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

+

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)

22 2 24

2 2 2

( , ) 6 ( , ) ( , ) ( , ) ( , )( , )

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