Microfabricated High Aspect Ratio Silicon Flexures

Author: Dr. Christopher G. Keller

publisher: MEMS Precision Instruments

Table of Contents

1. Introduction to Flexures for MEMS

1.1 Background of the Problem
1.1.1 MEMS
1.1.2 Flexures
1.1.3 Silicon
1.2 Mechanical Failure Modes in Silicon
1.2.1 Brittle Fracture
1.2.2 Fatigue Crack Growth
1.3 Historical Limitations of Silicon Microfabrication
1.3.1 Bulk Etching with Aqueous KOH
1.3.2 Reactive Ion Etching
1.3.3 Surface Micromachining
1.4 Hexsil: A New Class of MEMS
1.5 Other Methods for making High Aspect Ratio Flexures
1.5.1 Sidewall Beams
1.5.2 Boron Doping of Deep Trench Sidewalls
1.5.3 Hinged Structures
1.5.4 SCREAM
1.5.5 Photoelectrochemical Etching
1.5.6 LIGA
1.5.7 Photodefinable Glass
1.6 The Big Picture

2. Hexsil

2.1 Introduction
2.2 Fabrication Strategy
2.3 Mold
2.4 Protective Layer
2.5 Sacrificial Oxide
2.6 Polysilicon
2.7 Planarization
2.8 HF Release
2.9 Critical Point Drying
2.10 Mold Extraction
2.11 Hexsil Flexures
2.12 Limits of Hexsil

3. Hexsil Processing Issues

3.1 Obsolete Challenges
3.2 Mold
3.2 Protective Layer
3.3 Sacrificial Oxide
3.5 Polysilicon
3.6 Planarization
3.7 HF Release
3.7 Mold Extraction
3.8 Self-Extracting Structures
3.9 Molding Some Special Shapes
3.9.1 Array of Spikes
3.9.2 Corrugated Sheets
3.9.3 Molded Oxide Structures
3.9.4 Inverse of Hexsil
3.9.5 Tubing

4. Hexsil Flexures: microtweezers

4.1 Introduction
4.2 Hexsil Tweezers with Nickel Filled Beams
4.2.1 Design
4.2.2 Fabrication
4.2.3 Performance
4.3 Hexsil Tweezers with an External Actuator
4.3.1 Design
4.3.2 Fabrication
4.3.3 Performance
4.4 Hexsil Tweezers with an Integrated Actuator
4.4.1 Design
4.4.2 Fabrication
4.4.3 Performance
4.5 How to do Pick and Place on the Microscale
4.6 Micro Parts for Assembly
4.7 Results
4.8 Conclusions

5. Hexsil Flexures: Stages for TEM Specimens

5.1 Introduction
5.2 Hexsil TEM Tensile Tester (Active)
5.2.1 Design
5.2.2 Fabrication
5.2.3 Performance
5.3 Hexsil TEM Tensile Tester (Passive)
5.3.1 Design
5.3.2 Fabrication
5.3.3 Performance

6. New Designs for Thin Film Fracture Specimens

6.1 Introduction
6.2 Thin Film Fracture Toughness Specimens
6.3 Fabrication
6.4 Hexsil Tensile Test Specimens
6.5 Results
6.6 Conclusions

7. Single Crystal Silicon Flexures by Deep RIE

7.1 Processing Issues with Deep RIE
7.2 Design
7.3 Fabrication
7.4 Performance

8. In-Plane Silicon Flexures by Timed Bulk Etching

8.1 Introduction
8.2 Fabrication of Flexures
8.2.1 Etch conditions
8.2.2 Alignment to Crystal Direction
8.2.3 Corner protection
8.2.4 Front to Back Side Alignment
8.2.5 End point Detection
8.2.6 Metal compatibility
8.2.7 Smoothing Internal Corners

9. Bulk Etched Tester Design

9.1 Introduction
9.2 Statement of the Problem
9.3 The Basic Principle of the Tester
9.3.1 Stiffness of the Force Measuring Beams
9.3.2 Stiffness of the Displacement Measuring Beams
9.3.3 Out-of-plane Deflection Due to Moments
9.4 The Specimen Support Substrate
9.5 Strain Gages
9.6 Actuator
9.7 Heat Sink
9.8 Fixturing
9.9 Displacement Calibration
9.10 Force Calibration
9.11 Control Algorithms
9.12 Test Results
9.13 Conclusion

10. Plated Solder Specimen Design and Fabrication

10.1 Introduction
10.2 Plated Structures
10.2.1 Copper Plating Bath
10.2.2 Solder Plating Bath
10.2.3 Removing the Ti Adhesion Layer
10.2.4 Removing the Copper Seed Layer
10.2.5 Removing the Chrome Adhesion Layer
10.2.6 Solder Reflow
10.3 Solder Specimen Alignment
10.4 Conclusion

11. Economics: Bulk Etching vs. RIE vs. Hexsil

11.1 Introduction
11.2 Example Cost Calculation

12. Conclusion

12.1 Bulk Etched Flexures
12.2 RIE Flexures
12.3 Hexsil Flexures

References

Appendix A: Practical Tips on Fabrication

A.1 Basic Procedures

Plasma Hardening Procedure
Sputtering Procedure
Plating Procedure
Protecting the Backside of a Double Polished Wafer
Procedures to Strip Hardbaked Photoresist
Crystal Alignment Procedure

A.2 Solder Shear Tester Fabrication

Processing steps for KOH etch method

A.3 Specimen Fabrication:

Options for solder bump preparation:

A.4 Tester Fabrication Method 2: STS Etcher

Appendix B: Discrete TEM Specimens

B.1 Introduction
B.2 Design

Appendix C: Membrane Particle Filters

C.1 Introduction
C.2 Design of Long Channel Filter
C.3 Fabrication of the Long Channel Filter
C.4 Design of Short Channel Filter
C.5 Fabrication of Short Channel Filter

Appendix D: Bulk Etched Structures for TEM

D.1 Introduction
D.2 Design
D.3 Fabrication
D.4 Results

APPENDIX E: A Vision of Microassembly

E.1 Introduction
E.2 Micro Robotics With Hexsil
E.3 The Basic Building Blocks of an Infrastructure