Ghodssi R., Lin P., MEMS Materials and Processes Handbook
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xx Contents
8.3.3 Safety Concerns . . .................. 483
8.3.4 Training ........................ 483
8.4 IC-CompatibleMaterialsandWetEtching........... 484
8.4.1 OxideandDielectricEtching ............. 484
8.4.2 Silicon, Polysilicon, and Germanium Isotropic
Etching......................... 492
8.4.3 Standard Metal Etching . . . ............. 495
8.4.4 Photoresist Removal Techniques and Wafer
Cleaning Processes . .................. 501
8.4.5 Examples: Wet Chemical Etching of
IC-CompatibleMaterials ............... 513
8.4.5.1 Example 1: Wet Etch of
Low-TemperatureOxide ......... 514
8.4.5.2 Example 2: Wet Etch of Silicon
NitrideonSilicon............. 515
8.4.5.3 Example 3: Sacrificial Etch of
Deposited Polysilicon Under
a Structural Layer of
Stress-ControlledSiliconNitride..... 515
8.4.5.4 Example 4: Aluminum Etching
over Patterned Nitride, Oxide,
andSilicon ................ 515
8.4.5.5 Example 5: Junction Depth
Determination for an Integrated
MEMSDevice .............. 515
8.5 Nonstandard Materials and Wet Etching ............ 516
8.5.1 Nonstandard Dielectric, Semiconductor, and
MetalEtching ..................... 517
8.5.2 PlasticandPolymerEtching.............. 517
8.5.3 Examples: Wet Chemical Etching of
Nonstandard Materials ................. 570
8.5.3.1 Example 1: BCB Patterning and Etching . 570
8.5.3.2 Example 2: COC Patterning and
Solvent Bonding . ............. 579
8.5.3.3 Example3:LIGAMoldRemoval..... 579
8.6 AnisotropicSiliconEtchingandSiliconEtchStops ...... 579
8.6.1 AnisotropicEtchingofSilicon............. 581
8.6.2 Heavily Doped Silicon Etch Stops . . . ........ 582
8.6.3 Lightly Doped Silicon and Silicon–Germanium
EtchStops ....................... 589
8.6.4 Ion-ImplantedSiliconEtchStops ........... 589
8.6.5 Electrochemical Etching and Electrochemical
EtchStops ....................... 595
8.6.6 Photoassisted Silicon Etching and Etch Stops . .... 597
8.6.7 Thin-FilmEtchStops ................. 601
Contents xxi
8.6.8 Examples: Wet Chemical and Electrochemical
EtchStops ....................... 603
8.6.8.1 Example 1: Anisotropic Silicon
EtchingofanSOIWafer ......... 603
8.6.8.2 Example 2: Heavy Boron-Doped
EtchStop ................. 604
8.6.8.3 Example 3: Electrochemical Etch Stop . . 604
8.7 Sacrificial Layer Etching . . .................. 608
8.7.1 Sacrificial Layer Removal Techniques . ........ 610
8.7.2 Sacrificial Oxide Removal for Polysilicon
Microstructures .................... 611
8.7.3 Alternative Sacrificial and Structural Layer
Combinations ..................... 611
8.7.4 Etch Accelerator Layers for Enhanced
Sacrificial Layer Removal . . ............. 617
8.7.5 Rinse Liquid Removal and Antistiction Coatings . . . 619
8.7.6 Examples: Sacrificial Layer Removal and
Structural Layer Release . . . ............. 620
8.7.6.1 Example 1: Fine-Grain Stress-
Controlled Polysilicon with an
Oxide Sacrificial Layer . . ........ 620
8.7.6.2 Example 2: Poly-SiGe on a
PatternedOxide/NitrideLaminate .... 620
8.7.6.3 Example 3: Silicon Nitride on a
Polysilicon Sacrificial Layer . . . .... 623
8.7.6.4 Example 4: Aluminum on Photoresist . . 623
8.8 PorousSiliconFormationwithWetChemistry......... 623
8.8.1 Nanoporous, Mesoporous, and Macroporous
SiliconFormation ................... 624
8.8.2 SelectivePorousSiliconRemoval........... 625
8.8.3 Examples:PorousSiliconFormation ......... 625
8.8.3.1 Example 1: Chemical Porous
SiliconFormation............. 625
8.8.3.2 Example 2: Nanoporous Silicon
Formation................. 628
8.8.3.3 Example 3: Mesoporous Silicon
Formation................. 628
8.8.3.4 Example 4: Macroporous Silicon
Formation................. 629
8.9 Layer Delineation and Defect Determination
with Wet Etchants . ....................... 629
8.9.1 Dopant Level and Defect Determination with
Wet Etchants ...................... 630
8.9.2 Layer Delineation with Wet Etchants . ........ 636
xxii Contents
8.9.3 Examples: Layer Delineation and Defect
Determination ..................... 637
8.9.3.1 Example 1: Metallurgical Junction
Determination............... 637
8.9.3.2 Example 2: Cross-Sectioning and
Layer Delineation ............. 637
References . . . ............................ 638
9 MEMS Lithography and Micromachining Techniques ....... 667
9.1 Overview ............................ 667
9.2 UV Lithography . . ....................... 672
9.2.1 Photo Masks ...................... 672
9.2.2 OpticalProjectionSystems .............. 677
9.2.2.1 Contact Aligner . ............. 677
9.2.2.2 Stepper .................. 681
9.2.3 Photoresist ....................... 682
9.2.3.1 Positive Photoresist ............ 684
9.2.3.2 Negative Photoresist ............ 686
9.2.3.3 Image Reversal for Positive
Resist (Converting Positive Resist
intoaNegativeResist) .......... 687
9.2.4 Substrate . ....................... 688
9.2.5 Processing Steps for UV Lithography . ........ 688
9.2.5.1 Deposit Photoresist ............ 688
9.2.5.2 Expose Photoresist ............ 690
9.2.5.3 Develop Photoresist ............ 691
9.2.5.4 TransferPattern.............. 691
9.2.5.5 Remove Photoresist ............ 692
9.3 Grayscale Lithography . . . .................. 693
9.3.1 Photomask Pixelation ................. 696
9.3.2 Photoresist Properties for Grayscale Lithography . . . 697
9.3.2.1 Contrast and Thickness . . ........ 697
9.3.2.2 Exposure and Developing Times . .... 697
9.3.2.3 EtchSelectivity .............. 698
9.4 X-Ray Lithography ....................... 698
9.4.1 X-RayMasks ..................... 700
9.4.2 X-Ray Photoresists . .................. 702
9.4.3 Exposure . ....................... 702
9.4.4 Development...................... 703
9.5 Direct-Write Lithography . . .................. 704
9.5.1 E-Beam Lithography .................. 704
9.5.2 Ion Beam Lithography and Focused Ion Beam (FIB) . 708
9.5.3 Gas-Assisted Electron and Ion Beam Lithography . . 710
9.5.4 Dip-Pen Lithography (DPN) . ............. 711
Contents xxiii
9.5.5 Direct-WriteLaser................... 712
9.5.6 Stereolithography and Microstereolithography .... 714
9.6 Print/Imprint Lithography . . .................. 716
9.6.1 InkjetPrinting ..................... 719
9.6.2 Soft Lithography . . .................. 720
9.6.3 Nanoimprint Lithography (NIL) ............ 720
9.6.4 TransferPrinting.................... 722
9.7 CaseStudies........................... 725
9.7.1 Case Study 1: Substrate Cleaning-RCA Clean(s) . . . 725
9.7.1.1 RecipeSteps................ 726
9.7.1.2 Notes ................... 727
9.7.2 Case Study 2: Substrate Cleaning, O
2
Plasma Clean . 727
9.7.2.1 RecipeSteps................ 727
9.7.2.2 Note.................... 727
9.7.3 Case Study 3: Substrate Cleaning, Solvent Clean . . . 728
9.7.3.1 RecipeSteps................ 728
9.7.3.2 Note.................... 728
9.7.4 Case Study 4: Positive Photoresist Processing:
General Processing for Shipley 1800 Series
Photoresist ....................... 728
9.7.4.1 RecipeSteps................ 728
9.7.5 Case Study 5: Positive Photoresist Processing:
Specific Processing for Shipley S1813 ........ 729
9.7.5.1 RecipeSteps................ 729
9.7.6 Case Study 6: Positive Photoresist Processing:
Specific Processing for OiR 906-10 . . ........ 730
9.7.6.1 RecipeSteps................ 730
9.7.6.2 Notes ................... 731
9.7.7 Case Study 7: Negative Photoresist
Processing: Specific Processing for NR7-1500PY . . 731
9.7.7.1 RecipeSteps................ 731
9.7.7.2 Note1................... 732
9.7.7.3 Note2................... 733
9.7.8 Case Study 8: E-Beam Lithography . . ........ 733
9.7.8.1 Notes on Using the NPGS Software . . . 735
9.7.9 CaseStudy9:FabricationofPDMSTemplates.... 735
9.7.10 Case Study 10: Photomask Fabrication [226, 227] . . 736
9.7.10.1 Photomask Defects ............ 738
9.7.10.2 Grayscale Lithography Pixelated
Photomasks . . . ............. 739
9.7.10.3 Mask Manufacturers . . . ........ 740
9.7.11 Case Study 11: Multiphoton Absorption
Polymerization(MAP)................. 740
xxiv Contents
9.7.12 Case Study 12: Lithography Using Focused
IonBeams ....................... 741
References . . . ............................ 743
10 Doping Processes for MEMS ..................... 755
10.1 Overview ............................ 755
10.2 Applications........................... 756
10.2.1 Electrical Properties .................. 756
10.2.2 Etch Stop Techniques ................. 765
10.2.3 Materials and Process Selection Guidelines:
Etch Stop Techniques ................. 771
10.3 InSituDoping.......................... 774
10.3.1 Chemical Vapor Deposition . ............. 774
10.3.2 CrystalGrowthandEpitaxy.............. 777
10.4 Diffusion ............................ 781
10.4.1 Gas Phase Diffusion .................. 783
10.4.2 SolidStateDiffusion.................. 784
10.4.3 MaskingMaterials................... 786
10.4.4 Modeling........................ 787
10.5 IonImplantation......................... 788
10.5.1 Equipment....................... 790
10.5.2 MaskingMaterials................... 792
10.5.3 Modeling........................ 793
10.5.4 CrystalDamage .................... 793
10.5.5 Buried Insulator Layers . . . ............. 795
10.5.6 Case Study: Heavily Doped Polysilicon ........ 795
10.6 PlasmaDopingProcesses.................... 798
10.7 Dopant Activation Methods . .................. 800
10.7.1 Conventional Annealing Methods . . . ........ 800
10.7.2 RapidThermalProcesses ............... 802
10.7.3 Low-TemperatureActivation ............. 803
10.7.4 Process Selection Guide: Dopant Activation . .... 803
10.7.5 Case Study: Rapid Thermal Anneal Versus
Conventional Thermal Anneal ............. 804
10.8 Diagnostics ........................... 805
10.8.1 ElectricalMeasurements................ 806
10.8.2 Junction Staining Techniques ............. 809
10.8.3 SIMS.......................... 810
10.8.4 Case Study: Characterizing J unctions and
Diagnosing Implant Anomalies ............ 810
References . . . ............................ 812
11 Wafer Bonding ............................ 817
11.1 Introduction ........................... 817
11.2 Direct Wafer Bonding ...................... 821
11.2.1 Background and Physics . . . ............. 822
Contents xxv
11.2.2 Parameters for Successful Direct Wafer Bonding . . . 824
11.2.2.1 Surface Roughness ............ 824
11.2.2.2 Waviness or Nanotopography . . . .... 826
11.2.2.3 Wafer Shape . . . ............. 826
11.2.3 Recommendations for Successful Direct
Wafer Bonding . . . .................. 826
11.2.4 Procedure of Direct Wafer Bonding . . ........ 828
11.2.4.1 Surface Preparation for Direct
Wafer Bonding . ............. 828
11.2.4.2 Bonding Step – By Hand or by
Using a Wafer Bonding Tool . . . .... 832
11.2.4.3 Basic Operation Principle of a
Wafer Bonding Tool ............ 835
11.2.4.4 InspectionBeforeHeatTreatment .... 837
11.2.4.5 Thermal Treatment to Increase the
Bond Strength . . ............. 838
11.2.4.6 Remaining Fabrication Process
forMEMSDevice............. 840
11.2.5 Anodic Bonding . . .................. 840
11.2.6 Silicon–Glass Laser Bonding ............. 845
11.3 Wafer Bonding with Intermediate Material . . . ........ 846
11.3.1 Thermocompression Bonding ............. 846
11.3.2 Eutectic Bonding . . .................. 846
11.3.3 Polymer Bonding . .................. 847
11.4 Direct Comparison of Wafer Bonding Techniques . . . .... 854
11.5 Bonding of Heterogeneous Compounds ............ 854
11.6 Wafer Bonding Process Integration . . ............. 856
11.6.1 Localized Wafer Bonding . . ............. 856
11.6.2 Through Wafer via Technology ............ 857
11.7 Characterization Techniques for Wafer Bonding ........ 863
11.8 Existing Wafer Bonding Infrastructure ............. 866
11.8.1 Wafer Bonding Services . . . ............. 867
11.8.2 Bonding Tool Vendors ................. 867
11.8.2.1 Applied Microengineering Ltd
(AML),UK ................ 868
11.8.2.2 EVGroup(EVG),Austria ........ 869
11.8.2.3 Mitsubishi Heavy Industries Ltd.
(MHI), Japan . . ............. 870
11.8.2.4 SUSSMicroTecAG,Germany...... 871
11.9 SummaryandOutlook ..................... 872
References . . . ............................ 873
12 MEMS Packaging Materials ..................... 879
12.1 MEMS Packages and Applications . . ............. 879
12.1.1 Packaging Classes . .................. 880
xxvi Contents
12.1.2 MEMS Versus Microcircuit or Integrated
Circuit Packaging . .................. 881
12.1.3 Application Drivers and Interfaces . . ........ 881
12.1.4 Interfaces to Other System Components ........ 882
12.1.4.1 Power and Signals Interface ........ 883
12.1.4.2 OpticalInterface.............. 883
12.1.4.3 Microfluidic Interface . . . ........ 884
12.1.4.4 EnvironmentalInterface.......... 885
12.2 Package Selection . ....................... 885
12.2.1 Metal.......................... 886
12.2.2 Ceramic ........................ 888
12.2.3 Plastic ......................... 891
12.2.4 Array Packaging Materials/Wafer Level Packaging . . 892
12.2.5 Custom Packaging . .................. 892
12.2.6 Silicon Encapsulation ................. 892
12.2.7 Glass Encapsulation .................. 893
12.3 Lids and Lid Seals . ....................... 893
12.3.1 OpticalApplications.................. 894
12.4 DieAttachMaterialsandProcesses............... 894
12.4.1 Conductive Die Attach ................. 895
12.4.2 Metal-Filled Glasses and Epoxies . . . ........ 896
12.4.3 OtherDieAttachMaterials .............. 896
12.4.4 Flip-Chip Bonding . .................. 897
12.4.5 Tape Interconnects . .................. 898
12.5 Wire Bonding . . . ....................... 899
12.5.1 Gold Wire Bonding .................. 899
12.5.1.1 Au-AlSystem............... 900
12.5.1.2 Au-AgSystem............... 901
12.5.1.3 Au-AuSystem............... 901
12.5.1.4 Au-CuSystem............... 901
12.5.2 AluminumSystems .................. 901
12.5.2.1 Al-AlSystem ............... 902
12.5.2.2 Al-AgSystem............... 902
12.5.2.3 Al-NiSystem ............... 902
12.5.3 Copper Systems . . .................. 902
12.6 Electrical Connection Processes . . . ............. 902
12.7 Encapsulation . . . ....................... 903
12.7.1 Polyurethane ...................... 903
12.7.2 Polyimide ....................... 903
12.7.3 Polydimethylsiloxane (PDMS) ............ 904
12.7.4 Epoxy . . ....................... 904
12.7.5 Fluorocarbon (Polytetrafluoroethylene) ........ 905
12.7.6 Acrylic(PMMA).................... 905
12.7.7 Parylene ........................ 905
12.7.8 LiquidCrystalPolymer ................ 906
Contents xxvii
12.8 Electrical and Thermal Requirements . ............. 906
12.8.1 ElectricalConsiderations ............... 906
12.8.2 ThermalConsiderations................ 907
12.9 HermeticityandGetterMaterials................ 908
12.9.1 Hermeticity and Pressurized Packaging ........ 908
12.9.2 Hermeticity and Vacuum Packaging . . ........ 908
12.10 Quality and Reliability . . . .................. 908
12.10.1 MEMS Packaging Reliability Concerns ........ 909
12.10.1.1 ThermalEffects.............. 910
12.10.1.2 Shock and Vibration ............ 911
12.10.1.3 Humidity ................. 911
12.10.2 MEMS Packaging and Quality Assurance . . . .... 912
12.11CaseStudies........................... 912
12.11.1 MEMS Accelerometer ................. 914
12.11.2Micro-mirrorArray .................. 915
12.11.3 MEMS Microphone .................. 916
12.11.4MEMSShutters .................... 916
12.12Summary ............................ 918
References . . . ............................ 920
13 Surface Treatment and Planarization ................ 925
13.1 Release Processes and Surface Treatments to Prevent Stiction . 926
13.1.1 Wet Chemical Release Techniques . . ........ 928
13.1.2 Dry Release Techniques . . . ............. 928
13.2 SurfaceAnalysis ........................ 929
13.2.1 Surface Chemical Composition ............ 929
13.2.1.1 X-Ray Photoelectron
Spectroscopy (XPS or ESCA) . . .... 929
13.2.1.2 Scanning Auger Electron
Spectroscopy (AES) ............ 930
13.2.1.3 Energy Dispersive X-Ray
Spectroscopy (EDS or EDX) . . . .... 931
13.2.1.4 Secondary Ion Mass Spectroscopy
(SIMS)................... 932
13.2.2 Surface Structure and Morphology . . ........ 932
13.2.2.1 AtomicForceMicroscopy(AFM) .... 932
13.2.2.2 Scanning Electron Microscopy (SEM) . . 933
13.2.3 SurfaceEnergyMeasurements............. 933
13.3 Adhesion and Friction of MEMS . . . ............. 934
13.3.1 Measurements of Adhesion and Friction . . . .... 934
13.3.1.1 Cantilever Beam Array Technique .... 934
13.3.1.2 Double-Clamped Beam Array Technique . 935
13.3.1.3 FrictionTestStructures.......... 936
13.3.2 Effects of Surface Roughness ............. 936
13.4 Chemical Modification of MEMS Surfaces . . . ........ 936
xxviii Contents
13.4.1 TreatmentsforLowSurfaceEnergy.......... 936
13.4.2 Siloxane and Silane Treatments ............ 937
13.4.3 Weakly Chemisorbed Surfactant Films ........ 938
13.4.4 Materials Properties and Process Selection Guidance . 939
13.5 SurfaceConsiderationsforBiologicalApplications ...... 939
13.5.1 Surface Modification Techniques . . . ........ 941
13.5.2 Modification of Pristine Substrate Surfaces . . .... 942
13.5.2.1 PlasmaTreatment............. 942
13.5.2.2 PhysicalAdsorption............ 943
13.5.2.3 Covalent Linkage ............. 943
13.5.3 Modification of Pre-treated Substrate Surfaces .... 947
13.5.3.1 Chemistry of Hydroxyl Groups
(R-OH: Alcohols) ............. 948
13.5.3.2 Chemistry of Amino Groups
(R–NH
2
:Amines)............. 950
13.5.3.3 Chemistry of Carboxyl Groups
(R–COOH: Carboxylic Acids) . . .... 954
13.5.3.4 Chemistry of Mercapto Groups
(R–SH;Thiols) .............. 955
13.5.3.5 Chemistry of Formyl Groups
(R–CHO: Aldehydes) . . . ........ 959
13.5.4 CaseStudies...................... 962
13.5.4.1 Case Study 1: Promotion of
Immobilized Bioactive Proteins’
BiologicalActivity ............ 963
13.5.4.2 Case Study 2: Effective
Enhancement of Fluorescence
Detection Efficiency Using
Alternative Blocking Process in
ProteinMicroarrayAssays ........ 964
13.5.4.3 Case Study 3: Control of Specific
Reaction Kinetics Involving
Bifunctional Cross-Linkers ........ 964
13.5.4.4 Case Study 4: Surface
Modification Using Elaborately
Derivatized Functional Groups . . .... 967
13.5.4.5 Case Study 5: Surface Patterning
by Microcontact Printing . ........ 968
13.6 SurfaceCoatingforOpticalApplications............ 969
13.6.1 Fundamentals of Optical Phenomena on
SurfaceCoatings.................... 970
13.6.1.1 Index Variation of Materials
Versus Wavelength [108] . ........ 970
13.6.1.2 Fresnel Equation for Reflection [108] . . . 976
13.6.1.3 Principle of Antireflection (AR) [108] . . 978
Contents xxix
13.6.1.4 Principle of Absorption [108, 109] .... 982
13.6.1.5 Surface Plasmon Resonance . . . .... 983
13.6.2 Material Properties and Process Selection Guidelines . 985
13.6.2.1 High Reflection Applications . . . .... 985
13.6.2.2 Antireflection Applications ........ 986
13.6.2.3 Considerations for Surface
Smoothness and Roughness ........ 990
13.6.2.4 Polymer Materials for Optical
Applications................ 992
13.6.2.5 Surface Coatings for Polymer Materials . 992
13.6.2.6 ApplicationsforLightAbsorption .... 998
13.7 Chemical Mechanical Planarization . ............. 1002
13.7.1 Overview........................ 1002
13.7.1.1 ChemistryofCMP ............ 1002
13.7.1.2 Mechanics of CMP ............ 1004
13.7.2 Applications ...................... 1008
13.7.2.1 Smoothing and Local Planarization .... 1009
13.7.2.2 Global Planarization ............ 1010
13.7.2.3 Trench Fill ................. 1011
13.7.3 PadsandSlurry .................... 1011
13.7.3.1 SummaryofSlurryandPad........ 1015
13.7.4 Polishing Considerations for Different Materials . . . 1015
13.7.4.1 RateComparisonandSelectivity..... 1015
13.7.4.2 Dielectrics................. 1020
13.7.4.3 Metals................... 1021
13.7.4.4 Polymers ................. 1023
13.7.5 Cleaning and Contamination Control . ........ 1023
13.7.6 CaseStudy....................... 1025
13.7.6.1 Case Study 10: Magnetic Microdevice . . 1026
13.7.6.2 Case Study 11: A Drug-Delivery
ProbewithanIn-lineFlowMeter..... 1026
13.7.6.3 Case Study 12: Nanomechanical
OpticalDevices.............. 1029
13.7.6.4 Case Study 13: CMP of
SU-8/Permalloy Combination in
MEMSDevices.............. 1031
References . . . ............................ 1032
14 MEMS Process Integration ...................... 1045
14.1 Introduction ........................... 1045
14.2 What Is Process Integration? .................. 1046
14.3 WhatIsanIntegratedMEMSProcess?............. 1050
14.4 Differences Between IC and MEMS Fabrication ........ 1050
14.5 Challenges of MEMS Process Integration . . . ........ 1052
14.5.1 Topography ...................... 1054