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Optically flat arrays of micromirrors
June Yu
James A. Folta
William Cowan (AFRL)
to improve the mirror surface quality and optical fill-factor
of existing MEM DM prototypes
JY/11/15/99
MTC
There are a number of technical issues to be addressed
for MEM DMs for adaptive optics applications
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Wavefront Quality: < 20nm surface error
Fill Factor: > 99%
# of actuators: > 2000
Stroke: > 0.5 µm for single l, >4µm for multi- l
Speed:>1KHz
Packaging
Addressing
Coating: > 80% broad-band,
>95% narrow-band reflectivity
• Damage threshold: > 2J/cm2 (pulsed),
up to 1 KW (average)
• Size
• Interface electronics
JY/11/15/99
MTC
The two most critical issues limiting the application of current
prototype MEM DM’s are surface quality and fill factor
Two factors affect the optical surface figure of MEM
DM’s
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Residual stress in the fabrication material
curvature of mirror surface
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Topography induced by the underlying layers in the surface
micromachining process
print-through
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DMs fabricated with the MCNC MUMPs process
no metallization: ~ 150 nm PV curvature
with reflective Au: ~ 300 nm PV curvature
COWAN DMs with AFRL coating: 55.6nm to 98.3 nm PV
curvature
JY/11/15/99
MTC
Foundry-fabricated MEM DM’s exhibit stress induced
curvature and “print-through”
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Microscope image of AFRL
MEM DM array fabricated in the
MUMPs process showing printthrough of underlying layers
Lineout of a white-light
interferometer image of a
released MEM DM - mirror
surface has a PV curvature on
the order of 300 nm across a
single pixel. Unreleased
mirrors - 2.3 nm P-V flatness
(ignoring print-through)
JY/11/15/99
MTC
We are developing a process to bond flat mirror
arrays to foundry actuator arrays
Silicon
substrate
Sacrificial
layer
Released
interface
Au bond
posts
mirror
Mirror
array on
handle
wafer
Actuator
array
JY/11/15/99
MTC
Post-foundry addition of mirrors has a number of
advantages
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By separating mirror elements and the actuators, we can fine tune the
mirror surface figure independent of underlying actuator and circuit
layers
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Reduction or elimination of etch access holes from mirror surface
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Can incorporate a variety of application-specific optical coatings
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Possibly lower cost than CMP
JY/11/15/99
MTC
We have selected the Au bump compression bonding
technique
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Low temperature process
Does not require atomically clean and flat interfaces
Does not require large bond bumps as does solder bump technology.
suitable for fabrication of MEMS structures with small features.
Au is inert
Able to work with single dies
greatly reduces the cost and lowers the development risk by maximizing
the number of experiments that can be performed at reasonable cost.
JY/11/15/99
MTC
BSAC has successfully used the Au-to-Au compression
bonding technique to transfer micromirrors onto foundry
fabricated devices
Photo courtesy of Michel M. Maharbiz, Roger T. Howe, and Kristofer S. J. Pister
JY/11/15/99
MTC
We are applying the Au-to-Au bonding technique for
bonding mirrors to the foundry fabricated actuator
arrays
AFRL actuator arrays: 12 x 12 arrays, 203 mm center-to-center spacing, up to 0.7 mm
vertical stroke. 90 µm circular pads are designed to accept the bonding of a continuous or
pixilated mirrors.
Photo of 12x12 actuator array
JY/11/15/99
MTC
SEM image of one micro-actuator
Au-to-Au compression bonding technique requires
uniform arrays of electroplated Au-bumps
Arrays of electroplated Au bumps,
height = 7 µm±100nm,
Au bumps are compressed by 1.1 µm under 70Kg load during bonding
JY/11/15/99
MTC
We have selected a controlled stress film as the
mirror materials
Pixilated mirror array (before bonding)
• 197 µm square
• 1.4 µm thick
• with Au bumps
Experimental data show we can tune the mirror film stress
JY/11/15/99
MTC
Tensile Strength of Au-to-Au compression bonding
is comparable to that for bulk Au
Bond failed at 15.3 Newtons
104 Mpa
JY/11/15/99
MTC