1. 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.

2. What are MEMS? Micro-Electro-Mechanical Systems Surface Micromachining LIGA (electroforming) Deep Reactive Ion Etching Electrostatic attraction Electromagnetic force Electrostriction Resistive heating Photos courtesy Sandia National Labs … manufactured using technology created for VLSI electronics to build micron-scale devices “released” by selective etching …& electrically controlled by Note: “MEMS” = passive silicon V-grooves

3. Mass commercial application: Acceleration Sensors http://www.analog.com/library/techArticles/mems/xlbckgdr4.html 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.” Capacitive Accelerometer Silicon substrate Elastic hinge Proof Mass Spacer Force

4. Mass commercial application: Pressure Sensors Capacitive Pressure Sensor Silicon substrate P int P ext Spacer Membrane Force Measure RC time NovaSensor’s piezo-resistive pressure sensors Disposable medical sensor High-pressure gas sensor (ceramic surface-mount) Piezo-resistive pressure sensor

5. substrate magnetic layer EM coil conductive substrate conductive layer insulator substrate patterned resistive layer substrate electrostrictive layer Force Apply Current Apply Voltage Electrical actuation of active MEMS devices Electrostatic attraction Resistive heating Electrostriction Electromagnetic force Apply Current Apply Voltage

6. Surface Micromachining: Layer by layer addition Starting from bare silicon wafer, deposit & pattern multiple layers to form a (shippable) MEMS wafer From Cronos/JDSU MUMPS user guide at www.MEMSRUS.com Assembly = mechanical manipulation of structures (e.g., raising and latching a vertical mirror plate) Various techniques used, some highly proprietary Release = isotropic chemical etch to remove oxides Special techniques may be used to remove liquid (e.g., critical point drying) Diced and released MEMS device Completed MEMS wafer ~ 10 mask steps

7. Texas Instruments Digital Light Projector & DLP PROJECTOR TM 1 st Optical MEMS device

8. Bulk MEMS Fabrication: Pattern & selective etch (2) DRIE vertical etch samlab bulk silicon substrate Example: Bulk silicon DRIE: start with unpatterned wafer stack – a wafer-bonded SOI (silicon on insulator) wafer-bonded silicon sacrificial silicon oxide (1) Pattern photoresist photoresist (4) Gold evaporation Gold mirrors on top and potentially sides (3) SiO 2 isotropic etch Narrow features released, Wide features just undercut

9. “Bulk Silicon” MEMS Devices Comb-drive switch photo courtesy IMT (Neuchatel)Single-axis tilt-mirror photo courtesy R. Conant, BSAC

10. MEMS reliability? 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 40,000G impact test Ceramic package destroyed MEMS survives (!) Micromotor test device Comb-drive actuator Flexural contact to gears Failure by rubbing contact Wear on silicon surface Submicron particles generated “MEMS Reliability: Infrastructure, Test Structures, Experiments and Failure Modes” 171 page report by D. M. Tanner et al, SAND2000-0091, January 2000. www.sandia.gov

11. Optical MEMS Devices Classical vs Resonant

12. Lucent’s “LambdaRouter” Device Sir Isaac Newton (1642-1727) …and his Corpuscular Theory of Light “Classical” optical MEMS 1 st -surface reflection

13. Sir Isaac Newton (1642-1727) …and his Corpuscular Theory of Light Silicon Light Machine’s Grating Light Valve “Resonant” Optical MEMS Interference / Diffraction Thomas Young (1773-1829) … and his 1801 theory of Interference Christiaan Huygens (1629-1695) … and his 1687 Wave Theory of Light

14. Resonant Optical MEMS: Tunable Photonic Bandgap Resonant Optical MEMS Variable gap multilayer 5 - 30 V drive ~ 200 nm actuation ~ 10 us response Lucent’s MARS modulator Variable phase grating ~ 10 V drive ~ 200 nm actuation ~ 10 us response Stanford’s grating light valve VdVd

15. The “MARS” Resonant MEMS Modulator

16. Fabry-Perot etalon reflectivity Reflectivity = --------------------- F sin 2 (  d/d o ) 1+ F sin 2 (  d/d o ) F = 4R s /(1-R s ) 2 R s = top interface reflectivity = 30.6% d = gap between plates d o = gap @ minimum reflectivity ( /2) Resonant optics = Sub-wavelength actuation d Incident ReflectedTransmitted Operation 220 nm Initial gap Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998 Incident ReflectedTransmitted d’

17. 1163 nm Wavelength (microns) 0.00 0.50 1.00 1.21.31.41.51.61.7 Reflectivity air gap o 3 o /2 Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998 1163 nm 900 nm 820 nm 750 nm o /2 750 nm operation Fabry-Perot etalon spectral uniformity Resonant Optics = Wavelength dependence

18. The “MARS” resonant MEMS modulator MARS (Membrane Anti-Reflection Switch) analog optical modulator /4 Silicon Nitride “drumhead” suspended over a Silicon substrate 150  m etch access holes membrane edge PSG 0 < V drive < 30V 3 /4 < gap < /2 input /4 SiN x Silicon PSG reflect transmit V drive 0 < V drive < 30V 3 /4 < gap < /2 input /4 SiN x Silicon PSG reflect transmit V drive Goossen, Arney & Walker, IEEE Phot. Tech. Lett. 6, 1994

19. MARS dielectric multilayer structures Dielectric Silicon Nitride Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998 Conductive Polysilicon + Nitride

20. Lucent’s MARS “bulk” MEMS fabrication Walker, Goossen & Arney, J. MEMS 5(1), 1996 etch holes for HF access metal deposition HF release Silicon NitrideDouble Polysilicon etch via to bottom poly

21. MARS time & voltage response Temporal Response Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998 Greywall, Busch & Walker, Sensors & Actuators A A72, 1999. 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) Voltage Response theory measured Drive voltage (V)

22. MARS Applications: - Data modulator - Variable attenuator - Dynamic spectral equalizer - Dispersion compensator (see references or other presentations)