Optical MEMS: Overview & MARS Modulator Joseph Ford, James Walker, Keith Goossen References: “Silicon modulator based on mechanically-active antireflection layer with 1 Mbit/sec capability” K. Goossen, J. Walker and S. Arney, IEEE Photonics Tech. Lett. 6, p.1119, 1994 "Micromechanical fiber-optic attenuator with 3 microsecond response" J. Ford, J. Walker, D. Greywall and K. Goossen, IEEE J.of Lightwave Tech. 16(9), 1663-1670, September 1998 "Dynamic spectral power equalization using micro-opto-mechanics" J. Ford and J. Walker, IEEE Photonics Technology Letters 10(10), 1440-1442, October 1998 "Micromechanical gain slope compensator for spectrally linear power equalization" K. Goossen, J. Walker, D. Neilson, J. Ford, W. Knox, IEEE Photonics Tech. Lett.12(7), pp. 831-833, July 2000. "Wavelength add/drop switching using tilting micromirrors" J. Ford, V. Aksyuk, D. Bishop and J. Walker, IEEE J. of Lightwave Tech. 17(5), 904-911, May 1999. "A tunable dispersion compensating MEMS all-pass filter" Madsen, Walker, Ford. Goossen, Nielson, Lenz, IEEE Photonics Tech. Lett. 12(6), pp. 651-653, June 2000. What are MEMS? Micro-Electro-Mechanical Systems … manufactured using technology created for VLSI electronics to build micron-scale devices “released” by selective etching • Surface Micromachining • LIGA (electroforming) • Deep Reactive Ion Etching …& electrically controlled by • Electrostatic attraction • Electromagnetic force • Electrostriction • Resistive heating Photos courtesy Sandia National Labs Note: “MEMS” = passive silicon V-grooves Mass commercial application: Acceleration Sensors Elastic hinge Spacer Proof Mass Force Silicon substrate Capacitive Accelerometer Analog Devices' ADXL50 accelerometer Surface micromachining capacitive sensor 2.5 x 2.5 mm die incl. electronic controls Cost: $30 vs ~$300 bulk sensor (‘93) Cut to $5/axis by 1998 Replaced by 3-axis ADXL150 “Every new car sold has micromachined sensors on-board. They range from MAP (Manifold Absolute Pressure) engine sensors, accelerometers for active suspension systems, automatic door locks, and antilock braking and airbag systems. The field is also widening considerably in other markets. Micromachined accelerometer sensors are now being used in seismic recording, machine monitoring, and diagnostic systems - or basically any application where gravity, shock, and vibration are factors.” http://www.analog.com/library/techArticles/mems/xlbckgdr4.html Mass commercial application: Pressure Sensors Pext Membrane Measure RC time Force Pint Spacer Silicon substrate Capacitive Pressure Sensor Piezo-resistive pressure sensor High-pressure gas sensor (ceramic surface-mount) NovaSensor’s piezo-resistive pressure sensors Disposable medical sensor Electrical actuation of active MEMS devices magnetic layer conductive layer Force Force Apply insulator Voltage substrate conductive substrate Apply Current Electrostatic attraction Electromagnetic force Apply Voltage electrostrictive layer patterned resistive layer Force substrate Electrostriction EM coil Apply Current substrate Resistive heating Force Surface Micromachining: Layer by layer addition Starting from bare silicon wafer, deposit & pattern multiple layers to form a (shippable) MEMS wafer ~ 10 mask steps Completed MEMS wafer Diced and released MEMS device Release = isotropic chemical etch to remove oxides Special techniques may be used to remove liquid (e.g., critical point drying) Assembly = mechanical manipulation of structures (e.g., raising and latching a vertical mirror plate) Various techniques used, some highly proprietary From Cronos/JDSU MUMPS user guide at www.MEMSRUS.com st Optical MEMS device 1 Texas Instruments Digital Light Projector TM & DLP PROJECTOR Bulk MEMS Fabrication: Pattern & selective etch Example: Bulk silicon DRIE: start with unpatterned wafer stack – a wafer-bonded SOI (silicon on insulat (1) Pattern photoresist (2) DRIE vertical etch photoresist wafer-bonded silicon sacrificial silicon oxide bulk silicon substrate (4) Gold evaporation (3) SiO2 isotropic etch Narrow features released, Wide features just undercut Gold mirrors on top and potentially sides samlab “Bulk Silicon” MEMS Devices Single-axis tilt-mirror photo courtesy R. Conant, BSAC Comb-drive switch photo courtesy IMT (Neuchatel) MEMS reliability? “MEMS Reliability: Infrastructure, Test Structures, Experiments and Failure Modes” 171 page report by D. M. Tanner et al, SAND2000-0091, January 2000. Micromotor test device 40,000G impact test Failure by rubbing contact Comb-drive actuator Flexural contact to gears Ceramic package destroyed MEMS survives (!) Wear on silicon surface Submicron particles generated Conclusions: (1) Properly designed MEMS devices are remarkably shock resistant (2) Flexural failures due to fatigue were not apparent (3) Rubbing wear (& resulting debris) was their primary failure mechanism www.sandia.gov Optical MEMS Devices Classical vs Resonant “Classical” optical MEMS Sir Isaac Newton (1642-1727) …and his Corpuscular Theory of Light Lucent’s “LambdaRouter” Device 1st-surface reflection “Resonant” Optical MEMS Christiaan Huygens Sir Isaac Newton (1629-1695) (1642-1727) … andhis hisCorpuscular 1687 Wave Theory of Light …and Thomas Young Silicon Light Machine’s Grating Light Valve (1773-1829) … and his 1801 theory of Interference Interference / Diffraction Resonant Optical MEMS MEMS: Tunable Photonic Bandgap Variable phase grating ~ 10 V drive ~ 200 nm actuation ~ 10 us response Stanford’s grating light valve Variable gap multilayer 5 - 30 V drive ~ 200 nm actuation ~ 10 us response Lucent’s MARS modulator Vd The “MARS” Resonant MEMS Modulator Fabry-Perot etalon reflectivity 0.9 Incident 0.8 Initial gap 1.0 Operation 220 nm Reflected Transmitted dd’ F sin2(pd/do) Reflectivity = --------------------1+ F sin2(pd/do) F = 4Rs/(1-Rs)2 Rs = top interface reflectivity = 30.6% d = gap between plates do = gap @ minimum reflectivity (l/2) reflection 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Gap reduction (microns) Resonant optics = Sub-wavelength actuation Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998 Fabry-Perot etalon spectral uniformity 1.00 1163 900 820 750 nm nm air gap 1163 nm operation Reflectivity 3lo/2 0.50 900 nm lo/2 750 nm 820 nm 0.00 1.2 1.3 1.4 1.5 l o 1.6 1.7 Wavelength (microns) Resonant Optics = Wavelength dependence Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998 The “MARS” resonant MEMS modulator MARS (Membrane Anti-Reflection Switch) analog optical modulator l/4 Silicon Nitride “drumhead” suspended over a Silicon substrate input reflect l/4 SiNx Vdrive membrane edge etch access holes PSG Silicon 0 < Vdrive < 30V 3l/4 < gap < l/2 PSG transmit 150 mm Goossen, Arney & Walker, IEEE Phot. Tech. Lett. 6, 1994 MARS dielectric multilayer structures Dielectric Silicon Nitride Conductive Polysilicon + Nitride Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998 Lucent’s MARS “bulk” MEMS fabrication Silicon Nitride Double Polysilicon etch via to bottom poly etch holes for HF access metal deposition HF release Walker, Goossen & Arney, J. MEMS 5(1), 1996 MARS time & voltage response Temporal Response Voltage Response measured theory Drive voltage (V) 500 um DPOL drum w/ 300 um window has 1.1 microsecond response 110 um SiNx drum w/ 30 um window has 85 nanosecond response (used for 16 Mb/s digital data modulation) Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998 Greywall, Busch & Walker, Sensors & Actuators A A72, 1999. MARS Applications: - Data modulator - Variable attenuator - Dynamic spectral equalizer - Dispersion compensator (see references or other presentations)