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Case Studies in MEMS Case study Technology Transduction PackagingPressure sensor Bulk micromach. Piezoresistive sensing Plastic
+ bipolar circuitry of diaphragm deflection
Accelerometer Surface micromach. Capacitive detection of Metal canproof of mass motion
Electrostatic Surface micromach. Electrostatic torsion of Glass bondedprojection displays + XeF2 release suspended tensile beams
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Optical MEMS
Courtesy: H. Toshiyoshi
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. 1000 X 1000 displays as in the Digital Micro-mirror device)
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Applications of Electrostatic projection displays
Courtesy: H. Toshiyoshi
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Applications of Electrostatic projection displays
Control of light through:
(1)Reflection : Texas Instruments (DMD: Digital Micromirror Device)
(2) Diffraction: Silicon light Machines (GLV: Grating Light Valve)
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Texas Instruments’ Digital Micro-mirror Device (DMD)
- Each rotatable mirror is a pixel - 1024 shades of gray and 35 trillion colors possible- use in projection systems, TV and theaters
The most advanced display technology to date
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Distinguishing features of a DMD
• 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
H. T
oshiyoshi
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History (1): Si cantilever based light modulator
• SiO2 structural layer• Si sacrificial layer
Petersen, K.E., “Micromechanical light modulator array fabricated on Silicon”, Applied Physics Letters, 31, pp. 521-523, 1977
• Electrically actuated, individually addressable cantilevers• Pull -in
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History(2): Torsional electrostatic light modulator
• Bulk micromachining of Silicon
Petersen, K.E., “Silicon torsional scanning mirror”, IBM Journal of Research & Dev., 24, pp. 631-637, 1980
• Electrically actuated torsion mirrors• 1012 cycles, with ± 1o rotation
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Elastomer based
History (3): Deformable Mirror DevicesL. Hornbeck, “Deformable Mirror Spatial Light Modulator”,SPIE, vol. 1150, p.86, 1989
Cantilever based
Membrane basedTorsion: Amplitude dependent modulation
Cantilever based: Phase dependent modulation
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Digital Micro-mirror device www.dlp.com
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DMD Fabrication (6 photomask layers)
Courtesy: H. Toshiyoshi
• Surface micromachining process
• Hinge: Aluminum alloy (Al, Ti, Si) (50-100 nm thick)
• Mirror: Aluminum (200-500 nm thick)
• Aluminum : structural material• DUV hardened photoresist: sacrificial material• Dry release (plasma etching) reduces stiction
DMD superstructure on CMOS circuitry
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Texas Instruments DMD characteristics
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Digital Micro-mirror device www.dlp.com
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Principle of Operation
Balancing electrical torque with mechanical torque
Telectrical is proportional to (voltage)2
Tmechanical is proportional to (deflection
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Electrostatic model of a torsion mirror
)sin
(
x)θsinθ
d(rθa
xdV
aVE
-Neglect fringing electric field-Neglect any residual stress
Arc length
Electric field
V
x
d
Mirror
Torsion beam
ra
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Electrostatic model of a torsion mirror
Electrostatic torque (Telec) = dxx)-
sind(
xWεV21 Wxdx εE
21
2
22
V
x
d
Mirror
Torsion beam
ra
W: widthL: lengtht: thickness
Mechanical torque (Tmech) =
e.g. polysilicon, G = 73 GPag/cm3
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Graph Courtesy, M. Wu
Balancing electrical and mechanical Torques
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Operation of torsion mirror based DMD
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Energy domain modelThe torsion mirror as a capacitive device
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Calculation of capacitance
From: M. Wu and S. Senturia
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From: M. Wu and S. Senturia
Approximate solution- stable angle and pull-in voltage
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V
x
dra
V+v
x
dra
V-v
Schemes of Torsion Mirror operationPull-in voltage Scan angle Angle-voltage
Single side drive
Push-pull drive
Low Small Non-linear
High Large Linear
V.v α v)(Vv)(V 22
V.v α v)(Vv)(V 22 Bias voltages
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Digital Micro-mirror Device (Texas Instruments)
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1-DMD chip system
- Can create 1024 shades of gray- used in projectors, TVs and home theater systems
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2-DMD chip system
- Can create 16.7 million shades of color- used in projectors, TVs and home theater systems
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3-DMD chip system is used for higher resolutions
-For movie projection and other high end applications (35 trillion colors can be generated)
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Grating Light Valve (GLV)- Silicon Light Machines (www.siliconlight.com)
Reflection : broad band Diffraction :Wavelength dependent
1 mirror/pixel (2-D array) 6 ribbons/pixel (1-D array)
Larger displacements Displacement: (sec time response) (nanosecond response)
Voltage controlled A fixed angle
Constant intensity Diffracted intensity varied by voltage
Cou
rtes
y: M
.C. W
u
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A diffraction grating of 6 beams 1 pixel
Mode of Operation
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1 pixel in the GLV: 6 ribbons wide
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By using a different spacing between ribbons, one can createdifferent color-oriented pixels
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MEMS in Optical Communications
1 X 2 Optical switchOptical fibers
Optical Micro-mirrors used with Add-Drop multiplexers
Bell Labs research
- Very quick switching (> 100 kHz), low losses, - Low cost, batch fabrication
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MEMS Micro Optical BenchIntegrable Micro-Optics MEMS Actuators Opto MEMS
Slide courtesy: H. Toshiyoshi
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Scratch Drive ActuatorA
kiya
ma,
J. M
EM
S, 2
, 106
, 199
3
- Large total displacements can be achieved (1 mm) @ 100 Hz – 100 KHz- Increments / forward movement as small as 10 nm - voltages required are large
Scratch actuator movement
Voltage applied
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MEMS in 3-dimensions“Microfabricated hinges”, K. Pister et al, Sensors & Actuators A, vol. 33, pp. 249-256, 1992
Surface micromachining based
-Assembly of three-dimensional structures- Large vertical resolution and range
H. T
oshiyoshi
Other variants of the hinge
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MEMS in Optical Communications
1 X 2 Optical switchOptical fibers
Optical Micro-mirrors used with Add-Drop multiplexers
Bell Labs research
- Very quick switching (> 100 kHz), low losses, - Low cost, batch fabrication