Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected]) Micromachined Deformable Mirrors for Adaptive.

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected])

Micromachined Deformable Mirrors for Adaptive Optics

Thomas BifanoProfessor and ChairmanManufacturing Engineering DepartmentBoston University15 Saint Mary’s St.Boston, MA [email protected]

3 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 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.


µ-DM TeamBoston University Photonics Center

Adaptive Optics Associates

Fabrication Optical Testing

Cronos Integrated Microsystems


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 ConceptElectrostatically 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


µDMs in DevelopmentDelft 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


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

θ =λr0

θ =λD


Adaptive Optics with MEMS-DM

Deformablemirror

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

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)

Elasticityd 4 y(x)

dx 4+PEId2y(x)dx2

=q(x)EI

B.C. y(0) =y(L) =0′ y (0) = ′ y (L) = 0

Electrostatics

q(x ) =κ 2ε0w

2(d−y(x))2V2

(d −y(x))2d4y(x)dx4

+PEI

(d−y(x))2d2y(x)dx2

=κ 2ε0w2EI

V2

Non-linear ODE


Critical deflection is a function of initial gap only

@ critical voltage, dFR

dw=dFEdw

−k=−3cV2

h3 so: hcr =3cV2

k

substitute eqn for Vcr :hcrg

= 827

3 ≈0.67

∴wcr ≈0.33g

Vcr =4kg3

27c


Characterization of actuators

0 50 100 150 200 250 300-2

-1.5

-1

-0.5

0

0.5

Voltage (Volts)Act

uato

r ce

nte

r d

efle

ctio

n (m

)

200 m

250300350

Measured deflection versus voltage

100 m

Single point displacement measuring interferometer

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


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

0 1 2 3 4 5 6 7 8 9 100

1000

2000

3000

4000

5000

6000

7000

Lateral Dimensions (micrometers)

Top

ogra

phy

(nan

omet

ers)

Oxide1

Poly1

Oxide2

Poly2

SEM Photo Numerical Model of Growth


Surface Micromaching Topography Problem


A design-based planarization strategy


Narrow anchors reduce print-through to nm scale

0 1 2 3 4 5 6 7 8 9 10

0

1000

2000

3000

4000

5000

6000

7000


Top

ogra

phy

(nan

omet

ers)

Topography generation for 3 um micron anchor in Oxide1, h

t

=351.0691nm, h

n

=413.3069nm

Oxide1

Poly1

Oxide2

Poly2

0 1 2 3 4 5 6 7 8 9 10

0

1000

2000

3000

4000

5000

6000

7000


Top

ogra

phy

(nan

omet

ers)

Topography generation for 5 um micron anchor in Oxide1, ht

=1071.6054nm, hn

=1112.7103nm

Oxide1

Poly1

Oxide2

Poly2

0 1 2 3 4 5 6 7 8 9 10

0

1000

2000

3000

4000

5000

6000

7000


Top

ogra

phy

(nan

omet

ers)

Topography generation for 2 um micron anchor in Oxide1, h

t

=152.2509nm, hn

=209.018nm

Oxide1

Poly1

Oxide2

Poly2

0 1 2 3 4 5 6 7 8 9 10

0

1000

2000

3000

4000

5000

6000

7000


Top

ogra

phy

(nan

omet

ers)

Topography generation for 1.5 um micron anchor in Oxide1, ht

=84.9445nm, hn

=134.7378nm

Oxide1

Poly1

Oxide2

Poly2

5 2.5 2 1.5


Design-based planarization concept

Polycrystalline SiliconSilicon Substrate

Released Oxide

Captured Oxide


Nine-actuator prototype MEMS-DM

Center deflected

Edge deflected

Corner deflected

Number of actuators 9Mirror dimensions 560 x 560 x 1.5 µmActuator dimensions 200 x 200 x 2 µmActuator gap 2.0 µmInter-actuator spacing 250 µm


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

– 2 µm stroke

– 10 nm repeatability

– 7 kHz bandwidth λ/10 to λ/20 flatness

– <1mW/Channel

444.6 nm

-1512.6 nm

0 m 31610 m

3161

(a)

700 nm

-1455 nm

0 m 29290 m

2929

(b)

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


Mirror Deformation

2248.4 m

2318.5 m

671.2 nm

-364 nm

0.0

0.0

Interior dome shape created in a 100 zone continuous mirror.


MEMS-DM Bandwidth

Bandwidth 6.99 kHz

Frequency (Hz)

Response (dB)

130

1231 100 10,000

Tip-Tilt µ-DM,

250 µm actuator


µDM vs. Macro DM

Specification BU MEMS-DM Commercial Macro-DMNumber of actuators 100, 336 37, 97, or 350Actuation Integrated Electrostatic Discrete PiezoelectricPackage size 10 cc 1000 ccPower consumption 1 mW/actuator 7000 mW/actuatorActuator spacing 0.3 mm 7.0 mmActuator stroke 2 µm 4 µmHysteresis 0% >5%Settling time 0.2 ms 15.0 msSurface roughness 35 nm Rq ~30 nm RqNominal cost $5000 (100 actuator) ~$100,000 (97 actuator)


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

2

1

0

-1

-2

-3-3 -2 -1 0 1 2 3 4

Tilt Angle (mrad)


AO Experimental Setup

HV electronics

Data acquisition and control (WaveLab)

Point source

Hartmann wavefront sensor

µDM

Static aberration


AOA-testing: removal of static aberration

Aberrated Flattened (21st iteration)

Strehl = 0.0034

Wavefront

Point Spread

Strehl = 0.1950


AOA-testing: removal of static aberration

Number of Cycles

Number of Cycles

(m

)(V

)

Error signals

Drive signals 0.04 0.004

0.52 0.057

0.10 0.008

Nulled

Aberrated

Corrected

P-V error µm

RMS error µm


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


Deformable Micromirrors - The Future

2178 m

2297 m

831.6 nm

-616 nm 0.0

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 DAAH04-96-1-0250DARPA support DABT63-95-C-0065ARO Support through MURI: Dynamics and Control of Smart Structures DAAG55-97-1-0144Fabrication by Cronos Integrated MicrosystemsAO Experimental support by Boston Micromachines Corporation

Boston University Photonics Center: Precision Engineering Research Laboratory, Thomas Bifano ([email protected]) Micromachined Deformable Mirrors for Adaptive.

Documents

boston university dms

communication slide

deformable mirror aberrat

tilt slide

use of adaptive optics

prototype mirror systems

macroscopic dms

afit surface micromachined