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 bifano@bu.edu 617-353-5619 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)

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.

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

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.

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)

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

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.

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

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

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.

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

Critical deflection is a function of initial gap only

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

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

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)

Surface Micromaching Topography Problem

A design-based planarization strategy

Narrow anchors reduce print-through to nm scale 1 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 =351.0691nm, h n =413.3069nm Oxide1 Poly1 Oxide2 Poly2 Topography generation for 5 um micron anchor in Oxide1, h =1071.6054nm, h =1112.7103nm Topography generation for 2 um micron anchor in Oxide1, h =152.2509nm, h =209.018nm Topography generation for 1.5 um micron anchor in Oxide1, h =84.9445nm, h =134.7378nm 5 2.5 2 1.5

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

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

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.

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

Mirror Deformation Interior dome shape created in a 100 zone continuous mirror. 2248.4 mm 2318.5 mm 671.2 nm -364 nm 0.0

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

µDM vs. Macro DM

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 -3 -2 -1 0 1 2 3 4 Tilt Angle (mrad)

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

AOA-testing: removal of static aberration Aberrated Flattened (21st iteration) Wavefront Strehl = 0.0034 Strehl = 0.1950 Point Spread

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

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

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.

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