1. Chapter 14: Fundamentals of Microelectromechanical Systems
Jon Mah
Eric Wilson

2. 14.1 What are MEMS? Micro-electro-mechanical systems Examples Benefits
Need for fabrication technologies

3. What are Sensors and Actuators?
Physical input
Weak Signal
Actuator
Output or processing
Some physical change

4. 14.2 What are MEMS Applications?
NOW
Accelerometer
Pressure and chemical flow analysis
Inkjet print heads
mm-μm
FURURE
Medical diagnostics
Drug delivery
(No more Medellin cartel!!!)
(Just kidding, different drugs)
μm-nm

5. Fundamentals of MEMS Devices
Silicon
Already in use
Manipulatable conductivity
Allows for integration
Thin-Film Materials
Silicon dioxide
Silicon nitride

6. Micromachining Fabrication
Thin Films
Layers (μm) put on Si
Photomask
Positive or negative
Wet Etching
Isotropic
Anisotropic
KOH

7. Micromachining Fabrication II
Dry Etching
RIE
DRIE
Rate-Modified Etching
Cover with Boron
Wet etch with KOH

8. Lift-Off Process Lift-off process Excimer laser technique Noble metals
For unetchable materials
Acetone
Excimer laser technique
Burn with UV

9. Surface Micromachining
Grow silicon dioxide
Apply photoresist
Expose and develop
Etch silicon dioxide
Remove photoresist
Deposit polysilicon
Remove silicon dioxide
Bulk micromachining
Same, except not

10. LIGA Technique Lithographie, Galvanoformung, and Abformung
Or, lithography, plating and molding
High aspect ratio
Many materials
X-Rays

11. MEMS Packaging Wafer stack thickness Wafer dicing concerns
Before
After
Thermal management
Unique considerations
Protective coating

12. Hermetic Packaging and Die Attach Process
Prevents diffusion of water
Materials
No organics of plastics
Die Attach Process
Thermal considerations
Cracking or creep

13. Wiring and Interconnects and Flip Chip
Gold > Aluminum
Thermocompression Bonding
Thermosonic Gold Bonding
Flip Chip
Intimate attachments
Cram everything together

14. MEMS Packaging Purposes Reduce EMI Dissipate Heat Minimize CTE
Deliver Required Power
Survive Environment

15. Types of MEMS Packages Ceramic Packaging Plastic Packaging
Hermetic when sealed
High Young’s Modulus
Flip Chip or Wirebonding
Plastic Packaging
Not Hermetic
Postmolding
Premolding
Metal Packaging
Easy to assemble
Low Pin Count

16. Typical MEMS Devices Sensors Actuators Pressure Sensors Accelerometers
Gyroscopes
High Aspect Ratio Electrostatic Resonators
Thermal Actuators
Magnetic Actuators
Comb-drives

17. Pressure Sensors Gauge Pressure Sensors Differential Pressure Sensors
Absolute Pressure Sensors

18. Accelerometers Applications: Units of mV/g Air bag crash sensors
Active suspension systems
Antilock brake systems
Ride control systems
Units of mV/g

19. Actuators High aspect ratio electrostatic resonator
Piezoelectric crystals
Thermal actuators
Comb-drives
Magnetic actuators

20. Failure Mechanisms Failure by Stiction and Wear Delamination
Cause of most MEMS failures
Microscopic adhesion
Corrosion
Delamination
Due to bonding between dissimilar materials
Environmentally Induced Failures
Thermal cycle, shock, vibration, humidity, radiation
Cyclic Mechanical Fatigue
Critical for comb and membrane MEMS
Causes changes in elasticity
Mechanical Dampening Effect
Moving parts at resonance
Loss of Hermeticity

21. MEMS Accelerometer
Mass, Spring, Damper Model

22. MEMS Accelerometer (cont’d)

23. MEMS Accelerometer (cont’d)

24. MEMS Gyroscopes Typically Vibratory Gyroscopes
Utilize Coriolis Acceleration (“fictional force”)
Due to rotating reference frame

25. Types of Vibratory Gyroscopes
Vibrating Beam, Vibrating Disk, Vibrating Shell

26. Vibrating Ring Gyroscope
Capacitive drive and sense uses perturbations to the resonance of the ring structure to measure rate

27. Vibrating Ring Gyroscope (cont’d)
qsense – amplitude of secondary flexural mode
Ag – angular gain of ring structure
Q – quality factor of the structure
ω0 – angular flexural resonance frequency
qdrive – vibration amplitude of the primary flexural mode
Ωz – rotation rate around the normal axis

28. Flexural Modes of Vibrating Ring Gyro
First Mode Second Mode

29. Polysilicon Ring Gyro 80μm thick, 1mm wide with 1.2μm gap
capacitance changes on order of 10-18F!

30. Fabrication of HARPSS
High Aspect ratio combined poly- and single-crystal silicon
Utilizes Deep RIE of Si

31. Interface and Control Electronics for Vibrating Ring Gyro
Open Loop gyros have bandwidth of a few hertz
Closed Loop gyros bandwidth limited by readout and control electronics

32. Brownian Noise Due to Brownian motion of ring structure
Random movement caused by molecular collisions
Fundamental limit on resolution
Microstructures with large mass and high resonance frequencies reduce Brownian noise in vibratory gyros

33. Summary and Future Trends
Current MEMS devices are used most in automotive, medical, consumer, industrial and aerospace applications
Bulk micromachining, microfabrication, and surface micromachining technologies drive MEMS size and shapes
Packaging requires design for environment (i.e. pressure sensors in oil)
Mechanical fatigue, stiction, and hermeticity are main failure mechanisms
Vibrating ring gyro case study (fabrication, operation, control electronics, and Brownian noise)