SILICON CARBIDE PRESSURE SENSORS FOR HIGH TEMPERATURE APPLICATIONS by SHENG JIN Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Dissertation Adviser: Dr. Mehran Mehregany Department of Materials Science and Engineering CASE WESTERN RESERVE UNIVERSITY May, 2011 Copyright © 2011 by Sheng Jin & Mehran Mehregany All rights reserved To my wife and my son Contents CONTENTS I LIST OF TABLES IV LIST OF FIGURES V ACKNOWLEDGEMENT X ABSTRACT XII CHAPTER 1 INTRODUCTION 1 1.1 Problem to be addressed 1 1.2 Objective 5 1.3 Review of SiC materials and technology 6 1.3.1 Material properties of SiC 6 1.3.2 Processing technologies 8 1.3.2 SiC surface micromachining 11 1.3.3 SiC reactive ion etching 12 1.3.4 MEMS packaging 13 CHAPTER 2 HIGH-TEMPERATURE SIC PRESSURE SENSOR DEVELOPMENT 18 2.1 Literature review 18 2.1.1 State-of-the-art, high-temperature pressure sensors 18 2.2 Pressure sensing mechanisms 25 2.3 Prior related work in our group 28 2.4 Challenges and limitations 32 CHAPTER 3 DESIGN OF SIC CAPACITIVE PRESSURE SENSOR35 3.1 Introduction 35 3.2 Pressure sensor design 37 3.2.1 Small-deflection mode, circular diaphragm 37 3.2.2 Contact mode, circular diaphragm 46 3.2.3 Analytical design of 50 mmHg pressure sensor 52 CHAPTER 4 FINITE ELEMENT ANALYSIS 57 4.1 Introduction 57 4.2 COMSOL simulation for diaphragm deflection 59 4.2.1 Small-deflection mode simulation 59 4.2.2 Contact mode simulation 60 4.3 Thermal mismatch stress effect 64 4.3.1 Background 64 4.3.2 Analytical model 65 4.3.3 COMSOL simulation 70 CHAPTER 5 FABRICATION OF SIC CAPACITIVE PRESSURE SENSORS 82 5.1 Introduction 82 5.2 Fabrication Process 82 5.2.1 Process Flow Overview 82 5.3 Fabrication challenges 95 5.3.1 Mask 1 Photolithography 95 5.3.2 Etch control of RIE to precisely stop on underlying film 96 5.3.3 Non-uniformity of BOE etch 98 5.3.4 RIE of SiC films 100 5.4 Fabrication verification 104 CHAPTER 6 METALLIZATION DEVELOPMENT 111 6.1 Introduction 111 6.2 Experimental 119 6.2.1 Selection of measurement techniques 119 6.2.2 Experiment 121 6.3 Result and discussion 125 CHAPTER 7 PRESSURE SENSOR CHARACTERIZATION 151 7.1 Introduction 151 7.2 Test setup 153 7.2.1 Basic theory 153 7.2.2 Measurement setup 154 7.2.3 Packages 156 7.3 Dielectric property study of SiNx 160 7.4 Sensor array testing 165 7.4.1 Analytical calculation versus experimental data 166 7.4.2 Comparison between sensors with different sensing gaps 170 7.4.3 Comparison between sensors fabricated on SiC and Si substrates 173 7.4.4 Comparison between small deflection mode and contact mode sensors 180 7.4.5 Testing of contact mode SiC substrate sensors for different pressure ranges 189 7.5 Conclusion 199 CHAPTER 8 CONCLUSION 200 APPENDIX A 204 BIBLIOGRAPHY 206 List of Tables Table 1.1 Material properties of SiC, GaAs, Silicon, and Diamond at 300K. 8 Table 1.2 Properties of some ceramic substrates. 17 Table 3.1 Target sensor specifications for high temperature applications. 35 Table 3.2 Parameters and materials constants used in modeling the sensors. 37 Table 3.3 Performance comparison between pressure sensor arrays for different range and process parameters (small-deflection mode). 45 Table 3.4 Performance comparisons between pressure sensor arrays for different range and process parameters (contact mode). 52 Table 3.5 Analytical design of 0.108 MPa (15.7 psi) small deflection mode absolute pressure sensor. 53 Table 3.6 Analytical design of 0.108 MPa (15.7 psi) contact mode absolute pressure sensor. 54 Table 5.1 Process parameters of 9245 Photoresist. 86 Table 5.2 Test process and the corresponding criteria. 97 Table 6.1 Experimental methods for specific contact resistance ρc measurement. 119 Table 6.2 Atomic concentration of elements at line scan points. 149 Table 7.1 Performance comparison between SiC-S-15-54-500 and SiC-S-05-37-500. 173 Table 7.2 Performance comparison between SiC-S-15-47-700 and Si-S-15-47-700 at room temperature. 175 Table 7.3 Performance comparison between SiC-S-15-47-700 and Si-S-15-47-700 at 500 ºC. 176 Table 7.4 Performance of Si-T-15-68-1200 at different temperatures. 179 Table 7.5 Performance of SiC-S-05-37-500 at different temperatures. 183 Table 7.6 Performance of SiC-T-05-70-500 at different temperatures. 188 Table 7.7 Performance comparison between SiC-S-05-37-500 and SiC-T-05-70-500. 189 Table 7.8 Sensor array performance of SiC-T-15-70-1000 at different temperatures. 194 Table 7.9 Performance of SiC-T-15-95-300 at different temperatures. 198 List of Figures Figure 2.1 Sensor fabrication process ............................................................................29 Figure 2.2 Type A die layout ..........................................................................................31 Figure 2.3 Type B die layout ..........................................................................................31 Figure 2.4 Plan view micrograph of Type B die ............................................................32 Figure 2.5 Delamination of metal pads and interconnections of the sensor array .........33 Figure 3.1 Simulation plot of capacitance versus pressure for r=17 μm ........................40 Figure 3.2 Simulation plot of capacitance versus pressure for r=20 μm ........................40 Figure 3.3 Simulation plot of capacitance versus pressure for r=28 μm ........................41 Figure 3.4 Simulation plot of capacitance versus pressure for r=41 μm ........................41 Figure 3.5 Simulation plot of capacitance versus pressure for r=45 μm ........................42 Figure 3.6 Simulation plot of capacitance versus pressure for r=54 μm ........................42 Figure 3.7 Simulation plot of capacitance versus pressure for r=68 μm ........................43 Figure 3.8 Simulation plot of capacitance versus pressure for r=80 μm ........................43 Figure 3.9 Simulation plot of capacitance versus pressure for r=95 μm ........................44 Figure 3.10 Schematic drawing of an equivalent capacitor model in contact mode ........48 Figure 3.11 Plot of capacitance versus pressure for r=70 μm ..........................................49 Figure 3.12 Plot of capacitance versus pressure for r=58 μm ..........................................50 Figure 3.13 Plot of capacitance versus pressure for r=47 μm ..........................................50 Figure 3.14 Plot of capacitance versus pressure for r=38 μm ..........................................51 Figure 3.15 Plot of capacitance versus pressure for r=31 μm ..........................................51 Figure 3.16 Performance of a 0.5 μm-gap, 90 μm-radius, 1 μm-thick diaphragm pressure sensor with 0.46fF/Pa (3.2 pF /psi) sensitivity and 0.94% nonlinearity ...................................................................................................56 Figure 4.1 A schematic of the 3D COMSOL model ......................................................58 Figure 4.2 Deflection of a r=48 μm diaphragm under 3.4 MPa (500 psi) pressure from FEA. r=0 always at the diaphragm center ............................................59 Figure 4.3 FEA compared with analytical model for deflection of a diaphragm under pressure (r=48 μm and gap=2 μm)......................................................59 Figure 4.4 Deflection of a diaphragm with r=58 μm under different pressures from FEA (0.4 μm gap) .........................................................................................61 Figure 4.5 Comparison between FEA and analytical models at 2.8 MPa (400 psi) pressure .........................................................................................................62 Figure 4.6 Comparison between FEA and analytical models at 3.4 MPa (500 psi) pressure .........................................................................................................62 Figure 4.7 Comparison between FEA and analytical models at 4.1 MPa (600 psi) pressure .........................................................................................................63 Figure 4.8 A simple structure for film stress calculation ...............................................65 Figure 4.9 Temperature dependent linear expansion coefficient of poly-SiC and Si .....67 Figure 4.10 Thermal expansion stress for a poly-SiC film on Si wafer as a function of temperature by analytical calculation .......................................................68 Figure 4.11 Thermal mismatch stress impact on a small-deflection mode sensor designed for a pressure range of 5.2 MPa (750 psi), wherein the substrate is Si (50 μm radius and 1.5 μm gap) ..............................................69 Figure 4.12 Thermal mismatch stress impact on a small-deflection mode sensor designed for a pressure range of 1.7 MPa (250 psi), wherein the substrate is Si (67 μm radius and 1.5 μm gap) ..............................................69 Figure 4.13 The initial gap size change caused by thermal expansion mismatch stress based on COMSOL simulation (58 μm radius and without the LSN layer) .....................................................................................................72 Figure 4.14 Thermal stress in the poly-SiC diaphragm based on COMSOL simulation (58 μm radius and 2 μm gap and without the LSN layer) ...........72 Figure 4.15 The performance of a SiC substrate sensor accounting for the thermal mismatch stress based on COMSOL simulation (58 μm radius and 2 μm gap and without the LSN layer) ....................................................................73 Figure 4.16 The performance of a Si substrate