Ghodssi R., Lin P., MEMS Materials and Processes Handbook
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Contents
1 The MEMS Design Process ...................... 1
1.1 Introduction ........................... 1
1.1.1 Design Process . . . .................. 5
1.2 Design Methods for MEMS . .................. 7
1.2.1 History of Design Methodologies . . . ........ 7
1.2.2 Structured Design Methods for MEMS ........ 8
1.3 Brainstorming.......................... 9
1.4 Microphone Case Studies . . .................. 10
1.4.1 Microphone Background . . ............. 10
1.4.2 TheAvagoStory.................... 11
1.4.2.1 Design Process and Methods . . . .... 11
1.4.3 TheKnowlesStory .................. 20
1.4.4 Summary of Key Concepts . ............. 22
1.5 Materials and Process Selection ................. 23
1.5.1 MaterialsSelection................... 23
1.5.2 Process Selection . .................. 23
1.6 Evaluate Concepts . ....................... 30
1.6.1 Modeling........................ 30
1.7 Optimization and Other Design Methods ............ 31
1.7.1 DesignOptimization.................. 31
1.7.2 Uncertainty Analysis .................. 31
1.7.3 FMEA ......................... 31
1.7.4 DesignMethodTiming ................ 32
1.8 Summary ............................ 33
References . . . ............................ 33
2 Additive Processes for Semiconductors and Dielectric
Materials ................................ 37
2.1 Overview ............................ 37
2.2 ThermalConversion....................... 38
2.2.1 Process Overview . .................. 38
xi
xii Contents
2.2.2 Material Properties and Process Selection
GuideforThermalOxidationofSilicon........ 43
2.2.3 CaseStudies...................... 45
2.3 Chemical Vapor Deposition . .................. 45
2.3.1 Process Overviews . .................. 45
2.3.1.1 Introduction . . . ............. 45
2.3.1.2 Low Pressure Chemical Vapor
Deposition ................. 47
2.3.1.3 Plasma-Enhanced Chemical
Vapor Deposition ............. 50
2.3.1.4 Atmospheric Pressure Chemical
Vapor Deposition ............. 52
2.3.1.5 Hot Filament Chemical Vapor
Deposition ................. 53
2.3.1.6 Microwave Plasma Chemical
Vapor Deposition ............. 53
2.3.2 LPCVD Polycrystalline Silicon ............ 53
2.3.2.1 Material Properties and Process
Generalities . . . ............. 53
2.3.2.2 Process Selection Guidelines . . . .... 55
2.3.2.3 CaseStudies................ 56
2.3.3 LPCVDSiliconDioxide................ 65
2.3.3.1 Material Properties and Process
Generalities . . . ............. 65
2.3.3.2 Process Selection Guidelines . . . .... 66
2.3.3.3 CaseStudies................ 67
2.3.4 LPCVDSiliconNitride ................ 69
2.3.4.1 Material Properties and Process
Generalities . . . ............. 69
2.3.4.2 Process Selection Guidelines . . . .... 70
2.3.4.3 CaseStudies................ 70
2.3.5 LPCVD Polycrystalline SiGe and Ge . ........ 73
2.3.5.1 Material Properties and Process
Generalities . . . ............. 73
2.3.5.2 Process Selection Guidelines . . . .... 75
2.3.6 LPCVD Polycrystalline Silicon Carbide . . . .... 75
2.3.6.1 Material Properties and Process
Generalities . . . ............. 75
2.3.6.2 Process Selection Guidelines . . . .... 79
2.3.6.3 CaseStudies................ 79
2.3.7 LPCVDDiamond ................... 85
2.3.7.1 Material Properties and Process
Generalities . . . ............. 85
2.3.7.2 Process Selection Guidelines . . . .... 86
2.3.7.3 CaseStudies................ 86
Contents xiii
2.3.8 APCVD Polycrystalline Silicon Carbide . . . .... 89
2.3.8.1 Material Properties and Process
Generalities . . . ............. 89
2.3.8.2 Process Selection Guidelines . . . .... 89
2.3.9 PECVDSilicon .................... 89
2.3.9.1 Material Properties and Process
Generalities . . . ............. 89
2.3.9.2 Process Selection Guidelines . . . .... 91
2.3.10 PECVDSiliconDioxide................ 91
2.3.10.1 Material Properties and Process
Generalities . . . ............. 91
2.3.10.2 Process Selection Guidelines . . . .... 91
2.3.11 PECVDSiliconNitride ................ 95
2.3.11.1 Material Properties and Process
Generalities . . . ............. 95
2.3.11.2 Process Selection Guidelines . . . .... 95
2.3.12 PECVDSiliconGermanium.............. 95
2.3.12.1 Material Properties and Process
Generalities . . . ............. 95
2.3.12.2 Process Selection Guidelines . . . .... 97
2.3.12.3 CaseStudies................ 97
2.3.13 PECVDSiliconCarbide................ 101
2.3.13.1 Material Properties and Process
Generalities . . . ............. 101
2.3.13.2 Process Selection Guidelines . . . .... 102
2.3.13.3 CaseStudies................ 102
2.3.14 PECVD Carbon-Based Films ............. 104
2.3.14.1 Material Properties and Process
Generalities . . . ............. 104
2.3.14.2 Process Selection Guidelines . . . .... 104
2.4 Epitaxy ............................. 105
2.4.1 Process Overviews . .................. 105
2.4.2 Epi-Polysilicon..................... 106
2.4.2.1 Material Properties and Process
Generalities . . . ............. 106
2.4.2.2 Process Selection Guidelines . . . .... 107
2.4.2.3 CaseStudies................ 107
2.4.3 EpitaxialSiliconCarbide ............... 108
2.4.3.1 Material Properties and Process
Generalities . . . ............. 108
2.4.3.2 Process Selection Guidelines . . . .... 109
2.4.3.3 CaseStudies................ 109
2.4.4 III-V Materials and Gallium Nitride . . ........ 111
2.4.4.1 Material Properties and Process
Generalities . . . ............. 111
xiv Contents
2.4.4.2 Process Selection Guidelines . . . .... 111
2.4.4.3 CaseStudies................ 111
2.5 Physical Vapor Deposition . .................. 114
2.5.1 Process Overviews . .................. 114
2.5.2 Sputter-Deposited Si .................. 115
2.5.2.1 Material Properties and Process
Generalities . . . ............. 115
2.5.2.2 Process Selection Guidelines . . . .... 116
2.5.3 Sputter-Deposited SiC ................. 116
2.5.4 Sputter-Deposited SiO
2
................ 117
2.5.5 Sputter-Deposited Diamondlike Carbon ........ 118
2.5.6 Carbon Films Deposited by Pulsed Laser Deposition . 118
2.6 Atomic Layer Deposition . . .................. 119
2.6.1 Process Overview . .................. 119
2.6.2 Process Selection Guidelines and Material Properties . 120
2.7 Spin-OnFilms.......................... 121
References . . . ............................ 123
3 Additive Processes for Metals .................... 137
3.1 Introduction ........................... 137
3.1.1 Overview........................ 138
3.1.2 Fabrication Tradeoffs ................. 139
3.2 Physical Vapor Deposition . .................. 140
3.2.1 Evaporation ...................... 140
3.2.1.1 Thermal Evaporation . . . ........ 141
3.2.1.2 E-Beam Evaporation . . . ........ 142
3.2.1.3 IssueswithAlloys............. 142
3.2.2 Sputtering ....................... 142
3.2.2.1 DCSputtering............... 143
3.2.2.2 RFSputtering............... 144
3.2.2.3 StepCoverage............... 144
3.2.2.4 OtherIssuesinSputtering......... 145
3.2.3 Pulsed Laser Deposition . . . ............. 146
3.3 Electrochemical Deposition . .................. 147
3.3.1 Electroplating ..................... 147
3.3.1.1 Electrochemical Reactions ........ 147
3.3.1.2 Deposition Process ............ 149
3.3.1.3 Overpotential ............... 152
3.3.1.4 Bath Composition ............. 153
3.3.1.5 CurrentWaveform............. 153
3.3.1.6 Equipment................. 155
3.3.1.7 Process Flow . . ............. 157
3.3.1.8 Nickel................... 158
3.3.1.9 Copper . .................. 159
3.3.1.10 Gold.................... 159
Contents xv
3.3.1.11 NickelAlloys ............... 161
3.3.2 ElectrolessPlating................... 162
3.3.2.1 Nickel................... 164
3.3.2.2 Copper . .................. 166
3.3.2.3 Gold.................... 168
3.3.3 Comparison of Electroplating and Electroless
Plating ......................... 169
3.4 LIGAandUV-LIGAProcesses................. 169
3.4.1 Process Explanation .................. 170
3.4.2 Electroplating in LIGA and UV-LIGA
Microstructures .................... 171
3.4.3 Multilevel Metal Structures . ............. 173
3.5 Materials Properties and Process Selection Guidelines
forMetals............................ 179
3.5.1 Adhesion . ....................... 179
3.5.2 Electrical Properties .................. 180
3.5.3 Mechanical Properties ................. 182
3.5.4 Thermal Properties . .................. 183
3.5.5 Magnetic Properties .................. 184
References . . . ............................ 186
4 Additive Processes for Polymeric Materials ............. 193
4.1 SU-8............................... 193
4.1.1 Material Properties . .................. 195
4.1.2 ProcessingVariations ................. 196
4.1.2.1 Partial Exposure . ............. 196
4.1.2.2 Direct Writing . . ............. 197
4.1.2.3 RemovalofSU-8 ............. 197
4.1.2.4 ReleaseofSU-8.............. 198
4.1.2.5 Bonding .................. 198
4.1.2.6 Transfer .................. 199
4.1.2.7 SU-8asanEtchMask........... 199
4.1.3 LessonsLearned.................... 199
4.1.4 ExamplesofSU-8Application ............ 201
4.2 PDMS.............................. 201
4.2.1 Material Properties . .................. 202
4.2.2 Processing Techniques ................. 203
4.2.3 BiologicalApplicationGuide ............. 205
4.2.3.1 Stamp Material for Protein
Transfer: Microcontact Printing . . .... 206
4.2.3.2 Microfluidic Devices . . . ........ 206
4.2.4 CaseStudy....................... 208
4.3 Polyimide ............................ 212
4.3.1 Material Properties . .................. 212
4.3.2 ProcessingVariations ................. 213
xvi Contents
4.3.2.1 RemovalofPolyimide .......... 213
4.3.2.2 ReleaseofPolyimide ........... 213
4.3.2.3 Bonding .................. 215
4.3.3 LessonsLearned.................... 215
4.3.4 CaseStudy....................... 216
4.4 Hydrogels ............................ 216
4.4.1 Gelatin......................... 216
4.4.2 Chitosan ........................ 218
4.4.3 Polyethylene Glycol .................. 220
4.4.4 CaseStudies...................... 222
4.5 Parylene............................. 223
4.5.1 Material Properties . .................. 224
4.5.2 Processing Techniques ................. 225
4.5.3 LessonsLearned.................... 226
4.5.4 CaseStudy....................... 226
4.6 Conductive Polymers ...................... 227
4.6.1 Material Properties . .................. 228
4.6.2 Actuation Mechanism and Theories . . ........ 229
4.6.3 Applications ...................... 230
4.6.3.1 Actuators ................. 230
4.6.3.2 Conducting Polymer as a Strain
Gauge Material . ............. 231
4.6.4 Processing Techniques ................. 231
4.6.4.1 Deposition ................. 231
4.6.4.2 Patterning ................. 232
4.6.4.3 Release .................. 232
4.6.4.4 Process Considerations . . ........ 233
4.6.5 CaseStudy....................... 233
4.7 OtherPolymers ......................... 235
4.7.1 Benzocyclobutene . .................. 235
4.7.2 LiquidCrystalPolymer ................ 238
4.8 PolymersforEmbossingandMolding ............. 239
4.8.1 Technical Overview .................. 239
4.8.2 Substrate Material Selection . ............. 241
4.8.2.1 Polymethylmethacrylate . ........ 241
4.8.2.2 Polycarbonate . . ............. 242
4.8.2.3 Polytetrafluoroethylene . . ........ 242
4.8.2.4 Cyclic Olefin Copolymer . ........ 242
4.8.3 ToolSelection ..................... 242
4.8.4 MoldMaterialSelectionandFabrication ....... 243
4.8.4.1 Silicon................... 243
4.8.4.2 Nickel................... 244
4.8.4.3 SU-8.................... 245
4.8.5 Conventional Machining of Molds . . ........ 246
4.8.5.1 Milling .................. 246
Contents xvii
4.8.5.2 Laser.................... 247
4.8.5.3 Focused Ion Beam ............. 247
4.8.5.4 FixtureofMolds ............. 247
4.8.5.5 ReleaseCoatings ............. 247
4.8.6 Process Development ................. 247
4.8.7 Minimum Substrate Thickness ............ 249
4.9 Materials Properties ....................... 250
References . . . ............................ 250
5 Additive Processes for Piezoelectric Materials:
Piezoelectric MEMS .......................... 273
5.1 Introduction to Piezoelectric Thin Films ............ 273
5.1.1 Direct and Converse Piezoelectricity . ........ 275
5.1.2 Materials – Ferroelectrics and Nonferroelectrics . . . 276
5.1.3 Fundamental Design Equations and Models . . .... 281
5.1.3.1 Linear Constitutive Equations of
Piezoelectricity . ............. 281
5.1.3.2 Electromechanical Coupling Factors . . . 282
5.1.3.3 Influence of Boundary Conditions .... 284
5.1.3.4 Device Configurations . . ........ 285
5.1.3.5 Free Strain and Blocking Force . . .... 287
5.1.3.6 Cantilever Unimorph Model . . . .... 288
5.1.3.7 Actuator Force Generation
Against External Loads . . ........ 291
5.1.3.8 Piezoelectric Sensing . . . ........ 292
5.1.3.9 Equivalent Circuit Models ........ 294
5.1.3.10 Thin-Film Ferroelectric Nonlinearity . . . 295
5.1.3.11 Heat Generation . ............. 299
5.1.4 MaterialsSelectionGuide............... 299
5.1.5 Applications ...................... 300
5.2 PolarMaterials:AlNandZnO ................. 301
5.2.1 Material Deposition .................. 301
5.2.2 Patterning Techniques ................. 305
5.2.3 Device-Design Concerns . . . ............. 307
5.2.4 DeviceExamples ................... 309
5.2.5 CaseStudy....................... 313
5.3 Ferroelectrics:PZT ....................... 318
5.3.1 Material Deposition .................. 318
5.3.2 Patterning Techniques ................. 324
5.3.3 Device Design Concerns . . . ............. 328
5.3.4 DeviceExamples ................... 333
5.3.5 Case Study on the Design and Processing of a
RF MEMS Switch Using PZT Thin-Film Actuators . 338
5.4 Summary ............................ 343
References . . . ............................ 344
xviii Contents
6 Materials and Processes in Shape Memory Alloy .......... 355
6.1 Introduction and Principle . . .................. 355
6.1.1 BasicPrinciple..................... 355
6.1.2 Introduction of TiNi and TiNi-Base Ternary Alloys . . 357
6.1.3 Super-Elasticity . . .................. 359
6.1.4 One-Way Type, Two-Way Type,
All-Round-Way Type ................. 359
6.2 Materials Properties and Fabrication Process
ofSMAActuators........................ 360
6.2.1 BulkMaterial ..................... 361
6.2.2 ThinFilm ....................... 361
6.2.2.1 Sputtering................. 362
6.2.2.2 Evaporation . . . ............. 363
6.2.2.3 Non-planar Thin Film Deposition . .... 363
6.2.3 Micromachining . . .................. 364
6.2.4 EtchingandLift-Off.................. 365
6.2.4.1 CaseandExample............. 365
6.2.5 Assembly........................ 369
6.2.5.1 Mechanical Fixation ............ 369
6.2.5.2 Adhesion ................. 369
6.2.5.3 Welding .................. 369
6.2.5.4 Soldering ................. 371
6.2.6 Materials and Processes Selection Guidance . .... 371
6.2.6.1 Materials(Bulk/ThinFilm)........ 371
6.2.6.2 Process .................. 375
6.3 ApplicationsandDevices.................... 378
6.3.1 Medical ........................ 378
6.3.1.1 Stents ................... 378
6.3.1.2 Endoscopes . . . ............. 378
6.3.1.3 Catheters ................. 379
6.3.1.4 Micro Clips and Grippers . ........ 384
6.3.2 FluidicDevices .................... 387
6.3.3 OpticalFiberSwitch.................. 390
6.3.4 Tactile Pin Display . .................. 390
6.3.5 AFM Cantilever . . .................. 392
6.3.6 CaseStudiesandLessonsLearned .......... 393
6.3.6.1 Designs .................. 393
6.3.6.2 HeatingandCooling ........... 394
6.4 Summary ............................ 397
References . . . ............................ 398
7 Dry Etching for Micromachining Applications ........... 403
7.1 DryEtching........................... 404
7.1.1 EtchMetrics...................... 404
7.2 PlasmaEtching ......................... 407
Contents xix
7.2.1 Types of Etching . . .................. 408
7.2.2 Plasma Sources . . .................. 412
7.3 Plasma Process Parameters and Control ............ 418
7.3.1 Energy-DrivenAnisotropy............... 419
7.3.2 Inhibitor-DrivenAnisotropy.............. 420
7.3.3 SelectivityinPlasmaEtching ............. 421
7.4 Case Study: Etching Silicon, Silicon Dioxide,
andSiliconNitride ....................... 422
7.5 Case Study: High-Aspect-Ratio Silicon Etch Process . .... 427
7.5.1 Cryogenic Dry Etching . . . ............. 428
7.5.2 Bosch Process . . . .................. 429
7.5.3 Understanding Trends for DRIE Recipe
Development...................... 432
7.6 High-Aspect-Ratio Etching of Piezoelectric Materials . .... 434
7.6.1 Case Study: High-Aspect-Ratio Etching of
Glass (Pyrex
R
)andQuartz .............. 434
7.6.2 High-Aspect-Ratio Etching of Piezoelectric
Materials........................ 438
7.7 Etching of Compound Semiconductors ............. 441
7.7.1 Case Study: Etching of GaAs and AlGaAs . . .... 441
7.7.2 Case Study: Etching of InP, InGaAs, InSb,
andInAs........................ 444
7.8 Case Study: Ion Beam Etching ................. 446
7.9 Summary ............................ 449
References . . . ............................ 452
8 MEMS Wet-Etch Processes and Procedures ............ 457
8.1 Introduction ........................... 458
8.2 Principles and Process Architectures for Wet Etching . .... 460
8.2.1 Surface Reactions and Reactant/Product Transport . . 464
8.2.2 Etchant Selectivity and Masking Considerations . . . 467
8.2.3 Direct Etching and Liftoff Techniques . ........ 469
8.2.4 Sacrificial Layer Removal . . ............. 470
8.2.5 Substrate Thinning and Removal . . . ........ 471
8.2.6 Impact on Process Architecture ............ 472
8.2.7 Process Development for Wet Etches . ........ 473
8.2.8 Additional Considerations and Alternatives . . .... 476
8.3 Evaluation and Development of Wet-Etch Facilities
and Procedures . . ....................... 479
8.3.1 Facility Requirements ................. 479
8.3.1.1 General Facilities ............. 479
8.3.1.2 Wet-Bench Services ............ 480
8.3.1.3 Wet-Bench Equipment . . ........ 480
8.3.1.4 Safety ................... 481
8.3.2 WaferHandlingConsiderations............ 482