................................................. 2.2.4 BUIn Resists 21 ................................................................................... 2.2.4.1 Laser Exposure of BUIn Resist 21

.................................................................. 2.2.4.2 Development and Etching of Bi/h Resist 21 2.2.4.3 B i h Direct-Write and Greyscale Masks .................................................................. 22

....................................................................................... 2.3 Optical Model of Bimetallic Films 22 .......................................................................................................... 2.4 Wavelength Invariance 25

............................................................................................................................... 2.5 Summary 27

Chapter 3: Bimetallic Film Creation and Experimental Setup ................................................ 29 .......................................................................................................................... 3.1 Introduction 29

.................................................................................................. 3.2 Bimetallic Film Preparation 29 ........................................................................................................ 3.3 Laser Exposure System 32

................................................................................................................ 3.3.1 Laser Systems 32 3.3.2 X-Y-Z Table .................................................................................................................. 34 3.3.3 Mechanical and Electro-optical Shutter ........................................................................ 35

.................................................................................... 3.3.4 Control Computer and Software 35 3.4 Measurement and Analysis Instruments ............................................................................... 37

.............................................................................. 3.4.1 Tencor Alphastep 500 Profilometer 37 3.4.2 Varian Cary 3E UV-Visible Spectrophotometer ........................................................... 37 3.4.3 X-ray Diffractomety ...................................................................................................... 38 3.4.4 Scanning Electron Microscopy (SEM) .......................................................................... 39

3.4.5 Transmission Electron Microscopy (TEM) ................................................................... 39 3.5 Summary ............................................................................................................................... 39

Chapter 4: Laser Exposed SdIn Film Optical and Electrical Properties .............................. 40 .......................................................................................................................... 4.1 Introduction 40

4.2 The Process of Laser Exposure ............................................................................................ 40 4.3 Optical Property of SdIn Films ........................................................................................... 41

4.3.1 SnIIn Film Optical Property .......................................................................................... 42 4.3.2 Optical Density of the SdIn Film ................................................................................. 43

.................................................... 4 . 3.3 SdIn Optical Changes with Composition Variation 46 4 . 4 SdIn Film Electrical Properties .......................................................................................... 48

4.4.1 Detailed Research on S n h Film Electronic Properties ................................................ 49 ...................................................................................... 4.4.2 S n h Film Hall Measurements 52

............................................................................................................................... 4.5 Summary 53

Chapter 5: Laser Converted SnIIn Film Structure Analysis .................................... ................ 54 .......................................................................................................................... 5.1 Introduction 54

.................................................................................................................. 5.2 Profilometry Test -54 5.3 X-ray Diffraction of Exposed and Unexposed Film ............................................................. 56

5.3.1 Single Layer Sn and In Film XRD Analysis ................................................................. 57 5.3.2 XRD Analyses of Sn/In Bilayer Films .......................................................................... 59

.................................................. 5.3.3. Structure Comparison of Exposed SdIn Film to IT0 62 5.4 S n h Film TEM analysis ...................................................................................................... 67

............................................................................................. 5.4.1 . TEM Sample Preparation 67 ....................................................................................... 5.4.2. Electron Diffraction Patterns -69

5.4.3. TEM Calibration by Using GaAs Standard Samples ................................................... 70 5.4.4. TEM Electron Diffraction Patterns of Exposed SdIn Film ......................................... 71

5.5 EDX Composition Analysis ................................................................................................. 74 5.6 Post Annealing Film in Different Environments ................................................................. -75

............................................................................................................ 5.6.1 Annealing in Air 75 5.6.2 Annealing in Steam ....................................................................................................... 76

5.7 Summary ............................................................................................................................... 77

Chapter 6 Applications of SdIn Film ....................................................................................... 79 .......................................................................................................................... 6.1 Introduction 79

6.2 SdIn Laser Direct Write Binary Mask ................................................................................. 79 ........................................................................................................... 6.3 Sn/In Greyscale Mask 81

6.4 Development of SdIn to Leave Patterned IT0 Layers ........................................................ 84 ............................................................................................... 6.5 SdIn Anisotropic Etch Mask 87

6.6 Photovoltaic Property of Exposed SdIn with p-Si .......................................................... 90 6.7 Summary ............................................................................................................................... -93

Chapter 7: Conclusion and Future Work .................................................................................. 95 ............................................................................................................... 7.1 Thesis Conclusions 95

7.2 Further Work ........................................................................................................................ 96 7.2.1 Clarify the Mechanism of the Oxidation Process .......................................................... 96 7.2.2 Improving As-deposited Film Quality ........................................................................... 97 7.2.3 Improving Film Transparency ...................................................................................... -97

Reference List ............................................................................................................................... 98

Appendix: .................................................................................................................................... 102

List of Figures

Figure 1.1 : Microlithographic processes for micromachining and micro fabrication ............................................................................................. -4

Figure 1.2. Binary photomask ............................................................................................ 7 Figure 1.3. Process of binary mask manufacturing ............................................................ 9 Figure 1.4. A direct write binary photomask making process ......................................... 1 Figure 1.5: Using a greyscale mask to create a 3D structure . (a) shows the grey

..................................................................................................... level ramp, 12 Figure 1.6: 4-greyscale level halftone greyscale mask. (b) to (f) shows the

................................... transparency percentage from 0% to 100% in 5 steps 13 .................................. Figure 2.1 : Typical binary phase diagram with one eutectic point 18

Figure 2.2. Bi-In binary phase diagram (After Tu's thesis Figure 2.4 [24]) .................... 19 Figure 2.3. Si-In binary phase diagram (After 'Tu's thesis Figure 2.5 [24]) .................... 19

.................................................... Figure 2.4. The bilayer structure of the inorganic film 20 Figure 2.5: Reflections and refractions of the laser beam at each of interfaces in a

.................................. . bilayer film (After Marinko's thesis Figure 2.4 [21]) 24 Figure 2.6: Simulation results for BiIIn films (50% Bi) at (a) 830 nm. (b) 514

................................................ nm. (c) 193 nm and (d) 13 nm wavelengths 25 Figure 2.7: Simulation for SdIn films (50% Sn) at (a) 830 nm. (b) 514nm. (c:)

......................... 248 nm. (d) 193 nm. (e)154 nm and (f) 13 nm wavelengths 26 .............................................................................. Figure 3.1 : Corona sputtering system 30

Figure 3.2. Bilayer thermal film structure ....................................................................... 31 Figure 3.3. Coherent Innova 300 Series Argon Ion Laser and lenses .............................. 33 Figure 3.4. Coherent Infinity TM Nd:YAG laser ............................................................ 33 Figure 3.5. X-Y-Z Table, computer and optical system setup ......................................... 34 Figure 3.6. Control computer and graph interface of WinLTC Software ........................ 36 Figure 4.1 : Laser exposure system and X-Y-Z table ....................................................... 41

Figure 4.2: Exposed lines on SdIn film . (a) simple lines front side lighting . 0)) mask pattern under back side lighting ........................................................... 42

Figure 4.3. SEM photo of exposed lines on SdIn 120 nm (Sn 10%) film ...................... 43 Figure 4.4: OD changes versus wavelength for SdIn (Sn 10%) 120 nm film as-

deposited, and after 0.4 W, 0.9 W, 1.1 W and 1.2 W laser exposures .......... 44 Figure 4.5: OD changes versus power for SdIn 120 nm (10% Sn) film for a

wavelength 365 nm ........................................................................................ 44 Figure 4.6: OD changes vs wavelength for Bi/In 120 nm (Bi 50%) film for as-

deposited, 0.1 W, 0.4 W and 1.2 W laser exposure powers ..................... 45

Figure 4.7. OD changes versus laser power for BiIIn 120 nm film ................................. 46

vii

Figure 4.8: Exposed SdIn (90 nm) film OD changes versus wavelength for Sn ratios of SO%, lo%, 5% and 0% at (a) 0.5 W and (b) 0.9 W. ....................... 47

Figure 4.9: The sheet resistance of Sn/In (40140 nm) and BiIIn (40140 nm) films. ......... 48 Figure 4.10: Sheet resistances for different Sn ratio films with at various exposure

powers. ........................................... ................................................................ 50 Figure 4.1 1 : plot of OD and Rs of SdIn 120 nm (SnlO%) vs exposure power. ............... 5 1

Figure 5.1 : A profilometry across the exposed and unexposed areas of a 120 nm SdIn sample. ................................................................................................. 55

Figure 5.2: A profilometry of the exposed and unexposed areas of a 120 nm BiIIn sample ................................................................................................... 56

Figure 5.5: XRD of pure In film on glass substrate at as-deposited, 0.4 W and 0.9 W laser exposure powers. .............................................................................. 58

Figure 5.4: XRD of Sn 45 nm film as-deposited and exposed with 0.4 W, 0.9 W laser.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 9

Figure 5.5: XRD of S d h (Sn 50%) 120 nm thick film exposed at 0 W, 0.085 W, 0.4 W and 0.9 W laser power. ............... ...... ..... ........... .. ............................ .... 60

Figure 5.6: XRD of S d h (Sn 10%) 120 nm film on glass exposed at 0 W, 0.1 W, 0.4 W and 0.9 W laser powers. ................................................................ 64

Figure 5.7: XRD pattern of IT0 with a thickness of 150 nm [38] ................................... 64 Figure 5.8: XRD of SdIn (Sn 30%) 120 nm thick film on glass exposed at 0 W,

0.1 W, 0.4 W and 0.9 W exposure powers. ................................................... 65 Figure 5.9: XRD of SdIn (Sn 5%) 120 nm thick film on glass exposed with 0 W,

0.1 W, 0.4W and 0.9 W laser powers. ......... .. ... ............... .. ........ .... ............ .. ..66 Figure 5.10: (a) and (b): SdIn TEM samples made on formvar grids: (a) is a

back lit picture of the sample after laser exposure. (b) is an enlarged picture, in which area (1) is as-deposited film, (2) is laser burned area, (3) is transparent area, in which the film was exposed properly. ......... 68

Figure 5.11: Electron diffraction patterns of as-deposited (a) and exposed (b) S d h film. ....................................................................................... ............... 69

Figure 5.12: The electron diffraction pattern of the GaAs standard sample ...................... 71 Figure 5.13: EDS of as-deposited (a) and 0.3 W laser exposed (b) SdIn (Sn 30%)

120 nm film. ..... .. ...... ... ... .. ....... ... ... .............. .. ... ... .. ....... ........ .... ...... ............... 74 Figure 5.14: Post annealing SnIIn film (120 nm, Sn 10%) in air at 550•‹C ........................ 75 Figure 5.15 : SdIn 120 nm (Sn 10%) film's OD before and after annealing in air

and steam. ...................................................................................................... 76 Figure 6.1 : (a) SdIn (Sn 10%) direct write binary mask back-lit: film thickness

is 120 nm, laser power is 0.5 W (50x objective lens), the feature line is 2 pm wide. (b ) Shipley SPR2FX-1.3 photoresist pattern from the SdIn mask after exposure and development. .................................. .............. 80

Figure 6.2: Relationship of OD changes with the laser power for SdIn (120 nm) film. .. . .. .. . . .. ... ... .. ... ..... ... ... . .. . .. . .. . . . . .. .. ........ ... .. ... .. ... ... ...... .... .. .... .. .. .. . . .. ..... .. ... .8 1

Figure 6.3: (a) Plot of intensity vs X-axis position, (b) data mask gray levels, (c) SdIn greyscale mask made with (b) data. ...................................................... 82

Figure 6.4: SdIn 45 nm (Sn 10%) greyscale mask with 3 grey level strips: (a) 8 bit greyscale bitmap file. (b) S d h greyscale mask (dark area with 0.72 OD at 365 nm). ....................................................................... ............... 83

Figure 6.5: The profile of a 2-step structure made on Shipley SPR2FX-1.3 photoresist.. .. .. ..... .. ... ..... ... . .....,. .. .... .... .... ........ .. ......... ......... ............. ,. . .. .. .. .. ..... 84

Figure 6.6: Etch rate curve of exposed SdIn film in dilute RCA2 solution ..................... 85

Figure 6.7: Exposed S d h film (1 20 nm, Sn 1 0%) after development: (a) different width patterns of SdIn film on glass (50 mm lens with spot size of 10 ym). (b) Patterns (5 ym lines) of SdIn film on Si(l00) wafer (made by using 50x lens with spot size of 2 ym). ................................ 86

Figure 6.8: Profile of developed SdIn film (Sn 10%) with 5 ym lines. ........................... 86 Figure 6.9: Profile of exposed SdIn (Sn 10%) film with thickness of 100 nm

(before KOH etching). ... ... .. ... . ........ .. ... ................. ..... ........ .. .... .............. .. .. .. ... 88 Figure 6.10: Profile of exposed SdIn (Sn 10%) film with thickness of 100 nm

(after KOH etching). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 8 Figure 6.1 1 : Etch rate curves of exposed SdIn film in KOH and TMAH. ........................ 89 Figure 6.12: SEM pictures of anisotropic etched trenches on Si(100) wafer, (a) is

KOH etched trenches using 120 nm SdIn film as etch mask (etched for 10 min), (b) is TMAH etched trenches (etched for 20 min). ................... 90

Figure 6.13: I-V characteristic test set-up of the heterojunction of laser converted SdIn on p-Si. ......... ... ..... .......... .... ...... ............ .... .... ..... ........ ........... ... .. ............ 91

Figure 6.14:I-V characteristic of a heterojunction of laser converted SdIn on p-Si. ........ 92 Figure 6.15: Typical I-V characteristic of the solar cell of laser converted Sdhi on

p-Si. . ... ... .......... ..... ..... ... ... . . .. ... ... .... ... ... ......... . ........... .. .... .. .... .. .... ....,,. . .. .... .. .. ..92

List of Tables

Table 3.1 : Table 3.2: Table 3.3 : Table 4.1 :

Table 4.2: Table 5.1:

Table 5.2:

Table 5.3:

Bi. In and Sn single metal film parameters [28] ............................................ 31 ............................................................... Sputter parameters of Bi. Sn and In 31

Optical density and coordinating transmission value .................................... 38 120 nm SdIn film sheet resistance. film resistivity and optical

............................................................................................................ density 50 ...................................................... Hall measurement conditions and results 53

Results from XRD for SdIn 120 nm (Sn 50%) film (shaded areas ..................................................................... match with a JCPDF data base) 61

Comparison of TEM data for as-deposited SdIn film with JCPDF data (shaded areas are data that matchs the JCPDF data base) ..................... 72 Comparison of TEM data for exposed SdIn film with JCPDF data (shaded areas are data that matches the JCPDF data base) ........................... 73

List of Abbreviations and Acronyms

MEMS

OD

PVD

CVD

CMOS

DUV

EUV

IC

NA

PAC

RIE

uv TMAH

EDP

JCPDF

LIGA

Microelectromechanical system

Optical density

Physical vapour deposition

Chemical vapour deposition

Complementary Metal Oxide Semiconductor

Deep ultraviolet

Extreme ultraviolet

Integrated circuit

Numerical aperture

Photo-active compound

Reactive ion etch

Ultraviolet

Tetramethyl ammonium hydroxide

ethylene diarnine pyrocatechol

Joint Committee for Powder Diffraction Files

Llthographie Galvanoformung Abfirmung (German), or

Lithography Electroforming Moulding

Chapter 1: Introduction

1.1 General

The Integrated Circuit (IC) fabrication industry is one of the fastest growing industries in

the modem world. From its birth in the 1950s, it has grown to command worldwide sales of over

$250 billion US dollars in 2003 [I]. In IC manufacturing, lithography is a key process,

determining the chip's minimum transistor dimension, quality, yield and cost. IC chips are built

by the repeated process of creating thin layers or films of materials on a wafer substrate. The

application of the lithography process is to pattern these films into functional layers.

The lithography process involves two important components: the photoresist and the

photomask. The photoresist is a light sensitive thin film applied to surfaces, which can be

exposed and then developed in order to transfer the designed pattern onto another film underneath

or to the substrate. Organic photoresists, consisting of resin, photo-active compounds and solvent,

currently dominate industrial lithography [2]. Although organic photoresists are highly sensitive,

easy to deposit and have high resolution, they entail many disadvantages, such as organic

contaminations, time-consuming cleans and high wavelength sensitivity [3]. These issues create

problems in their applications. In addition, when the device feature size shrinks and the exposure

light shifts into the short wavelength range, traditional organic photoresists show many problems.

Therefore, research into new photoresists has been conducted in recent years.

In 1999, Chapman, Marinko and Tu developed the idea of using a laser exposure process

to convert a highly absorbing metallic material into a nearly transparent oxide film that could be

used for rnicrolithography [4, 51. This new resist film is formed by depositing two thin metallic

layers of low melting point materials, such as Bismuth (Bi) and Indium (In) or 'Tin (Sn) and

Indium (In) on glass or Si wafer substrate. When a laser beam exposes the film in air, the film

will melt and oxidize. Consequently, its structure will change. Since the new resist film's melting

point is very low, the laser power required for the exposure is close to that of the conventional

organic photoresists [ 5 ] .

These bimetallic resists have many unique features: first, they are inorganic resists which

have the same sensitivity as that of the conventional organic photoresists (about 7 mJlcm2).

Second, the exposure process is a thermal process, in which the film is heated by the exposure

laser beam and then melted and oxidized in air finally, as will be shown by this thesis, they also

have the unique characteristic of being wavelength invariant, which means the wavelength of

exposure laser has little or no effect on the resist behaviour. The film after exposure show

transparent and conductive properties. This thesis will report new research results relating to one

of the bimetallic resists, SnIIn film. It will study the film's properties, analyze the film's

microstructure and discuss the film's possible applications.

As mentioned earlier, another important component of lithography is the photomask. A

photomask is a piece of glass with a pattern of absorbing film deposited on one side. A binary

photomask consists of two types of image regions: opaque and completely transparent areas. This

mask is used to create patterns by allowing light to pass through the transparent areas in the mask

and block the rest of the light from reaching the photoresist. The purpose of a photomask is to

transfer the pattern to the photoresist by strategically controlling the transmission of light to the

resist. The traditional photomask is made from Chromium (Cr) film deposited on quartz. The

Chromium photomask has been used for decades because it has high chemical stability and high

durability against irradiation. However it has some drawbacks. Firstly, it is difficult to minimize

defects since its manufacturing processes are quite complex. It includes Cr filrn deposition,

photoresist spin coating, e-beam patterning, photoresist developing, etching and photoresist

striping. Each step can introduce process defects, such as particles and pinholes. Secondly, the

electron beam (e-beam) or laser writing process is very expensive and has even lower throughput

when the designed patterns get smaller. Thirdly, it can easily be damaged by electrostatic

discharge [6] . Therefore, many new photomask materials have been investigated by researchers.

In this thesis, it has been found that SdIn film exhibits properties that are suitable for a new mask

making process. Since S n h film can be easily exposed by a modest power laser beam, and its

optical transparency changes are quite large after laser exposure, it can be used as an ideal direct

write mask material. This thesis will present a series of research results related to SnlIn film and

its applications in this field.

This chapter will briefly introduce the modem lithography processes and its components:

photoresist and photomask. Subsequent chapters will discuss Sn/In film-making processes,

structure studies and possible applications.

1.2 Introduction to Microlithography

Microlithography is a process to accomplish the pattern transfer from a mask to a resist

and then to a device. After the process, a patterned thin film or layer is created on the substrate or

bulk material used for microfabrication or micromachining applications. The process starts with a

photomask, which is located above the wafer (see Figure 1.1 ). Photoresist is spun onto the surface

of a substrate or a wafer. Since the photoresist is sensitive to UV light, when light is projected

through the mask, illumined areas are chemically changed. During the subsequent development

process, the exposed photoresist will be kept (if a negative photoresist is used) and the unexposed

photoresist will be removed. During an etching process the unremoved photoresist will protect the

functional layer underneath while the unprotected areas are etched. After the etching process, all

the photoresist is stripped away and the patterned functional film layer is left behind. Modem

integrated circuits and micro-electro-mechanical system devices are built with these patterned

thin layers or films. As many as 30 layers are needed in the manufacturing of a typical IC chip,

such as a logic memory-DRAM. Hence, the microlithographic process is very important, as it

determines the device dimensions, the production yield and the manufacturing cost.

Light

Photoresist

(a) Film deposition (b) Photoresist application

Etch niasli

(c) Exposure

(d) Development (e) Etching (f) Resist removal

Figure 1.1: Microlithographic processes for rnicrornachining and microfabrication.

Figure I. I shows in detail the typical steps of the microlithographic and etching processes

that are currently used in industry 16, 71. Ir Figure I . l(a), a thin film is deposited on the substrate.

In Figure 1. I(I)), liquid organic photoresist is droppcd onto the centre of wafer and tten the wafer

is spun, causing the liquid photoresist to thin down because of centrifugal force until at last a

uniformly thin resist film has covered the wafer. Typically, the thickness of the photo resist layer

is about 0.5 -1.5 p n ~ . Then the resist is sofi-baked (typically at 80•‹C for 20 minutcs) In an oven to

remove the resist solvent. In Figure l . l(c), the photomask is precisely aligned to [lie wafer and

the photoresisl is exposed with a UV light. The exposure systems commonly used in the induslry

are steppers, which project reduced pattern images on the photoinasks to the wafer. After

exposure, the mtterns are transferred from the photomask to the photoresist. The exposure causes

changes in the chemical properties of the cxposed area, inducing different dissolution rates in the

resist by the developer solution for the exposed areas and the unexposed areas. In Figure l.l(d),

the resist film is developed by using a developer to dissolve the unexposed area quickly and retain

the exposed area (for a negative photoresist). Then the wafer is hard baked to additionally

evaporate solvents. In Figure l.l(e), after the patterned photoresist has formed, dry or wet etching

is camed out to "carve" the patterns into the functional film or substrate underneath the

photoresist. As shown in Figure l.l(e), the area covered by the photoresist is protected from

being etched away as the photoresist is resistant to etching chemicals or plasma, while the "open"

area not covered by the resist is etched off by the chemicals or plasma. In Figure l.l(f), finally,

all photoresist is stripped from the wafer and the whole microlithography cycle ends.

1.2.1 Photoresists

The basic function of photoresist is to transfer designed patterns on a photomask into

films underneath the photoresist. The photoresists currently widely used in the semiconductor

industry are organic photoresists. Organic photoresists consist of three components: (1) the resin,

which serves as the binder of the film, (2) the photo-active compound, which is the light sensitive

material, and (3) the solvent, which keeps the resist in liquid state until it is processed. Generally,

photoresists have two technical features: precise pattern formation and protection of the

underlying films from chemical attack during the etch process.

However, when the device feature sizes shrink, shorter wavelength exposure light have to

be used in order to avoid diffraction effects. At short UV wavelength the organic photoresist

encounters problems. When the light wavelength moves to Deep Ultra Violet (DUV) range, the

high-energy photons are highly absorbed by organic compounds in the top thin layer of the resist.

Thus, the resist can not be completely exposed since the light can not penetrate through the whole

resist [8]. Organic photoresists are also sources of organic contaminants in wafer processing

which require many wet cleans and water rinses. Therefore, organic resists block the process

switching to all-dry process flow in which the wafer are kept in a cleaner vacuum

microfabrication environment. In order to overcome these organic resist problems, people started

research on inorganic resists as alternatives.

There are some inorganic researches reported in literature. J. M. Lavine and M. J.

Buliszak researched a silver-based photoresist which has the advantage of requiring low energy

exposure (3-10 d l c m 2 at 248 nm) [9], which is about the level of the existing organic photoresist

exposure level (-10 mJlcm2). However, this silver based film cannot work with silicon processes

because silver is a deadly poison to silicon devices due to its high diffusion rate into silicon where

it kills the carrier lifetime.

In the 1980s, S. W. Pang conducted research on a metal-oxide thermal resist: A110 [lo].

Instead of being sensitive to light, thermal resists are activated by heat, which is converted from

the exposure light. The as-deposited AVO thermal resist is a mixture of metal A1 and oxide A1203

of about 30 nm thick. It is a shiny, smooth and conductive film. When exposed to a single 20 ns

UV laser pulse at 100 mJlcm2 energy density, the exposed area absorbs a portion of the laser

energy and the film is heated above a certain threshold temperature, which will cause a phase

change of the materials. Although the A110 thermal resist was more sensitive than previous

inorganic thermal resists, it still requires about 40 - 100 mJlcm2 for film conversion, several

times higher than conventional organic photoresist values of 10 mJlcm2 [lo].

A laser activated thermal resist, BiIIn film, was explored by Chapman, Marinko and

Tu [4]. It is the first reported inorganic thermal resist that can be exposed by laser light sources

with a wide range of wavelengths. Its laser exposure energy was around 7 mJlcm2 for film

conversion, which was close to the conventional organic photoresist values. It has the potential to

become a new practical inorganic thermal resist [4,5].

1.2.2 Photomask

As was noted, another important component of the microlithographic process is the

photomask. Traditional photomasks are made from very flat pieces of quartz or glass with a

patterned layer of chrome on one side. IC chips and MEMS devices are manufactured layer by

layer, and each layer requires a unique photomask with specific design patterns on it.

1.2.2.1 Binary Photomask

There are two types of photomasks: binary and greyscale photomasks. The binary

photomask is the traditional photomask; it consists of both fully transparent and fully absorbing

sections. Figure 1.2 shows a typical binary mask.

Figure 1.2: Binary photomask.

A blank mask consists of an opaque film on a substrate. Quartz is usually used as a

substrate material due to its high transmission rate at short wavelengths, its low thermal

expansion coefficient (0.52 x ~ o - ~ I K ) , and its chemical stability. The opaque film should have an

optical density (OD) > 3. Equation 1.1 shows the formula to calculate optical density from

transmittance:

where T is the transmittance of the material. The opaque film should also have the following

properties [11,12,13,14]: (1) high chemical stability; (2) high durability against irradiation;

(3) strong adhesion to the substrate; (4) moderate electrical conductivity; and (5) ease of

preparation and patterning. Chromium and its compounds are the most widely used materials for

photomasks, since they possess these properties.

The manufacturing process for conventional binary photomasks is similar to that of the

microlithography process, which involves steps such as resist coating, resist patterning,

development, etching, etc.

Figure 1.3 shows a typical process flow for the preparation of a binary photomask [6,7]:

(a) Chromium and Chromium Oxide are deposited to form an absorbing layer on a glass or quartz

plate. (b) Photoresist with a thickness of about 0.5 pm is spun on the Chromium surface.

(c) Resist patterning: e-beam writers are mainly used in the patterning process. (cl) Photoresist

developing. (e) Etching. Dry etching is mostly used to remove unprotected Chromium film.

(f) Finally, all photoresist is stripped away. In summary, the making of a conventional Cr

photomask requires 6 main process steps.

(a) Cr deposition I (13) IDhotorcsist coating

(c) I<-heam patterning

(d) Ihl-elopment

(e) Etching

(0 ICcsist removal

1 Substrate

Figure 1.3: Process of binary mask manufacturing.

1.2.2.2 Some Ih-awbacks of Conventional Masks

There are some drawbacks to the conventional photomask and its preparation

processes [15,16,17]. Firstly, i t is difficult to minimize defects as there are 5 - 9 opuation steps

involved in m k i n g a photomask and each slep can introduce process defects ar~d particles.

Secondly, although Cr dry etching has become a standard process in mask making, at current

geometries a loading effect becomes a problem. The chrome etch rate changes with the ratio of

clear area to opaque area on the mask, which can seriously affect the precise Critical

Dimensions (CD) control. Thirdly, the e-beam writing systems are expensive and have even

lower throughput when design patterns get smaller. Finally, mask damage from Electrostatic

Discharge (ESD) has long been a concern, although much effort has been spent on making the

photomask conductive by adding conductive films or conductive frames. ESD damage can be

more problematic due to the shrinking of the feature size [15,16].

In order to solve these problems, people have started to explore a direct-write photomask

process. Laser ablation is one of the direct-write processes that can potentially shape high-

resolution structures in a single processing step. Direct writing by photoablation of thin chrome

layers for photomasks has been studied by Venkatakrishnan using a Ti:sapphire chirped-pulse

amplified ultra fast laser [17]. It takes advantage of short-pulse interactions to avoid damage to

the underlying quartz substrate. However, an expensive short-pulse laser system is needed.

Furthermore, material re-deposition and damage threshold control are still the ma-jor unsolved

issues which prevent this direct-write method from being used industrially.

Laser light

Sensitive layer

I Glass I (a) sensitive layer deposition (b) laser direct-writing (c) Binary mask created

Figure 1.4: A direct write binary photomask making process.

Another direct-write method is to use a special material whose transparency can be

changed by writing it with a laser light or e-beam. Figure 1.4 shows a laser direct-write binary

photomask making process. High Energy Beam Sensitive (HEBS) glass [18] is such a material, as

its optical density changes with the electron beam dosage and acceleration voltage. However, the

writing process takes an extremely long time and very powerful e-beams have to be used to

expose the glass. Hence, it is very expensive.

1.2.2.3 Greyscale Photomask

Greyscale photomasks have attracted much attention recently for the application of

making three-dimensional micro-machined mechanical, electrical and optical devices by using a

modification of conventional IC manufacturing photolithography and reactive Ion Erching (RE).

In contrast to binary masks, greyscale photomask's optical transmission is gradually variable.

Figure 1.5 shcws a greyscale mask and the 3D structure fabrication process. The thickness of

photoresist after development is a function of the resist exposure level. The greyscale mask in

Figure 15(a), which has a linear change on absorption when used to expose a photoresist film

(Figure 16(b)), will generate a ramp in the thickness of the resist after development (see Figure

15(c)), which can be transferred by etching to thin layers below. There are two main categories of

greyscale pho~omasks: binary

118,19,20j.

Grayscale mask

(or digital) greyscale masks and ana log~e greyxale masks

' r

Figure 1.5: Using a greyscale mask to create a 3D structure. (a) shows the grey level ramp,

(b) mask exposure of photoresist, and (c) developed ramp in photoresist.

A bina-y (or chrome digital) greyscale mask usually consists of 0.5 p m round, equally-

sized spots wh ch are completely transparent on the Cr covered glass photcmask substrate. The

greyscale in this chrome mask is created bj. the ratio of the number of the transparent spots to Cr

covered area within a greyscale resolution unit on thz mask. Figure 1.6 shows a 4 greyscale level

halftone greyscale photomask resolution unit with 4 spots. If all the 4 binary spots in the

greyscale resolution unit are opaque, and the greyscale resolution unit is totally opaque, we have

0% transmission (see Figure 1.6(b)). When one spot is transparent, the transmission is 25% or 114

(as shown in Figure 1.6(c)). With an increase in the number of transparent spots, the

transmittance increases linearly in steps of 25% (shown in Figures 1.6(d), (e) and (f)

respectively). Thus, a 2-bit, 4 level greyscale photomask is obtained. This kind of greyscale mask

needs to be used in a defocusing mode.

(a) A unit of a 4-greyscale level halftone greyscale photomask

(b) 0% (c) 25% (d) 50% (e) 75% (f) 100%

Figure 1.6: 4-greyscale level halftone greyscale mask, (b) to (f) shows the transparency percentage from 0 % to 100 % in 5 steps.

In this thesis we are more interested in analogue greyscale masks. Compared with binary

or digital greyscale masks, analogue greyscale masks have continuous tones of grey. Each spot in

an analogue greyscale mask can be at any grey level. The total number of greyscale levels is

dependent on the analogue greyscale mask material.

The difficulty in making analogue greyscale masks is in finding a medium whose

transparency can be controlled gradually from highly absorbing to fully transparent. Wu [15,18]

found that the optical density of High Energy Beam Sensitive (HEBS) glasses change with the

electron beam dosage and acceleration voltage. He claimed that more than 1000 grey levels could

be assigned to each spot. But as already mentioned the writing process takes an extremely long

time and very powerful e-beams have to be used to expose the glass, making it costly. It is

difficult to expose the glass to a high transparent level [18]. Also, the mask's best working

wavelength is around 500 nm. Therefore, its application in the microfabrication industry is

limited.

1.3 Thesis Objective

This thesis will present some new research results on SdIn films, a laser oxidized

bimetallic film. The research includes the study of SdIn film's optical and electrical properties,

SdIn film's micro-structure and the possible applications of SdIn film. Some comparisons with

BiAn films, which were studied in our lab earlier, are also presented.

Chapter 2 introduces previous work on laser oxidized bimetallic films, including the

material and film structure selection, some basic property investigations, and the results of optical

simulations.

Chapter 3 gives the experimental setup, which includes the film sputter deposition

machine, laser exposure system ( laser source, X-Y-Z table, and control computer), and

measurement instruments.

Chapter 4 demonstrates the optical and electrical property changes of SdIn films before

and after laser exposure. It is found that exposed SdIn film shows higher OD changes compared

to Bi/In film, showing that S n h film is a good film for laser direct write photomask applications.

The film's electrical properties will also be investigated in this chapter.

Chapter 5 discusses studies of the material micro-structure and composition. In order to

understand the mechanism behind the laser exposure process, a series of experiments including

X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray

Spectroscopy (EDX) and Transmission Electron Microscopy (TEM). have been carried out. The

results show that the laser conversion process is an oxidation process. The optical and electric

properties of the exposed film are closely related to the oxidation level, which in turn is related to

the laser power. It is also found that the exposed Sn4n film has the same structure as directly

deposited ITO.

Chapter 6 explores the possible applications of SnlIn film, including direct write,

greyscale and anisotropic etch masks. It also shows that exposed SdIn film can replace deposited

I T 0 film in heterojunction solar cell structures.

Chapter 7 presents the conclusions and also discusses some future work.

1.4 Summary

In this chapter, an introduction to conventional photolithography has been provided.

Photolithography is an important process in today's microelectronic and micromachining

industries. It determines the IC or device's dimensions, cost and yield. In the lithography process,

the photoresist and the photomask are two very important components. However, conventional

photoresists and photomasks have many disadvantages. Organic photoresists largely depend on

the exposure wavelength and encounter challenges when the wavelength shrinks into the Deep

Ultraviolet (DUV) or (Extreme Ultraviolet) EUV range. Traditional chrome photomasks and

mask making processes also have drawbacks, such as too many manufacturing stzps and high

cost. Therefore, searching for new kinds of lithographic materials and processes is necessary.

This thesis work will explore SdIn film as a new type of lithographic film which can be used as a

direct-write photomask, greyscale mask, anisotropic etch resist, and for patterned IT0 films.

Chapter 2: Introduction to Laser Oxidized Bimetallic Thermal Resists

2.1 Introduction

As discussed in Chapter 1, there are many benefits to using an inorganic material to

replace the organic photoresists in the lithography process. In this chapter, an introduction to the

previous work on laser oxidized inorganic thermal resists will be given. Also this chapter

introduces the requirements for the inorganic resist, plus research on resist material and structure

characteristics. Finally, an Airy optical simulation method is introduced and results for SdIn

resists are discussed.

2.2 Studies of Bimetallic Thermal Resist

2.2.1 Requirements for the Inorganic Resists

There are two current applications for inorganic resists: as a classical resist, and where a

large change in optical density is required (eg. photomasks). The ideal requirements for an

inorganic thermal resist are as follows: (1) laser exposure should induce significant chemical or

physical changes in the film, (2) it should have organic resist level of sensitivity (I 10 mJlcm2 ),

(3) an effective developer method has to be found so a clear pattern can be obtained after the

developing process, (4) the developed film should have a lower etch rate than the film

underneath, such as Si or Si02 film, so that patterns can be transferred into those films through the

use of etching solutions, (5) the exposed film should be easily removed (stripped) after the

etching process is done, and (6) finally, the resist should not be poisonous to silicon device

operation. In some applications the sensitivity is less important than an enhancement of the etch

resistance (eg. etch resist).

Previous work by Chapman, Marinko and Tu suggests that laser activated inorganic

BiIIn and SdIn films possess the required properties. We will now summarize how BiIIn

inorganic resists have demonstrated those characteristics, and introduce how the SdIn resist is

expected to exhibit similar behaviour which will be explored in this research.

2.2.2 Material of the Bimetallic Resist

The first priority in considering an inorganic lithographic film is its sensitivity. The film

should have enough sensitivity to meet the application requirements. The concept of the

bimetallic thermal resists was to use two metallic films in a ratio at their eutectic point. The

eutectic phase of binary metals creates a melting point that is by definition lower than that of

either of the individual metals. Figure 2.1 shows a typical binary phase diagram, with one eutectic

point, of metals A and B. The eutectic point is lower than the melting points of both MA and MB.

In modem lithographic exposure systems, short (-10 nsec) pulse high intensity lasers create the

patterns. This light will be absorbed, heating the film to the eutectic point. This requires much

less energy than previous inorganic films.

0 1 1 0 20 40 60 80 100

A Element percentage (%) B

Figure 2.1: Typical binary phase diagram with one eutectic point.

The materials for the inorganic thermal resist were chosen by Chapman and Marinko in

1999 [21,22]. It was found that Bismuth is the most attractive metal as it has a low melting point,

low thermal conductivity and also low reflectivity. Indium is another candidate metal with a low

compatible melting point and moderate thermal conductivity. From the Bi-In phase diagram in

Figure 2.2, it can be seen that Bi and In can form 3 eutectic alloys with melting points at 72"C,

89•‹C and 1 10•‹C. Therefore, the bimetallic film will require less exposure energy than either Bi or

In by themselves. Furthermore, since Bi is a Group V element and In is a Group I11 element, if

properly controlled, there is no unwanted doping effect in Si: the p-type In doping can be

cancelled by the n-type Bi doping. According to these considerations, the composition of 50% by

weight Bi was chosen to form the Bi/In inorganic thermal resist. The research shows that this

inorganic resist has a light sensitivity (about 7 mJlcm2) close to the organic resist [21.,22,23,24].

0 10 20 30 40 50 60 70 80 90 100 In Atomic Per cent Bismuth 0 i

Figure 2.2: Bi-In binary phase diagram (After Tu's thesis Figure 2.4 [24]).

0 10 20 30 40 50 60 70 80 90 100 In Atormc percenl Tin S n

Figure 2.3: Si-In binary phase diagram (After Tu's thesis Figure 2.5 [24]).

This mearch explores another possible pair of metals: tin-indium. From the Sn-In binary

phase diagram shown in Figure 2.3, il can be seen that Sn-In has a eutectic point at 120•‹C, which

is a little higher than 110•‹C of Bi/In. Preliminary research by Tu [24] has shown that Sn/In would

react like Bi/In with laser exposure. Since Indium Tin Oxide (ITO) is a widely studied transparent

conductor, par1 of this thesis is to explore the creation of I T 0 using S d I n thermal resists.

2.2.3 Structure of the Bimetallic Resists

In our earlier research [:!I], il was found that a bilayer structure is a good choice for a

bimetallic resizt. From the point of view of setting [he material ratio, the two layer structure can

be more accurately controlled to get the dcsired cornposition of bimetallic films when compared

to a single mixed layer in the sputtering prxess . I t is also more efficient to deposit tXn!o layers of

different metal films onto the substrate from two separate targets than to co-sputter two targets to

get onc mixed metal layer. Furthermore, wc: can depxi t a low reflecting metell layer on the lop of

the film which will increase the absorption of laser light during the exposure t ine, thereby

increasing the sensitivity of the film. Therefore, .he two-layer structure was chosen for the

bimetallic film formation under s t~~cly 121, 241. F i g ~ ~ r e 2.4 shows the structure of this inorganic

bilayer film.

Figure 2.4: Thc bilayer structure of the inorganic film.

2.2.4 Binn Resists

Bilayer Bi/In thin films have been successfully used as the inorganic thermal resists in

our research [5].

2.2.4.1 Laser Exposure of Binn Resist

BiIIn bilayer film with Bi 50% was used in these experiments. The resist can be exposed

by either an Argon laser in continuous wave, or pulsed mode, or as a pulsed Nd:YAG laser [22,

241. Research shows that BiIIn bimetallic resist can be exposed by a low energy level laser beam.

For a thinner Bi/In sample (15 nmll5nm thick) deposited on glass, the minimum exposure energy

of a pulsed Nd:YAG laser at 533 nm was measured to be about 7 d l c m 2 [24]. This energy is

close to organic resist exposure energy levels (c10 d lcm2) . With this low level exposure energy,

the BilIn resist has a potential to be used in practical lithography processes.

2.2.4.2 Development and Etching of Binn Resist

In the BiIIn inorganic thermal resist, previous analysis had shown several changes occur

during laser exposure: (1) a melted of the eutectic alloy forms in exposed areas, (2) oxygen, either

from the atmosphere via diffusion or oxygen trapped within the film, reacts with the film to form

an oxide. A special solution or developer has to be found, to which the exposed Bi/In resists

should have enough resistance to ensure that unexposed area can be efficiently removed, leaving

behind clear patterns. It was found that dilute RCA2 (HC1:H202:H20 =1:1:48) meets this

requirement [22,23,24]. The solution of dilute RCA2 demonstrated a greater than 1:60 etching

selectivity ratio of exposed area to unexposed area of BiAn film, which allows retention of the

exposed area and efficient removal of the unexposed film [24].

After development, the next process is etching. In one application, anisotropic etching

solution will etch unprotected Si or Si02, and leave the protected area un-etched. It was found that

exposed BiIIn resist demonstrates good etch resistance properties. The exposed BiIIn film has an

etch ratio 500 times lower than Si wafer in alkaline based solutions such as KOH [24].

Anisotropically-etched trenches on Si(100) wafers were successfully created by using the BVIn

etch resist [24].

It was also found that full strength RCA2 (HC1:H202:H20 = 1: 1:6) at 80•‹C can be used as

a stripper after the patterning is done [24]. The solution can also remove metallic contaminants in

the process.

2.2.4.3 BiIIn Direct-Write and Greyscale Masks

Since BVIn film can be easily exposed by a low power laser beam and the exposed film

shows high transparency [23], direct-write masks have been successfully created by using Bi/In

resists [23,24]. Further research showed that BVIn resist's optical density decreases almost

linearly with increase of laser power in a certain power range. This property indicates that the

film can be used to make greyscale photomasks. A software called WinLTC was developed for

this purpose in our early research [23]. A 8-bit greyscale bitmap file is used to control the power

level of the exposure laser and has been successfully used to create greyscale masks.

2.3 Optical Model of Bimetallic Films

Since these bimetallic films are converted by heat generated from the laser exposure, it is

necessary to know how much of the laser energy is absorbed and changed into heat, and how the

energy absorption rate changes with film thickness, laser wavelength, film structure and type of

material. In order to answer these questions, Marinko [21] created an optical model by using the

Airy summation method for these highly absorbing materials. The model computed the intensity

of the laser reflected (R), absorbed ( A ) and transmitted (T) by the as-deposited thermal resist

during the laser exposure. This model will be briefly described here. A detailed description is

given in Marinko's Master's thesis [21].

The model was developed for bilayer metallic films deposited on a thick substrate. The

refractive index of an absorbing material is defined in its complex form, ii :

where n is the real component of the refractive index and k is the absorption index, the imaginary

component of the complex refractive index [27]. For a completely transparent (non-absorbing)

medium, k is zero, and for an absorbing medium, k is greater than zero.

In its simplest form, light absorption in a medium is governed by Beer's Law. The light

intensity has the following relationship between the thickness of the film, light wavelength and

the material properties:

where If is the light intensity after the light travels through a distance d in the film, lo is the

4 d original light intensity, h is the wavelength of the light, k is the absorption index, and a , = - A

is called the absorption coefficient. This equation can be rewritten in the following format to

represent the attenuation rate of the electric field of light inside the medium:

where Ef is the electric field of light after it travels through a distance of d in the film, Eo is the

electric field of the original light, and a the electric field attenuation rate.

The bimetallic layers exhibit both absorption and reflectionlrefraction at each layer

interface (see Figure 2.5). Marinko used the Airy summation method to add up all the electric

field vectors reflected and refracted at each interface of a multilayer film to calculate the electric

field magnitude at all points within the film. As shown in Figure 2.5, the laser beam reflects and

refracts at each of the interfaces inside the bilayer film on a glass substrate. Each beam is

absorbed as it passes to the next interface where each reflected and refracted bean1 will further

reflect and refract. The program steps through small increments Ad of depth of the film,

calculating the total E field vector from all the transmitted or reflected beams. At each Ad

increment, it calculates the beam intensity and the energy deposed in that increment. The program

then sums up the results at the film top and bottom giving the total light escaping the top surface

(Reflection R), the total energy absorbed (Absorption A) and the total transmitted through the

bottom interface (Transmission T). It also shows the total light intensity and energy absorbed at

each depth, and how these change when the total film thickness changes

Top layer h

Figure 2.5: Reflections and refractions of the laser beam at each of interfaces in a bilayer film. (After Marinko's thesis Figure 2.4 [21]).

Assuming I0 is the original incident light intensity, lR is the reflected light intensity at the

top surface, IT is the transmitted light intensity at the bottom and IA is the absorbed energy

respectively, we have the following RAT (Reflection, Absorption and Transmission) curve which

satisfies the law of the conservation of energy:

Bi on In on Glass 830 nm

0 20 40 60 80 :a>

Thickness (nm) each layer

Bi on In on Glass 193 nm

(CJ Thickness (nm) each layer

Bi on In on Glass 514 nm

:b) 0 20 40 60 80

Thickness (nm) each layer

Bi on In on Glass 13 nm

. . Thickness (nm) each layer

Figure 2.6: Simulation results for Binn films (50% Bi) at (a) 830 nm, (b) 514 nm, (c) 193 nm and (d) 13 nm wavelengths.

2.4 Wavelength Invariance

An organic photoresist is very dependent on the wavelength of the exposure light. When

the exposure wavelength changes, the photoresist properties or structures need to change in

response to the specific wavelength. The inorganic bimetallic film in this case is activated by

heat; therefore it should respond to changes in wavelength only as the optical energy absorbed (or

deposited) in the film changes. Thus, a simulation was conducted to obtain Reflection,

Absorption and Transmission (RAT) for different wavelengths of BiIIn film calculated from

Marinko's program [21]. The results are shown in Figure 2.6.

Sn on In on Glass 830 nm

(4 0 20 40 60 80

Thickness (nm) each layer

Sn on In on Glass 193 nrn

(d) Thickness (nm) each layer

Sn on In on Glass 514 nm

0.000 --

(b) 0 20 40 60 80

Thickness (nm) each layer

Sn on In on Glass 154 nm

(e) Thickness (nm) each layer

Sn on In on Glass 248 nm

(c> 0 20 40 60 80

Thickness (nm) each layer

Sn on In on Glass 13 nm

(f) Thickness (nm) each layer

Figure 2.7: Simulation for SnIIn films (50% Sn) at (a) 830 nm, (b) 514nm, (c) 248 nm, (d) 193 nm, (e)154 nm and (f) 13 nm wavelengths.

It was found that for films with the same thickness, the amount of light absorbed varies

by less than 10% over a wavelength range of 514 nm to 193 nm. At 830 nm wavelength, the

absorbed light is around 30% of the incident value and when wavelength goes to the

Extreme UV (EUV), 13 nm, the light is almost fully absorbed. Since the light absorbed can be

directly related to the temperature the film reaches, the relative exposure needed for a thermal

resist can be calculated from these RAT curves for different wavelengths [24]. 'The research

shows that BVIn film can be exposed over a wide wavelength range extending fiom infrared

(830 nm) to EUV (13 nm). Therefore, BiIIn films are nearly wavelength invariant compared to

highly wavelength dependent organic resists.

For S d I n film, the same simulation was conducted and the results are shown in

Figure 2.7. It was found that the amount of light absorbed by Sn/In film varies only about 10%

from 830 nm to 154 nm. This means S d I n shows similar wavelength invariance to Bib. Again

when the wavelength decreases to 13 nm, nearly all incident light is absorbed. From Figure 2.6

and 2.7, it is also found that S d I n films show a lower absorption rate and higher reflectance than

B i h film. This means that higher power is needed to expose S d I n film than BiIIn films.

In Marinko's earlier research, the wavelength invariance was previously confirmed by

experiments with different wavelength exposures. The Bi/In films were successf~~lly exposed

using wavelengths of 213 nm, 266 nm and 533 nm from an Nd:YAG laser and 514 nm and

488 nm from an Argon laser with the same level of laser power [21].

2.5 Summary

This chapter introduced the ideas of inorganic thermal resists, and provided a discussion

of previous work which created high sensitivity bimetallic resists with low melting point and low

thermal conductivity. Bi/In resist shows almost the same sensitivity as organic resist do. The

resist film also demonstrated enough chemical property changes after the laser exposure, such

that the exposed pattern can be developed and etched by developing solutions. The optical model

analysis shows BilIn and SnlIn films exhibit wavelength invariant characteristics. This is a big

advantage over the current organic photoresists.

In the next chapter, the inorganic thermal film's preparation process and the experimental

set-up will be presented. The instruments for film property measurement and strucl.ura1 analysis

will also be introduced.

Chapter 3: Bimetallic Film Creation and Experimental Setup

3.1 Introduction

This chapter describes the preparation process for laser activated bimetallic I henna1 resist

and the exposure system setup. The first section describes the major steps involved in preparing

the bimetallic thermal films, including substrate cleaning and the deposition of metals on the

substrate using DC sputtering. In the second section, the exposure system is described, giving the

details of the argon and Nd:YAG laser sources, lenses, mirrors, shutters, X-Y-Z table, control

computer and software. The last section introduces the tools and methods used to measure and

analyse film thickness, optical property, and structure. These methods include X-ray diffraction

(XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDX)

and Transmission Electron Microscopy (TEM).

3.2 Bimetallic Film Preparation

The sputter deposition of single and bilayer metallic films was the first step in preparing

the inorganic resists tested in this thesis. Film preparation began with cleaning the substrates.

Glass substrates were used when transmission or Optical Density (OD) tests were required for the

films. The glass substrates were slides with 1" x 3" rectangular dimensions and a thickness of

-1.0 mm. A silicon wafer was used for analyses of the films' composition or structure because of

its pure composition and excellent crystal orientation. Before the deposition, the substrates were

first cleaned using a standard RCA clean [7] starting with a RCA1 solution (NI&0H:H202:H20 =

1: 1:5) at 80•‹C for 10 minutes to remove organic contaminants. Then after 10 min deionised water

(DI) rinsing, the substrates were cleaned in RCA2 solution (HC1:H202:H20 = 1:1:6) at 80•‹C for

10 minutes to remove metal contaminants [7 ] . After that the substrates were rinsed in DI water

again for about 10 minutes and blown dry using compressed N2 gas. They were then baked at

100•‹C for 20 minutes. When they had cooled down, the substrates were loaded into the sputtering

chamber.

The mztallic films were deposited by using a DC sputtering process. Thc sputlcring

machine is matie by Corona Vacuum Coaters I,td (Model: 600). Its vacuum system uses a two-

stage mechanical roughing pump and a Varian 6" oil-based diffusion pump. Initially, the system

was first pumpcd down to a base pressure of 8 x 10.' Torr. During the deposition time, argon gas

was introduced and Ihc sputter pressure was typically kept at 3 mTorr.

Figure 3.1: Corona sputtering system.

The control system included a DC power supply (Advanced Energy MDX-IK), a

substrate bias voltage controlles. a substr;~te heating controller, a mass controller, a bulterfly

throttle valve, and a control computer. Figure 3.1 shows the DC sputtering system.

The three target disks were 2" in diameter with the purity of 99.99% Bi, 99.11506 Sn and

99.99% In. During the deposition, the first layer (typically indium) was sputtered on the substsnLe.

Without breaking vacuum, the second layer (Bi or Sn) was deposited on the first layer. Table 3.1

shows the physical parameters of the three metal materials. Table 3.2 gives the typical sputter

parameters of Sn, In and Bi. Figure 3.2 shows the typical film structure created.

Table 3.1: Bi, In and Sn single metal film parameters [28]

Metal Melt Temp Thermal Conductivity Density Density 1 1 (g/cm3) 1 (Oc) (w1m.K)

or Sn layer

Table 3.2: Sputter parameters of Bi, Sn and In

In layer

/

Metal

Bi

In

Sn

-

Substrate \

Figure 3.2: Bilayer thermal film structure.

Deposition rate (h Min) 12.0 * 2.0

4.0 * 0.6

6.4 + 1.0

DC bias

500V

500V

500V

Deposition Power (W)

32

42

22

3.5

4.2

One advantage of using the sputtering method is that it allows precise control of film

characteristics by balancing sputtering parameters pressure, deposition rate, and target material.

After one layer is deposited, it is possible to switch to another target and deposit another layer.

The thickness of each layer was controlled by programming the number of Watt-min for that

layer. These were calculated from previous deposition tests on single layer films of Bi, In and Sn.

In these experiments, the deposited film thickness was checked with a Tencor Alphastep-500

profilometer. For the bimetallic films, only the combined layer thickness was measured.

Section 3.4 will describe how the optical characteristics of the deposited films were measured.

3.3 Laser Exposure System

The purpose of the exposure system was to enable the film to be exposed properly and

precisely so that the designed patterns could be formed on the film. The exposure system

consisted of three sets of equipment: laser sources, the X-Y-Z table, and a control system that

included a control computer and software.

3.3.1 Laser Systems

There were two laser sources used as part of this work. One was a Coherent Innova 300

Series Argon Ion Laser, which is shown in Figure 3.3. It is a 5 W Continuous Wave (CW) laser.

This laser is operated in multi-wavelength mode, with main output wavelengths of 518 nm and

488 nm. The other laser used was the Coherent Infinity TM Nd:YAG laser (Figure 3.4). The

Nd:YAG laser is a 4 nsec pulse duration laser, with a fundamental laser wavelength of 1064 nm.

By sending this base wavelength laser light through a second harmonic crystal within the laser,

the laser frequency can be doubled and laser wavelength decreased to 533 nm. If a fourth

harmonic crystal is added after the second harmonic crystal, the laser wavelength will be further

shortened to 266 nm. The pulse repetition rate can be changed from 0.1 - 30 Hz. In inost cases in

this study, the argon laser was used since its power is stable for the long duration exposure and

the power is easily controlled. In both cases the laser beam is directed onto the sample by using a

set of mirrors. In this work, 50 mm and 75 mrn simple convex lenses and 5x and 50x objective

lenses were uscd to focus the laser light.

Argon laser

\

Figure 3.3: Coherent Innova 300 Series Argon Ion Laser and lenses.

Nd:'iAG laser Harnon c crystal

Figure 3.4: Coherent Infinity TM Nd:YAG laser.

3.3.2 X-Y-Z Table

The X-Y-Z stage is mounted on an air-vibration isolated 2.5 ton granite table. The stage

positioning consists two axis laser interferometers, using a 633 nm Hewlett-Packartl HP-5517B

He-Ne laser head, two linear X-Y induction motors and an Anorad controller, which uses the

laser interferometer measurements. The table has a k0.2 pm position repeatability over a

25 x 25 cm area. The Intelligent Dual Axis Contrdler (IDAC) uses the computer software to

control the X-Y table. The table moving speed along the X and Y directions can bc set from 1

pmds to 25 cnds. Figure 3.5 shows the X-Y-Z Table and the control system.

Figure 3.5: X-Y-7, Table, computer and optical system setup.

The sample holder (Z-axis table) i:; able to move upward or downward, and is used to

bring the sample into the focus of the objective lens. The Z-axis can position the resist film to

within 0.05 pm over a 2.5 cm range. The film samples are placed on an alurniniurn plate, which is

mounted on the top of the Z-axis table holcler. The laser beam is re-directed to this Z-axis stage

using three dielrxtric mirrors. The X-Y-Z table is suitable for creating structures with sub-micron

geometry. A video camera, which is mounted above the focus lens set, sends live video to the

control computer screen, allowing capture of real time images of patterns as they are created.

3.3.3 Mechanical and Electro-optical Shutter

For the argon laser system two beam shutters were used in our exposure system: a

mechanical shutter and an electro-optical shutter. The mechanical shutter (Model LS200, NM

laser products Inc) consists of a solid beam blocker and an actuator, which can either open or

close. This makes it possible to turn the beam on or off. However the mechanical shutter is slow

(-1 msec to open), so it is only used for low speed control. The electro-optical shutter used was

from Conoptics (Model 302). This shutter allows the user to vary the intensity of the beam, by

controlling an input control voltage, which is supplied by a function generator under computer

control. This voltage controls an electro-optical Pockels cell polarizer, which acts as polarizer

whose phase angle is a function of the applied electric field and thus varies the intensity of the

polarized laser beam. The electro-optical (EO) shutter switches at a speed of 0 1 psec, thus

allowing a very fast laser power control. Since the controller for the EO shutters is set by digital

signals from the control program, this gives a sub microsecond ability to set the laser power.

However, the electro-optic shutter cannot fully turn off the beam so the mechanical shutter

supplies this feature.

The mechanical shutter (Model LS0.55) used for the Nd:YAG laser is made by NM laser

products Inc. A solid laser beam blocker inside the shutter can either open or close, which is

controlled by WinLTC control software. A second mirror system, which by-passes the Argon

laser mirrors, was used to direct the Nd:YAG laser beam on to samples.

3.3.4 Control Computer and Software

The X-Y-Z table, shutters and Argon laser can be controlled by the control computer

under a Microsoft Windows XP operating system. A laser table control software program called

WinLTC was written by James ~Methven Dykes [23]. The WinLTC program has a friendly

Graphical User Interface (GUI), which allows easy input of all exposure parameters (see

Figure 3.6). Because the diame~er of the laser beam is small, a raster-scan was often used to

expose large areas and make patterns on the bimetallic films. In raster scan mode a constant X

and Y speed bere used and a continuous wave argon laser was employed. The X-Y table moved

back and forth along the X direction, and took a small increment step along the Y direction after

each X swipe. The control computer can lake either command scripts or an 8-bit bitmap image

file as its input (see Figure 3.5).

The h e r power is controlled from a console, which also monitors the light intensity of

the laser outpul. Since there are power losses in the optical system, such as the mirrors, the lenses

and the electro.optical shutter, the power that reaches the sample is about 70% of the laser output

power indicated on the console. Only part 3f the power that reaches the sample is converted into

heating energy; part of the power is reflected light and the other part passes throu;;h the glass

substrate.

mzl W W I C tquipmnc mornallon 2pecE4zsd lolmlnd

Ccnlroller S t a d Rasult ol La* C m a n d , l dln I A

1 Stop Comnana

_d ' !. . . . .. . . . . . . . .

I Clear Rwulrs I

1 LUM Put S l a m

I TLDLC r o n T

/ AHOUNPUHT

FLNCGEN PORT

1 NDYAGPORT mm m ? q

SHUTTiR PORT

FIELDMASTER PCRT p C _ I %

O P [ . ' I_ CLOSE -- OPEN 1 CLOSE I - OPEA CLOSE J - -- O P E ~ CLOSE J - - OPEN I CLOSE 1 flPFN I ClnqF 1 OrCN 1 uosc j -

Figure 3.6: Control computer and graph interface of WinLTC Software.

3.4 Measurement and Analysis Instruments

After film deposition, the thickness and optical density of the deposited film were

measured with a profilometer and a W-visible spectrophotometer. After exposure, the film's

properties and structures were studied using XRD, EDX, SEM and TEM instruments. In this

section, these instruments and their specifications are introduced.

3.4.1 Tencor AlphaStep 500 Profilometer

Profilometry is an important measurement method of the film thickness and surface

roughness in this thesis. The Tencor AlphaStep 500 profilometer measures the 2-dimensional

vertical profile of structures on a solid surface. The maximum horizontal scan distance is 5 cm.

The vertical profile accuracy can reach up to +10A. The profilometer is equipped with a camera

and a TV monitor, which allows users to look at the sample features while they are being

measured. It provides up to 210x optical zoom on the samples measured.

3.4.2 Varian Cary 3E UV-Visible Spectrophotometer

Since the unexposed and exposed bimetallic films' optical characteristics iire important

for masks, the optical characteristics were measured over a wide wavelength rang(=. The films'

optical density was measured using a Varian Cary 3E spectrophotometer located in the Chemistry

department at SFU. The Varian Cary 3E spectrophotometer utilizes a dual-beam measurement

with a baseline correction function. It measures the sample's OD versus wavelength ranging from

300 nm to 900 nm. The instrument measures a film's optical density by calculating the amount of

the light transmitted by one beam through the film medium and comparing that to the

transmission through an identical un-deposited substrate. The W-Visible spectrophotometer has

a precision of 4 significant digits.

The optical density is related to the transmission (T) by the following mathematical

relationship:

where OD is the optical density and T is the light transmission. Table 3.3 shows some typical

optical densities and their related transmission values.

Table 3.3: Optical density and coordinating transmission value

3.4.3 X-ray Diffractomety

To characterize the material phase and microstructure, X-ray diffraction 8-20 scans using

a Cu Ka, source was conducted. X ray diffraction peaks occur when Bragg's law is satisfied:

where d(h,k,l) is the distance between atomic planes with Millar indices (h, k, l ) , 8 is the X-ray

incident angle and h is the wavelength of the X-ray. A certain diffraction peak appears

corresponding to a certain interplanar distance.

A Philips PW1730 x-ray generator with Cu Ka, (h = 1.5418 A) and. a Norelco

diffractometer (Model No. 3-202) were used. The slits to the x-ray source side and on the counter

side had a width of lo. The XRD spectra were obtained over a 28 range of 10" - SO0, with a step

size of 0.05O, counting 2 seconds per step.

3.4.4 Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM, FEI 235 Dualbeam with Focused Ion Beam) was

used to study the film surface morphology. X rays generated by the e-beam in an EDX system

were used to analyse the elemental composition of the film.

3.4.5 Transmission Electron Microscopy (TEM)

Transmission Electron Microscopy (TEM) is a common technique used for analyzing the

micro-structure of a material. In our research, the 'TEM (Hitachi 8000, 200 keV) in the Physics

Department at SFU was used to investigate the film's crystal structure.

3.5 Summary

This chapter describes the film-making, laser exposure and film analysis facilities.

Specifically, the film was sputtered onto RCA-1 and RCA-2 cleaned glass or quartz substrates by

a Corona DC-sputtering system. The sputtering system sputtered two single metallic layers

without an air break where the thickness of the film can be controlled precisely. The laser

exposure system consisted of an argon laser, an Nd:YAG laser, an X-Y-Z table, and a computer

control system. Film optical properties were measured before and after exposure.

Film structure analysis tools have been frequently used throughout this research in order

to understand what happens during the laser exposure processes. XRD, SEM, EDAX and TEM

analyses played important roles in understanding the laser conversion mechanism and gave

precise composition information.

In the next chapter, the film's optical and electrical properties will be discussed. The

relationship between changes to the film and laser annealing conditions are also studied.

Chapter 4: Laser Exposed Sn/In Film Optical and Electrical Properties

4.1 Introduction

As mentioned in Chapter 3, bilayer film (SnIIn and BVIn) was deposited by DC

sputtering. The as-deposited film is shiny, reflective, and electrically conductive metallic film.

However, after laser exposure, SnlIn and Bi/In film optical, electrical, and structural properties

changed. This chapter will describe experimental measurements of the changes in SnlIn film

optical and electrical properties, the relationship of these changes to the exposur'e laser beam

power, and the relationship of Sn percentage in SnIIn film to the film's optical dens'ity and sheet

resistance.

4.2 The Process of Laser Exposure

Using the setup discussed in Chapter 3, the Argon laser beam was focused cln the sample

by a 50 mm converging lens or a 50x objective lens (Figure 4.1). A raster scan mode was used to

make a pattern or to scan a large area. The X-Y table moved back and forth along the X direction,

and took a small incremental step along the Y direction after each X swipe. The table moving

speed in the X direction was usually set at lcrnls unless otherwise specified. The Y direction

incremental step was dependent on the laser spot size and the laser power. When using a 50 mm

converging lens, the Y-step was typically set to 10 pm, as in this case the diameter of the spot

size was about 10 p m (l/e2 diameter). When using a 50x objective lens, the Y-step was set to

2 p m as the laser spot size was reduced to about 2 pm.

Shutter Laser Beam

4

\ , /objective Lens

Figure 4.1: Laser exposure system and X-Y-Z tablc showing raster scan pattenis.

When [he laser beam was focused on the surface of the film, it separated into three parts:

one part of the light energy was reflected, :he second part of i t transmitted through the film, and

the third part ol'the light energy was absorbed by the film. It is the absorbed part of the laser light

that makes the film's properties change. As more laser power is absorbed by the film. the

temperature of the exposed area will increase. When the temperature exceeds a threshold

temperature (7:!"C for BiIIn film with Bi 25% and 120•‹C for SnIIn film with Sn 50%), a eutectic

alloy will form. At the same time, previous research found that some oxide material will be

crealed because of the reaction of metal with the oxygen in the air 1211.

4.3 Optical Property of Sn/In Films

In the Ili/In thermal resist study, carried out by Yuqiang Tu [24], it was found that I3iIIn

film Optical Dtmity (OD) changed from 3 OD to 0.26 OD (for 120 nm Bi SO% film) after Iaser

exposure. The lilm changed from a shiny. highly reflective film into an almost transparent one (in

visible and UV range) when the laser (above a certain threshold power) exposed the film. This

phenomenon is very interesting since it co~lld be used to make a direct-write binary photomask or

greyscale photomask (see Chap~er 6). In this section, detailed research on S d I n film's optical

properties and its relationships with laser power and film composition will be discussed.

Figure 4.2: Esposcd lines on SnAn film. (a) cinlple lines front side lighting. (b) mask pattern under back side lighting.

4.3.1 SnIIn Film Optical Property

Figure 4.2 shows optical pictures of exposure-created lines and patterns on a S n h (~otal

120 nm) samplc with Sn 10%. The lines uere exposed by the argon laser beam at C.4 W (scan

rate 1 c d s e c ) , which was focused by a 50x objective lens onto the sample. Figure 4.2(a) shows

that under front-lighting conditions the exposed line!; have changed in reflectivity. Figure 4.2(b)

shows that under back-lighting conditions, the lines become almost transparent compared with

the unexposed I~lack area. Figure 4.3 show:; a SEM photo of the laser exposed lines on a SnIIn

120 nm (Sn 10%) film on a Si wafer. A 50x objective lens was used (power: 0.3 M', scan rate

1 cn~lsec) and lines less than 2 pm wide were created on the film. In the SEM photo, the exposed

areas become dark, showing that the film's ropert ties have changed in those areas.

Figure 4.3: SEM photo of exposed Iines on SnIIn 120 nm (Sn 10%) film.

4.3.2 Optical Density of the SnIIn Film

Optical changes in the film can be measured in terms of Optical Density (011). For this,

SdIn film deposited with a thickness of 121108 nrn on a glass substrate. The CW argon laser

scanning at pokvers that varied from 0.1 W to 1.2 \Y (scan rate of I cmls). 1 cm\smple areas

were processec by the laser scanning at each different power. Optical properties were measured

using a UV-visible spectrometer (CARY 31; spectrometer). All measurements were normalized to

the absorption value of a bare glass s~bstrnte. Figure 4.4 shows the OD changes vcrsus

wavelength in ;I 120 nm SnIIn film (Sn 10%) after different laser power exposures.

The top curve in Figure 4.4 is the OD of the as-deposited 120 nm SnIIn film with about 3

OD at the 365 nm wavelength (I-line). I-line is a commonly used wavelength for microfabrication

exposure [GI. The OD decreases (from about 3 to 2.4) when the wavelenglh changes from UV

(365 nm) to infrared (850 nm). The second, third ard fourth curves are those exposed by 0.4 W,

0.9 W, 1.1 W and 1.2 W argon lasers, respectively. With 0.4 W laser exposure, the OD changes

from 3 to 0.7. As the laser exposlire power increases from 0.4 W to 0.9 W, the OD of the exposed

area decreases from 0.7 to 0.45. When the laser power is increased to 1.2 W, the 01) of the film

drops to 0.24 at 365 nm ( the C 4 R Y 3E spectrometer has accuracy of 4 significant digits after

decimal point, and the spread in the measured OD value for 3 samples is less than k0.002 OD).

From the spectrum, it can be seen that when the laser power reaches 1.2 W, the minimum OD

value is less than 0.15 in the visible range and about 0.24 at a wavelength of 365 nrn. This means

that an OD change larger than 2.5 was obtained.

- __T___ \- as-deposited

300 400 500 600 700 800 900 wavelength (nm)

Figure 4.4: OD changes versus wavelength for Snfln (Sn 10%) 120 nm film as-deposited, and after 0.4 W, 0.9 W, 1.1 W and 1.2 W laser exposures.

10 100 1000 laser power (mw)

Figure 4.5: OD changes versus power for SnDn 120 nm (10% Sn) film for a wavelength 365 nm.

Figure 4.5 shows the relationship of optical density versus laser exposure power at

365 nm wavelength (I-line) for SdIn 120 nm film (Sn 10%). It can be noted that when the laser is

lower than 100 mW the optical density (OD) of the film does not change. The OD drops sharply

over 100 mW exposure power and quickly saturates at about 900 mW. The slo:pe of optical

density to exposure laser power is about -0.003 k 10% (OD/ mW) in the linear range. The optical

density stabilizes around 0.24 k 0.002 at the I-line wavelength for SdIn (120 nm thickness) on a

glass slide. Repeatable results were obtained for other 120 nm films with the same composition.

Figure 4.6 shows the variation of optical density in relation to the transmission light

wavelength for a BiIIn 120 nm film (Bi 50%) after laser exposure. The exposure laser power

changes from 0.1 W, 0.4 W to 1.2 W. As a result, the film's OD drops from nearly 3 at 365 nm

down to 0.26 k 0.002. (This is the lowest OD for 120 nm Bi/In film we obtained so fiir [24]).

wave length (nm)

3.5

3

2.5 -

8 2 -

1

0.5 -

0

Figure 4.6: OD changes vs wavelength for Binn 120 nm (Bi 50%) film for as-deposited, 0.1 W, 0.4 W and 1.2 W laser exposure powers.

-- - As-deposi ted

0.1W

1.2W -- 5 . \ O e 4 Y * * -

- - - - -- - - --

350 450 550 650 750

--

Wavelength 365 nm

-- I

laser power (mw)

Figure 4.7: OD changes versus laser power for Binn 120 nm film.

Figure 4.7 shows the relationship of optical density versus laser exposure power for BiAn

120 nm film (Bi 50%). The slope of the Bi/In film is -0.004 ? 10% (OD/ mW), which is steeper

than that of SdIn film, showing that SdIn film has less OD change for a given increase in laser

power.

4.3.3 Sn/In Optical Changes with Composition Variation

After the initial tests of SnlIn (Sn 10%) film, we then investigate how these OD changes

vary with the tin-indium ratio. Figure 4.8(a) and (b) give the OD changes for 90 nm SdIn films

with different Sn ratios. The samples are 90 nm SdIn films with Sn ratios of 0%, 5%, 10% and

50% (exposed by 0.5 W and 0.9 W laser powers). Figure 4.8(a) shows the film's Sn ratio and OD

at the 365 nm wavelength, which are O%(OD: 0.29), 5%(OD: 0.28), 10%(OD: 0.42) and

50%(OD: 0.56), respectively. This result shows thal the Sn content in the film has a large impact

to the transparency. Figure 4.8(b) shows the OD changes for films exposed by 0.9 W laser power.

Note that with the same laser exposure conditions, the 5% Sn films exhibit the lowest OD value,

0.22 at a 365 nm wavelength. By comparison, other films were more absorbing, showing that 5%

ratio Sn films are the best choice for those applications in which transparency at UV range is the

first priority. (Note the typical Indium Tin Oxide (ITO) film has a Sn ratio of 5% -10% in the

literature [3 1,32,33]).

(a) Laser power 0.5 W

350 450 550 650 750 1350 wavelength (nrn)

0.35 50% S n

(b) Laser power 0.9 W

0.05 - O%Sn

0 8 7

350 450 550 650 750 ,350 wavelength (nm)

Figure 4.8: Exposed S d n (90 nm) film OD changes versus wavelength for Sn ratios of SO%, lo%, 5% and 0% at (a) 0.5 W and (b) 0.9 W.

With such a low OD level, the SdIn films are close to meeting the needs of ii direct-write

photomask in which the converted areas can be used as the mask "openings" without

development. This possibility will be discussed in Chapter 6.

4.4 SdIn Film Electrical Properties

Another attractive issue of the laser oxidized bimetallic films are th'cir electrical

properties. The SdIn and BiIIn oxide films exhibit metal-like conductivity both before and after

laser exposure. Indium Tin Oxide (ITO) film is well known to be a conductive transparent film

from the literature [32,35]. We investigated the laser converted SdIn for the same characteristics.

This characteristic is very useful in many applications; for example, the I T 0 films ciin be used as

a transparent and conductive electrode in optical devices (such as solar cells) or simply used as a

conductive layer in device structures [34,35].

The electrical conductivity or resistivity can be measured in terms of sheet resistance or

film resistance. In this work, the sheet resistances (Rs) were measured using a four-point probe

system, a Model MP0705A made by Wentworth Labs, which was connected to an HP 3478A

multi-meter. The 4 probes were arranged in a line with a 1 mm space between them. This type of

method eliminates any contact resistance on the film surface area.

0 0.05 0.1 0.15 0.2 0.25 0.3

laser power (w)

Figure 4.9: The sheet resistance of S d n (40140 nm) and Bi/In (40140 nm) films.

Figure 4.9 shows the sheet resistance of SdIn (80 nm with Sn 50%) and BiIIn (80 nm

with Bi 50%) films at various laser powers under the condition of section 4.2. The sheet

resistances range from 10 o h d s q to 380 o h d s q as the exposure laser power increases from 0 to

0.3 W. It is noted that as the laser exposure power increases, the sheet resistance becomes higher.

Besides laser power, the composition of the film also affects its electrical ;properties. In

Figure 4.9, the SdIn film (80 nm with Sn 50%) demonstrates lower sheet resistance compared to

BiIIn (80 nm with Bi 50%) film. The sheet resistance of SdIn film with other Sn ratios are

studied in the next section.

4.4.1 Detailed Research on SnIIn Film Electronic Properties

In order to conduct detailed research into the electrical properties of exposed SdIn film, a

series of sheet resistance tests have been performed for films of 120 nm thickness with different

Sn ratios and varying laser exposure power using 50 mm lens and raster scan conditions of the

section 4.2. Figure 4.10 shows the sheet resistances of films with Sn ratios of 0%, 5%, 10% and

50% at different exposure powers. In the experiments, the laser exposure power changed from

0 W to 0.4 W. The average sheet resistance test data were obtained with an accuracy of +5%. It

was obvious that the film with Sn 10% gives the lowest sheet resistance. This result is consistent

with the literature [30,31,32], in which deposited I T 0 (Indium Tin Oxide) films show lower sheet

resistance (around 10 o h d s q - 200 o h d s q ) at a SdIn ratio of 10%. These reports also explain

that film resistance is related to the Sn content of the film because Sn acts as an intentional dopant

[31,32,35]. The high conductivity of I T 0 film comes from the oxygen vacancy and the excess

metal In ions which form in the structure of the film [35].

0 0.1 0.2 0.3 0.4

Laser Power (w )

Figure 4.10: Sheet resistances for different Sn ratio films with at various exposure powers.

Assuming that the thickness of the films are uniform, the film resistivity (p) was

determined using the simple relation p = I i , x d, where d is the film thickness and R., is the sheet

resistance.

Table 4.1: 120 nm SdIn film sheet resistance, film resistivity and optical density

Table 4.1 shows the sheet resistance, resistivity and optical density of film versus laser

power and SdIn ratio. The data indicate that the film's resistivity increases with a rise in

exposure power. On the other hand, the different Sn ratios of 0%, 59'0, 10% and 50% exhibit

different resistivity values even when scanned by the same laser power. This gives evidence that

Laser scan power (W)

Sheet resistance (L2 1 sq) *5%

Film resistivity (x10-4 L2 cm) *5%

Snlln(60/60 nm, Sn 50%)

Optical Density at 365 nm k0.002

0.4

353

42

0

17.2

2.06

Sn/ln (61114 nm, Sn 5%) Pure In (120nm, Sn 0%)

.085

17.5

2.1

Sn/ln(12/108nm, Sn 10%)

2,98

0.4

741

88.9

0

16.8

2.01

0.4

774

93

0

32.4

3.88

.9

17.4

2.09

.085

32.8

3.9

.085

17.5

2.1

.085

17.5

2.1

0.4

135

16

the electrical properties are influenced both by the laser induced oxidation state and the

composition of the film. It is also noted that the film's resistivity (2.1 ~ 1 0 . ~ Q cm:) for Sn 10%

film (exposed by 0.085 W) is close to the resistivity of IT0 films, which is around 1 x10-~ to

40 x10-~ Q cm [31, 32, 331. From the data for the Sn 5% film, the resistivity and related OD

changes can be seen to occur simultaneously. When the film is exposed by a 0.085 W laser beam,

the resistivity value is almost the same as unexposed film; however, the optical density drops

from 2.99 to 1.8 (at 0.085 W). When the exposure power reaches 0.4 W, the resistivity goes to

88.9 x10-~ Q cm, while the OD drops to 0.52. This means that in some exposure power ranges,

low OD and low resistivity can be obtained at the same time. Figure 4.11 shows the plot of OD

and R, of SdIn (121108 nm, Sn 10%) vs laser exposure power.

0 0.1 0.2 0.3 0.4 laser power (W)

Figure 4.11: Plot of OD and Rs of SdIn 120 nrn (SnlO%) vs exposure power.

A further increase in Sn ratio (larger than 10%) did not produce any marked improvement

in film conductivity. According to a study by Radhouane Be1 [33], increasing the IT0 films'

conductivity via doping has its limitations. At a higher doping level, the conductivity of the film

is lower than expected. The reason is that part of the dopant in the heavily doped film becomes

inactive and decreases the mobility of carriers. This explains the result where Sn 50% ratio film

shows higher resistivity than Sn 10% film in Figure 4.10.

In experiments, it was noted that the electrical properties of the SdIn film largely

depended on the film's thickness. The thinner film shows much higher sheet resistance. In

Table 4.1 it is shown that the resistivity of the thin pure In films (2 x10'Q cm) is higher than that

of In bulk materials (8 x 10.~ Q cm). Research in the literature suggests that the surface of a thin

film affects the conduction of charges by interrupting carrier transit along their mean free path.

They may either be diffusely scattered or be reflected so that only their velocity component

perpendicular to the surface is reversed 132, 331. Another possible reason is that thinner films

contain more defects than thicker films. What is important here is that the laser corwerted SdIn

behaves very similarly to other deposited I T 0 in terms of the SdIn ratio that shows fhe minimum

resistivity and the sheet resistance obtained. Hence, this confirms that laser converted SdIn films

are another candidate for creation of IT0 layers.

4.4.2 Sn/In Film Hall Measurements

A Hall measurement facility, located in the SFU physics department, was used to

measure the film's Hall mobility and carrier concentration. The facility consists of an Electro-

Magnet (MAGNET, B-M6) and an ER088 Power supply, which is connected to a field controller

(BRUKER, B-H15) and a desktop computer.

Hall measurements were obtained by using a Van der Pauw structure [34] at room

temperature. The sample film was Sn 10% with a thickness of 120 nm and exposed by a laser at

0.4 W. The sample current was 4 rnA with a magnetic field strength of 5 kG. The results show

that the film is an n type material and its Hall mobility is around 5.8 cm2 N . S arid its carrier

concentration is 7 x 1018 ~ m - ~ . These results are close to the values of I T 0 film deposited by other

methods [32,33,34,35], in which the Hall mobility ranges from 5 cm2 N . S to 7.2 cm2 N . S and

carrier concentration is from 7 x 1018 ~ m ' ~ to 9 x 1018 ~ m - ~ . Table 4.2 shows the measurement

conditions and results.

Table 4.2: Hall measurement conditions and results

Magnetic field Sample Hall mobility 1 kG 1 mA 1 Sheet 1 Carrier 1 Sample 1 strength current concentration concentration

Snlln film (120 nm, Sn 10%) 1.3~10 '5 cm-2 7x 10 l a cm-3 5.8 cm2 I V.S

4.5 Summary

SdIn films showed a large change in transparency after laser exposure. The transparency

is related to optical density (OD) by log (l/T) = OD. It is noted that OD drops significantly with

increase of exposure laser power. The minimum OD obtained was 0.24 for SdIn (121108 nm)

film. The maximum OD change was from 3 to 0.24. These changes demonstrate that SdIn films

are potential materials for direct-write mask making and other optical applications.

At the same time, SdIn film demonstrates good electrical conductivity after laser

exposure. It was found that the film's sheet resistance was influenced by thickness, composition

and laser exposure power. High exposure power caused more oxidation in the film, thus

decreasing film conductivity. Sn concentration in the SdIn films has a significant impact on

conductivity.

In general, laser converted laser SdIn films show unique properties, combining high

transparency and good conductivity. These properties indicate that the films may be used in

lithography and optical device manufacturing in similar ways that IT0 is currently used. In the

next chapter, a series of micro-structural and c:ompositional analysis experiments will be

presented and the details of the investigation into the causes of the film's property changes will be

discussed. In Chapter 6, we will also show that laser converted SnIIn film enables many new

applications not possible with existing IT0 deposition methods.

Chapter 5: Laser Converted Sn/In Film Structure Analysis

5.1 Introduction

Previous chapters noted that when laser light is absorbed by inorganic thermal resists it

creates a temperature increase in the exposed area. The heat energy, in turn, causes. both optical

and structural changes in the film. The iilm changes from a shiny and reflecting film into an

almost transparent one. At the same time, its conductivity changes a little. This chapter

investigates the mechanisms that cause and influence those changes. The purpose is to confirm

that the laser exposure is an oxidation process of the bimetallic films. Results from surface

profilometry, XRD, SEM, TEM and EDX will be presented and the detailed analysis will be

discussed. These results will help to optimise the film making process and to develop the desired

films for our applications. As a comparison, some BiIIn structure analysis results will also be

presented.

5.2 Profilometry Test

Surface profilometry is a technique in which a diamond stylus, in contact with a sample,

can measure minute physical surface variations as a function of position. Surface profilometry is

commonly used to measure film thickness in thin film deposition and processing. As noted in

section 3.4.1, the Tencor Alphastep 500 profilometer was used to measure the film's vertical

profile, with accuracy up to 210 A.

In earlier research [4,5], it was not clear what caused the exposed BUIn and SnIIn film to

become transparent. Is it an ablation process, an oxidation process or other processes:' What kinds

of materials were formed and what kind of structural change happened? In order to help answer

these questions, a surface profilometry was used to test the film thickness changer; before and

after laser exposure. From thickness changes, some information related to the film's structure can

be obtained, and the ablation possibility can be confirmed or rebuted.

Figure 5.1 shows the profilometry picture of a SdIn (Sn 10%) film with a thickness of

120k10 nm exposed by a 0.4 W power laser (50 r n m lens, spot size 10 pm). In Figure 5.1, the

average height of the exposed area increases in thickness by about 25 +5 nm (about a 20%

increase). This same percentage increase was obtained in three other SdIn samples. This

phenomenon is very important because it shows that the laser exposure process developed here is

not a laser ablation process. If an ablation process was the cause of the OD changes of Chapter 4,

it would require about 94% of the material to be removed in order to have an OD drop from 3 to

0.2. Clearly, such a thickness change is not seen. The increased thickness in our samples would

be consistent with an increase in film material occurring as in an oxidation process, when the

laser beam heats the film. This will be confirmed by XRD analysis later on.

X- Direction (pm) Figure 5.1: A profilometry across the exposed and unexposed areas of a 120 nm Snfln sample.

B

6

4

LI] LI]

0 2 F= z a c

- 2

X- Direction (pm)

Figure 5.2: A profilometry of the exposed and unexposed areas of a 120 nm Binn sample.

Similar results were found for Bi/In films. Figure 5.2 shows the profilome.try scan of a

120 nm Bi/In (Bi 50%) film exposed by a 0.6 W power laser (spot size 10 pm). An average

increase in thickness of about 40+5 nm (average increase 33%) was observed. As with SdIn film,

a possible reason for this result is that Bi/In oxidized and the material volume increased.

5.3 X-ray Diffraction of Exposed and Unexposed Film

X-ray Diffraction (XRD) is a powerful non-destructive technique for ana.lysing solid

materials, especially for crystalline materials. It provides information on structure, phase,

preferred crystal orientation (texture), and other structural parameters such as average grain size

and crystal defects. The purpose of XRD measurements is to confirm that the laser exposure

process is an oxidation process and to discover what happens for a partial exposure condition.

XRD diffraction peaks will appear if certain inter-planar distances and incident angles

satisfy Bragg's law (equation 5.1). It can be described as

where d(h,k, l) is the distance of crystal planes with Millar indices (h, k, l ) , 8 is the X-ray incident

angle, 1 is the wavelength of the incident X-ray and n is an integer. The standard XRD data for

different materials are available in the JCPDF (Joint Committee for Powder Diffraction Files)

database [29]. The possible materials and structures can be identified if the peak positions match

the data in the database.

As noted in section 3.4.3, a Philips PW 17130 x-ray generator with a CuKa, (=IS418 A)

x-ray source was used to perform the XRD test. The sequence settings were: initial angle lo0,

final angle 80•‹, step size 0.05", and counting for 1.00 seconds (the time to collect X-ray

diffraction), incident slit lo, receiving slit lo, voltage 45 KeV and filament current 40 mA. Before

the XRD test, the diffractometer's system errors were calibrated by using a poly-Si sample (see

Appendix A-1 to A-4).

5.3.1 Single Layer Sn and In Film XKD Analysis

The indexing of exposed SnIIn encountered difficulties since many XRD peaks

overlapped with each other. Considering that XRD indexing for a single metal film is relatively

simple and the XRD data are well documented, the single Sn and In film XRD indexings were

conducted first. Since there is only one metal element in the film and no other process than

oxidation, the peak indexing is relatively easy.

In order to get the XRD peaks from single metal films, pure Sn and In were IIC-sputtered

(as shown in Chapter 3) on glass slides. The argon laser system of Chapter 3 with different power

levels (focused by a 50 rnm lens, spot size 10 pm, scan rate 1 cmls) was used to raster-scan the

samples to make large laser-converted areas, upon which XRD could be performed.

10 30 50 70

2 theta (Degree)

Figure 5.3: XRD of pure In film on glass substrate at as-deposited, 0.4 W and 0.9 W laser exposure powers.

5.3.1.1 Laser Scanned Pure Indium Films

Figure 5.3 shows XRD curves for three In (60 nm) samples: as-deposited film (0 W), film

scanned with a 0.4 W laser (50 mm lens, spot size 10 pm), and at 0.9 W. As expected, the as-

deposited film (1.2 OD) displays the typical diffraction peaks of indium powder. All I-he expected

major peaks show up. From left to right, the peaks are (101), (002) and (112), respectively. No

preferred orientation is observed. When exposed with a 0.4 W laser (where the OD drops to 0.86),

the strong In (101) peak remains but now In203 peaks of (222), (400) and (440) are observed.

This indicates partial oxidization of the In film. When the laser scan power increases to 0.9 W

(with a 0.24 OD), all the In peaks disappear. It is interesting to note that even the irtdium oxide

peaks no longer exist. This may be due to crystalline orientation change of the converted film, or,

more likely, the film became an amorphous material. Appendix Tables Al-1 and A1 -2 show the

index data for the single In film.

2 theta (degree)

Figure 5.4: XRD of Sn 45 nm film as-deposited and exposed with 0.4 W, 0.9 W laser.

5.3.1.2 Laser Scanned Pure Sn Films

Figure 5.4 shows XRD curves for three Sn (45 nm) samples: as-deposited film (0 W), a

film scanned with a 0.4 W laser (50 rnrn lens, spot size 10 pm), and a film scanned a.t 0.9 W. The

as-deposited film (with a 0.98 OD) displays three main diffraction peaks of tin: Sn(2.00), Sn(211)

and Sn(112). When exposed with a 0.4 W laser (the OD drops to 0.4 OD), two Sn oxide peaks,

SnO(201) and Sn02(200), appeared. This indicated that tin was oxidized. When the laser scan

power increased to 0.9 W (0.18 OD), all peaks disappeared, suggesting that the film was

completely converted into an amorphous structure or misorientation. The detailed XRD index

data are shown in Appendix Tables A2-1 and A2-2.

These results clearly suggest that pure films of In and Sn undergo at least partial

oxidation with sufficient laser exposure.

5.3.2 XRD Analyses of SdIn Bilayer Films

XRD was performed on SdIn bilayer films and compared to the single material film

structure analysis. With the addition of another metal layer, the situation becomes more complex.

In a bilayer film system, in addition to an oxidation process, alloying and doping processes

happen at the same time. In most cases, the argon laser system of Chapter 3 with different power

levels (50 mm lens, spot size 10 pm, scan rate 1 crnls) was used to expose the films.

Figure 5.5 shows the XRD curves for SdIn film (Sn 50%), 120 nm thick, exposed with

different laser powers using 50 rnrn lens with a spot size 10 pm, unless otherwise stated these

same parameters will be used for all exposure in this section. Four curves in Figure 5.5 represent

XRD for as-deposited (2.87 OD), 0.085 W (2.1 OD), 0.4 W (0.6 OD) and 0.9 W (0.24 OD) laser

exposed film, respectively. The XRD index analysis was done and the result is shown in

Table 5.1. -8

20 3 0 40 50 6 0 7 0 80 2 Theta (degree)

Figure 5.5: XRD of SdIn (Sn 50%) 120 nm thick film exposed at 0 W, 0.085 W, 0.4 W and 0.9 W laser power.

Table 5.1: Results from XRD for S n , n 120 nm (Sn 50%) film (shaded areas match with a JCPDF data base).

1: as-deposited film (Snnn 120 nm, Sn 50%)

Measured data

* 0.05

2: 0.085 W scanned film

3: 0.4 and 0.9 W scanned films 2 theta

(0.4,O.g W) * 0.05 31.05 1001 35.49 [20]

ImSn In203

k\*oa (22i?#IOO]

4447 {;100)[@1

In Sn

31.45

Table 5.1 shows the detailed indexing of XRD scans for S n h 120 nm (Sn 50%) film. In

Table 5.1-1, the pure In and Sn peaks appeared, such as Sn(200), Sn(211), Sn(112), In(lO1) and

In(002). No Sn oxide or In oxide material peaks appeared in the as-deposited film, showing that

no oxidation happened before laser exposure. Also, no In-Sn alloy peaks appeared, suggesting the

film consisted of separated, unalloyed materials. After 0.085 W laser exposure, those metallic

peaks still appeared (see Table 5.1-2), showing that the metallic structure was changed very little,

which is consistent with the very small change in OD (2.1 OD). After 0.4 W or 0.9 W laser

exposure (see Table 5.1-3), those peaks became weaker or disappeared and at the same time In

oxide peaks appeared, such as In203(222) and In203(400), while no tin oxide peaks appeared.

Meanwhile the transparency significantly changed (to 0.6 OD and 0.24 OD). These peaks indicate

that oxidation happened during the laser exposure process.

Some single metallic film XRD results were used in the indexing process. For example,

in Table 5.1-2, the peak at 32.96' represents two possible materials, one is Sn2O3(130) and

another is In(lO1). Because Sn2O3(130) never appeared in single Sn XRD analysis (see Appendix

Table A2-2) and its intensity is lower than In(101), In(lO1) was chosen as the structural material.

The results show that after high laser power exposure, material structure in the film is a

metal oxide structure.

5.3.3. Structure Comparison of Exposed SnDn Film to IT0

Since the exposed SdIn film has the same tin-indium ratio as Indium Tin Oxide (ITO)

film, which is widely used in industry, it is interesting to compare both films' structures. In a

typical I T 0 film sputter-deposition process, Sn 10%-In 90% high purity (99.99%) alloy target

was used. The I T 0 layers were deposited in pure O2 ambient. The oxygen gas pressure during the

deposition was Torr. The substrates could be heated or kept at room temperature, depending

on the different applications [34,35,36]. Now let us compare the XRD results of laser formed

material to the XRD of IT0 in literature.

Figure 5.6 shows XRD curves for a 120 nm thick, SdIn (Sn 10%) film. The bottom curve

is the as-deposited SnIIn film (with a 2.87 OD). The curves above it belong to films exposed by

0.1 W (with a 1.32 OD), 0.4 W (with a 0.34 OD), and 0.9 W (with a 0.24 OD) respectively. The

pure In(lOl), In(002) and Sn(112) peaks appeared in the as-deposed film. When the film was

exposed to 0.1 W, indium oxide peaks In203(222) and In2o3(400) started to appear in addition to

In(002), which agrees with the large drop in OD from 2.87 to 1.32 due to oxidatilon. With the

increase in laser power (0.4 W), the indium oxide peaks In203(222) and In2o3(LC40) became

stronger and the film became very transparent (0.34 OD). However, with a further increase in the

laser power (0.9 W), only a weak In203(222) peak was seen, possibly indicating strong

orientation along the < I l l > direction. However it is more likely to indicate that an amorphous

structure was formed. Yet the transparency increased further to 0.24 OD. The detailed index data

are shown in Appendix tables A3-1, A3-2 and A3-3. Clearly these results were consistent with

film oxidation.

20 30 40 50 60 70 80

2 theta

Figure 5.6: XRD of S d n (Sn 10%) 120 nm film on glass exposed at 0 W, 0.1 W, 0.4 W and 0.9 W laser powers.

2 theta

Figure 5.7: XRD pattern of I T 0 with a thickness of 150 nm [38].

Figure 5.7 gives the typical XRD pattern of' deposited I T 0 from the literature [38]. The

In203(222) peak at 31.05 degrees and In203(440) peaks at 52.1 degrees are two typical I T 0 peaks

(the intensity of the peak is sensitive to preparation process), as reported in Ming-Huei Yang's

paper [38]. By comparing the 0.4 W curve of Figure 5.6 to the I T 0 curve in Figure 5.7, it is

clearly seen that the laser exposed SdIn film has a similar structure to the deposited IT0 film.

The same peaks are present (except for the (21 1) IT0 peak) in almost the same intensity ratios.

However, at 0.4W, there is the addition of a In(002) peak which indicates that full oxidation has

not yet occurred. As noted in Chapter 4, these films showed resistivities similar to ITO. That

suggests the laser exposed SdIn film can be used as an alternative to deposited IT0 l'ilm.

10 20 30 40 50 60 70 80 2 theta (degree)

Figure 5.8: XRD of Sn/In (Sn 30%) 120 nm thick film on glass exposed at 0 W, 0.1 W, 0.4 W and 0.9 W exposure powers.

Figure 5.8 and Figure 5.9 show the XRD curves of SdIn films with 30% Sn and Sn 5%

compositions. In Figure 5.8, the bottom curve is the as-deposited SdIn film (with i i 2.91 OD).

The curves above it belong to films exposed by 0.1 W (with a 1.35 OD), 0.4 W (with a 0.36 OD),

and 0.9 W (with a 0.28 OD) laser power respectively. The pure Sn(lOl), In(lOl), In(002),

In(112) and Sn(112) peaks appeared in as-deposed film. When the film was exposed by O.lW,

similar peaks appeared, showing that there were no big changes in the film's structure and that

the absorption was still high at 1.35 OD. When laser power increased to 0.4 W, indium oxide

peaks In203(222) and In203(400) started to appear. However, with a further increase in the laser

power (0.9 W), no peaks were seen, again possibly indicating that an amorphous structure was

formed. Appendix tables A4-1, A4-2 and '44-3 shows the peak index data for the filrn.

2 theta (degree)

Figure 5.9: XRD of SnDn (Sn 5%) 120 nm thick film on glass exposed with 0 W, 0.1 W, 0.4W and 0.9 W laser powers.

Figure 5.9 shows the XRD curves of a SnIIn film with Sn 5% (120 nm thick). The bottom

curve is the as-deposited Sn/In film (with a 2.80 01)). The curves above belong to films exposed

to 0.1 W (with a 1.31 OD), 0.4 W (with a 0.29 OD), and 0.9 W (with a 0.22 OD), respectively.

The pure Sn(lOl), In(lOl), In(002) and Sn(211) peaks appeared in as-deposited film. When

exposed by 0.1 W, a Sn304 peak appeared in addition to other same peaks, showing the beginning

of oxidation. With the increase in laser power (0.4 W and 0.9 W), the indium oxide peaks

In203(222) and In2o3(440) appeared strongly. This again suggests again an IT0 like structure was

formed. Appendix Tables A5-1, A5-2 and A5-3 show the peak index data for the film.

From Figure 5.6, 5.8 and 5.9 XRD results, it is noted that the exposed films with Sn 10%

and 5% have similar structures to that of IT0 films. Therefore, those exposed Sn/In lilms may be

used to replace IT0 film in some applications.

5.4 SnIIn Film TEM analysis

Transmission Electron Microscopy (TEM) is another common technique used for

analysing solid film's structure. TEM uses electron diffraction to get information about the

structure and can provide a resolution on the order of 0.2 nm.

5.4.1. TEM Sample Preparation

Generally, a TEM sample must be thinned down to less than 30-40 nm. The quality of the

prepared sample contributes greatly to whether the TEM picture will be good or not. So in these

experiments, great effort was taken in the preparation of TEM samples.

Attempts were made at using ion-milling to thin down exposed SdIn and BVIn films on

silicon substrate, but it was later found that the silicon had a negative influence on the diffraction.

An effort was made to use an HF solution to remove the exposed film from the glass substrate,

but the film chemically reacted with the solution. The films were also directly DC-sputtered on

Teflon tape in order to allow the film to be peeled off after laser exposure. However, because the

film was too thin, it was impossible to remove from the tape as a useful layer.

In the end, Lacay-Carbon-coated-formvar TEM grids (300 mesh copper) were

successfully used to make TEM samples. Sn and In with a thickness of 12/12 nm were DC

sputtered on the grids. Second, an Nd:YAG laser operating at the 2nd harmonic (532 nm) with

very low laser power (less than 1 rnT/cm2) exposed the film for a very short duration (one 4 ns

pulse). This short exposure was used because the regular argon laser conversion of SnIIn resulted

in long exposures destroying the carbon, due to the high thermal conduction to the substrate.

Previous work had shown short Nd:YAG exposures would convert the film properly [24].

Figures 5.10(a) and 5.10(b) show a SdIn sample made by this method. Figure 5.10(a) shows a

back lit picture of the sample. Figure 5.10(b) is an enlarged picture, in which the exposed area is

labelled. The power distribution of the laser beam followed a Gaussian distribution. Thus,

although the central section of the exposed area was totally burned because of the higher laser

power, the oufer section was successfully exposed and became transparent. Looking at Figure

5.10(b), i t is clear that there were three areas: (1) an as-deposited area which showed even grains

on the surface; (2) high level laser exposed areas i n which the films were burned completely and

holes left; and (3) transparent areas be twxn thc regions described above where t l e film was

properly exposed and their structures were zhanged.

Figure ,lo: (a) and (b):Sn/In TEM samples made on formvar grids: (a) is a back lit picture of the sample after laser exposure. (b) is an enlarged picture, in which area (1) is as-deposited film, (2) is laser burned area, (3) is transparent area, in which the film was e~posed properly.

In Figuse 5.11, a series of diffraction patterns were obtained from area (3). Thcse patterns

were different "roo the diffraction patterns that came from the as-deposited films (area (I)),

indicating the film's structure has changed after laser exposure.

Figure 5.11: Electron diffraction patterns of as-deposited (a) and exposed (b) Snlln film.

5.4.2. Electron Diffraction Patterns

Electron diffraction happens when the Bragg condition is satisfied. Below are the

related formulas:

Here, A is the wavelength of electron, as determined by TEM acceleration voltage. At 200 kV,

A=2.5080 x A. C is the velocity of light. The crystal lattice constant at 300 K temperature is

given as a,. The R represents the radius of rings on the diffraction pattern. The letters h, k, I are

the Miller indices of crystal plates. L is the TEM camera length that is not measured directly; it

can be determined by calibration with a substance with a known lattice constant.

5.4.3. TEM Calibration by Using GaAs Standard Samples

In order to precisely index the TEM diffraction patterns, calibration is needed to get the

correct camera length L. Only substances with the following properties can be used for

calibration: (1) a sharp diffraction pattern with known dhkl. (2) no change in lattice parameters

under electron irradiation, and (3) a previously determined lattice constant a,..

In our case, a GaAs standard sample was used to calibrate the TEM camera length. GaAs

has a Zinc Blende structure, which is an fcc lattice with a 2 atom basis. Figure 5.12 shows the

diffraction pattern of GaAs film. The GaAs lattice constant, a,, is 5.65325 A at 300 K:. The radius

of the diffraction pattern for lattice-planes spacing of [040] is R = 14.5 + 0.5 mm. From the

formula

2 112 d = a o l ( h 2 + k 2 t - 1 )

comes the crystal plate spacing d: d = a, / 4 = 1.4133125 A.

Figure 5.12: The electron diffraction pattern of the GaAs standard sample.

The camera length can be calculated by using the following formula:

From the obtained data comes the camera length: L = 0.8008 +. 0.0280 m.

5.4.4. TEM Electron Diffraction Patterns of Exposed SnIIn Film

Figure 5.11 shows the TEM diffraction patterns and related Bright Field (BF) images from

laser exposed and as-deposited SnIIn films. Table 5.2 and Table 5.3 show the TEM diffraction

data compared with JCPDF data. The shaded area shows possible matching compounds. It can be

clearly seen that for as-deposited SnIIn film the possible structures are: InSn4, In, In!Sn and Sn.

This result shows only small differences from our XRD analysis, in which Sn and tn have not

formed any type of alloy structures before laser exposure. The reason for this is that the TEM

sample has very thin Sn and In films, and it is easy to form an alloy during the sputter deposition

process. In the tables, the average error includes measurement errors and camera length errors.

Table 5.2: Comparison of TEM data for as-deposited S d n film with JCPDF data (shaded areas are data that matches the JCPDF data base).

Calculated d value (A) possibly matched compounds

Compared with as-deposited film, the exposed film diffraction pattern shows more rings,

indicating that other structures formed after laser exposure. Analysis shows the most likely

material structures are In203, Sn02, Sn203, SnO, Sn304, Sn and In (see shaded areas in Table 5.3).

This result was consistent with the XRD result. It again shows that the laser exposure process is a

laser induced oxidation process, and metal oxide structures are the main structures of the film

after laser exposure.

From BF images the grain information was also obtained. Before the laser exposure, the

grain boundary was not sharp, after laser exposure the boundary changed to a sharp one.

Table 5.3: Comparison of TEM data for exposed SnAn film with JCPDF data (shaded areas are data that matches the JCPDF data base).

Calculated d value (A) possibly matched compounds

5.5 EDX Composition Analysis

To help identify the converted films, a FEI 235 SEM (Dual beam with Focused Ion

Beam) with an Energy Dispersive X-ray (EDAX) analysis function was used to investigate the

film's element composition and distribution. The system has an EDS spectrum database, which

can be used to conduct peak identification and quantitative analysis. The purpose of this

experiment is to obtain the elemental composition about the as-deposited and exposed films.

SnIIn (Sn 30%) film of thickness 120 nm was deposited on a Si(100) wafer. A 1 cm2

window was created by 0.3 W laser scanning (50 rnm lens with spot size 10 pm) The measured

spectra was compared with the database, peaks were matched, and quantitative information was

obtained. Figure 5.13(a) shows that only 3 wt% oxygen exists in as-deposited film, which may

come from the surface of the silicon wafer or film. However, after 0.3 W laser scanning, the

amount of oxygen increases to 14 wt%, as shown in Figure 5.13(b). These results again suggest

that the laser exposure increases the extent of oxidation in the film, indicating that the process is

an oxidation process.

Figure 5.13: EDS of as-deposited (a) and 0.3 W laser exposed (b) SdIn (Sn 30%) 120 nm film.

5.6 Post Annealing Film in Different Environments

Since SdIn film is being oxidized by the laser rather than in a furnace, furnace annealing

tests may show if the same results would be produced by both processes. A series of annealing

experiments were designed for this purpose. The films were annealed in air and steam

environments at different temperatures.

5.6.1 Annealing in Air

As-deposited SdIn 120 nm (Sn 10%) films on glass substrates were annealed in air at

different temperature in a furnace. For films annealed at temperatures below 350•‹C there were no

obvious OD changes [24].

O I I

350 450 550 650 750 850 wavelength (nm)

Figure 5.14: Post annealing SdIn film (120 nm, Sn 10%) in air at 550•‹C.

After the as-deposited (120 nm, Sn 10%) SdIn film (about 2.7 OD at 365 nm

wavelength) was annealed at 550•‹C for 30 min, the film become more transparent with an OD of

1.2 at a 365 nm wavelength (see Figure 5.14). This shows that high temperature annealing can

cause film's OD to drop. But detailed research shows that the OD drop (about 1.5 OD) caused by

furnace annealing is much smaller than the OD drop (2.7 OD) caused by laser exposure, even

though the annealing temperature (550•‹C) is much higher than the simulated laser exposure

temperature (40O0C)[39]. This means that the laser exposure process is different from high

temperature annealing. The reason for this is still not fully understood; therefore, more research is

needed.

5.6.2 Annealing in Steam

As-deposited SnIIn 120 nm (Sn 10%) films on glass substrates were also annealed in a

steam environment at 5 5 0 ' ~ for 30 rnin. It was found that steam annealing yields even higher OD

drop in the short wavelength range compared with annealing in air. Figure 5.15 shows the

difference in the OD variation after the films were annealed in different environments.

g 1 . 5 4 Annealed in air

0.5 Annealed in steam

350 450 550 650 750 850 wavelength (nm)

Figure 5.15: SnAn 120 nm (Sn 10%) film's OD before and after annealing in air and steam.

For as-deposited films annealed (550•‹C) in air for 30 min, the OD drops to 1.2 OD at

365 nm (I-line), but for films annealed in steam with the same conditions, the OD can drop to

1.0 OD at 365 nm compared to air. Steam (water) allows faster oxidation.

5.7 Summary

This chapter investigated the mechanisms that cause and influence the SdIn film's

property changes. Surface profilometry, XRD, SEM, TEM and EDX analyses werl? used in the

investigation.

The surface profilometry test showed that the SdIn film's thickness increaozd after laser

exposure. This indicates that the laser exposure process developed here is not a laser ablation

process. A possible reason for thickness increase is that an oxidation process happened in the

film. This has been confirmed by XRD and TEM analysis.

XRD experiments were conducted for Sn and In single metal films. It was found that all

single metal films were partially oxidized or fully oxidized after the laser exposure. SdIn binary

metallic film's XRD peaks were indexed and results showed that no oxidation happened before

laser exposure. When the film was exposed with low laser power exposure (c 0.9 W), the most

likely material structures were In203, Sn02 and SnO. After sufficient laser exposure (:>0.9 W), the

film's structure changed to an In203 structure or formed an amorphous structure. This indicated

that metallic material was oxidized after laser exposure. Detailed research showed that exposed

SdIn film had similar structure to that of typical deposited IT0 film, showing that the laser

exposed SdIn film can be used as an alternative to deposited IT0 film, which is widely used in

industry.

TEM analysis was also conducted for SnIIn films. The samples were made on Lacy-

Carbon-coated-formvar TEM grids. The TEM results confirmed that an oxidation process

happened during the laser exposure process.

Research found that high temperature furnace annealing in air and steam can cause film

OD to drop as well. But detailed research showed that the OD drop (about 1.5 OD) caused by

furnace annealing is less than the OD drop (2.7 OD) caused by laser exposure. This means that

the laser exposure process is different from high temperature annealing.

The next chapter will demonstrate some applications of SdIn films. These applications

will show that the laser activated SdIn film is a promising film, which could be used in future

lithographic and device making processes.

Chapter 6: Applications of Sn/In Film

6.1 Introduction

Laser exposed SdIn film shows a high transparency in the W and visible wavelength

range and possesses good conductivity at the same time. This unique combination of properties

suggests that the films may have many potential applications. Since SdIn film showed a large

transparency change after laser exposure, its potential as a direct-write photomask and greyscale

mask material was investigated. Because the etch rate of exposed film is much smaller than that

of Si, an investigation into using the film as an anisotropic etch mask was also conducted. Since

exposed SdIn film has a similar structure to deposited ITO, applications such as patterned I T 0

and the photovoltaic effects were also investigated.

6.2 SnIIn Laser Direct Write Binary Mask

Photomasks with smaller features, fewer defects, larger areas and lower cost are required

by the microfabrication and micromachining industries. From the previous study, it is noted that

SdIn film has a large OD change from 3 into 0.24 after laser exposure. This OD change is much

larger than other existing direct-write materials such as HEBS (High Energy Bea.m Sensitive

Glass), a glass that darkens with e-beam exposure, whose OD changes from 1.9 to 0.4 at the

365 nm [15,18]. Therefore, SdIn film has the potential for making a direct-write (no

development and chemical etch) binary photomask.

Figure 6.1: (a) Snhn (Sn 10%) direct write binary

I mask back-lit: film thickness is 120 nm, laser

power is 0.5 W (50x objective lens), the feature line is 2 pm wide. (b ) Shiplc:i SPR2FX- 1.3 photoresist pattern from the SnIIn mask after exposure and development.

Figure 6.l(a) shows a simple binary SdIn photomask consisting of a group of 2 p m

lines on 6 ,urn spaces made under the following conditions. A laser system script file was uscd to

control the X - Y table, which was described in Chapter 3, section 3.3, to create the rr~ask using a

raster scan procedure. A 50x objective lens was used to focus the argon laser beam (0.5 W) to the

spot size of 2 pm. The X-Y-Z table speed along the X direction was lcmdsec. The raster-scan

increment (along the Y direction) was 6 ,urn. Sn/In film (Sn 10%) with a total thicknes:; of 120 nm

was used, whic i gave a maximum transparency of 0.24 OD (at 365 nnl) at the expos,:d area and

high absorption of 3 OD at the urlexposed area. The feature line width was 2 ,urn. Since the line

width was limited by the laser exposure system used, structures smaller than 2 ,urn could be

produced if a higher power objective lens was used. This simple mask shows that Snlln film can

produce a direct-write binary mask.

To test )he Sd In mask, photoresist was exposed with the mask seen in Figure 6. I(a). The

Shipley SPR2FY-1.3 photoresist (about 1 pm thickness) was coated on a Si wafcr and soft

baked 171. The resist was successfully expo:;ed by using the Quintel mask aligner wit1 the SnIIn

direct-write mask. The exposure time was 10 seconds at 10 niw/cm2 light intensity, which is

typical for chrome masks. Figure 6.l(b) shows the photoresist pattern after developrient in the

Shipley MF-319 for 60 seconds. This demonstrates that the SdIn masks work as direct-write

photomasks.

6.3 SnIIn Greyscale Mask

The results of Chapter 5 showed that SdIn (120 nm) film's OD does not change when the

laser power is lower than 100 mW (see Figure 6.2). The OD then drops sharply from the 100 mW

exposure power and until it saturates at about 900 mW. In the power range of 200 mW to about

500 mW, the OD decreases almost linearly with the increase of laser power (see Figure 6.2), as

described in Chapter 4. Therefore, one can achieve different absorption in the exposed area and

hence different greyscale levels by controlling the laser power. This property can be used to make

greyscale photomasks.

10 1 00 1000 laser power ( m w )

Figure 6.2: Relationship of OD changes with the laser power for Snnn (120 nm) film.

In the greyscale mask making process, as shown in Chapter 3 [40,41], an 8-bit greyscale

bitmap file is loaded into the laser table control computer. According to the greyscale value of

each pixel of the bitmap pattern, the computer sends out a signal to the optical shutter which in

turn controls the amount of laser light that passes through it. The computer also controls the table

movement in accordance with the shutter so that the aspect ratio of the bitmap image is

maintained. The shutter is fully open when the greyscale value is O (black in the bilmap image)

and the shutter is fully closed when the value is 255 (white in the bitmap image).

0 100 200 300 400

X-axis distance

Figure 6.3: (a) Plot of intensity vs X-axis position, (b) data mask gray levels, (c) S d l n greyscale mask made with (b) data.

Figure 0.3(b) is an S-bit greyscale bitmap image used as the input file by the laser mask

writing system. The pattern's greyscale changes linearly from left to right. Figure 6.3(a) is the

plot of two optical density percentage versus X-axis position curves (labelled 1 and 2). Using the

8-bit greyscale ti tmap file in Figure 6.3(b), the two greyscale patterns are created. Figure 6.3(c) is

the back-lit image of a greyscale mask made on Sn/In 45 nm (Sn 1096, 1.1 OD) film. Note that

the witten patk rn is the negative of the data file. These results suggest tha: similar grey level

changes have bezn achieved.

The detailed writing pammeters are as follows: the Argon laser (514 nm) power was

0.15 W. A 50 mm lens created a spot size of 10 pm. The laser writing speed was 500 pmls in the

Y direction and the raster-scan X direction increment was 5 pm. The S d I n film showed different

transparency dependent on the exposed laser power, and finally the greyscale mask was created

(see Figure 6.3(c)).

Figure 6.4: Snhn 45 nm (Sn 10%) greyscale mask with 3 grey level strips: (a) 8 bit greyscale bitmap file. (b) SnIIn greyscale mask (dark area with 0.72 OD at 365 nm).

T o demonstrate the greyscale characteristics, Figure 6.4(a) shows a greyscale bitmap file

with 3 grey level strips ( loo%, 3096, and 111%). Figure 6.4(b) shows the Sn/In (45 nrn, 1.1 OD)

greyscale mask made by using the Figure 6.4(a) greyscale bitmap file. By using this greyscale

mask, a 3D st~ucture has been made on a conventional photoresist. Figure 6.5 shows the

profilometry of a 2-step structure made on the Shipley SPR2FX-1.3 photoresist with he 3 grey-

scale strip photomask. The 3D structure was mdde under the following conditions: (1) Shipley

SPR2FX-1.3 photoresist was spin-coated (at the speed of 2500 rpm) on a bare silicon wafer with

a thickness of about 1.75 pm a d soft baked for 10 m.n. (2) A Quintel 4 inch mask aligner with a

Hg source (wavelength: 365 nm, light intensity: 10 m ~ l c m " was used to expose tlie resist with a

10 seconds exposure time. (3) After the development (using Shipley MF-319 developer for 60

seconds) good patterns were created in the photoresist as shown in Figure 6.5.

Figure 6.5 shows profilometry of the resist pattern. The step hights are 1.75 pm, 0.7 pm

and 0.2 pm, which correspond to loo%, 40% and 11%, close to the target of loo%, 30% and

10%. The difference between the target height ratio and the measured results is po:~sibly due to

the relative exposure development rates of the resist. This demonstrates SnAn greyscale

operation.

Figure 6.5: The profile of a 2-step structure made on Shipley SPR2FX-1.3 photoresist.

6.4 Development of Sn/In to Leave Patterned I T 0 Layers

Typical I T 0 film patterning is a complex process [35,43]. It includes: (a) IT0 film

sputtering deposition (described in Section 5.33 of the Chapter 5). (b) photoresist being spun on

the I T 0 film surface. (c) photoresist exposure. (d) photoresist development. (e) unprotected IT0

film being etched away by using the solution (HC1:H20:HN03 = 4:2:1 by volume) at room

temperature, (f) and finally, photoresist striping. Furthermore, the typical I T 0 chemical etching is

sensitive to the IT0 stoichiometry and deposition method, so the process is difficult to

control [48].

Compared with typical IT0 patterning process, the process of laser patterning SdIn film

is relatively simple. It only requires three steps: ( I ) the SdIn film with required thickness and

ratio is deposited on the substrate, (2) a focused laser beam writes the designed patterns on the

film, and (3) use of a etchant (developer) etches away the unexposed SdIn film and leaves the

patterned IT0 like film on the substrate.

This research found that SdIn oxide films can be developed in the same solution [24] as

BiIIn (dilute RCA2: HCI:H202:H20 =1: 1:48, at room temperature). In order to calculate the

etching rate and selectivity, the Tencor AS500 profiler was used to measure the thickness of the

film before and after development at different durations. Figure 6.6 shows the etch rate curve of

exposed SdIn film in dilute RCA2 solution. Table 6.1 shows the comparison of etching rate and

selectivity of BiIIn [24] and SdIn films. It is found that SdIn film's etching selectivity is higher

(> 62:l) than that of BiIIn film (> 60:l) (see Table 6.1). The exposed film etches at only

0.12 nmls while the unexposed film etches at 7.5 nrnls.

I I

0 2 4 6 8 10 etch time (min)

Figure 6.6: Etch rate curve of exposed SnIIn film in dilute RCA2 solution.

Table 6.1: the development etch ratio of BiIIn and S n h film in the diluted RCA 2

By u s i ~ ~ g the dilute RCA2 etch, ~ h e unexposed SnIIn can be removed rapidly. Therefore,

the patterned exposed SnIIn film can be created, which has similar properties to typical deposited

I T 0 film (shown in Chapter 5). This kind of I'TO patterning process is simple and fast (typical

development times are less than a minute).

Figure 6.7: Exposed SndIn film (120 nm, Sn 10%) after development: (a) different width patterns of S d l n film on glass (50 mm lens vith spot size of 10 ,urn). (b) Patterns (5 ,um lines) of Srdln film on Si(100) wafer (made by using 50x lens with spot size o f 2 ,urn).

Figure 6.8: Profile of developed S d I n film (Sn 10%) with 5 pm lines.

Figure 6.7 shows the exposed SdIn film on glass and silicon wafer after development.

Figure 6.7(a) shows the different width patterns (10 pm and 30 pm lines) of SdIn film with a

thickness of 120 nm (Sn 10%) on glass. The laser power used to create the patterns is 0.3 W

(50 rnrn lens with spot size of 10 pm) and development time is 40 sec. Figure 6.7(b) shows

patterns (5 pm lines) of SdIn film with a thickness of 120 nm (Sn 10%) on Si(100), the laser

power used is 0.5 W (50x lens with spot size of 2 pin) and development time is 50 sec. Figure 6.8

shows the profile of developed SdIn film (Sn 10%) with 5 pm lines. The lines iire not fully

etched at the central areas due to those areas being over-exposed by the laser beam.

The full strength RCA2 (HC1:H202:H20 = 1: 1:6) at 80•‹C can be used as the resist stripper

after the patterning is done. The exposed SdIn film can be easily stripped away and it can also

remove other metallic contaminants in the process.

6.5 Sn/In Anisotropic Etch Mask

Silicon anisotropic etching is widely used in microelectronic and micromachining fields.

But standard organic photoresists cannot work as etch masks for anisotropic alkaline etching

processes, using enchants such as KOH (Potassium Hydroxide) and TMAH

(TetraMethylAmmonium Hydroxide) [6, 71. Prev:ious research by Y. Tu [24] showed that

exposed Bi lh film can be used as a resist for anisotropic etching. In this research, similar

properties were found for the exposed SdIn films, which have a high resistance to anisotropic

etching solutions. In order to find out the SdIn film's etch rates in KOH and TMAH, a SdIn film

(100 nm) was DC-sputtered on glass slides, which previous research has shown is resistant to

KOH and TMAH [24]. Laser raster-scanning was then carried out to create an exposed area. The

slide samples were developed in dilute RCA2 to remove the unexposed film. A Tencor AS500

profiler was used to measure the thickness of the e,xposed film before and after the etching in

different etchants with different durations (20 seconds to 1 hour).

IG CALIB DIAG EXIT 28 Jan 04 13 0 W 1

Figure 6.9: Profile of exposed Sn/In (Sn 10%) film with thickness of 100 nm (before KOH etching).

-G ClLIB DIlG EXIT 28 Jan 04 13 1

xposed ?ea

I Unexppsed area MhA

0 j I

/.

X- Direction (pm)

Figure 6.10: Profile of exposed Sn/In (Sn 10%) film with thickness of 100 nm (after KOH etching).

Figure 6.9 shows the thickness profile of exposed SdIn film on the glass slide after

development. In Figure 6.9 the exposed film had an average thickness of 103 nm and the

unexposed SdIn film was completely etched away by the developer. Figure 6.10 shows the

thickness profile of the film after 1 hour KOH (50 wt%) etching at 80•‹C. As one can see the

exposed SdIn film thickness is reduced to 43 nm. However, the etching was very slow: about

60 nm of the developed film was removed after 1 hour of KOH etching. The etch rate is about

1 ndrnin (which is almost the same as BilIn film [24]). The silicon etch rate in the same KOH at

85•‹C is about 420 n d m i n [7] . The etch ratio of Si to exposed SdIn film can reach 420: 1. With

this high etch ratio, 100 nm thick SdIn film can act as a protective resist for etching of Si wafer

up to 42 pm deep with KOH.

1 KOH

0 20 40 60 80 Etch time (min)

Figure 6.11: Etch rate curves of exposed Sn/In film in KOH and TMAH.

In similar experiments it was found that the etch rate of exposed SnlIn film in

TMAH (25%) at 85•‹C is about 0.5 nmlmin. By comparison, exposed SdIn etch rate in TMAH is

about half that of KOH (100 nm thick SdIn film can be used for etching of Si wafer up to 84 pm

deep with TMAH).

Figure 6.1 1 shows the etch rate curves of exposed SdIn film in KOH and TMAH. The

glass slide substrate etch rate (5 h m i n ) in all the etchants has been compensated for when

calculating the SdIn etch rate. After the patterning i,s done, the exposed Sn/In etch resist can be

easily stripped by using RCA:! (HC1:H202:HZ0 = 1:1:6) at 80•‹C. This demonstrates SnIIn resist

operation as ar. anisotropic etch mask.

Figure 6.12(a) is a SEM picture of' trenches anisotropically etched by KOH on a Si(100)

wafer. using 1:!0 nm SnIIn film as etch mask and with an etch time of 10 min. The !$ELM picture

shows that the etch depth is about 4.5 pnl. Figure 6.12(b) is a SEM picture of TMAH etched

trenches on a Si(100) wafer, using 120 nm SnIIn film as etch mask and with a etch time of

20 min. The SElM picture shows the etch depth is about 4.5 pm.

Figure 6.12: SEM pictures of anisotropic etched t r e n c h on Si(100) wafer, (a) is KOH etched trenches using 120 nni SnIIn film as etch mask (etched for 10 min), (b) is Tb1,iH etched trenches (etched for 20 min).

6.6 Photovoltaic Property of Exposed SidIn with p-Si

According to the literature 14.3, 44, 4.51. transparent and conductive films like I T 0 can

form a heteroju ~c t ion structure with p-Si substrate (used in solar cell). This ITOIp-Si structure

creates a heterojunction diode that operates differently than regular Si photodiode. The top I T 0

layer is a transpxent n-type semiconductor which crt:ates a p-ti junction with the p-tjpe silicon,

and thus the space charge layer needed for photo-generation operation. However, as I T 0 is

transparent film, it does not absorb light, so that all the electron-hole pairs are created in the Si

layer. The I T 0 does collect carries from the Si due to the junction electric field and acts as a

transparent cortductor for the device. Thus this struclure really acts like a silicon pholodiode even

though it is a hetero-junction device.

Curve Tracer

light \

Figure 6.13: I-V characteristic test set-up of the heterojunction of laser converted Sn/h on psi.

Since the work in earlier chapters suggested our laser exposed Sn/In film had similar

properties to tr,lditional ITO, we investigated whether laser formed I T 0 showed photovoltaic

effects in a heterojunction. In our experiment, the laser exposed S d I n film was used to replace

the traditional I T 0 in the structure. The experiment was as follows: An aluminium Iiiyer with a

thickness of 600 nm was deposited on the backside of a wafer (p-Si, 2 ohm-cm resistivity,

measured by a 4 point probe) to create an ohmic contact. After this, the wafer was lmded into a

furnace with a temperature of 400•‹C and annealed for 20 mi11 for creation of the ohmic contact

laycr in Nitrogen gas (at a rate of 4-.0 SLPM:. Afier removal from the furnace, the bilayer of Sn./In

film (Sn layer 011 the top) with a thickness 01' 120 nm was sputtered on the front side ol'the wafer.

An argon laser (0.4 W) with 50 mm lens was used to expose the SdIn film, creating a 1 cm2

window on the surface (scan rate was I c~nls) and followed by a dilute RCA developing process

to remove unexposed SdIn. Finally, a heterojunction was built by this laser expose:d SdIn film

and the p-type silicon wafer, in which the exposed SdIn film acts as a transparent n-type

material.

Under illumination 1 -

n - -0.8 -0.6 -0.4 -0.a 0

-2 -

? -

_ A - d Voltage (V)

Figure 6.14:I-V characteristic of a heterojunction of laser converted S d n on p-Si.

Isc Maximum power point Wmp, Imp)

2 -

1 -

v o c

Voltage (V)

Figure 6.15: Typical I-V characteristic of the solar cell of laser converted S d n on p-Si.

Figure 6.13 shows how a Tektronix 17'7 curve tracer was used to test the I-V

characteristic. The data was collected by Labview software. The sample was loaded on a probe

table with the aluminium ohmic contact side down. A test probe was put on the exposed SdIn

film. An incandescent light bulb was used to illuminate the test area. The test condic.ions were as

follows: the curve tracer polarity was set to AC, Horiz voltsldiv: 0.1 V, Vertical currentldiv:

1 mA. The sample was first tested in a dark condition (using a cover to block of the room light)

and then under the illumination of the light bulb (illumination intensity is 10 m~lcrn'!).

Figures 6.14 and 6.15 show the plots of I-V curves of the heterojunction. From the curve

it was noted that the p-n junction exhibits a typical solar cell I-V characteristic. The open-circuit

voltage (Voc) is 0.63 V and short-circuit current density (Jsc) is 3.1 mAlcm2 when the junction

was illuminated (10 mw/cm2). The maximum power point: Vmp = 0.40 V, Imp = 2.5 mA. The

fill factor (FF) is calculated as 72.9%. The open-circuit voltage of our sample is about 50% higher

than that of a typical deposited I T 0 on Si from the literature [47], which is about 0.4 V. The Jsc

from our sample is proportional to that of typical deposited I T 0 on Si, which is about 32 r n ~ / c m ~

under 100 r n ~ l c r n ~ illumination (10 times higher than our illumination condition) [47]. That

scales to 3.2 mAkm2 under our 10 mJlcm2 conditions and is within the expected error of our

3.1 mAlcm2 measure value. This indicates that laser exposed S d I n film has similar properties to

deposited IT0 film and can be developed to create new devices (such as solar cells).

6.7 Summary

Laser exposed SdIn film showed high transparency in UV and visible wavelength ranges

and possessed good conductivities at the same time. This unique combination of the properties

enables the film to have many potential applications.

Since the SdIn film showed a large transparency change after laser exposure, it has been

shown that it can be used as a direct-write photomiisk material. More importantly, since S n h

oxidation and thus change in transparency is a function of the laser exposure, it ~cnables the

creation of greyscale masks. This chapter showed the direct-write binary mask anti greyscale

mask that were successfully created by using the SnIIn film.

Because typical I T 0 film patterning is a complex process, this chapter investigated laser

patterning SdIn film. Compared with the typical I T 0 patterning process, the process of laser

patterning SdIn film is relatively simple. A dilute KCA2: HCI:H202:H20 =1:1:48 can be used to

develop exposed SdIn film and leave the patterned I T 0 like film on the substrates.

SdIn film as an anisotropic etch mask was also studied because the etch rate of exposed

SdIn film is much slower than that of Si (ratio is 1: 420). Anisotropic etched trenches on Si(100)

wafer were created using the exposed SnIIn films.

Laser exposed SdIn film on p-Si can also form a heterojunction, which shows a

photovoltaic property when the film is illuminated. This property may be used in making solar

cells or optical detectors.

Chapter 7: Conclusion and Future Work

7.1 Thesis Conclusions

The laser oxidized SdIn films discussed in this thesis have the potential to be developed

into a new type of photosensitive material. The films demonstrate many unique properties, which

are important in lithography and microelectronics applications. Many of these properties and

applications have been explored in this thesis.

The S n h films showed a large change in transparency after laser exposure. The film's

OD can drop from 3 for as-deposited film to 0.24 (Sn 10% with thickness 120 nm) after laser

exposure. This OD change is bigger than that from HEBS glass, a currently marketed product.

Therefore, SdIn film can be used as a potential material for direct-write mask applications. In

addition, with part of the laser power range, the film's OD change is a function of the laser

exposure power. Thus, it can be used to make greyscale masks. In Chapter 6, we demonstrated

this application by creation of a 3 step structure in photoresist.

Surface profilometry tests show that the film's thickness increased after laser exposure.

XRD and TEM analysis confirmed the reason for this thickness increase is due to oxidation. The

results show that with low laser power exposure, the metallic films become partially oxidized,

and with sufficient laser power, all metallic material was oxidized. TEM analysis showed the

same results. At low laser power exposure, the most likely material structures are In203, Sn02,

and SnO. When the film is exposed by a high laser power (0.9 W), the film's structure. changes to

an amorphous oxide.

SdIn film demonstrates good electrical conductivity after laser exposure. This opens the

possibility for using SdIn film as a functional layer in some devices. Detailed research shows that

exposed S d h films have a similar structure to that of typical deposited IT0 film in the literature.

It is found that dilute RCA2 (HCL:H202:H20 =1: 1:48) can be used to develop the exposed SdIn

film and leave a patterned I T 0 like film on substrates. With this developer, different patterns are

successfully created on glass and Si wafer substrates. This direct exposure and etch process is

much less complicated than classical I T 0 depositions, patterning and etch processes. In the

research, a heterojunction structure made by laser exposed SdIn film on p-Si wafer is created.

The structure demonstrated a photovoltaic characteristic which is similar to the ITOISi

heterojunctions.

In this research, using exposed SdIn film as an anisotropic etch mask was also

investigated. The research shows that the exposed Sn/In film's etch rates in KOH and TMAH are

1 nmlmin and 0.5 nrnlmin, respectively. The etch ratio of Si to exposed SdIn film can reach

420: 1 for KOH and 840: 1 for TMAH. With this high etch ratio, deep trenches ('4.5 pm) on

Si(100) wafer can be created by using exposed SnIIn films as etch masks.

All of these research results show that laser oxidized SdIn film is a promising film in

future lithographic and optical device making processes. It is worthwhile to conduct further

research in this field.

7.2 Further Work

As mentioned earlier, the laser activated SdIn films showed many unique properties

compared with other reported lithographic materials. But, in order to use it in industry, further

improvements are needed.

7.2.1 Clarify the Mechanism of the Oxidation Process

Material analyses show that laser exposure of S n h film is an oxidation process. But it is

still not clear how we can control this process and optimise the parameters to get the best exposed

film. Further research work should be focused on these issues. Some important topics are:

(1) Continue the XRD analysis to precisely index the exposed film's material structure.

(2) Continue to improve TEM sample preparation to get more precise TEM analysis results.

(3) Use other advanced analysis methods to figure out the detailed structure changes in the

exposure process.

7.2.2 Improving As-deposited Film Quality

In order to use this film in the industry, I.he quality of as-deposited film needs to be

improved. One issue is to make the film smoother. This is a requirement for making high

resolution patterns and clear sidewall profiles. Some possible improvement methods are:

(1) Control the deposition parameters, (2) control the substrate temperature during deposition to

get a smaller grain size.

7.2.3 Improving Film Transparency

In order to meet the industrial photomask requirement, which requires OD changes from

3 to 0.1, the transparency of the exposed area should be increased. This may be achieved through

the following methods: (1) improving as-deposited film's quality, (2) Choosing a different

exposure environment; optimising the exposure environment so as to enhance the oxidation

process during exposure time may make the exposed area more transparent.

This research demonstrates that laser oxidized Sn/In films have the potential to be

developed into a new kind of lithographic film. With further research, this film could be used in

future microelectronic and micromachining industries.

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Appendix:

A-1: XRD calibration (1).

9'8L

L'CL

8'89

6'€9

6'3

1 'P'3

Z'6P

C'PP

P'6C 8 '3w 9'6 Z

L'PZ

8'6 1

6 3 1

01

A-1: XRD calibration (2).

E'6L

P'PL

P'69

s379

Gm6G

937s

9'6P

L'PP Ei L'6E

8'PC

8'62

6 3 2

6'6 1

s 1

0 1

A-1: XRD calibration (3).

A-1: XRD calibration (4).

Table Al-1: XRD index data of as-deposited (0 W) In (60 nm) film (relate to Figure 5.3) (shaded areas are data that matches the JCPDF data base).

Measured

2 theta (0 W) 2 theta * 0.05 [Intensity]

31.03

Table A1 -2: XRD index data of as-deposited (0.4 and 0.9 W) In (60 nm) film (relate to Figure 5.3) (shaded areas are data that matches the JCPDF data base).

Table A2-1: XRD index data of as-deposited (0 W) Sn (60 nm) film (relate to Figure 5.4) (shaded areas are data that matches the JCPDF data base).

I

2 theta I 2 theta I

Table A2-2: XRD index data of as-deposited (0.4 W) Sn (60 nm) film (relate to Figure 5.4) (shaded areas are data that matches the JCPDF data base).

Table A3-1: XRD index data of as-deposited (0 W) SnAn (120 nm, Sn 10%) film (relate to Figure 5.6) (shaded areas are data that matches the JCPDF data base).

I I I I I

Table A3-3: XRD index data of (0.4 W, 0.9 W) SnAn (120 nm, Sn 10%) film (relate to Figure 5.6).

Table A3-2: XRD index data of (0.085 W) SnIIn (120 nm, Sn 10%) film (relate to Figure 5.6). (shaded areas are data that matches the JCPDF data base).

(shaded areas are datr

(0.4,O.g W) t 0.05

:hat matches t

ln&n

SnO

31.45 (1 1 I)[IOO]

2 theta (0.085 W) t 0.05 30.67 [201

32.92 [80]

36.34 [I 001

e JCPDF data base).

31.03

54.46 52.23 (201 )POI

lnoSn

32.95 (21 1)[20] 33.01 POI

ln2O3

31 .03 [I 001

67.03

In

[lo"$!!O] 36.21 (31 2)[21

a31 (ooZXso1

Sn

@*65. (e%m oo]

Snln4

30.59 [201

Table A4-2: XRD index data of (0.1 W) Sn/In (120 nm, Sn 30%) film (relate to Figure 5.8). (shaded

Table A4-1: XRD index data of as-deposited (0 W) Sn/In (120 nm, Sn 30%) film (relate to Figure

- areas are data that matches the JCPDF data base).

30.67

32.92 32.95

5.8). (shaded areas are

Measured data

2 theta (0 W) i 0.05 30.68 [51 32.92 [I 001 36.34 [20]

44.93 [25]

54.48 [5]

data that matches

ln3Sn

2 theta (hkl) [intensity]

33.01 [20]

45.20 (41 0)[20]

Table A4-3: XRD index data of (0.4 W, 0.9 W) Sn/In (120 nm, Sn 30%) film (relate to Figure 5.8). (shaded areas are data that matches the JCPDF data base).

SnO

31.45 (I 1 1)[100]

52.23 (201 )[20]

the JCPDF

In203

30.58 (222)[100]

54.31 [24]

2 theta (0.4,O.g W)

* 0.05 31.05 1001

35.49 1201

data base).

lnrSn

In

%$as { I O l l f W ]

~ g n i r(k);?)1601

36.33

In203

3 ~ 1 3 @2aL1 35343

4400)/401

Sn Snln4

In Sn

Table AS-2: XRD index data of (0.1 W) SdIn (120 nm, Sn 5%) film (relate to Figure 5.9). (shaded

Table AS-1: XRD index data of as-deposited (0 W) SdIn (120 nm, Sn 5 %) film (relate to Figure 5.9).

areas are data that matches the JCPDF data base).

SnO

31.45 (1 1 1 )[loo]

52.23 (201 )[201

(shaded areas are data that matches the JCPDF data base).

SnO

31.45 (1 1 1)[100]

52.23 (20 1 )[20]

Table AS-3: XRD index data of (0.4 W, 0.9 W) SdIn (120 nm, Sn 5 %) film (relate to Figure 5.9). (shaded areas are data that matches the JCPDF data base).

2 theta (0.4,O.g W)

r 0.05 31.05 1001

35.49 POI

Sn Snln4 SnO

31.45 (1 1 1)[100]

ln3Sn h2O3

9$$3 (am 001 Cgzg

(400X401

In