DESIGN AND FABRICATION OF MEMS CANTILEVER AND OTHER BEAM STRUCTURES

by

SHANKAR DUTTA

Department of Physics

Submitted

in fulfillment of the requirements of the degree of Doctor of Philosophy

to the

INDIAN INSTITUTE OF TECHNOLOGY DELHI

FEBRUARY 2012

This thesis is dedicated to

my Father Shri Apur6a Kumar cDutta

CERTIFICATE

This is to certify that the thesis entitled, "Design and Fabrication of MEMS Cantilever and

Other Beam Structures", being submitted by Shankar Dutta for the award of the degree of

Doctor of Philosophy (Ph.D.) to the Indian Institute of Technology Delhi, New Delhi, is a

record of bonafide research work carried out by him under my guidance and supervision. In

my opinion, the thesis has reached the standard of fulfilling the requirements of all the

regulations related to the degree. The results contained in this thesis have not been submitted

in part or full, to any other University or Institute for the award of any degree or diploma.

Dr. Ratnamala Chatterjee Professor

Physics Department Indian Institute of Technology Delhi

New Delhi — 110016, INDIA

ACKNOWLEDGEMENTS

The thesis is based on the research work performed between the years 2007 and 2011 at

MEMS Division, Solid State Physics Laboratory (DRDO), Delhi and Physics Department

(Magnetics and Advanced Ceramics Lab.), Indian Institute of Technology Delhi, India. It is

now my sincere duty and also an opportunity to express my gratitude to all people who

contributed through their guidance, experience, support and friendship to my research work.

First of all, I would like to express my deepest gratitude to my supervisor Prof.

Ratnamala Chatterjee, Physics Department, IIT, Delhi for her intellectual guidance,

continuous interest, generous support and constant encouragement throughout this research

work. She has devoted her invaluable time for me for discussions, writing papers and thesis.

I take privilege to express my gratitude to Director, SSPL for his kind support,

allowing me to work for Ph.D. and providing me a lot of opportunity for the interaction with

national and international scientists in the various conferences and seminars held during the

tenure of my Ph.D.

I also express my sincere gratitude to Director, IIT Delhi for his kind support and

allowing me to pursue Ph.D. at IIT, Delhi.

I express my sincere thanks and heartfelt gratitude to Dr. P. Datta, Scientist `G', Dr.

R.K. Bhan, Scientist `G', Dr. Ramjay Pal, Scientist `F', Mr. Akhilesh Pandey, Scientist `C',

Mr. Brijesh Yadav, Scientist `C', Ms. Isha Yadav, Scientist `C', Mohd. Imran, Scientist `B'

and all other members of MEMS Division for helping me during the tenure of the research.

I am also liked to thankful to Dr. Manoj Kumar Sharma, Dr. Abhishek Pathak, Mr.

Alok Kumar Jha for their co-operation during my research work at IIT Delhi.

Last but not the least I am indebted to my parents, my wife and my lovely kids for their moral

support and encouragement.

SHANKAR DUTTA

ii

ABSTRACT Micromachined beams (cantilever and other beams) are the most ubiquitous structures in the

field of MEMS. They can act as physical, chemical or biological sensors by detecting

changes in bending or vibrational frequency. From literature it has been found that the

characteristic properties of the micromachined beams are dependent on (i) material properties

and (ii) on the specific design of the beam.

The motivation behind this work is selection of (i) processes and (ii) materials for

successful development of MEMS based cantilever and other beam structures. Proper design

of cantilever and other beam structures have been studied using FEM based simulation.

Fabrication of these structures have also been realized in this work.

Generation of residual stress is a reality associated with the thin film deposition and

consequently also in MEMS structures. It has been found that residual stress can affect the

MEMS devices performances in many ways. The effect of residual stress on three most

common MEMS materials — (a) p+ + silicon, (b) electroplated gold and (c) PZT — is reported

in the thesis. Other properties of these materials are also characterized. Effect of the residual

stress on micromachined cantilever and other beam structures, based on above materials,

have also been discussed in this thesis.

Moreover, design and fabrication issues of development of wet etching based comb-

type microaccelerometer structure (inter-digitated cantilever beam array), with vertical comb

walls and narrow torsion beams, is also discussed in this thesis.

Thus, this thesis offers design, simulation and fabrication of different MEMS

cantilever and other beam structures by surface and bulk micromachining techniques.

Chapter wise description of the theis is briefed below:

Chapter 1 contains a comprehensive literature survey of research in the MEMS field

and the importance of the cantilever and other beam structures.

iii

Chapter 2 describes the fabrication unit processes used in MEMS devices. The

techniques used to characterize the MEMS materials are also discussed in this chapter.

Chapter 3 presents the optimization of deep boron diffusedp++ silicon layer (>10gm

thick) of boron concentration above 5x1019atoms/cm3. Detailed characterization of the p++

silicon layers, by using HRXRD, SIMS, SEM, FTIR in mid-IR range are reported. Stress

generated due to the deep diffusion is estimated to be — 800MPa using Raman spectroscopy.

The chapter includes fabrication ofp++ silicon cantilever structure.

The effects of residual stress on surface micromachined gold beams are discussed in

chapter 4. Bending of the surface micromachined gold cantilever beams due to residual stress

is studied using FEM simulation. The residual stresses of the deposited electroplated gold

layers are estimated by XRD. The simulated results are correlated with the fabricated gold

cantilever beams. The behavior of a double sided clamped beam structure under residual

stress, for RF MEMS switch is also simulated.

Chapter 5 contains the studies on Lead Zirconium Titanate (PbZrO.52TiO.4803) thin

films deposited on platinized as well as SiO2 coated silicon wafers. The PZT films showed

polycrystalline perovskite phase in XRD data on both the substrates. The samples are

characterized for their ferroelectric properties. The residual stresses generated in the PZT

films are estimated by XRD and wafer curvature measurement techniques. Finally, PZT

cantilever with IDT structures is successfully realized by using DRIE.

Chapter 6 contains design and simulations of wet etching based comb type

microaccelerometer structure. A wet etching based comb-type structure (inter-digitated

cantilever beam array) is modeled using close form equations. Thus calculated results are

then compared with the simulated results obtained using FEM based software (Intellisuit).

In chapter 7, fabrication challenges and process flow of the wet etching based

suspended comb-type microaccelerometer structure are discussed. In this work we

iv

demonstrated that surface roughness can be minimized by optimizing the wet etching process

in different etchants — KOH, TMAH and EDP. Suspended comb-type accelerometer structure

with vertical comb walls, suspended by two narrow torsional beams is finally realized.

Chapter 8 summarizes the complete research work of the thesis and highlights the

important conclusions drawn from the present research work. The suggestions for further

research in this area are also part of this chapter.

v

TABLE OF CONTENTS

Page No.

Certificate i

Acknowledgement ii

Abstract iii

Table of contents vi

List of figures xii

Chapter 1: Introduction to MEMS

1.1 Processes involved in MEMS Technology 3

1.1.1 Bulk Micromachining 3

1.1.2 Surface Micromachining 5

1.1.3 Dissolved Wafer Process (DWP) 7

1.1.4 LIGA 8

1.2 Materials for MEMS 10

1.2.1 Silicon 10

1.2.2 Silicon Dioxide 13

1.2.3 Silicon Nitride 14

1.2.4 Quartz 14

1.2.5 Piezoelectric materials 15

1.2.6 Metals 16

1.2.7 Polymers 17

1.2.8 Shape Memory Alloy 18

1.3 Classification of Micromachined Devices 18

1.4 Brief History of MEMS 19

1.5 Advantages of MEMS 22

ix

1.5 Advantages of MEMS 22

1.6 Applications of MEMS 23

1.7 Micromachined Cantilever and Other Beam Structures 23

1.8 Effect of Residual Stress in MEMS 25

1.9 Motivation/ Objective 27

2.0 Organization of the Thesis 29

Chapter 2: MEMS Fabrication and Characterization Process Steps

2.1 Design and Modeling of MEMS Structures and Devices 32

2.2 Cleaning of Silicon wafer 34

2.2.1 Wet cleaning 35

2.2.2 Dry cleaning 36

2.3 Thermal Oxidation 37

2.4 Doping 39

2.5 Optical lithography 42

2.5.1 UV light source 42

2.5.2 Photo-mask 43

2.5.3 Exposure system 44

2.5.4 Photoresist 45

2.6 Thin Film Deposition Techniques 46

2.6.1 Physical Vapour Deposition (PVD) 46

2.6.2 Chemical Vapour Deposition (CVD) 49

2.6.3 Electroplating 51

2.6.4 Spin-coating 53

2.7 Etching 55

2.7.1 Wet etching 57

vii

2.7.2 Deep Reactive Ion Etching 59

2.7.2 Added value of wet etching 61

2.8 Planarization 62

2.9 Wafer Bonding 62

2.9.1 Direct Wafer Bonding 63

2.9.2 Anodic Bonding 64

2.9.3 Intermediate layer assisted bonding 65

2.10 Characterization Techniques 66

2.10.1 Scanning Electron Microscopy (SEM) 66

2.10.2 Secondary Ion Mass Spectrometry (SIMS) 67

2.10.3 Fourier Transform Infrared Spectroscopy 68

2.10.4 High Resolution X-Ray Diffraction 69

2.10.5 Dielectric Measurements 71

2.10.6 Hysteresis (P-E loop) Measurement 72

Chapter 3: Study of deep (>10pm) boron diffused p++ silicon layers for MEMS

cantilever structure

3.1 Introduction 73

3.2 Brief Overview of Diffusion in Silicon 75

3.2.1 Fick's Laws 76

3.2.2 Non-Constant Diffusivity 79

3.2.3 Redistribution of Impurities during Oxidation 80

3.3 Theoretical Estimation of Diffusion Parameters 81

3.4 Deep Boron Diffusion Experiment 82

3.5 Experimental Results 84

3.5.1 SIMS 84

viii

3.5.2 X-ray Rocking Curve and Reciprocal Space Mapping Study 85

3.5.3 FTIR study 87

3.5.4 Raman Spectroscopy Study 89

3.5.5 SEM study 91

3.6 Fabrication of p++ Silicon Cantilever Structure 93

3.7 Conclusion 98

Chapter 4: Study of the effect of residual stress on surface micromachined gold beams

4.1 Introduction 99

4.2 Effect of residual stress gradient on gold cantilever beam structure 101

4.2.1 Model for Beam curvature due to Intrinsic Stress Gradient 101

4.2.2 Simulation of effect of intrinsic residual stress 103

on micromachined cantilever beam

4.3.2 Fabrication gold cantilever structure 106

4.3.3 Results and Discussion 110

4.3 Effect of Residual Stress Gradient on RFMEMS Switch 114

4.3.1 RF MEMS Switch Principles 114

4.3.2 Simulation 117

4.3.2.1 Mechanical Analysis 117

4.3.2.2 Electromechanical analysis 121

4.4 Conclusion 125

Chapter 5: PZT Thin Films on Platinized and Oxidized Silicon — Design issues for

MEMS structures

5.1 Introduction 127

5.1.1 Brief literature survey on PZT thin films on various 128

substrates by various methods

ix

5.1.2 Piezoelectric MEMS structures and operation modes 130

5.2 Deposition of PZT thin film on Pt and oxidized Si wafers 131

5.3 Characterization of PZT films on Pt and oxidized Si Substrates 132

5.3.1 PZT on Pt/TiO2/SiO2/Si 132

5.5.1.1 XRD analysis 132

5.3.1.2 SEM Study 134

5.3.1.3 P — E Loop 135

5.3.1.4 Estimation of residual stress generated 136

during deposition of PZT on Pt/ SiO2/ Si from

wafer curvature

5.3.2 PZT on Si02/Si 137

5.3.2.1 XRD Study 137

5.3.2.2 SEM Study 139

5.3.2.3 P — E Loop 140

5.3.2.3 Estimation of residual stress generated 142

during deposition of PZT on SiO2/Si from

wafer curvature

5.4 Fabrication of PZT Cantilever Structure 143

5.7 Conclusions 148

Chapter 6: Design and Simulation of wet etching based comb type microaccelerometer

structure

6.1 Introduction 149

6.1.1 Micromachining in Si (110) wafer 153

6.2 Accelerometer Model 154

6.3 Modelling of the comb type capacitive accelerometer structure 156

V1

6.4 FEM Analysis of the comb-type microaccelerometer structure 163

6.4.1 Modal Analysis 163

6.4.2 Stress Analysis 164

6.4.3 Static Analysis 165

6.4.4 Dynamic Analysis 166

6.5 Critical Dimensional Analysis of Comb-type Structure 167

6.6 Conclusion 169

Chapter 7: Fabrication of Wet Etching Based Comb Type Capacitive

Microaccelerometer Structure

7.1 Introduction 170

7.2 Optimization of anisotropic wet etching of Si (110) 171

7.1.1 Results and Discussions 173

7.2.1.1 Etching rate 173

7.2.1.2. Surface roughness 174

7.2.1.3 Activation energy 178

7.3 Fabrication of wet etching based comb-type accelerometer structure 181

7.3.1 Fabrication process of the comb-type structure 181

7.3.2 Fabrication Challenges and Results 190

7.4 Conclusion 198

Chapter 8: Conclusions and Future scope of work

8.1 Conclusions 200

8.2 Future scope of work 201

References 203

Brief Bio-data of the author 211

List of publications 212

xi