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Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Micromachined Deformable Mirrors for Adaptive Optics
Thomas BifanoProfessor and ChairmanManufacturing Engineering DepartmentBoston University15 Saint Mary’s St.Boston, MA [email protected]
3 mm 0 µm
2 µm
Micromachined Deformable Mirror (µDM)
A new class of silicon-based micro-machined deformable mirror (µDM) is being developed. The devices are approximately 100x faster, 100x smaller, and consume 10000x less power than macroscopic DMs.
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Boston University µDMs
At Boston University’s new Photonics Center, a core project is to develop technology for µDMs for adaptive optics and optical correlation.
Funded by DARPA and ARO, our project goals are to design prototype mirror systems, fabricate them using standard foundry processes, and test them in promising optical compensation applications.
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
µ-DM TeamBoston University Photonics Center
Adaptive Optics Associates
Fabrication Optical Testing
Cronos Integrated Microsystems
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
What are µDMs
A promising new class of deformable mirrors, called µDMs, has emerged in the past few years.
These devices are fabricated using semiconductor batch processing technology and low power electrostatic actuation.
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
µ-DM ConceptElectrostatically actuated diaphragm
Attachment post
Membrane mirror
Continuous mirror
Segmented mirrors (piston)
Segmented mirrors (tip-and-tilt)
• Concept: Micromachined deformable mirrors (µDM)
• Fabrication: Silicon micromachining (structural silicon and sacrificial oxide)
• Actuation: Electrostatic parallel plates
• Applications: Adaptive optics, beam forming, communication
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
µDMs in DevelopmentDelft University (OKO)
Underlying electrode array
Continuous membrane mirror
JPL, SY Tech., AFIT
Surface micromachined, segmented mirror
Lenslet cover for improved fill factor
Boston University
Surface micromachined
Continuous membrane mirrors
Texas Instruments
Surface micromachined
Tip and tilt
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Potential Applications/ Imaging & Beamforming
Such devices offer new possibilities for use
of adaptive optics. Their widespread
availability in the next few years will
transform the fields of imaging, beam
propagation, and laser communication.
Lightweight, high resolution imaging systems
Point-to-point optical communication through turbulence
Compact optical beam-forming systems
θ =λr0
θ =λD
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Adaptive Optics with MEMS-DM
Deformablemirror
Aberrated Incoming Image
Image camera
Wavefront sensor
Control system
Beamsplitter
Shape signals
Tilt signals
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
µ-DMs vs. macro DMs
• Why MEMS?
– Compact mirror and electronics
– High bandwidth
– Low power consumption
– Mass producible
• Challenges
– Development of optical coatings
– Reduction of residual strains in films
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Electrostatic Microactuator
Optical microscope image (top view) of a single microactuator actuated through instability point. Membrane is 300 µm x 300 µm, with 5 µm gap between membrane and substrate. Actuation requires 100V.
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Actuator deflection vs. applied voltage
Deflection v(x) as a function of Applied Voltage V can be modeled as a 4th order nonlinear ODE
+
–
x
q(x)
v(x)
d(x)
Elasticityd 4 y(x)
dx 4+PEId2y(x)dx2
=q(x)EI
B.C. y(0) =y(L) =0′ y (0) = ′ y (L) = 0
Electrostatics
q(x ) =κ 2ε0w
2(d−y(x))2V2
(d −y(x))2d4y(x)dx4
+PEI
(d−y(x))2d2y(x)dx2
=κ 2ε0w2EI
V2
Non-linear ODE
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Critical deflection is a function of initial gap only
@ critical voltage, dFR
dw=dFEdw
−k=−3cV2
h3 so: hcr =3cV2
k
substitute eqn for Vcr :hcrg
= 827
3 ≈0.67
∴wcr ≈0.33g
Vcr =4kg3
27c
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Characterization of actuators
0 50 100 150 200 250 300-2
-1.5
-1
-0.5
0
0.5
Voltage (Volts)Act
uato
r ce
nte
r d
efle
ctio
n (m
)
200 m
250300350
Measured deflection versus voltage
100 m
Single point displacement measuring interferometer
Yield: ~95%Repeatability: 10 nm (for 99% probability)Bandwidth: >66kHz
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Fabrication Issues for Surface Micromachined Mirrors
• Planarization: Conformal thin film deposition results in large topography
• Residual Strain: Fabrication stresses result in out-of-plane strain after release
• Stiction: Adhesion occurs between released polysilicon layers
• Release Etch Access Holes: Holes to allow acid access cause diffraction
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Unintended topography generation is a problem in MEMS
0 1 2 3 4 5 6 7 8 9 100
1000
2000
3000
4000
5000
6000
7000
Lateral Dimensions (micrometers)
Top
ogra
phy
(nan
omet
ers)
Oxide1
Poly1
Oxide2
Poly2
SEM Photo Numerical Model of Growth
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Surface Micromaching Topography Problem
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
A design-based planarization strategy
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Narrow anchors reduce print-through to nm scale
0 1 2 3 4 5 6 7 8 9 10
0
1000
2000
3000
4000
5000
6000
7000
Lateral Dimensions (micrometers)
Top
ogra
phy
(nan
omet
ers)
Topography generation for 3 um micron anchor in Oxide1, h
t
=351.0691nm, h
n
=413.3069nm
Oxide1
Poly1
Oxide2
Poly2
0 1 2 3 4 5 6 7 8 9 10
0
1000
2000
3000
4000
5000
6000
7000
Lateral Dimensions (micrometers)
Top
ogra
phy
(nan
omet
ers)
Topography generation for 5 um micron anchor in Oxide1, ht
=1071.6054nm, hn
=1112.7103nm
Oxide1
Poly1
Oxide2
Poly2
0 1 2 3 4 5 6 7 8 9 10
0
1000
2000
3000
4000
5000
6000
7000
Lateral Dimensions (micrometers)
Top
ogra
phy
(nan
omet
ers)
Topography generation for 2 um micron anchor in Oxide1, h
t
=152.2509nm, hn
=209.018nm
Oxide1
Poly1
Oxide2
Poly2
0 1 2 3 4 5 6 7 8 9 10
0
1000
2000
3000
4000
5000
6000
7000
Lateral Dimensions (micrometers)
Top
ogra
phy
(nan
omet
ers)
Topography generation for 1.5 um micron anchor in Oxide1, ht
=84.9445nm, hn
=134.7378nm
Oxide1
Poly1
Oxide2
Poly2
5 2.5 2 1.5
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Design-based planarization concept
Polycrystalline SiliconSilicon Substrate
Released Oxide
Captured Oxide
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Nine-actuator prototype MEMS-DM
Center deflected
Edge deflected
Corner deflected
Number of actuators 9Mirror dimensions 560 x 560 x 1.5 µmActuator dimensions 200 x 200 x 2 µmActuator gap 2.0 µmInter-actuator spacing 250 µm
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Nine-element mirror performance
Surface map and x-profile through the center of a nine-element continuous mirror, pulled down by 155V applied to the center actuator. The mirror and actuator system exhibited ~7kHz frequency bandwidth, when driven by a custom designed electrostatic array driver.
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
100 Actuator MEMS Deformable Mirrors
– 2 µm stroke
– 10 nm repeatability
– 7 kHz bandwidth λ/10 to λ/20 flatness
– <1mW/Channel
444.6 nm
-1512.6 nm
0 m 31610 m
3161
(a)
700 nm
-1455 nm
0 m 29290 m
2929
(b)
Interferometric surface maps of different 10x10 actuator arrays with a single actuator deflected
Performance Testing in an adaptive optics
test-bed currently underway at United
Technologies
Fastest, smallest, lowest power DM ever made
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Mirror Deformation
2248.4 m
2318.5 m
671.2 nm
-364 nm
0.0
0.0
Interior dome shape created in a 100 zone continuous mirror.
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
MEMS-DM Bandwidth
Bandwidth 6.99 kHz
Frequency (Hz)
Response (dB)
130
1231 100 10,000
Tip-Tilt µ-DM,
250 µm actuator
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
µDM vs. Macro DM
Specification BU MEMS-DM Commercial Macro-DMNumber of actuators 100, 336 37, 97, or 350Actuation Integrated Electrostatic Discrete PiezoelectricPackage size 10 cc 1000 ccPower consumption 1 mW/actuator 7000 mW/actuatorActuator spacing 0.3 mm 7.0 mmActuator stroke 2 µm 4 µmHysteresis 0% >5%Settling time 0.2 ms 15.0 msSurface roughness 35 nm Rq ~30 nm RqNominal cost $5000 (100 actuator) ~$100,000 (97 actuator)
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Dynamic optical correction
A/D
Voltage signals to mirror
Dynamic aberration
MEMS Deformable mirror
He Ne LASER
Quad cell (tilt sensor)
Mirror driver
Computer
Controller
Two axis wavefront tilt due to a candle flame corrected in real time
using the MEMS-DM
2
1
0
-1
-2
-3-3 -2 -1 0 1 2 3 4
Tilt Angle (mrad)
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
AO Experimental Setup
HV electronics
Data acquisition and control (WaveLab)
Point source
Hartmann wavefront sensor
µDM
Static aberration
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
AOA-testing: removal of static aberration
Aberrated Flattened (21st iteration)
Strehl = 0.0034
Wavefront
Point Spread
Strehl = 0.1950
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
AOA-testing: removal of static aberration
Number of Cycles
Number of Cycles
(m
)(V
)
Error signals
Drive signals 0.04 0.004
0.52 0.057
0.10 0.008
Nulled
Aberrated
Corrected
P-V error µm
RMS error µm
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Adaptive compensation using BU µDM and AOA sensor/controller:
0.8µm
4 mm
Measured wavefront error due to a static aberration (bent glass plate) and compensation by µDM
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Deformable Micromirrors - The Future
2178 m
2297 m
831.6 nm
-616 nm 0.0
0.0
Further development planned by Boston University in collaboration with Boston Micromachines Corporation
121 element arrays, bare silicon or with gold overlayer, are currently available for testing.
Novel design based on lessons learned in prototype Phases I and II is complete. Fabrication in planning stages.
Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])
Acknowledgements
AASERT program DAAH04-96-1-0250DARPA support DABT63-95-C-0065ARO Support through MURI: Dynamics and Control of Smart Structures DAAG55-97-1-0144Fabrication by Cronos Integrated MicrosystemsAO Experimental support by Boston Micromachines Corporation