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Barron, Lance, "High-reflectance, sputter-deposited aluminum alloy thin thin films for micro-electro-
mechanical systems" (2005). Thesis. Rochester Institute of Technology. Accessed from
This Thesis is brought to you for free and open access by RIT Scholar Works. It has been accepted for inclusion in
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[email protected].
High-Reflectance, Sputter-Deposited Aluminum Alloy Thin
Films for Micro-Electro-Mechanical Systems
A thesis submitted in
partial fulfillment of the requirements for the degree of
Master of Science in
Materials Science & Engineering
By
Lance W. Barron
Materials Science & Engineering Department
Rochester Institute of Technology
September 2005
Dr. Santosh K. Kurinec ________________________ Date: ________________
(Thesis Advisor)
Dr. S. K. Gupta ________________________ Date: ________________
(Committee Member)
Prof. Dale E. Ewbank ________________________ Date: ________________
(Committee Member)
Jason Neidrich ________________________ Date: ________________
Texas Instruments Inc.
(Committee Member)
Dr. K.S.V. Santhanam ________________________ Date: ________________
(Department Head)
High-Reflectance, Sputter-Deposited Aluminum Alloy Thin
Films for Micro-Electro-Mechanical Systems
By
Lance W. Barron
I, Lance W. Barron, hereby grant permission to the Wallace Library, of the Rochester
Institute of Technology, to reproduce this document in whole or in part, with the
knowledge that any reproduction will not be for commercial use or profit.
Signature of Author: _________________________ Date: ___________________
ii
This work is dedicated to my family,
who has always been a source of love,
support, encouragement, and inspiration
iii
Acknowledgments
I would like to sincerely thank the people who have contributed to this work with
their ideas, suggestions, and support. They truly made this project possible.
Dr. Santosh Kurinec, for providing relentless advice, encouragement, and support
throughout the project. Without Dr. Kurinec’s support, this project would not have been
possible.
Dr. S.K. Gupta, for serving on my committee and providing support for both XRD and
AFM instrumentation.
Prof. Dale Ewbank, for serving on my committee, and providing valuable insight and
feedback for optical measurement.
Jason Neidrich, for serving on my committee, his tireless efforts in securing the
relationship with Texas Instruments Inc., and as a bottomless source of optimism and
encouragement for this project.
Pat Warner, for assisting in SMFL processing and data collection.
Texas Instruments Inc., for providing material support for the entire course of the
project.
Dr. Stephen Burns and Chris Pratt, University of Rochester, for providing assistance
with nano-indentation analysis and glancing angle XRD.
Dr. Thomas Blanton, Eastman Kodak, for XRD analysis of selected films.
The Staff of the Semiconductor and Microsystems Fabrication Laboratory, without
your support, none of this work would have been possible.
Fellow Graduate Students, particularly Dan Jaeger, Eric Woodard, and Bob Mulfinger,
for the encouragement, the laughs, and the occasional commiseration during the last 18
months.
The Faculty and Staff of the Microelectronic Engineering Department, for support
and motivation during my 4+ years in Rochester.
iv
Abstract
This study comprises an investigation of highly reflective thin film for micro-
electro-mechanical systems (MEMS) consisting of reflective components. For these
applications, the desired film system must have (1) low stress, (2) high specular
reflectivity, (3) improved nano-hardness relative to pure aluminum, (4) compatibility
with traditional semiconductor fabrication techniques. This study is an in depth
investigation of both the specular reflectance and mechanical properties of thin film
reflectors formed by low-thermal (<200°C) processing. Six different aluminum films
(containing Cu, Ti, Cr) were chosen based on extensive literature research. Each film was
characterized by a variety of optical, electrical, and mechanical analytical techniques to
obtain data relating microstructure to the film’s reflectivity and mechanical properties. A
complete dielectric function for each deposited aluminum alloy has been developed
utilizing Drude-Lorentz parameters and effective media approximations (EMA) to
account for film surface topography.
Results show that copper solute addition generate films that maintain much of the
bulk reflectance of pure aluminum while refining surface morphology to create a more
specular surface consisting of smaller, more uniform grains. Electron and atomic force
microscopy and x-ray diffraction studies reveal that copper inclusion into the aluminum
lattice cause both a reduction in the preferred orientation of the film and change in the
lattice parameter. Copper concentrations of 1.0% and above lead to spatial variation in
copper content within the films, with copper precipitating out of solution. Film hardness,
resistivity, and stress increase as a function of copper content, with the absolute
magnitude of stress being acceptable for MEMS applications. The addition of titanium
and chromium to Al-1.0%Cu films cause further microstructure refinement as well as
increased stress, resistivity, and hardness. Al-2.0%Ti-1.0%Cu and Al-2.0%Cr-1.0%Cu
exhibit reflectivity spectra that differ significantly in both inter- and intraband absorption
from that of pure aluminum. The Drude-Lorentz/EMA dielectric function model has been
successfully applied to all alloy films explaining the observed reflectivity and showing an
excellent agreement between measured and Drude resistivity. This study will enable to
model and predict the optical response of thin films in terms of their intrinsic and
extrinsic properties.
v
Table of Contents
Section Title Page
Title Page............................................................................................................................ i
Library Release................................................................................................................... ii
Dedication...........................................................................................................................iii
Acknowledgements........................................................................................................... iv
Abstract............................................................................................................................. v
Table of Contents............................................................................................................... vi
List of Figures................................................................................................................... xi
List of Tables.................................................................................................................. xvii
Chapter 1 – Introduction and Motivation.............................................................................1
1.1 MEMS: An Emerging Market...........................................................................1
1.2 Optical Microdisplay Technology.....................................................................2
1.3 MEMS Processing Challenges...........................................................................4
1.3.1 Process Compatibility......................................................................5
1.3.2 Residual Film Stress........................................................................6
1.3.3 Environmental Stability...................................................................7
1.4 Organization of Thesis.......................................................................................7
Chapter References..................................................................................................9
Chapter 2 – Sputter Deposition and Microstructure of Metallic Thin Films.....................10
2.1 Sputter Deposition Fundamentals....................................................................10
2.2 Stress in Sputtered Thin Films.........................................................................12
2.3 Microstructure of Aluminum Thin Films........................................................14
2.3.1 Aluminum Properties........................................................................17
2.3.1.1 Mechanical Properties........................................................18
2.3.1.2 Electrical Properties...........................................................18
2.3.1.3 Optical Properties...............................................................19
2.3.2 Dependence of Microstructure on Sputter Parameters.........................21
2.4 Aluminum Alloys............................................................................................23
vi
2.4.1 Semiconductor Industry.....................................................27
2.4.2 Optical Disc Industry.............................................................30
2.5 As-Deposited Aluminum Alloy Characteristics........................................32
Chapter References................................................................................................34
Chapter 3 – Combined Surface & Bulk Optical Modeling................................................36
3.1 Reflection, Transmission, and Absorption.......................................................38
3.2 Relative Dielectric Function and the Lorentz-Drude Model...........................40
3.2.1 Drude Atomist Model.......................................................................42
3.2.2 Lorentz Oscillators............................................................................45
3.3 Effective Media Layers....................................................................................49
3.4 Historical Aluminum Optical Models..............................................................50
3.4.1 Bulk Models......................................................................................52
3.4.2 Combined Surface-Bulk Models.......................................................55
3.5 Summary..........................................................................................................58
Chapter References................................................................................................59
Chapter 4 – Thin Film Characterization Techniques.........................................................61
4.1 Morphology......................................................................................................61
4.1.1 Thickness Measurement....................................................................61
4.1.2 Atomic Force Microscopy................................................................63
4.1.3 Transmission Electron Microscopy..................................................65
4.2 Chemical Composition.....................................................................................67
4.2.1 Auger Electron Spectroscopy...........................................................67
4.2.2 Energy Dispersive X-ray Analysis....................................................69
4.3 Crystal Structure: X-ray Diffraction................................................................70
4.4 Physical Properties...........................................................................................73
4.4.1 Optical: Specular Reflectivity...........................................................73
4.4.2 Electrical: Resistivity........................................................................75
4.4.3 Mechanical........................................................................................77
4.4.3.1 Thin Film Stress.................................................................77
4.4.3.2 Nano-Indentation...............................................................77
Chapter References................................................................................................80
vii
Chapter 5 – Experimental Methodology............................................................................81
5.1 Target Materials...............................................................................................83
5.2 Deposition Methodology.................................................................................84
5.3 Sample Preparation.........................................................................................87
5.4 Analysis Details...............................................................................................90
5.5 Phase 1: Target Characterization.....................................................................94
5.6 Phase 2: Secondary Testing.............................................................................95
5.6.1 Variation of Process Pressure...........................................................96
5.6.2 Post-Deposition Anneal....................................................................97
5.5 Optical Modeling Approach............................................................................98
5.6 Summary........................................................................................................102
Chapter References..............................................................................................104
Chapter 6 – Characterization Results..............................................................................105
6.1 Film Thickness Summary..............................................................................105
6.2 Microstructure................................................................................................107
6.2.1 AFM Results...................................................................................107
6.2.1.1 AFM Images....................................................................108
6.2.1.2 Measured RMS Roughness..............................................118
6.2.2 TEM Images....................................................................................119
6.2.3 Grain Size Calculations...................................................................129
6.2.3.1 AFM.................................................................................129
6.2.3.2 XRD.................................................................................131
6.2.3.3 Comparison of Methods...................................................132
6.3 Crystal Structure............................................................................................133
6.4 Composition...................................................................................................145
6.4.1 Auger Electron Spectroscopy.........................................................145
6.4.2 Energy Dispersive X-ray.................................................................147
6.5 Film Stress.....................................................................................................156
6.5.1 Stress as a Function of Copper Content..........................................156
6.5.2 Stress in Al-Cu with Ternary Transition Metal Species.................158
6.6 Nano-Indentation Results...............................................................................158
viii
6.6.1 Elastic Modulus..............................................................................159
6.6.2 Nano-Hardness................................................................................160
6.7 Resistivity......................................................................................................162
6.8 Evolution of Material Properties Post-Anneal...............................................163
6.8.1 Resistivity.......................................................................................163
6.8.2 Grain Size........................................................................................165
6.8.3 XRD Results...................................................................................165
6.8.4 AFM Topography...........................................................................170
6.9 Evolution of Al-1.0%Cu Properties with Deposition Pressure......................176
6.9.1 AFM Morphology...........................................................................176
6.9.2 Evolution of Stress at Different Sputter Pressures..........................177
6.9.3 Resistivity.......................................................................................179
Chapter References..............................................................................................181
Chapter 7 – Reflectivity & Optical Models.....................................................................182
7.1 Specular Reflectivity......................................................................................182
7.1.1 Reflectivity as a Function of Copper Content.................................183
7.1.2 Reflectivity of Al-Cu with Cr and Ti Solute...................................184
7.1.3 Evolution of Reflectivity Post-Anneal............................................185
7.1.4 Evolution of Reflectivity with Sputter Pressure.............................188
7.2 Optical Model Results....................................................................................189
7.2.1 Model Fit.........................................................................................189
7.2.2 Drude-Lorentz Parameters..............................................................197
7.2.3 EMA Parameters.............................................................................198
7.3 Relative Dielectric Function..........................................................................199
7.3.1 Al-Cu Series....................................................................................199
7.3.2 Al-Ti-Cu & Al-Cr-Cu.....................................................................201
7.3.3 Plasma Frequency...........................................................................202
7.3.4 Free Electron Density.....................................................................202
7.3.5 Comparison of Drude and Measured Resistivity............................204
7.4 Bulk Reflectivity of Aluminum Alloys..........................................................205
Chapter References..............................................................................................208
ix
Description:reflectivity, (3) improved nano-hardness relative to pure aluminum, of both the specular reflectance and mechanical properties of thin film reflectors
