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Chapter 14: Fundamentals of Microelectromechanical Systems

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Due to bonding between dissimilar materials. Environmentally Induced Failures ... Current MEMS devices are used most in automotive, medical, consumer, industrial ... – PowerPoint PPT presentation

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Title: Chapter 14: Fundamentals of Microelectromechanical Systems


1
Chapter 14Fundamentals 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?
  • Sensors
  • 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
  • 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 andDie Attach Process
  • Hermetic packaging
  • Prevents diffusion of water
  • Materials
  • No organics of plastics
  • Die Attach Process
  • Thermal considerations
  • Cracking or creep

13
Wiring and Interconnects and Flip Chip
  • Wiring and interconnects
  • Gold gt 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
  • Hermetic when sealed
  • High Youngs Modulus
  • Flip Chip or Wirebonding
  • Plastic Packaging
  • Not Hermetic
  • Postmolding
  • Premolding
  • Metal Packaging
  • Hermetic when sealed
  • Easy to assemble
  • Low Pin Count

16
Typical MEMS Devices
  • Sensors
  • Pressure Sensors
  • Accelerometers
  • Actuators
  • 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
  • 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
  • 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 (contd)
23
MEMS Accelerometer (contd)
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 (contd)
  • qsense amplitude of secondary flexural mode
  • Ag angular gain of ring structure
  • Q quality factor of the structure
  • ?0 angular flexural resonance frequency
  • qdrive vibration amplitude of the primary
    flexural mode
  • Oz 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)
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