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V. Milanović, IEEE, 12/2002
MEMS Technologies for Optical Applications
Dr. Veljko MilanovićAdriatic Research Institute
2131 University Ave Suite 322Berkeley, CA 94704-1079
http://www.adriaticresearch.org
V. Milanović, IEEE, 12/2002
Outline
� Motivations and Background� Application Examples� Electrostatic actuators overview� Some micromirror examples
� High aspect ratio MEMS� Silicon on Insulator MEMS (SOI-MEMS)� Micromirrors in SOI-MEMS
� ARI Approach: Multilevel Beam SOI-MEMS� Fabrication and Results� Device examples
� Latest results with collaborators at UC Berkeley, demos
� Gimbal-less 2D Optical Scanner � Kyoto device
V. Milanović, IEEE, 12/2002
BSAC Motivation: cm3 Multisensor Optical Transceiver
V. Milanović, IEEE, 12/2002
Latest SALT ObjectivesFree-space laser communication between mobile nodes using MEMS transceivers
RD tθ≈θdiv: Laser beam divergence
R
D 5 mθdiv
5 km
1mrad
V. Milanović, IEEE, 12/2002
Applications: Automotive
� Demand for miniature and low-cost laser beam-steering systems
� High speed scanning/refresh rate
� Large deflection angles desirable
HUD � head-up display
Collision Avoidance
V. Milanović, IEEE, 12/2002
Fiber-optic switchesOptical cross-connects (OXC)
� Optical switches� 1xN and NxN switch arrays for network restoration/protection
(OADMs)� NxN large port-count optical cross-connects (OXC)
� Above e.g. 32x32 count beam-steering mirrors with 1DoF or 2DoF
V. Milanović, IEEE, 12/2002
Beam-steering OXC
� Analog mirrors � more complex implementation� 1DoF or 2DoF
� 2N Scaling � easier to achieve larger N
� Lucent, C-speed, Xros, �
V. Milanović, IEEE, 12/2002
Motivation: Adaptive Optical Arrays
� Array of micromirrors� High fill factor� High reflectivity� Analog Tip and tilt rotation� Analog piston (vertical)
actuation� Universal optical element
� Beam steering� Reflective lenses� Gratings, etc.� Yield breaks!
Digital micromirrors – TINo tip/tilt, nor pistonNot analog
V. Milanović, IEEE, 12/2002
Outline
� Motivations and Background� Application Examples� Electrostatic actuators overview� Some micromirror examples
� High aspect ratio MEMS� Silicon on Insulator MEMS (SOI-MEMS)� Micromirrors in SOI-MEMS
� ARI Approach: Multilevel Beam SOI-MEMS� Fabrication and Results� Device examples
� Latest results with collaborators at UC Berkeley, demos
� Gimbal-less 2D Optical Scanner � Kyoto device
V. Milanović, IEEE, 12/2002
Electrostatic Actuators
Plate-1
Plate-2
Plate-3
V (volts)
x
yd Consider parallel plate 1 & 2Force of attraction (along y direction)
Fp = (½ ε V2)(A/g2)
1nN@15Vdimensionless
Consider plate 2 inserted between plate 1 and 3(Popularly known as a COMB DRIVE)
Fc = (½ ε V2 )(2t/g)Force of attraction (along x direction)
Constant with x-directional translation
V. Milanović, IEEE, 12/2002
Parallel-Plate Electrostatic Actuator Pull-in
Electrostatic instability
mF = ε0V 2
2 s0 − x( )2 = kx ⇒
V = 2kxε0
s0 − x( )
∂V∂x
= 0 ⇒ xsnap = s03
Vsnap = 8ks03
27ε0
k
x
s0V
x
s0
1/3 s0
VVsnap
V. Milanović, IEEE, 12/2002
Outline
� Motivations and Background� Application Examples� Electrostatic actuators overview� Some micromirror examples
� High aspect ratio MEMS� Silicon on Insulator MEMS (SOI-MEMS)� Micromirrors in SOI-MEMS
� ARI Approach: Multilevel Beam SOI-MEMS� Fabrication and Results� Device examples
� Latest results with collaborators at UC Berkeley, demos
� Gimbal-less 2D Optical Scanner � Kyoto device
V. Milanović, IEEE, 12/2002
2x2 MEMS Fiber Optic Switches2x2 MEMS Fiber Optic Switches with Silicon Sub-Mount for Low-Cost Packaging
Shi-Sheng Lee, Long-Sun Huang, Chang-Jin “CJ” Kim and Ming C. WuElectrical Engineering Department, UCLA
63-128, engineering IV Building, Los Angeles, California 90095-1594Mechanical and Aerospace Engineering Department
Figure 3. SEM of the torsion mirror device.
Figure 1. SEM of the 2x2 MEMS fiber optic switch.
V. Milanović, IEEE, 12/2002
Lucent 2DoF Micromirror
V. Milanović, IEEE, 12/2002
Surface MEMS Micromirror Structure
Torsion Hinges
Mirror Surface
Electrostatic Combdrive
Frame Hinge
Combdrive Linkage
Support Frame
Substrate Hinge
Conant et al, MOEMS99
V. Milanović, IEEE, 12/2002
Optical Phased Arrays Of Micromirrors
Dual mode µmirror design Static top comb
electrodes
Static bottom comb electrodes
Movable mirror comb electrodes
TorsionalHinge
Moving mirror
Vertical Comb
Reference mirror
!
-1 -0.5 0 0.5 1
10
20
30
40
50
60
70
Distance (mm)
Inte
nsity
• Phased arrays of micromirrors allow directional control and optical phase correction for precision optical spatial and spectral scanning
• Applications:" Spectroscopy" Fiber optics" Free space optical communication
Courtesy of O. Solgaard
Far-field pattern of scanning mirror array with phase correctionSEM of phased array for spatial switching.
V. Milanović, IEEE, 12/2002
1D Phased Array with Tilt and Piston motion
Krishnamoorthy et al, Transducers 01, Munich, Germany
V. Milanović, IEEE, 12/2002
Outline
� Motivations and Background� Application Examples� Electrostatic actuators overview� Some micromirror examples
� High aspect ratio MEMS� Silicon on Insulator MEMS (SOI-MEMS)� Micromirrors in SOI-MEMS
� ARI Approach: Multilevel Beam SOI-MEMS� Fabrication and Results� Device examples
� Latest results with collaborators at UC Berkeley, demos
� Gimbal-less 2D Optical Scanner � Kyoto device
V. Milanović, IEEE, 12/2002
High Aspect Ratio MEMS (HARM)SOI-MEMS
2DoF Electrostatic actuator
Thermal Actuator
Comb-drive Actuator
� Height/width > ~3:1� Height/space > ~3:1� Increased capacitance for
actuation and sensing� Low-stress structures
� single-crystal Si only structural material
� Highly stiff in vertical direction� isolation of motion to wafer
plane� flat, robust structures
V. Milanović, IEEE, 12/2002
Courtesy Robert A. Conant, BSAC
• DRIE etching of SOI materials• High-quality, flat, and stiff
mirrors• High-speed deflection• Electrostatic actuators
• High force, Dual-mode and two-sided possible, All-conductive structure
• Flexible Process# Phased arrays, 2-D arrays,
Through-the-wafer interconnects
• Combs require accurate alignment# Yield, Reliability, Maximum
deflection• Front-side processing
# Improved reliability and yield, controlled damping, standard wafers, integration
MEMS Mirrors in SOI
O.SolgaardStanford
V. Milanović, IEEE, 12/2002
STEC Micromirror � Vertical Comb-drives
Conant et al, HH2000
Conant et al,HH2000
V. Milanović, IEEE, 12/2002
Coherent phased array Far-field diffraction patterns
O.SolgaardStanford
20
40
60
80
100
120
Inte
nsity
(a
.u.)
0-1 -0.5 0 0.5 1
Inte
nsity
(a.u
.)
20
40
60
80
100
120
Distance (mm)
0-1
20
40
60
80
100
120
Inte
nsity
(a.u
.)
0
Calculated far field
0.500.5 -0.5-1 -0.5 0 1 1
Distance (mm)Distance (mm)
K. Li, U. Krishnamoorthy, J.P. Heritage, O. Solgaard �Micromirror Array Phase Modulator for Ultrashort Optical Pulse Shaping�, Proceedings of the Solid-State Sensor and Actuator Workshop, pp. 15-18, Hilton Head, South Carolina, June 2-6, 2002.U. Krishnamoorthy, K. Li, K. Yu, D. Lee, J.P. Heritage, O. Solgaard, "Dual mode micromirrors for optical phased array applications", Sensors and Actuators: A. Physical, Vol. 97-98C, May 2002, pp 22-26.K. Li, U. Krishnamoorthy, J.P. Heritage, O. Solgaard �Coherent micromirror arrays�, Optics Letters Vol. 27, No. 5, March 1, 2002 p.366-368.
V. Milanović, IEEE, 12/2002
Outline
� Motivations and Background� Application Examples� Electrostatic actuators overview� Some micromirror examples
� High aspect ratio MEMS� Silicon on Insulator MEMS (SOI-MEMS)� Micromirrors in SOI-MEMS
� ARI Approach: Multilevel Beam SOI-MEMS� Fabrication and Results� Device examples
� Latest results with collaborators at UC Berkeley, demos
� Gimbal-less 2D Optical Scanner � Kyoto device
V. Milanović, IEEE, 12/2002
upperSCS highSCS
lowSCS
midSCS
middownSCS
Previous wafer surface, or surface of bonded SOI layer,
after removal of SOI oxide
midupSCS
New methodology and devices: Multilevel Beam SOI-MEMS � ARI-MEMS
� Front- and Back-side multilevel etching for advanced HARM� 2 front masks gives lowSCS, highSCS� 2 front + 1 back mask gives lowSCS, highSCS, upperSCS� 3 front + 2 back (or other combos) gives low, mid, upper, high,
midup, middown SCS
Previous wafer surface, or surface of bonded SOI layer,
after removal of SOI oxide5 masks
Milanović, to appear, JMEMSMilanović, ICECS’02, Dubrovnik, Croatia, Sep. 02
V. Milanović, IEEE, 12/2002
Conversion of in-plane motion to rotation - I
anch
or
xy
z
Back-side etched cavity
Low-massupperSCS mirror
Upper beamtorsional support
mirror
Lower beamactuation arm
anch
or
To lateralactuatorsLateral pull
Axial tension
Lateral push
Axial compression
Milanović, Transducers’01, Muenchen, Germany.
V. Milanović, IEEE, 12/2002
3-level ARI-MEMS
lowSCS
highSCS
upperSCS
lowSCS
highSCS
upperSCS
V. Milanović, IEEE, 12/2002
Push-mode 1DoF mirror
Device is actuated by SEM charging of the comb-driveMilanović et al, MOEMS 2001
V. Milanović, IEEE, 12/2002
Most recent 1DoF measurements � over 40° optical static deflection
Actuation voltage [V]0 20 40 60 80 100
Opt
ical
def
lect
ion
[deg
]
0
2
4
6
8
10
12
14
16
18
20
22
PUSH mode with NO displaced attachment
Actuation voltage [V]0 25 50 75 100 125 150
Opt
ical
def
lect
ion
[deg
]-22
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
PULL mode with 340 µm displaced attachment
Push-mode Pull-mode (with displaced attachment)
Milanović et al, MOEMS 2001
V. Milanović, IEEE, 12/2002
Conversion of in-plane motion to rotation - II
� Lower and Upper beams can be placed at minimum spacing that is DRIE aspect-ratio limited
� Lower and Upper beams self-aligned by same top mask
� Vertical comb-drives for rotation and piston motion
Upper beamtorsional support
mirror
Lowerbeams
Upperbeams
V. Milanović, IEEE, 12/2002
Device improvement example � pure torque actuation (both directions) and up/down piston motion
middleSCS
lowSCS
lowSCS
middleSCS
upperSCSupperSCS
mirror
lowSCS DC voltage
DC voltage groundlowSCS
ground
DC voltageDC voltage
Milanović, submitted to JMEMS
V. Milanović, IEEE, 12/2002
Fabricated 1DoF Mirror with Independent Pistoning Control
GND
V2 V4
V1 V3
500 µm
Up combs
Down combs
A
A’
B
B’
Mirror surface
Milanović et al, to appear in IEEE Photonics Tech. Lett.Milanović et al, MOEMS’02, Switzerland, Aug. 02
V. Milanović, IEEE, 12/2002
Multiple modes of actuation
Up combsDown combsV2 V1
V3V4
V2 V3
V4 V1
GND
GND
GND
GND
A-A’
B-B’
A’-B
A-B’
Upper HighLower
V. Milanović, IEEE, 12/2002
Fabrication: Embedded �Backup� mask
Wafer 1
Wafer 2
Silicon Wet ox.
SOI Wafer device layer
SOI Wafer handle
V. Milanović, IEEE, 12/2002
Fabrication: Mask Self-alignment process steps
Silicon Photoresist LTO Wet ox.
Mask TRENCH
Etch thermal oxide
Deposit LTO
Mask ALIGN
Etch LTO (RIE)
Continue etching (RIE)
thermal oxide to Si
Silicon Photoresist LTO Wet ox.Silicon Photoresist LTO Wet ox.
Mask TRENCH
Etch thermal oxide
Deposit LTO
Mask ALIGN
Etch LTO (RIE)
Continue etching (RIE)
thermal oxide to Si
LTO
LTO
WETOX+LTO
PHOTORESIST
6.13 6.20
LTO
LTO
WETOX+LTO
PHOTORESIST
6.13 6.20
V. Milanović, IEEE, 12/2002
Fabrication: masks and backside steps
� All masks prepared before DRIE steps
� Front-side two oxide masks� One embedded oxide mask� Back-side thick-resist mask
DRIE
Backup maskBackside mask
Trench and Align masks
DRIE
V. Milanović, IEEE, 12/2002
Fabrication: final, frontside steps
OX RIE
Lower beam
Upper beams
High beam
DRIE
Middle beam
� After backside, wafer diced� Front-side steps on chips� Good cooling for structures is
needed
DRIE
V. Milanović, IEEE, 12/2002
Resulting structures - II20 µm
Lower
Upper
High10 µm
MiddleHigh
� Self aligned combdrives� Compliant low aspect-ratio
beams� Isolated combdrives� Opposing actuation combdrives
100 µmUp combs
Down combs
V. Milanović, IEEE, 12/2002
4-mode device: Rotation Measurements
Voltage [V]0 10 20 30 40 50 60
Stat
ic o
ptic
al d
efle
ctio
n [d
egre
es]
0
2
4
6
8
10
12
14
16 Combs on both sides - "pure" rotationCombs on one side only
Voltage [V]0 10 20 30 40
Stat
ic o
ptic
al d
efle
ctio
n [d
egre
es]
-12
-10
-8
-6
-4
-2
0
Combs on both sides- pure rotationCombs on one side only
Milanović et al, MOEMS’02, Switzerland, Aug. 02
V. Milanović, IEEE, 12/2002
4-mode device: Up and Down Pistoning Measurements
Voltage [V]0 10 20 30 40 50 60
Stat
ic v
ertic
al m
otio
n [ µ
m]
0
2
4
6
8
10
Piston motion UP
Voltage [V]0 10 20 30 40 50 60
Stat
ic v
ertic
al m
otio
n [ µ
m]
-10
-8
-6
-4
-2
0
Piston motion DOWN
Milanović et al, MOEMS’02, Switzerland, Aug. 02
V. Milanović, IEEE, 12/2002
Two-axis rotation and independent pistoning- future adaptive optics arrays -
-10 0 10 20 30 40 50 60 70 80 90
0
2
4
6
8
10
12
14
Angl
e[de
gree
]
Driving Voltage [V]
% Inner mirror % Outter mirror
200 µm
V1
V1
V2
V2
V3
V3
GND
V4
+ - + - + - + - + -10° at 68V -10° at 72V 8.0 at 60V -7.6 at 60V 7.0° at 50V -12.0 at 50 V 7.6° at 50V n/a n/a n/a
24° at 130V -20° at 123V 12 at 120V -11 at 120V 15.6° at 100V -20.3° at 100V 11.9° at 100V n/a 7.0 at 100V -6.0 at 100V
Pistoning [µm]2DoF Micromirror1DoF Micromirror
Rotation [opt. deflect.] Pistoning [µm] Outter axis [opt. deflect.] Inner axis [opt. deflect.]
Kwon, et al, Optical MEMS 2002
V. Milanović, IEEE, 12/2002
Low-voltage pistoning actuator for imaging applications
V1V2
GND
Upper
Down combs
Down pistoning 8 µm < 10V
Up/Down pistoning, -8 µm to +8 µm, <15V
At 8 µm of piston, only 0.035° of tilt
Tilt correction demonstrated
Kwon et al, Hilton Head Workshop 2002
V. Milanović, IEEE, 12/2002
Demonstration video
V. Milanović, IEEE, 12/2002
Outline
� Motivations and Background� Application Examples� Electrostatic actuators overview� Some micromirror examples
� High aspect ratio MEMS� Silicon on Insulator MEMS (SOI-MEMS)� Micromirrors in SOI-MEMS
� ARI Approach: Multilevel Beam SOI-MEMS� Fabrication and Results� Device examples
� Latest results with collaborators at UC Berkeley, demos
� Gimbal-less 2D Optical Scanner � Kyoto device
V. Milanović, IEEE, 12/2002
UAV (MLB Bat) Specifications
� 5 ft. wingspan� Weight 13 lbs (including video equipment)� Speed 25-50 miles/hr� Cost $35,000 � Laser comm UAV to be
� Gasoline engine, 18 lb at takeoff, 2 hr flight time
V. Milanović, IEEE, 12/2002
Pod Assembly
MEMSLaser
ADI Gyro
CanisterImaging Receiver
Cap
V. Milanović, IEEE, 12/2002
Outline
� Motivations and Background� Application Examples� Electrostatic actuators overview� Some micromirror examples
� High aspect ratio MEMS� Silicon on Insulator MEMS (SOI-MEMS)� Micromirrors in SOI-MEMS
� ARI Approach: Multilevel Beam SOI-MEMS� Fabrication and Results� Device examples
� Latest results with collaborators at UC Berkeley, demos
� Gimbal-less 2D Optical Scanner � Kyoto device� Toward high-frequency beam-steering micro-optical
devices!
V. Milanović, IEEE, 12/2002
Gimbal-less 2D scanners � Kyoto device
X-axis
Y-axis
Rotation actuator A
Rotation actuator A’
Rotation a
ctuato
r B
Rotation ac
tuator B
’
Backsideetched cavity
X-axis
Y-axis
Rotation actuator A
Rotation actuator A’
Rotation a
ctuato
r B
Rotation ac
tuator B
’
Backsideetched cavity
Milanović et al, MEMS, Kyoto, Japan, Jan. 2003
Rotationactuator A
Rotationactuator A’Micromirrorplate
Rotationtransformer
Rotationtransformer
Axisof rotation
(x-axis)
Insidelinkage
OutsidelinkageRotation
actuator ARotation
actuator A’Micromirrorplate
Rotationtransformer
Rotationtransformer
Axisof rotation
(x-axis)
Insidelinkage
Outsidelinkage
V. Milanović, IEEE, 12/2002
1st generation Kyoto device SEM
200 µm
V 2
V 2V 1
V 1
GND
BACKSIDECAVITY
Milanović et al, MEMS, Kyoto, Japan, Jan. 2003
V. Milanović, IEEE, 12/2002
Mechanical transformer / linkages
100 µm
Torsionally compliantbeam
Rotationtransformer
Milanović et al, MEMS, Kyoto, Japan, Jan. 2003
V. Milanović, IEEE, 12/2002
Characterization of a Kyoto deviceX-axis 4.9kHz lowest res.Y-axis 6.52 kHz lowest res.
0
2
4
6
8
10
12
0 10 20 30 40 50 60 70 80 90
DC Actuation Voltage [V]
Stat
ic O
ptic
al B
eam
Def
lect
ion
[Deg
] x-axis reflector
x-axis actuator
y-axis reflector
y-axis actuator
Single actuator rotation Complete 2D scanner
Milanović et al, MEMS, Kyoto, Japan, Jan. 2003
V. Milanović, IEEE, 12/2002
Demonstration video
V. Milanović, IEEE, 12/2002
Latest tape-out with improved stabilitySupport beams shorterand farther apart
V. Milanović, IEEE, 12/2002
Characterization of a large angle device � latest tape-out
0
5
10
15
20
25
0 20 40 60 80 100
VDC
De
gre
es
y-axis
x-axis
X-axis 1.03 kHz lowest res.Y-axis 1.36 kHz lowest res.
V. Milanović, IEEE, 12/2002
V. Milanović, IEEE, 12/2002
Summary� New class of High Aspect Ratio MEMS is possible by
providing additional degrees of freedom� Especially useful for Optical Applications!
� Many mechanical, optical, etc. applications can benefit from the technology� Microoptics, microrobotics, inertial sensing
� Technologies are maturing� variety of mechanical designs are being tested� models and tools (CAD) are being developed for beams,
structures, layout� ARI seeks research grants and commercial partners to
further the work and apply to commercial applications� Near the goal of >10kHz two-axis scanners with large
angle static scanning� Working toward large arrays (above + pistoning)