Case Studies in MEMS Case study Pressure sensor
- Slides: 36
Case Studies in MEMS Case study Pressure sensor Technology Bulk micromach. + bipolar circuitry Transduction Packaging Piezoresistive sensing of diaphragm deflection Plastic Accelerometer Surface micromach. Capacitive detection of proof of mass motion Metal can Electrostatic projection displays Surface micromach. + Xe. F 2 release Electrostatic torsion of suspended tensile beams Glass bonded
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. 1000 X 1000 displays as in the Digital Micro-mirror device) Courtesy: H. Toshiyoshi
Applications of Electrostatic projection displays Courtesy: H. Toshiyoshi
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)
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
Distinguishing features of a DMD • 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. Toshiyoshi • Higher brightness and contrast
History (1): Si cantilever based light modulator 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 • Si. O 2 structural layer • Si sacrificial layer
History(2): Torsional electrostatic light modulator Petersen, K. E. , “Silicon torsional scanning mirror”, IBM Journal of Research & Dev. , 24, pp. 631 -637, 1980 • Electrically actuated torsion mirrors • 1012 cycles, with ± 1 o rotation • Bulk micromachining of Silicon
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
Digital Micro-mirror device www. dlp. com
DMD Fabrication (6 photomask layers) DMD superstructure on CMOS circuitry • 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 Courtesy: H. Toshiyoshi
Texas Instruments DMD characteristics
Digital Micro-mirror device www. dlp. com
Principle of Operation Balancing electrical torque with mechanical torque Telectrical is proportional to (voltage)2 Tmechanical is proportional to (deflection, a) a
Electrostatic model of a torsion mirror Arc length Electric field x r q a Mirror d V Torsion beam -Neglect fringing electric field -Neglect any residual stress
Electrostatic model of a torsion mirror Electrostatic torque (Telec) = Mechanical torque (Tmech) = e. g. polysilicon, G = 73 GPa r= 2. 35 g/cm 3 x r q a Mirror d V Torsion beam W: width L: length t: thickness
Balancing electrical and mechanical Torques Graph Courtesy, M. Wu
Operation of torsion mirror based DMD
DMD bias cycles
Energy domain model The torsion mirror as a capacitive device
Calculation of capacitance From: M. Wu and S. Senturia
Approximate solution - stable angle and pull-in voltage From: M. Wu and S. Senturia
Schemes of Torsion Mirror operation Pull-in voltage Single side drive x r a q Scan angle Angle-voltage Low Small Non-linear High Large Linear d V Push-pull drive x r d a q V+v Bias voltages V-v
Digital Micro-mirror Device (Texas Instruments)
1 -DMD chip system - Can create 1024 shades of gray - used in projectors, TVs and home theater systems
2 -DMD chip system - Can create 16. 7 million shades of color - used in projectors, TVs and home theater systems
3 -DMD chip system is used for higher resolutions -For movie projection and other high end applications (35 trillion colors can be generated)
Courtesy: M. C. Wu Grating Light Valve (GLV) - Silicon Light Machines (www. siliconlight. com) Reflection : broad band Diffraction : Wavelength (l) dependent 1 mirror/pixel (2 -D array) 6 ribbons/pixel (1 -D array) Larger displacements (msec time response) Displacement: l/4 (nanosecond response) Voltage controlled A fixed angle Constant intensity Diffracted intensity varied by voltage
Mode of Operation A diffraction grating of 6 beams 1 pixel
1 pixel in the GLV: 6 ribbons wide
By using a different spacing between ribbons, one can create different color-oriented pixels
MEMS in Optical Communications - Very quick switching (> 100 k. Hz), low losses, - Low cost, batch fabrication 1 X 2 Optical switch Optical fibers Optical Micro-mirrors used with Add-Drop multiplexers Bell Labs research
MEMS Micro Optical Bench Integrable Micro-Optics MEMS Actuators Opto MEMS Slide courtesy: H. Toshiyoshi
Scratch Drive Actuator Akiyama, J. MEMS, 2, 106, 1993 - 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
MEMS in 3 -dimensions “Microfabricated hinges”, K. Pister et al, Sensors & Actuators A, vol. 33, pp. 249 -256, 1992 -Assembly of three-dimensional structures - Large vertical resolution and range Surface micromachining based Other variants of the hinge H. Toshiyoshi
MEMS in Optical Communications - Very quick switching (> 100 k. Hz), low losses, - Low cost, batch fabrication 1 X 2 Optical switch Optical fibers Optical Micro-mirrors used with Add-Drop multiplexers Bell Labs research
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