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

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: 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 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

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 SiO2 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 ± 1o 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 Mirror r d a q 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/cm3 q V x d Mirror Torsion beam r a 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

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

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

Digital Micro-mirror Device (Texas Instruments)

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

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

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

Grating Light Valve (GLV) - Silicon Light Machines (www.siliconlight.com) 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

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 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

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

Scratch Drive Actuator 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 Akiyama, J. MEMS, 2, 106, 1993 Voltage applied

MEMS in 3-dimensions Other variants of the hinge “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 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