Micromachined Deformable Mirrors for Adaptive Optics 3 mm

Micromachined Deformable Mirrors for Adaptive Optics 3 mm 0 µm 2 µm Micromachined Deformable Mirror (µDM) 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 micromachined deformable mirror (µDM) is being developed. The devices are approximately 100 x faster, 100 x smaller, and consume 10000 x less power than macroscopic DMs. Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

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 ( bifano@bu. edu)

µ-DM Team Boston University Photonics Center Fabrication Optical Testing Cronos Integrated Microsystems Adaptive Optics Associates Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

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 ( bifano@bu. edu)

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 ( bifano@bu. edu)

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 Surface micromachined Tip and tilt Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

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 ( bifano@bu. edu)

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

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 ( bifano@bu. edu)

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 100 V. Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

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

Critical deflection is a function of initial gap only Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

Characterization of actuators Measured deflection versus voltage Single point displacement measuring interferometer Actuator center deflection ( m) 0. 5 0 -0. 5 200 m -1 350 -1. 5 -2 0 50 300 150 200 250 300 Voltage (Volts) Yield: ~95% Repeatability: 10 nm (for 99% probability) Bandwidth: >66 k. Hz 100 m Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

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 ( bifano@bu. edu)

Unintended topography generation is a problem in MEMS SEM Photo Numerical Model of Growth Topography (nanometers) 7000 6000 Poly 2 5000 Oxide 2 4000 Poly 1 3000 2000 Oxide 1 1000 0 0 1 2 3 4 5 6 7 8 Lateral Dimensions (micrometers) Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu) 9 10

Surface Micromaching Topography Problem Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

A design-based planarization strategy Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

Narrow anchors reduce print-through to nm scale 5 2. 5 =1112. 7103 nm Topography generation for 3 um micron anchor in Oxide 1, h n t 2 Poly 2 Poly 1 Topography (nanometers) 2000 3000 2000 1000 2000 Oxide 1 0 0 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 1 2 3 4 10 Lateral Dimensions (micrometers) 2000 1000 0 2 3000 Oxide 1 1000 1 Poly 1 3000 Oxide 1 0 Oxide 2 4000 Poly 1 Topography (nanometers) 5000 4000 3000 Poly 2 Oxide 2 4000 6000 Poly 2 5000 Oxide 2 n t 7000 6000 Poly 2 =134. 7378 nm =84. 9445 nm, h Topography generation for 1. 5 um micron anchor in Oxide 1, h n t 6000 5000 =209. 018 nm =152. 2509 nm, h 7000 6000 1. 5 Topography generation for 2 um micron anchor in Oxide 1, h n t 7000 =413. 3069 nm =351. 0691 nm, h Topography (nanometers) =1071. 6054 nm, h Topography generation for 5 um micron anchor in Oxide 1, h Lateral Dimensions (micrometers) 5 6 7 8 9 10 0 1 2 3 4 Lateral Dimensions (micrometers) Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu) 5 6 7 8 9 10

Design-based planarization concept Released Oxide Polycrystalline Silicon Substrate Captured Oxide Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

Nine-actuator prototype MEMS-DM Number of actuators Mirror dimensions Actuator gap Inter-actuator spacing 9 560 x 1. 5 µm 200 x 2 µm 2. 0 µm 250 µm Center deflected Edge deflected Corner deflected Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

Nine-element mirror performance Surface map and x-profile through the center of a nineelement continuous mirror, pulled down by 155 V applied to the center actuator. The mirror and actuator system exhibited ~7 k. Hz frequency bandwidth, when driven by a custom designed electrostatic array driver. Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

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

Mirror Deformation Interior dome shape created in a 100 zone continuous mirror. 671. 2 nm -364 nm 2248. 4 mm 0. 0 2318. 5 mm 0. 0 Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

MEMS-DM Bandwidth 130 Tip-Tilt µ-DM, Response (d. B) 250 µm actuator Bandwidth 6. 99 k. Hz 123 1 100 10, 000 Frequency (Hz) Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

µDM vs. Macro DM Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

Dynamic optical correction He Ne LASER MEMS Deformable mirror Quad cell (tilt sensor) 2 1 Dynamic aberration Controller A/D Two axis wavefront tilt due to a candle flame corrected in real time using the MEMS-DM Computer Voltage signals to mirror Mirror driver 0 -1 -2 -3 -3 -2 -1 0 1 2 3 Tilt Angle (mrad) Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu) 4

AO Experimental Setup Data acquisition and control (Wave. Lab) HV electronics µDM Static aberration Hartmann wavefront sensor Point source Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

AOA-testing: removal of static aberration Flattened (21 st iteration) Aberrated Wavefront Strehl = 0. 1950 Strehl = 0. 0034 Point Spread Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

AOA-testing: removal of static aberration Error signals ( m) Number of Cycles Drive signals (V) P-V error µm RMS error µm Nulled 0. 04 0. 004 Aberrated 0. 52 0. 057 Corrected 0. 10 0. 008 Number of Cycles Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)

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 ( bifano@bu. edu)

Deformable Micromirrors - The Future 831. 6 nm -616 nm 0. 0 2178 mm 2297 mm 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 ( bifano@bu. edu)

Acknowledgements AASERT program DAAH 04 -96 -1 -0250 DARPA support DABT 63 -95 -C-0065 ARO Support through MURI: Dynamics and Control of Smart Structures DAAG 55 -97 -1 -0144 Fabrication by Cronos Integrated Microsystems AO Experimental support by Boston Micromachines Corporation Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ( bifano@bu. edu)