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Micromachined MEMS Deformable Mirror Systems

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					                 Micromachined Deformable
                 Mirrors for Adaptive Optics
                                                               Thomas Bifano
                3 mm                          0 µm
                                                               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
                                              2 µm
                                                           being developed. The devices are
                                                           approximately 100x faster, 100x smaller,
Micromachined Deformable Mirror
                                                           and consume 10000x less power than
            (µDM)
                                                           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                                                                          Adaptive Optics
Microsystems                                                                               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
                                              Membrane
                                                                  • Concept: Micromachined
actuated                   Attachment
diaphragm                                     mirror                deformable mirrors (µDM)
                           post

                                                                  • Fabrication: Silicon
                                                                    micromachining (structural
                 Continuous mirror
                                                                    silicon and sacrificial oxide)

                                                                  • Actuation: Electrostatic parallel
           Segmented mirrors (piston)                               plates

                                                                  • Applications: Adaptive optics,
                                                                    beam forming, communication
         Segmented mirrors (tip-and-tilt)



   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
                                                                              Lightweight, high
                                                                              resolution imaging
Such devices offer new possibilities for use                                  systems
of adaptive optics. Their widespread
availability in the next few years will                                       Point-to-point optical
transform the fields of imaging, beam                                         communication
propagation, and laser communication.                                         through turbulence

                                                                            Compact optical
                                                                          beam-forming
        r0                                              D
                                                                              systems

  Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano (bifano@bu.edu)
                       Adaptive Optics with
                          MEMS-DM
                     Aberrated Incoming Image



                                                            Beamsplitter
                        Deformable
                                                                                            Image
                        mirror
                                                                                            camera
                                           Shape
                                           signals
                                 Control                       Wavefront
       Tilt signals              system                        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 100V.

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano (bifano@bu.edu)
                       Actuator deflection vs.
                          applied voltage
                                                 Deflection v(x) as a function of
                                                 Applied Voltage V can be modeled
 Elasticity                                      as a 4th order
 d 4 y( x) P d 2 y( x) q(x )                                                      q(x)
                                               nonlinear ODE
       4           2
   dx       EI dx       EI
  B.C. y(0)  y( L)  0
      y  y  L)  0
        (0)     (                                                v(x)
 Electrostatics
                 20w                                                                           +
 q(x )                      V2                                                          d(x)
           2(d  y(x ))    2
                                                                                                  –
 Non-linear ODE
                4
             2 d y(x)  P              2
                                   2 d y( x)    20w 2                                                 x
 (d  y( x))           (d  y(x))                  V
                dx4    EI             dx 2      2EI



Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano (bifano@bu.edu)
                 Critical deflection is a
               function of initial gap only
       cV 2                                                      dFR dFE
kw                                         @ critical voltage,       
     (g  w)2                                                     dw     dw
          cV 2                                   3cV 2                 3cV 2
k(g  h)  2                                k             s o: hc r 
           h                                        h3                    k
         cV 2                                                       h       8
h  gh 
 3         2
                       0                   substitute eqn for Vr : c r  3
                                                                c               0.67
          k                                                          g      27
                                               cr 
                                           w4kg30.33g
Critical : (two        equal real roots) V =
                                               27c
               4kg3
  Vc r 
               27c


Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano (bifano@bu.edu)
                              Characterization of
                                  actuators
                                                                                                     Measured deflection versus voltage

                                                                                              0.5




                                                            Actuator center deflection (mm)
                                                                                               0

                                                                                              -0.5
                                                                                                                                       200 mm
                                                                                              -1
   Single point displacement measuring                                                                      350
                                                                                              -1.5                      300       250
              interferometer
                                                                                              -2
                                                                                                     0    50      100   150      200    250     300
                                                                                                               Voltage (Volts)

                                                          Yield: ~95%
                                                          Repeatability: 10 nm (for 99% probability)
                                                          Bandwidth: >66kHz
                                         100mm

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
                                                                                       7000




                                                             Topography (nanometers)
                                                                                       6000                                   Poly2
                                                                                       5000
                                                                                                                              Oxide2
                                                                                       4000
                                                                                                                              Poly1
                                                                                       3000
                                                                                       2000
                                                                                                                              Oxide1
                                                                                       1000
                                                                                        0
                                                                                              0   1   2   3   4   5   6   7   8      9   10

                                                                                                  Lateral Dimensions (micrometers)



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



                                                                                                      5m                                                                                                                                                            2.5m                                                                                                                                                                     2m                                                                                                                                                                           1.5m
                                         Topography generation for 5 um micron anchor in Oxide1, h                       =1071.6054nm, h       =1112.7103nm                                                    Topography generation for 3 um micron anchor in Oxide1, h                        =351.0691nm, h       =413.3069nm                                                         Topography generation for 2 um micron anchor in Oxide1, h                      =152.2509nm, h       =209.018nm                                                                                                                                       =84.9445nm, h       =134.7378nm
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            Topography generation for 1.5 um micron anchor in Oxide1, h
                                                                                                                     t                     n                                                                                                                                                t                    n                                                                                                                                                  t                    n                                                                                                                                                t                   n

                          7000                                                                                                                                                                  7000                                                                                                                                                                      7000                                                                                                                                                               7000




                          6000                                                                                                                       Poly2                                      6000                                                                                                                                                                      6000                                                                                                                                                               6000                                                                                                                      Poly2
                                                                                                                                                                                                                                                                                                                              Poly2                                                                                                                                                              Poly2




                          5000                                                                                                                                                                  5000                                                                                                                                                                      5000                                                                                                                                                               5000


                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 Oxide2
                                                                                                                                                     Oxide2                                                                                                                                                                   Oxide2                                                                                                                                                                                                                                                                                                                                   Oxide2


                                                                                                                                                                                                                                                                                                                                                                          4000
                          4000                                                                                                                                                                  4000                                                                                                                                                                                                                                                                                                                                         4000
                                                                                                                                                                                                                                                                                                                              Poly1                                                                                                                                                              Poly1
                                                                                                                                                     Poly1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              Poly1
                                                                                                                                                                                                                                                                                                                                                Topography (nanometers)




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   Topography (nanometers)
Topography (nanometers)




                                                                                                                                                                      Topography (nanometers)




                                                                                                                                                                                                                                                                                                                                                                          3000
                          3000                                                                                                                                                                  3000                                                                                                                                                                                                                                                                                                                                         3000




                                                                                                                                                                                                                                                                                                                                                                          2000
                          2000                                                                                                                                                                  2000                                                                                                                                                                                                                                                                                                                                         2000
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 Oxide1
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        Oxide1
                                                                                                                                                     Oxide1                                                                                                                                                                   Oxide1




                                                                                                                                                                                                                                                                                                                                                                          1000
                          1000                                                                                                                                                                  1000                                                                                                                                                                                                                                                                                                                                         1000




                                                                                                                                                                                                                                                                                                                                                                                 0
                                 0                                                                                                                                                                     0                                                                                                                                                                                                                                                                                                                                            0
                                                                                                                                                                                                                                                                                                                                                                                     0       1               2              3               4               5   6              7         8                9   10
                                     0        1               2               3               4              5   6                7        8                 9   10                                        0         1               2              3               4               5   6                7             8               9   10                                                                                                                                                                                                           0          1               2              3               4               5   6              7            8              9   10

                                                                                                                                                                                                                                                                                                                                                                                                                         Lateral Dimensions (micrometers)
                                                                          Lateral Dimensions (micrometers)                                                                                                                                       Lateral Dimensions (micrometers)                                                                                                                                                                                                                                                                                                              Lateral Dimensions (micrometers)




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




             Released Oxide
             Polycrystalline Silicon
             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                 9
Mirror dimensions                   560 x 560 x 1.5 µm
Actuator dimensions                 200 x 200 x 2 µm                                                    Center
Actuator gap                        2.0 µm                                                              deflected
Inter-actuator spacing              250 µm




                                                                       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 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 (bifano@bu.edu)
                                     100 Actuator MEMS
                                         Deformable Mirrors
           3161                                             2929                                               2218
                                                   700 nm                                        294.2 nm
  444.6 nm




 -1512.6    nm                                     -1455   nm                                    -388.8   nm

                                                           0    mm                                         0   mm
       0   mm
                                                                 0   mm                   2929    (c)
                0   mm                      3161                                                                 0    mm          2218
 (a)                                               (b)


Interferometric surface maps of different 10x10 actuator arrays with a single actuator deflected

             –           2 µm stroke                                                                      Performance Testing
             –           10 nm repeatability                         Fastest, smallest,                   in an adaptive optics
             –           7 kHz bandwidth                             lowest power DM                        test-bed currently
             –           /10 to /20 flatness                       ever made                             underway at United
             –           <1mW/Channel                                                                         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




                                 0.0
                    2318.5 mm




Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano (bifano@bu.edu)
                    MEMS-DM Bandwidth
        130



                                                                                       Tip-Tilt µ-DM,
Response                                                                               250 µm actuator
(dB)
                    Bandwidth
                    6.99 kHz


        123
                          1                100                   10,000
                                Frequency (Hz)


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

Specification                        BU MEMS- DM                                 Commercial Macro-DM
Number of actuators                  100, 336                                    37, 97, or 350
Actuation                            Integrated Electrostatic                    Discrete Piezoelectric
Package size                         10 cc                                       1000 cc
Power consumption                    1 mW/actuator                               7000 mW/actuator
Actuator spacing                     0.3 mm                                      7.0 mm
Actuator stroke                      2 µm                                        4 µm
Hysteresis                           0%                                          >5%
Settling time                        0.2 ms                                      15.0 ms
Surface roughness                    35 nm Rq                                    ~30 nm Rq
Nominal cost                         $5000 (100 actuator)                        ~$100,000 (97 actuator)




Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano (bifano@bu.edu)
                   Dynamic optical correction
                                                                      Two axis wavefront tilt due to a
                                                                    candle flame corrected in real time
    He Ne
                                                                          using the MEMS-DM
                                            MEMS
    LASER                                   Deformable            2
                                            mirror
Quad cell                                                         1
(tilt sensor)

                Dynamic                          Voltage          0
                aberration                       signals
                                                 to mirror       -1
                                Mirror
         Controller
                                driver                           -2
  A/D
            Computer
                                                                 -3
                                                                  -3 -2 -1 0 1 2 3                              4
                                                                        Tilt Angle (mrad)

 Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano (bifano@bu.edu)
            AO Experimental Setup
                                              Data acquisition and
                                              control (WaveLab)

                                                HV electronics


                                                                    µDM
                                                                                                          Hartmann
                                                                                                          wavefront
                                                                                                          sensor
                                                                          Static
                                                                          aberration             Point source




Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano (bifano@bu.edu)
                 AOA-testing: removal of
                    static aberration
Aberrated                                                     Flattened (21st iteration)



                                     Wavefront




                     Strehl = 0.0034                                                 Strehl = 0.1950


                                                              Point
                                                             Spread




Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano (bifano@bu.edu)
                 AOA-testing: removal of
                    static aberration
Error signals


(mm)



                                Number of Cycles                                            P-V error        RMS error
                                                                                              µm               µm
Drive signals                                                               Nulled              0.04             0.004

(V)                                                                        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
                                                               Further development planned by Boston
                                                               University in collaboration with Boston
                                                               Micromachines Corporation
                2297 mm    0.0
                                                               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 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




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

				
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