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

• Surface Micromachining• LIGA (electroforming)• Deep Reactive Ion Etching

• Electrostatic attraction• Electromagnetic force• Electrostriction• Resistive heating

Photos courtesySandia 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

Mass commercial application: Acceleration Sensors

http://www.analog.com/library/techArticles/mems/xlbckgdr4.html

Analog Devices' ADXL50 accelerometerSurface 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

Mass commercial application: Pressure Sensors

Capacitive Pressure Sensor

Silicon substrate

Pint

Pext

Spacer

Membrane

Force

MeasureRC time

NovaSensor’s piezo-resistive pressure sensors Disposable medical sensor

High-pressure gas sensor(ceramic surface-mount)

Piezo-resistive pressure sensor

substrate

magnetic layer

EM coil

conductive substrate

conductive layer

insulator

substrate

patterned resistive layer

substrate

electrostrictive layer

Force

Force

Force

Force

ApplyCurrent

ApplyVoltage

Electrical actuation of active MEMS devices

Electrostatic attraction

Resistive heatingElectrostriction

Electromagnetic force

ApplyCurrent

Apply Voltage

Surface Micromachining: Layer by layer additionStarting from bare silicon wafer, deposit & patternmultiple 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 oxidesSpecial techniques may be used to remove liquid (e.g., critical point drying)

Diced and released MEMS device

Completed MEMS wafer

~ 10 mask steps

Texas Instruments Digital Light Projector& DLP PROJECTOR

TM

1st Optical MEMS device

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 photoresistphotoresist

(4) Gold evaporation

Gold mirrors on topand potentially sides

(3) SiO2 isotropic etch

Narrow features released,Wide features just undercut

“Bulk Silicon” MEMS Devices

Comb-drive switch photo courtesy IMT (Neuchatel)Single-axis tilt-mirror photo courtesy R. Conant, BSAC

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 testCeramic package destroyed

MEMS survives (!)

Micromotor test deviceComb-drive actuator

Flexural contact to gears

Failure by rubbing contactWear 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

Optical MEMS Devices Classical vs Resonant

Lucent’s “LambdaRouter” DeviceSir Isaac Newton

(1642-1727) …and his Corpuscular Theory of Light

“Classical” optical MEMS

1st-surface reflection

Sir Isaac Newton (1642-1727)

…and his Corpuscular Theory of LightSilicon 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

Resonant Optical MEMS: Tunable Photonic BandgapResonant Optical MEMS: Tunable Photonic BandgapResonant Optical MEMS

Variable gap multilayer 5 - 30 V drive ~ 200 nm actuation ~ 10 us response

Lucent’s MARS modulatorLucent’s MARS modulator

Variable phase grating ~ 10 V drive ~ 200 nm actuation ~ 10 us response

Stanford’s grating light valveStanford’s grating light valve

Vd

The “MARS”Resonant MEMS Modulator

Fabry-Perot etalon reflectivity

Reflectivity = ---------------------F sin2(d/do)

1+ F sin2(d/do)

F = 4Rs/(1-Rs)2

Rs = top interface reflectivity = 30.6%d = gap between platesdo = gap @ minimum reflectivity (/2)

Resonant optics = Sub-wavelength actuationResonant optics = Sub-wavelength actuation

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

-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)

refl

ec

tio

n

d

Incident

Reflected Transmitted

Operation

220 nm

Initi

al g

ap

Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998

Incident

Reflected Transmitted

d’

1163 nm

Wavelength (microns)

0.00

0.50

1.00

1.2 1.3 1.4 1.5 1.6 1.7

Ref

lect

ivit

y

air gap

o

3o/2

Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998

1163 nm

900 nm

900 nm

820 nm

820 nm

750 nmo/2

750 nm

op

erat

ion

Fabry-Perot etalon spectral uniformity

Resonant Optics = Wavelength dependenceResonant Optics = Wavelength dependence

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 < Vdrive < 30V3/4 < gap < /2

input

/4 SiNx

Silicon

PSG

reflect

transmit

Vdrive

0 < Vdrive < 30V3/4 < gap < /2

input

/4 SiNx

Silicon

PSG

reflect

transmit

Vdrive

Goossen, Arney & Walker, IEEE Phot. Tech. Lett. 6, 1994

MARS dielectric multilayer structures

Dielectric Silicon Nitride

Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998

Conductive Polysilicon + Nitride

Lucent’s MARS “bulk” MEMS fabrication

Walker, Goossen & Arney, J. MEMS 5(1), 1996

etch holesfor HF access

metaldeposition

HF release

Silicon Nitride Double Polysilicon

etch viato bottom poly

MARS time & voltage response

Temporal Response

Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998Greywall, 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)

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