Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Micromachined Deformable Mirrors for Adaptive Optics Thomas Bifano Professor and Chairman Manufacturing Engineering Department Boston University 15 Saint Mary’s St. Boston, MA mm 0 µm 2 µm Micromachined Deformable Mirror (µDM) 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.

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano 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: Precision Engineering Research Laboratory, Thomas Bifano µ-DM Team Boston University Photonics Center Adaptive Optics Associates Fabrication Optical Testing Cronos Integrated Microsystems

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano 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.

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano µ-DM Concept Electrostatically actuated diaphragm Attachment post Membrane mirror Continuous mirror Segmented mirrors (piston) Segmented mirrors (tip-and-tilt) Concept: Micromachined deformable mirrors (µDM) Fabrication: Silicon micromachining (structural silicon and sacrificial oxide) Actuation: Electrostatic parallel plates Applications: Adaptive optics, beam forming, communication

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano µDMs in Development Delft University (OKO) Underlying electrode array Continuous membrane mirror JPL, SY Tech., AFIT Surface micromachined, segmented mirror L enslet cover for improved fill factor Boston University Surface micromachined Continuous membrane mirrors Texas Instruments Surface micromachined Tip and tilt

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Potential Applications/ Imaging & Beamforming 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. Lightweight, high resolution imaging systems Point-to-point optical communication through turbulence Compact optical beam-forming systems

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Adaptive Optics with MEMS-DM Deformable mirror Aberrated Incoming Image Image camera Wavefront sensor Control system Beamsplitter Shape signals Tilt signals

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano 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

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano 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.

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Actuator deflection vs. applied voltage Deflection v(x) as a function of Applied Voltage V can be modeled as a 4th order nonlinear ODE + – x q(x) v(x) d(x) Elasticity Electrostatics Non-linear ODE

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Critical deflection is a function of initial gap only

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Characterization of actuators Voltage (Volts) Actuator center deflection (  m) 200  m Measured deflection versus voltage 100  m Single point displacement measuring interferometer Yield: ~95% Repeatability: 10 nm (for 99% probability) Bandwidth: >66kHz

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano 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

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Unintended topography generation is a problem in MEMS Lateral Dimensions (micrometers) Topography (nanometers) Oxide1 Poly1 Oxide2 Poly2 SEM Photo Numerical Model of Growth

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Surface Micromaching Topography Problem

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano A design-based planarization strategy

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Narrow anchors reduce print-through to nm scale Lateral Dimensions (micrometers) Topography (nanometers) Topography generation for 3 um micron anchor in Oxide1, h t = nm, h n = nm Oxide1 Poly1 Oxide2 Poly Lateral Dimensions (micrometers) Topography (nanometers) Topography generation for 5 um micron anchor in Oxide1, h t = nm, h n = nm Oxide1 Poly1 Oxide2 Poly Lateral Dimensions (micrometers) Topography (nanometers) Topography generation for 2 um micron anchor in Oxide1, h t = nm, h n = nm Oxide1 Poly1 Oxide2 Poly Lateral Dimensions (micrometers) Topography (nanometers) Topography generation for 1.5 um micron anchor in Oxide1, h t = nm, h n = nm Oxide1 Poly1 Oxide2 Poly2 5  2.5  2  1.5 

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Design-based planarization concept Polycrystalline Silicon Silicon Substrate Released Oxide Captured Oxide

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Nine-actuator prototype MEMS-DM Center deflected Edge deflected Corner deflected Number of actuators9 Mirror dimensions560 x 560 x 1.5 µm Actuator dimensions200 x 200 x 2 µm Actuator gap2.0 µm Inter-actuator spacing250 µm

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano 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.

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano 100 Actuator MEMS Deformable Mirrors –2 µm stroke –10 nm repeatability –7 kHz bandwidth – /10 to /20 flatness –<1mW/Channel Interferometric surface maps of different 10x10 actuator arrays with a single actuator deflected Performance Testing in an adaptive optics test-bed currently underway at United Technologies Fastest, smallest, lowest power DM ever made

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Mirror Deformation  m  m nm -364 nm 0.0 Interior dome shape created in a 100 zone continuous mirror.

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano MEMS-DM Bandwidth Bandwidth 6.99 kHz Frequency (Hz) Response (dB) ,000 Tip-Tilt µ-DM, 250 µm actuator

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano µDM vs. Macro DM

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Dynamic optical correction A/D Voltage signals to mirror Dynamic aberration MEMS Deformable mirror He Ne LASER Quad cell (tilt sensor) Mirror driver Computer Controller Two axis wavefront tilt due to a candle flame corrected in real time using the MEMS-DM Tilt Angle (mrad)

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano AO Experimental Setup HV electronics Data acquisition and control (WaveLab) Point source Hartmann wavefront sensor µDM Static aberration

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano AOA-testing: removal of static aberration Aberrated Flattened (21 st iteration) Strehl = Wavefront Point Spread Strehl =

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano AOA-testing: removal of static aberration Number of Cycles (  m) (V) Error signals Drive signals Nulled Aberrated Corrected P-V error µm RMS error µm

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Adaptive compensation using BU µDM and AOA sensor/controller: 0.8µm 4 mm Measured wavefront error due to a static aberration (bent glass plate) and compensation by µDM

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Deformable Micromirrors - The Future 2178  m 2297  m 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.

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano Acknowledgements AASERT program DAAH 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