1. Micromachined Deformable Mirrors for Adaptive Optics
3 mm
0 µm
2 µm
Thomas Bifano
Professor and Chairman
Manufacturing Engineering Department
Boston University
15 Saint Mary’s St.
Boston, MA 02215
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)

2. 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.

3. Boston University Photonics Center
µ-DM Team
Boston University Photonics Center
Introduction
Stop me if you have questions
Fabrication
Optical Testing
Cronos Integrated Microsystems
Adaptive Optics Associates

4. 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.

5. µ-DM Concept Concept: Micromachined deformable mirrors (µDM)
Electrostatically actuated diaphragm
Attachment post
Membrane mirror
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)

6. 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
Tip and tilt

7. 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.

8. Adaptive Optics with MEMS-DM
Deformable
mirror
Aberrated Incoming Image
Image camera
Wavefront sensor
Control system
Beamsplitter
Shape signals
Tilt signals

9. 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

10. 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.

11. Actuator deflection vs. applied voltage+ – x
q(x)
v(x)
d(x)
Deflection v(x) as a function of Applied Voltage V can be modeled as a 4th order nonlinear ODE
Elasticity
Electrostatics
Non-linear ODE

12. Critical deflection is a function of initial gap only

13. Characterization of actuators
Measured deflection versus voltage 50
100
150
200
250
300 -2
-1.5 -1
-0.5
0.5
Voltage (Volts)
Actuator center deflection (mm)
200 mm
350
Single point displacement measuring interferometer
Yield: ~95%
Repeatability: 10 nm (for 99% probability)
Bandwidth: >66kHz
100 mm

14. 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

15. Unintended topography generation is a problem in MEMS
SEM Photo
Numerical Model of Growth
1000
2000
3000
4000
5000
6000
7000
Poly2
Oxide2
Topography (nanometers)
Poly1
Oxide1 1 2 3 4 5 6 7 8 9 10
Lateral Dimensions (micrometers)

16. Surface Micromaching Topography Problem

17. A design-based planarization strategy

18. Narrow anchors reduce print-through to nm scale1 2 3 4 5 6 7 8 9 10
1000
2000
3000
4000
5000
6000
7000
Lateral Dimensions (micrometers)
Topography (nanometers)
Topography generation for 3 um micron anchor in Oxide1, h t
= nm, h n
= nm
Oxide1
Poly1
Oxide2
Poly2
Topography generation for 5 um micron anchor in Oxide1, h
= nm, h
= nm
Topography generation for 2 um micron anchor in Oxide1, h
= nm, h
= nm
Topography generation for 1.5 um micron anchor in Oxide1, h
= nm, h
= nm
5   1.5

19. Design-based planarization concept
Released Oxide
Polycrystalline Silicon
Silicon Substrate
Captured Oxide

20. 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

21. 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.

22. 100 Actuator MEMS Deformable Mirrors
Interferometric surface maps of different 10x10 actuator arrays with a single actuator deflected
2 µm stroke
10 nm repeatability
7 kHz bandwidth
/10 to /20 flatness
<1mW/Channel
Performance Testing in an adaptive optics test-bed currently underway at United Technologies
Fastest, smallest, lowest power DM ever made

23. Mirror Deformation
Interior dome shape created in a 100 zone continuous mirror. mm mm
671.2 nm
-364 nm
0.0

24. MEMS-DM Bandwidth 130 Tip-Tilt µ-DM, 250 µm actuator
Response (dB)
Bandwidth 6.99 kHz
123 1
100
10,000
Frequency (Hz)

25. µDM vs. Macro DM

26. 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 -1 -2 -3
Quad cell (tilt sensor)
Dynamic aberration
Voltage signals to mirror
Mirror driver
Controller
A/D
Computer
Tilt Angle (mrad)

27. Data acquisition and control (WaveLab)
AO Experimental Setup
Data acquisition and control (WaveLab)
HV electronics
µDM
Hartmann wavefront sensor
Static aberration
Point source

28. AOA-testing: removal of static aberration
Aberrated
Flattened (21st iteration)
Wavefront
Strehl =
Strehl =
Point Spread

29. AOA-testing: removal of static aberration
Error signals
(m)
Number of Cycles
0.04
0.004
0.52
0.057
0.10
0.008
Nulled
Aberrated
Corrected
P-V error µm
RMS error µm
Drive signals
(V)
Number of Cycles

30. Adaptive compensation using BU µDM and AOA sensor/controller:
4 mm
Measured wavefront error due to a static aberration (bent glass plate) and compensation by µDM

31. Deformable Micromirrors - The Future
2178 mm
2297 mm
831.6 nm
-616 nm
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.

32. Acknowledgements AASERT program DAAH04-96-1-0250
DARPA support DABT63-95-C-0065
ARO Support through MURI: Dynamics and Control of Smart Structures DAAG
Fabrication by Cronos Integrated Microsystems
AO Experimental support by Boston Micromachines Corporation