MEMS Deformable Mirrors in Astronomical AO
Thomas Bifano
Director, Boston University Photonics Center (BUPC)
Chief Technical Officer, Boston Micromachines Corporation (BMC)
Paul Bierden President, BMC
Steven Cornelissen, VP, BMC
AO4ELT, Paris, 25 June 2009
Microelectromechanical (MEMS) DMs
Over the past decade, we’ve led an academic program at the Boston University Photonics Center (BU), and a technology development program at Boston Micromachines Corporation (BMC), to pioneer and demonstrate DMs made with semiconductor foundry processes.
Mirror Electrostatic actuator array
Attachment post
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Silicon wafer
Two DMs described in this talk
4096 actuator continuous membrane DM for Gemini Planet Imager
331 segment (993 actuator) hexagonal tip-tilt-piston DM for NASA TPF-C visible nulling coronagraph
Application: Gemini Planet Imaging (4K DM)
B. Macintosh, J. Graham, D. Palmer et al., “Adaptive optics for direct detection of extrasolar planets: the Gemini Planet Imager,” Comptes Rendus Physique, vol. 8, no. 3-4, pp. 365-373, Apr-May, 2007.
Gemini Planet Imager: 4096 actuator DM (BMC), with 3.5µm stroke, for Jovian exoplanet detection. Engineering mirror delivered, science mirror due.
Some DM Requirements for 4K GPI DM
Description Requirement
Actuators 4096 (64x64 array)
Stroke 3µm, after mirror is flattened
Active Aperture 19.2 mm (48 actuator diameter @ 400µm pitch)
Local nonflatness <10 nmRMS
Bandwidth ~2.5 kHz
Inter-Actuator Stroke >1µm
Yield 100% of actuators on a 48 actuator aperture
Operating Temperature -30C to +25C
4K DM Prototype Results
High spatial frequency print-through reduced to <10nm RMS
Previous DM: 21.5nm RMS Phase I DM: 5nm RMS
2.6mm
4.32µm
1.15µm Interactuator stroke achieved
175nm 80nm
0nm 0nm
1150nm
0nm
>4µm stroke achieved @ 210V
Measured Optical Quality
Top right zone (showing scallop at periphery)
Center zone
16RW013#001
~50nm PV
6µm
0µm
4.06µm PV707nm RMS48m ROC
Measured surface200nm
0nm
40nm PV4nm RMS
Filtered surface (uncontrollable)
~25nm PV
50nm
0nm
100nm
0nm
DM Static Cold Test
@ 24.7C @ -20.2C
Cycling & Hysteresis
Package and Driver
Form factor 3U Chassis (5.25” x19” x14”)
Frame rate 34 kHz / 60 kHz (Low Latency)
Resolution 14-bit
This MEMS DM architecture permits ultraprecise, repeatable control
1024 actuator MEMS DM• Controllable flatness <12nm• Actuator repeatability <1nm• Hysteresis <1nm
144nm Initial 12nm Controlled
Three research groups have developed precise models of MEMS DM behavior, including mechanical coupling through the mirror and nonlinear actuation electromechanics. Result: We can now achieve open-loop shape control within 25nm error in one step.
J. B. Stewart, A. Diouf, Y. P. Zhou, T. G. Bifano, Journal of the Optical Society of America 24, 3827 (Dec, 2007).
J. W. Evans et al., Optics Express 14, 5558 (2006)
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2 2 2
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q x y w x y w x y w x y w x yw x y
D h x x y y
331 Element Tip-Tilt-Piston MEMS DM
+/-6mrad tip-tilt2um piston
9.5 mm
600 µm
Silicon substrate
Piston motion
Tip/tilt motionMirror segment
Electrostatic actuator
9.5 mm
600 µm
Silicon substrate
Piston motion
Tip/tilt motionMirror segment
Electrostatic actuator
600µm
Hex Mirror SegmentsUse thick, eptiaxial-grown polysilicon layer (6-10µm) to achieve
surface figure requirement
Mirror segment
Flexure cuts
Actuator
Mirror post
Mirror segment
Flexure cuts
Actuator
Mirror post
5.9 nm ± 0.2nm RMS over DM aperture
Actual Segment Thickness: 7.5µm
35nm
0nm
Acknowledgements
MEMS DM Students: Y. Zhou, J. Stewart*, J. Perreault, R. K. Mali, Andrew LeGendre
BMC Technical Research Staff: A. Hartzell, P. Bierden, S. Cornelissen, J. Stewart, P. Woskov, C. Lam
Funding: CfAO, Gemini, NASA, DARPA