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 , Apr-May, 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 DescriptionRequirement Actuators4096 (64x64 array) Stroke3µm, after mirror is flattened Active Aperture19.2 mm (48 actuator 400µm pitch) Local nonflatness<10 nmRMS Bandwidth~2.5 kHz Inter-Actuator Stroke>1µm Yield100% 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 4.32 µm 1.15µm Interactuator stroke achieved 175nm 80nm 0nm 200µm 1000µm 1150nm 0nm >4µm stroke 210V

Measured Optical Quality Top right zone (showing scallop at periphery) Center zone 16RW013#001 ~50nm PV 6µm 0µm 4.06µm PV 707nm RMS 48m ROC Measured surface 200nm 0nm 40nm PV 4nm RMS Filtered surface (uncontrollable) ~25nm PV 50nm 0nm 100nm 0nm

DM Static Cold C

Cycling & Hysteresis

Package and Driver Form factor3U Chassis (5.25” x19” x14”) Frame rate34 kHz / 60 kHz (Low Latency) Resolution14-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)

331 Element Tip-Tilt-Piston MEMS DM +/-6mrad tip-tilt 2um piston 600µm

Hex Mirror Segments Use thick, eptiaxial-grown polysilicon layer (6-10µm) to achieve surface figure requirement 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