1. Case Studies in MEMS Case study Technology Transduction Packaging
Pressure sensor Bulk micromach. Piezoresistive sensing Plastic
+ bipolar circuitry of diaphragm deflection
Accelerometer Surface micromach. Capacitive detection of Metal can
proof of mass motion
Electrostatic Surface micromach Electrostatic torsion of Glass bonded
projection displays XeF2 release suspended tensile beams

2. Optical MEMS Why are MEMS used here?
- Structures are the same dimensions as the wavelength
- Small displacement has a large effect, can be used for SWITCHING
* Interferometric devices
* Scanning devices
- A photon has no mass, easy to deflect light
Can fabricate large-scale systems,
(e.g X 1000 displays as in the
Digital Micro-mirror device)
Courtesy: H. Toshiyoshi

3. Applications of Electrostatic projection displays
Courtesy: H. Toshiyoshi

4. Applications of Electrostatic projection displays
Control of light through:
Reflection : Texas Instruments
(DMD: Digital Micromirror Device)
(2) Diffraction: Silicon light Machines
(GLV: Grating Light Valve)

5. Texas Instruments’ Digital Micro-mirror Device (DMD)
The most advanced display technology to date
Each rotatable mirror is a pixel
1024 shades of gray and 35 trillion colors possible
use in projection systems, TV and theaters

6. Distinguishing features of a DMD
H. Toshiyoshi
Higher brightness and contrast
Gray scale achieved by digital and analog modulation
- Digital: Pulse Width Modulation (PWM)
- Analog: Spatial Light Modulation (SLM)
Compact, low weight and low power  Portable system

7. History (1): Si cantilever based light modulator
Petersen, K.E., “Micromechanical light modulator array fabricated on Silicon”,
Applied Physics Letters, 31, pp , 1977
Electrically actuated, individually addressable cantilevers
Pull -in
SiO2 structural layer
Si sacrificial layer

8. History(2): Torsional electrostatic light modulator
Petersen, K.E., “Silicon torsional scanning mirror”,
IBM Journal of Research & Dev., 24, pp , 1980
Electrically actuated torsion mirrors
1012 cycles, with ± 1o rotation
Bulk micromachining of Silicon

9. History (3): Deformable Mirror Devices
L. Hornbeck, “Deformable Mirror Spatial Light Modulator”,
SPIE, vol. 1150, p.86, 1989
Elastomer based
Cantilever based
Membrane based
Torsion: Amplitude dependent modulation
Cantilever based: Phase dependent modulation

10. Digital Micro-mirror device

11. DMD Fabrication (6 photomask layers)
DMD superstructure on CMOS circuitry
Surface micromachining process
Hinge: Aluminum alloy (Al, Ti, Si)
( nm thick)
Mirror: Aluminum ( nm thick)
Aluminum : structural material
DUV hardened photoresist: sacrificial material
Dry release (plasma etching) reduces stiction
Courtesy: H. Toshiyoshi

12. Texas Instruments DMD characteristics

13. Digital Micro-mirror device

14. Principle of Operation
Balancing electrical torque with mechanical torque
Telectrical is proportional to (voltage)2
Tmechanical is proportional to (deflection, a) a

15. Electrostatic model of a torsion mirror
Arc length
Electric field x
Mirror r d a q V
Torsion beam
Neglect fringing electric field
Neglect any residual stress

16. Electrostatic model of a torsion mirror
Electrostatic torque (Telec) =
Mechanical torque (Tmech) =
e.g. polysilicon, G = 73 GPa
r= 2.35 g/cm3 q V x d
Mirror
Torsion beam r a
W: width
L: length
t: thickness

17. Balancing electrical and mechanical Torques
Graph Courtesy, M. Wu

18. Operation of torsion mirror based DMD

19. DMD bias cycles

20. Energy domain model
The torsion mirror as a capacitive device

21. Calculation of capacitance
From: M. Wu and S. Senturia

22. - stable angle and pull-in voltage
Approximate solution
- stable angle and pull-in voltage
From: M. Wu and S. Senturia

23. Schemes of Torsion Mirror operation
Pull-in voltage Scan angle Angle-voltage
Single side drive q V x d r a
Low Small Non-linear
High Large Linear
Push-pull drive q
V+v x d r a
V-v
Bias voltages

24. Digital Micro-mirror Device (Texas Instruments)

25. Can create 1024 shades of gray
1-DMD chip system
Can create 1024 shades of gray
used in projectors, TVs and home theater systems

26. Can create 16.7 million shades of color
2-DMD chip system
Can create 16.7 million shades of color
used in projectors, TVs and home theater systems

27. 3-DMD chip system is used for higher resolutions
For movie projection and other high end applications
(35 trillion colors can be generated)

28. Grating Light Valve (GLV)
- Silicon Light Machines (
Courtesy: M.C. Wu
Reflection : broad band Diffraction :Wavelength (l) dependent
1 mirror/pixel (2-D array) 6 ribbons/pixel (1-D array)
Larger displacements Displacement: l/4
(msec time response) (nanosecond response)
Voltage controlled A fixed angle
Constant intensity Diffracted intensity varied by voltage

29. Mode of Operation
A diffraction grating of 6 beams  1 pixel

30. 1 pixel in the GLV: 6 ribbons wide

31. By using a different spacing between ribbons, one can create
different color-oriented pixels

32. MEMS in Optical Communications
Very quick switching (> 100 kHz), low losses,
Low cost, batch fabrication
1 X 2 Optical switch
Optical Micro-mirrors used
with Add-Drop multiplexers
Optical fibers
Bell Labs research

33. MEMS Micro Optical Bench
Integrable Micro-Optics MEMS Actuators Opto MEMS
Slide courtesy: H. Toshiyoshi

34. Scratch Drive Actuator
Large total displacements can be achieved (1 100 Hz – 100 KHz
Increments / forward movement as small as 10 nm
voltages required are large
Scratch actuator movement
Akiyama, J. MEMS, 2, 106, 1993
Voltage applied

35. MEMS in 3-dimensions Other variants of the hinge
“Microfabricated hinges”, K. Pister et al, Sensors & Actuators A, vol. 33, pp , 1992
Assembly of three-dimensional structures
Large vertical resolution and range
Surface micromachining based
Other variants of the hinge
H. Toshiyoshi

36. MEMS in Optical Communications
Very quick switching (> 100 kHz), low losses,
Low cost, batch fabrication
1 X 2 Optical switch
Optical Micro-mirrors used
with Add-Drop multiplexers
Optical fibers
Bell Labs research