DESIGN AND FABRICATION OF MEMS CANTILEVER AND OTHER BEAM STRUCTURES

Transcription

1 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

2 This thesis is dedicated to my Father Shri Apur6a Kumar cdutta

3 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 , INDIA

4 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

5 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 combtype 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

6 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

7 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

8 TABLE OF CONTENTS Page No. Certificate Acknowledgement Abstract Table of contents List of figures i ii iii vi xii Chapter 1: Introduction to MEMS 1.1 Processes involved in MEMS Technology Bulk Micromachining Surface Micromachining Dissolved Wafer Process (DWP) LIGA Materials for MEMS Silicon Silicon Dioxide Silicon Nitride Quartz Piezoelectric materials Metals Polymers Shape Memory Alloy Classification of Micromachined Devices Brief History of MEMS Advantages of MEMS 22 ix

9 1.5 Advantages of MEMS Applications of MEMS Micromachined Cantilever and Other Beam Structures Effect of Residual Stress in MEMS Motivation/ Objective Organization of the Thesis 29 Chapter 2: MEMS Fabrication and Characterization Process Steps 2.1 Design and Modeling of MEMS Structures and Devices Cleaning of Silicon wafer Wet cleaning Dry cleaning Thermal Oxidation Doping Optical lithography UV light source Photo-mask Exposure system Photoresist Thin Film Deposition Techniques Physical Vapour Deposition (PVD) Chemical Vapour Deposition (CVD) Electroplating Spin-coating Etching Wet etching 57 vii

10 2.7.2 Deep Reactive Ion Etching Added value of wet etching Planarization Wafer Bonding Direct Wafer Bonding Anodic Bonding Intermediate layer assisted bonding Characterization Techniques Scanning Electron Microscopy (SEM) Secondary Ion Mass Spectrometry (SIMS) Fourier Transform Infrared Spectroscopy High Resolution X-Ray Diffraction Dielectric Measurements Hysteresis (P-E loop) Measurement 72 Chapter 3: Study of deep (>10pm) boron diffused p++ silicon layers for MEMS cantilever structure 3.1 Introduction Brief Overview of Diffusion in Silicon Fick's Laws Non-Constant Diffusivity Redistribution of Impurities during Oxidation Theoretical Estimation of Diffusion Parameters Deep Boron Diffusion Experiment Experimental Results SIMS 84 viii

11 3.5.2 X-ray Rocking Curve and Reciprocal Space Mapping Study FTIR study Raman Spectroscopy Study SEM study Fabrication of p++ Silicon Cantilever Structure Conclusion 98 Chapter 4: Study of the effect of residual stress on surface micromachined gold beams 4.1 Introduction Effect of residual stress gradient on gold cantilever beam structure Model for Beam curvature due to Intrinsic Stress Gradient Simulation of effect of intrinsic residual stress 103 on micromachined cantilever beam Fabrication gold cantilever structure Results and Discussion Effect of Residual Stress Gradient on RFMEMS Switch RF MEMS Switch Principles Simulation Mechanical Analysis Electromechanical analysis Conclusion 125 Chapter 5: PZT Thin Films on Platinized and Oxidized Silicon Design issues for MEMS structures 5.1 Introduction Brief literature survey on PZT thin films on various 128 substrates by various methods ix

12 5.1.2 Piezoelectric MEMS structures and operation modes Deposition of PZT thin film on Pt and oxidized Si wafers Characterization of PZT films on Pt and oxidized Si Substrates PZT on Pt/TiO2/SiO2/Si XRD analysis SEM Study P E Loop Estimation of residual stress generated 136 during deposition of PZT on Pt/ SiO2/ Si from wafer curvature PZT on Si02/Si XRD Study SEM Study P E Loop Estimation of residual stress generated 142 during deposition of PZT on SiO2/Si from wafer curvature 5.4 Fabrication of PZT Cantilever Structure Conclusions 148 Chapter 6: Design and Simulation of wet etching based comb type microaccelerometer structure 6.1 Introduction Micromachining in Si (110) wafer Accelerometer Model Modelling of the comb type capacitive accelerometer structure 156 V1

13 6.4 FEM Analysis of the comb-type microaccelerometer structure Modal Analysis Stress Analysis Static Analysis Dynamic Analysis Critical Dimensional Analysis of Comb-type Structure Conclusion 169 Chapter 7: Fabrication of Wet Etching Based Comb Type Capacitive Microaccelerometer Structure 7.1 Introduction Optimization of anisotropic wet etching of Si (110) Results and Discussions Etching rate Surface roughness Activation energy Fabrication of wet etching based comb-type accelerometer structure Fabrication process of the comb-type structure Fabrication Challenges and Results Conclusion 198 Chapter 8: Conclusions and Future scope of work 8.1 Conclusions Future scope of work 201 References 203 Brief Bio-data of the author 211 List of publications 212 xi