Micromachined Deformable Mirrors for Adaptive Optics

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Micromachined Deformable Mirrors for Adaptive
Optics
Thomas Bifano Professor and Chairman Manufacturing
Engineering Department Boston University 15
Saint Marys St. Boston, MA 02215 bifano_at_bu.edu 61
7-353-5619
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.
Micromachined Deformable Mirror (µDM)
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Boston University µDMs
At Boston Universitys 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.
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µ-DM Team
Boston University Photonics Center
Fabrication
Optical Testing
Cronos Integrated Microsystems
Adaptive Optics Associates
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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.
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µ-DM Concept

• Concept Micromachined deformable mirrors (µDM)
• Fabrication Silicon micromachining (structural
  silicon and sacrificial oxide)
• Actuation Electrostatic parallel plates
• Applications Adaptive optics, beam forming,
  communication

Continuous mirror
Segmented mirrors (piston)
Segmented mirrors (tip-and-tilt)
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µDMs in Development
Delft 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
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Potential Applications/ Imaging Beamforming
Lightweight, high resolution imaging systems
Point-to-point optical communication through
turbulence Compact optical beam-forming systems
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.
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Adaptive Optics with MEMS-DM
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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

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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.
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Actuator deflection vs. applied voltage
Deflection v(x) as a function of Applied Voltage
V can be modeled as a 4th order nonlinear ODE
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Critical deflection is a function of initial gap
only
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Characterization of actuators
Measured deflection versus voltage
Single point displacement measuring interferometer
Yield 95 Repeatability 10 nm (for 99
probability) Bandwidth gt66kHz
100 mm
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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

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Unintended topography generation is a problem in
MEMS
SEM Photo
Numerical Model of Growth
Poly2
Oxide2
Topography (nanometers)
Poly1
Oxide1
0
1
2
3
4
5
6
7
8
9
10
Lateral Dimensions (micrometers)
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Surface Micromaching Topography Problem
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A design-based planarization strategy
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Narrow anchors reduce print-through to nm scale
5? 2.5? 2? 1.5?
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Design-based planarization concept
Released Oxide
Polycrystalline Silicon
Silicon Substrate
Captured Oxide
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Nine-actuator prototype MEMS-DM
Number of actuators 9 Mirror dimensions 560 x 560
x 1.5 µm Actuator dimensions 200 x 200 x 2
µm Actuator gap 2.0 µm Inter-actuator spacing 250
µm
Center deflected
Edge deflected
Corner deflected
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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.
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100 Actuator MEMS Deformable Mirrors
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

• 2 µm stroke
• 10 nm repeatability
• 7 kHz bandwidth
• ?/10 to ?/20 flatness
• lt1mW/Channel

Fastest, smallest, lowest power DM ever made
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Mirror Deformation
Interior dome shape created in a 100 zone
continuous mirror.
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MEMS-DM Bandwidth
130
Tip-Tilt µ-DM, 250 µm actuator
Response (dB)
Bandwidth 6.99 kHz
123
1
10,000
100
Frequency (Hz)
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µDM vs. Macro DM
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Dynamic optical correction
Two axis wavefront tilt due to a candle flame
corrected in real time using the MEMS-DM

He Ne LASER
MEMS Deformable mirror
2 1 0 -1 -2 -3
Quad cell (tilt sensor)
Dynamic aberration
Voltage signals to mirror
Mirror driver
Controller
A/D
Computer
-3 -2 -1 0 1 2 3 4
Tilt Angle (mrad)
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AO Experimental Setup
Data acquisition and control (WaveLab)
HV electronics
µDM
Hartmann wavefront sensor
Static aberration
Point source
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AOA-testing removal of static aberration
Flattened (21st iteration)
Aberrated
Wavefront
Strehl 0.1950
Strehl 0.0034
Point Spread
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AOA-testing removal of static aberration
Error signals
(?m)
Number of Cycles
Drive signals
(V)
Number of Cycles
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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
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Deformable Micromirrors - The Future
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.
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Acknowledgements
AASERT program DAAH04-96-1-0250 DARPA support
DABT63-95-C-0065 ARO Support through MURI
Dynamics and Control of Smart Structures
DAAG55-97-1-0144 Fabrication by Cronos Integrated
Microsystems AO Experimental support by Boston
Micromachines Corporation