A novel Microelectromechanical Systems (MEMS) deformable mirror (DM) technology for large, light weight, segmented space telescopes is being proposed. This technology is reported to provide an unprecedented imaging capability in a visible and near infrared spectral range. The MEMS-DM proposed in this paper consists of a continuous membrane mirror supported by electrostatic actuators with pixel-to-pixel spacing as small as 200 micrometer. An array of 4 X 4 electrostatic actuators for the DM has been successfully fabricated by a new membrane transfer technique. The fabricated actuator membrane has been characterized by using an optical surface profiler. The actuator shows a vertical deflection of 0.37 micrometer at 55 V. This device can also address requirements for smaller size and high resolution applications involving optical transmission through aberrating mediums such as imaging and optical communications through atmospheres, high resolution biometric retina signatures through the eye and endoscopic investigation of tissues and organs

Dekany, Richard

Wiberg, Dean V.

Yang, Eui-Hyeok

Caltech Authors - Main

PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spie
Design and fabrication of electrostatic
actuators with corrugated
membranes for MEMS deformable
mirror in space
Eui-Hyeok  Yang, Dean V. Wiberg, Richard G. Dekany
Eui-Hyeok  Yang, Dean V. Wiberg, Richard G. Dekany, "Design and
fabrication of electrostatic actuators with corrugated membranes for MEMS
deformable mirror in space," Proc. SPIE 4091, Imaging Technology and
Telescopes,  (31 October 2000); doi: 10.1117/12.405766
Event: International Symposium on Optical Science and Technology, 2000,
San Diego, CA, United States
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Design and Fabrication of Electrostatic Actuators with Corrugated
Membranes for MEMS Deformable Mirror in Space
Eui-Hyeok (EH) Yanga, Dean V. Wiberga, and Richard G. Dekanyb
aDevice Research and Applications Section
bftterferomety Systems and Technology Section
Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109
ABSTRACT
A novel Microelectromechanical Systems (MEMS) deformable mirror (DM) technology for large, light weight,
segmented space telescopes is being proposed. This technology is reported to provide an unprecedented imaging capability
in a visible and near infrared spectral range. The MEMS-DM proposed in this paper consists of a continuous membrane
mirror supported by electrostatic actuators with a pixel-to-pixel spacing as small as 200 tm. An array of 4x4 electrostatic
actuators for the DM has been successfully fabricated by a new membrane transfer technique. The fabricated actuator
membrane has been characterized by using an optical surface profiler. The actuator shows a vertical deflection of 0.37 jtm at
55 V. This device can also address requirements for smaller size and high resolution applications involving optical
transmission through aberrating mediums such as imaging and optical communications through atmospheres, high resolution
biometric retina signatures through the eye and endoscopic investigation oftissues and organs.
Keywords: deformable mirror (DM), Microelectromechamcal Systems (MEMS), adaptive optics, electrostatic actuator,
single crystal silicon (SCS) membrane, continuous mirror, wafer-level membrane transfer, Next Generation Space Telescope
(NGST)
1. INTRODUCTION
Deformable mirrors (DMs) have been used in the auxiliary adaptive optics for precision wave front correction. We are
building a MEMS-DM technology, which allows substantial reductions in cost over conventional space based instruments
such as Hubble Space Telescope (HST), while improving imaging capability and system robustness to adapt to unanticipated
mission conditions.
NASA and its industrial partners are formulating the design of the Next Generation Space Telescope (NGST) which, with
an estimated 6-8m diameter aperture and cryogenic operation, will reveal more distant structures of the Universe, but in
similar detail at infrared wavelengths. The NGST will likely consist of a segmented primary mirror, which breaks the scaling
law by utilizing an articulated array of lighter weight, and thus, lower cost glass mirror segments. A small, lightweight,
cryogenic DM has been identified as an enabling technology to achieving NGST science objectives. The MEMS technology
is being taken to explore the development of the continuous DM technology with the ultimate application as the active optical
surface of space based telescopes. This work paves the way for the development of a more sophisticated critical technology
complement for large lightweight space telescopes of the future, whose demanding requirements need the investment in
radically new mirror technologies today.
Currently, most DMs commercially available are macroscopic devices made with flat glass mirror plates supported by an
array of electrostrictive actuators, operational over a limited temperature range and susceptible to hysteresis and creep.
Because of these effects, they do not satisfy the requirements for an optical figure which should remain stable for a period of
weeks as required by space-based telescope. The electrostrictive lead magnesium mobate (PMN) technology has been
demonstrated with excellent surface stability for a period of weeks but is functional only near room temperature '. New
electrostrictive materials such as strontium titanate are under development for 30 °K operation but do not operate at a room
temperature, making system level testing difficult.
ImagingTechnology and Telescopes, James W. Bilbro, James B. Breckinridge, Richard A. Carreras,
Stanley R. Czyzak, Mark J. Eckart, Robert D. Fiete, Paul S. Idell, Editors, Proceedings of SPIE Vol. 4091 83
(2000) 2000 SPIE. . 0277-786X/OOI$l 5.00
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Micromachined designs have been developed by several research groups to improve the DM technology and offer the
potential to be scalable and cost effective. Segmented mirrors 2-4have been fabricated with individual pixel tip/tilt capability
but a continuous mirror surface is required for astronomical adaptive optics applications. Micromachined continuous
membrane DMs have been fabricated by Delft and JPL 6(the previous JPL design was substantially more rudimentary in
approach than that described in this paper). Both have excellent surfaces but the mirror membranes have high inter-actuator
coupling between individual pixels (influence function), and a more recent effort has produced a bulk micromachined DM
with still unacceptably high influence function '.
An electrostatically actuated surface micromachined DM 8 has been demonstrated to be one hundred times faster, one
hundred times smaller, and consume ten thousand times less power than macroscopic DMs. This micromachined DM has a
continuous mirror backed by parallel plate actuators, which is fabricated using the surface micromachining technology
embodied in the Multi-User MEMS Processes (MUMPS). Restriction to the MUMPS process creates design limitations and
marginal surface quality, which in turn limits the applicability ofthis approach.
A new design for the materials, structures and fabrication methods to meet the requirements of imaging astronomical
adaptive optics is necessary. To satisfy this requirement, a novel concept has been taken to develop an electrostatically
actuated MEMS-DM with a continuous single crystal silicon (SCS) membrane, which is targeted for insertion into NGST and
Gossamer Telescopes. A bulk-micromachining approach is utilized to overcome the limitation of the surface
micromachining embodied in the MUMPS process thus providing optical mirror surface quality with lower influence
function and higher stroke. The comparison results of the proposed DM with current state-of-the-art technologies are
summarized in Table 1 . In this paper, an array of 4x4 electrostatic actuators for the MEMS-DM has been fabricated and the
static deflection of an actuator has been measured.
2. TilE MEMS-DM WITH A CONTINUOUS SCS MEMBRANE
Astronomical applications call for optical-quality continuous mirror surfaces. A continuous membrane has the advantage
that it does not cause diffraction of the reflected beam, increasing the astronomical sensitivity for imaging and spectroscopic
applications. Such membrane also ensures smooth and continuous phase variations across the mirror. Moreover, a device
based upon SCS technology will result in a stress free membrane with excellent optical quality.
Table 1 Comparison ofthe designed MEMS-DM with current state-of-the-art technologies
Performance
comparison
MEMS-DM desi at JPL
Current SOA technologies
NGST
RequirementsBulk-MEMS Surface-MEMS 8 PMN'
Stroke/Mm.
actuator spacing
Qtm) 0.5/200
A floating
membrane with
2/300 0.4/1000
1. optical
underlying
electrodes
quality
mirrorMax.
stroke/Actuator
spacing Qim) 4/2000 2—3 (?) 2/7000
surface
2. 4 m stroke
with a lowSurface stability
(nm) 1* (goal) N/A N/A 1. influence
function;
influence function
(%) < 10 10 10 3. cryogenic
operation
4. surfaceApplied voltage (V) 160 200 300 100
stability of
0.1—10 nm in
a second
Issue
Complicated process steps
to have a single crystal
silicon mirror membrane
Influence function
unacceptably high
Marginal surface
quality
Room temp.
device
84 Proc. SPIE Vol. 4091
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Mirror
Indium post
11I1tI—j1I1F1
Corrugation _________________________ -.//
Fig. I The schematic view of the continuous SCS MEMS-DM backed by an array of electrostatic actuators with corrugated membranes.
The mirror membrane deflects downward by the pulling force of the underlying electrostatic actuators. The device is designed to have low
influence between adjacent pixels, while providing continuous optical quality mirror surface.
A key element of our approach is a MEMS-DM with a continuous SCS membrane illustrated in figure 1. The unit pixel is
electrostatically actuated and has a center-to-center spacing of 200 pm. A continuous SCS mirror membrane constitutesthe
reflective adaptive surface, which is supported by an array of electrostatic actuators. The actuator is also a deformable
membrane attached to the immobile base substrate. The corrugated actuator membrane structure is incorporated to release the
residual stress caused by wafer bonding and film deposition processes. The use of both bulk- and surface-micromachm.ing as
opposed to surface-only-micromachining allows a much broader design space to optimize actuation and minimize mfluence
function.
3. FABRICATION AND CHARACTERIZATION
A new technique for transferring wafer-level silicon membranes onto dissimilar substrates has been developed for the
fabrication of the continuous SCS MEMS-DM. This technique has been used for the fabrication of the actuators with
corrugated polysilicon membranes. The fabrication steps are shown m Fig. 2. A Silicon-On-Insulator (SOl) wafer acts as a
carrier wafer in the Indium bonding and subsequent wet etching processes. After depositing Indium layers on both SOI and
silicon wafers. the two wafers are precisely aligned and hermetically bonded. Then, the backside of SO! wafer is etched in
TMAH solution. The substrate silicon is etched away, exposing a buried oxide which is subsequently removed by a HF
solution. The SF6 plasma etches the remaining silicon to release the actuator membrane structures. Fig. 3 shows the SEM
photographs of a polysilicon membrane with 1 pm in thickness, which has been successfully transferred to a dissimilar
silicon wafer. The membrane with underlying electrodes constitutes the 4x4 electrostatic actuators.
The WYKO RST Plus optical profiler, a commercially available instrument, is used to accurately measure the surface
topography and the static deflection of actuator membranes. The images depicted in Fig. 4 show that the fabricated actuator
membranes have initial upward deformation due to the residual stress in the polysilicon. The actuator membrane with more
uniform surface profile is being fabricated.
Proc. SPIE Vol. 4091 85
IHftJIIIL
Ilcetrode
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Pad'H
Electrode
JIJUL___JIiIJI I
r
lu U U LII
1W ftf1ftflfU1
sFt plasma
Fig. 2 The process sequence of the wafer-level silicon membrane transfer.
Fig. 3 The SEM photographs of the fabricated actuators with a corrugated polysilicon membrane. The device is fabricated by the
membrane transfer technique utilizing the Indium bonding and subsequent dry/wet etching technologies. (a) The 4x4 actuators array
(b) Enlarged view of the corrugation profile
86 Proc. SPIE Vol. 4091
sl—
Si02
Si
Indium
111111
+
'I,
355 sm
5 tLnl
bonding
TA.l.111 c't(/?!flg
(a)
(b)
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/13/2018
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208.3 urn
60.0
20.0
80.0
40.0
0.0
(a)
urn
280.8
- IJIH
I.'
1.
-1. —- jrH
lCG 1
_________________
(b)
Fig. 4 Surface profiles of the fabricated actuator array. The center flat membranes show upward deformation by the constraint due to the
residual stress in the polysilicon film. This deformation can be reduced by modifying the membrane profile. (a) Surface profile of a
actuator membrane (b) 2-D view of the profile of the membrane
Fig. 5 shows the deflection curve of an actuator membrane over the applied voltages. The membrane deflection is
constrained by the surrounding corrugation, though the corrugation releases the residual stress in the polysilicon film. The
mirror membrane transfer technology is under development to create a stress-free high quality SCS membrane mirror over
the flexible actuator membranes.
4. DISCUSSION
The optical profile measurement results show that the fabricated actuator membrane has a non-uniform surface
topography and the profile of each actuator varies over its position in the array. The membrane is constrained by the
corrugated profile and deflection of a membrane affects the shape of adjacent pixels. A modified actuator design is under
development to fabricate actuators array with a uniform membrane profile. Each pixel is mechanically and electrically
isolated in the 2' generation design. The 4 .tm stroke design will be incorporated to provide the enabling capability for
astronomical adaptive optics (e.g. segmented space telescopes).
Proc. SPIE VoL 4091 87
[.0 50.0 100.0 150.0 200.0
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Fig. 5 Deflection ofthe membrane with the applied voltages.
A bulk-micromachining approach is being taken to overcome the limitations of the surface micromachining embodied in
the MUMPS process thus providing greater actuator stroke, surface quality, and non-planar topography. We are exploring
Indium bonding as a potential solution by forming a more compliant interface, which is an interim solution to demonstrate
the device using a silicon direct bonding approach in the fmal design to avoid possible fracture and/or distortion due to
thermal mismatch at cryogenic temperature.
Potential applications for the JPLMEMS-DM technology in addition to space telescopes include:
(a) Reduced transmitter power requirements for long distance space-to-space or space-to-ground optical communications
links
(b) Very large, non-optical (e.g. A cost-effective array of high bandwidth communication stations in geosynchronous orbit.),
space based apertures fabricated using Silicon telescope technology
(c) Imaging the human retina at high resolution as a biometric signature for security purposes using a very compact, portable
device. Although the turbulence structure of the human eye requires only 5-10 Hz closed-loop bandwidth correction,
very large numbers of actuators are required for diffraction-limited operation.
5. CONCLUSION
A new concept for building a continuous membrane SCS MEMS-DM has been proposed. The actuator membrane has
been fabricated by membrane transfer technology developed at JPL. The surface profile and static deflection of actuator
membranes have been measured. The transfer process of the SCS mirror membrane onto the existing actuator membrane is
under development. If our approach to build a continuous membrane SCS MEMS-DM is successful, it will be a
breakthrough for the development of large aperture, light-weight, space telescopes at a low cost (based on the power laws of
cost-savings vs. decreasing optical quality). This technology will reduce the volume and mass of a pixel by more than two-
orders-of-magnitude, which will promote the design of smaller optical systems by a factor roughly proportional to the pixel
size, while delivering a high quality, compact energy efficient system. This will also allow over a 10-fold increase in light-
gathering capability over NGST for astronomical capabilities at similar cost, and potentially, a 10-fold increase in theoretical
resolution of down looking surveillance telescopes over the HST-class observatories operating at visible wavelengths.
REFERENCES
1. M. A. Ealey and J. F. Washeba, "Continuous face sheet low voltage deformable mirrors," Optical Eng., (29) p.1 191, Oct.
1990
88 Proc. SPIE Vol. 4091
Applied Voltage (V)
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/13/2018
Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
68 6Ot' IOA 3IdS OOJd 
. L66T 
jçJd (ç) 'ioimu jquuojp uooqts purqtuoioriu-ojzns uiquiw-snonmuoj,, 'v a ouiji j g 106968 dd '8661 O1JJ '!!EMH 'UO 'iOOtUJOOj 
wsics AfldPV U •JUOJ IIdS 'JOLmU jquuopp uooips puqouoij,, 'iA 'j 'j prn jjsuj,sj f L 9661 'LTZITZdd 'çç i°A 'V siomoy pu siosug 'sJoJ.mu 
jqauojp purqouioiau rn tuoid poip 1omD-otsoipp &imsp ioj potpw y,, './V a 'UE 3j 606Z06 dd '8661 -J'\I '!'"H 'UO{ 'Aoouijoj UISAg ioidO oAdEpy uo jtI IIdS 'sioiutu jqauojp puapmoiortujo uoiido posq-uotzuxrud,, 'uIAopA 0 
170806L dd '8661 -1°IAI '!MEH 'UO{ 'OjOuqoj usAg oqd AdpV 
'o juo IIdS 'suisAs ionau qiuuopp pouqjoioaujo UOUfljA,, '/v ja uoyrj •j7 .cTg-c08 
dd '8661 1TON 'H '1UO{ '2OjOuqoj wsAg joidO Atdpy u w°c AIdS 'suotoIjddE sotdo aAidEpE doosp ioj sotAp ioimu oiiui pamqotuoioiui ojxns uoijnuiis pu u2tsQ,, '1v a poijqoiJ,s y 
L661 qd 'EIZ-90Z dd ' °M '•D 
-O8 j0A 'uoxp3j suiJ jjpjj 'susis jorn w-oj-odo-oioiui paUUjOIUOJ.OTW oJms 'i i UHWA 
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/13/2018
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Design and fabrication of electrostatic actuators with corrugated membranes for MEMS deformable mirror in space

PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spie
Design and fabrication of electrostatic
actuators with corrugated
membranes for MEMS deformable
mirror in space
Eui-Hyeok  Yang, Dean V. Wiberg, Richard G. Dekany
Eui-Hyeok  Yang, Dean V. Wiberg, Richard G. Dekany, "Design and
fabrication of electrostatic actuators with corrugated membranes for MEMS
deformable mirror in space," Proc. SPIE 4091, Imaging Technology and
Telescopes,  (31 October 2000); doi: 10.1117/12.405766
Event: International Symposium on Optical Science and Technology, 2000,
San Diego, CA, United States
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/13/2018  Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
Design and Fabrication of Electrostatic Actuators with Corrugated
Membranes for MEMS Deformable Mirror in Space
Eui-Hyeok (EH) Yanga, Dean V. Wiberga, and Richard G. Dekanyb
aDevice Research and Applications Section
bftterferomety Systems and Technology Section
Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109
ABSTRACT
A novel Microelectromechanical Systems (MEMS) deformable mirror (DM) technology for large, light weight,
segmented space telescopes is being proposed. This technology is reported to provide an unprecedented imaging capability
in a visible and near infrared spectral range. The MEMS-DM proposed in this paper consists of a continuous membrane
mirror supported by electrostatic actuators with a pixel-to-pixel spacing as small as 200 tm. An array of 4x4 electrostatic
actuators for the DM has been successfully fabricated by a new membrane transfer technique. The fabricated actuator
membrane has been characterized by using an optical surface profiler. The actuator shows a vertical deflection of 0.37 jtm at
55 V. This device can also address requirements for smaller size and high resolution applications involving optical
transmission through aberrating mediums such as imaging and optical communications through atmospheres, high resolution
biometric retina signatures through the eye and endoscopic investigation oftissues and organs.
Keywords: deformable mirror (DM), Microelectromechamcal Systems (MEMS), adaptive optics, electrostatic actuator,
single crystal silicon (SCS) membrane, continuous mirror, wafer-level membrane transfer, Next Generation Space Telescope
(NGST)
1. INTRODUCTION
Deformable mirrors (DMs) have been used in the auxiliary adaptive optics for precision wave front correction. We are
building a MEMS-DM technology, which allows substantial reductions in cost over conventional space based instruments
such as Hubble Space Telescope (HST), while improving imaging capability and system robustness to adapt to unanticipated
mission conditions.
NASA and its industrial partners are formulating the design of the Next Generation Space Telescope (NGST) which, with
an estimated 6-8m diameter aperture and cryogenic operation, will reveal more distant structures of the Universe, but in
similar detail at infrared wavelengths. The NGST will likely consist of a segmented primary mirror, which breaks the scaling
law by utilizing an articulated array of lighter weight, and thus, lower cost glass mirror segments. A small, lightweight,
cryogenic DM has been identified as an enabling technology to achieving NGST science objectives. The MEMS technology
is being taken to explore the development of the continuous DM technology with the ultimate application as the active optical
surface of space based telescopes. This work paves the way for the development of a more sophisticated critical technology
complement for large lightweight space telescopes of the future, whose demanding requirements need the investment in
radically new mirror technologies today.
Currently, most DMs commercially available are macroscopic devices made with flat glass mirror plates supported by an
array of electrostrictive actuators, operational over a limited temperature range and susceptible to hysteresis and creep.
Because of these effects, they do not satisfy the requirements for an optical figure which should remain stable for a period of
weeks as required by space-based telescope. The electrostrictive lead magnesium mobate (PMN) technology has been
demonstrated with excellent surface stability for a period of weeks but is functional only near room temperature '. New
electrostrictive materials such as strontium titanate are under development for 30 °K operation but do not operate at a room
temperature, making system level testing difficult.
ImagingTechnology and Telescopes, James W. Bilbro, James B. Breckinridge, Richard A. Carreras,
Stanley R. Czyzak, Mark J. Eckart, Robert D. Fiete, Paul S. Idell, Editors, Proceedings of SPIE Vol. 4091 83
(2000) 2000 SPIE. . 0277-786X/OOI$l 5.00
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Micromachined designs have been developed by several research groups to improve the DM technology and offer the
potential to be scalable and cost effective. Segmented mirrors 2-4have been fabricated with individual pixel tip/tilt capability
but a continuous mirror surface is required for astronomical adaptive optics applications. Micromachined continuous
membrane DMs have been fabricated by Delft and JPL 6(the previous JPL design was substantially more rudimentary in
approach than that described in this paper). Both have excellent surfaces but the mirror membranes have high inter-actuator
coupling between individual pixels (influence function), and a more recent effort has produced a bulk micromachined DM
with still unacceptably high influence function '.
An electrostatically actuated surface micromachined DM 8 has been demonstrated to be one hundred times faster, one
hundred times smaller, and consume ten thousand times less power than macroscopic DMs. This micromachined DM has a
continuous mirror backed by parallel plate actuators, which is fabricated using the surface micromachining technology
embodied in the Multi-User MEMS Processes (MUMPS). Restriction to the MUMPS process creates design limitations and
marginal surface quality, which in turn limits the applicability ofthis approach.
A new design for the materials, structures and fabrication methods to meet the requirements of imaging astronomical
adaptive optics is necessary. To satisfy this requirement, a novel concept has been taken to develop an electrostatically
actuated MEMS-DM with a continuous single crystal silicon (SCS) membrane, which is targeted for insertion into NGST and
Gossamer Telescopes. A bulk-micromachining approach is utilized to overcome the limitation of the surface
micromachining embodied in the MUMPS process thus providing optical mirror surface quality with lower influence
function and higher stroke. The comparison results of the proposed DM with current state-of-the-art technologies are
summarized in Table 1 . In this paper, an array of 4x4 electrostatic actuators for the MEMS-DM has been fabricated and the
static deflection of an actuator has been measured.
2. TilE MEMS-DM WITH A CONTINUOUS SCS MEMBRANE
Astronomical applications call for optical-quality continuous mirror surfaces. A continuous membrane has the advantage
that it does not cause diffraction of the reflected beam, increasing the astronomical sensitivity for imaging and spectroscopic
applications. Such membrane also ensures smooth and continuous phase variations across the mirror. Moreover, a device
based upon SCS technology will result in a stress free membrane with excellent optical quality.
Table 1 Comparison ofthe designed MEMS-DM with current state-of-the-art technologies
Performance
comparison
MEMS-DM desi at JPL
Current SOA technologies
NGST
RequirementsBulk-MEMS Surface-MEMS 8 PMN'
Stroke/Mm.
actuator spacing
Qtm)
0.5/200 A floating
membrane with
2/300 0.4/1000
1. optical
underlying
electrodes
quality
mirrorMax.
stroke/Actuator
spacing Qim)
4/2000 2—3 (?) 2/7000
surface
2. 4 m stroke
with a lowSurface stability
(nm) 1* (goal) N/A N/A 1. influence
function;
influence function
(%) < 10 10 10 3. cryogenic
operation
4. surfaceApplied voltage (V) 160 200 300 100
stability of
0.1—10 nm in
a second
Issue
Complicated process steps
to have a single crystal
silicon mirror membrane
Influence function
unacceptably high
Marginal surface
quality
Room temp.
device
84 Proc. SPIE Vol. 4091
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Mirror
Indium post
11I1tI—j1I1F1
Corrugation _________________________ -.//
Fig. I The schematic view of the continuous SCS MEMS-DM backed by an array of electrostatic actuators with corrugated membranes.
The mirror membrane deflects downward by the pulling force of the underlying electrostatic actuators. The device is designed to have low
influence between adjacent pixels, while providing continuous optical quality mirror surface.
A key element of our approach is a MEMS-DM with a continuous SCS membrane illustrated in figure 1. The unit pixel is
electrostatically actuated and has a center-to-center spacing of 200 pm. A continuous SCS mirror membrane constitutesthe
reflective adaptive surface, which is supported by an array of electrostatic actuators. The actuator is also a deformable
membrane attached to the immobile base substrate. The corrugated actuator membrane structure is incorporated to release the
residual stress caused by wafer bonding and film deposition processes. The use of both bulk- and surface-micromachm.ing as
opposed to surface-only-micromachining allows a much broader design space to optimize actuation and minimize mfluence
function.
3. FABRICATION AND CHARACTERIZATION
A new technique for transferring wafer-level silicon membranes onto dissimilar substrates has been developed for the
fabrication of the continuous SCS MEMS-DM. This technique has been used for the fabrication of the actuators with
corrugated polysilicon membranes. The fabrication steps are shown m Fig. 2. A Silicon-On-Insulator (SOl) wafer acts as a
carrier wafer in the Indium bonding and subsequent wet etching processes. After depositing Indium layers on both SOI and
silicon wafers. the two wafers are precisely aligned and hermetically bonded. Then, the backside of SO! wafer is etched in
TMAH solution. The substrate silicon is etched away, exposing a buried oxide which is subsequently removed by a HF
solution. The SF6 plasma etches the remaining silicon to release the actuator membrane structures. Fig. 3 shows the SEM
photographs of a polysilicon membrane with 1 pm in thickness, which has been successfully transferred to a dissimilar
silicon wafer. The membrane with underlying electrodes constitutes the 4x4 electrostatic actuators.
The WYKO RST Plus optical profiler, a commercially available instrument, is used to accurately measure the surface
topography and the static deflection of actuator membranes. The images depicted in Fig. 4 show that the fabricated actuator
membranes have initial upward deformation due to the residual stress in the polysilicon. The actuator membrane with more
uniform surface profile is being fabricated.
Proc. SPIE Vol. 4091 85
IHftJIIIL
Ilcetrode
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/13/2018
Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
Pad'H
Electrode
JIJUL___JIiIJI I
r
lu U U LII
1W ftf1ftflfU1
sFt plasma
Fig. 2 The process sequence of the wafer-level silicon membrane transfer.
Fig. 3 The SEM photographs of the fabricated actuators with a corrugated polysilicon membrane. The device is fabricated by the
membrane transfer technique utilizing the Indium bonding and subsequent dry/wet etching technologies. (a) The 4x4 actuators array
(b) Enlarged view of the corrugation profile
86 Proc. SPIE Vol. 4091
sl—
Si02
Si
Indium
111111
+
'I,
355 sm
5 tLnl
bonding
TA.l.111 c't(/?!flg
(a)
(b)
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Fig. 4 Surface profiles of the fabricated actuator array. The center flat membranes show upward deformation by the constraint due to the
residual stress in the polysilicon film. This deformation can be reduced by modifying the membrane profile. (a) Surface profile of a
actuator membrane (b) 2-D view of the profile of the membrane
Fig. 5 shows the deflection curve of an actuator membrane over the applied voltages. The membrane deflection is
constrained by the surrounding corrugation, though the corrugation releases the residual stress in the polysilicon film. The
mirror membrane transfer technology is under development to create a stress-free high quality SCS membrane mirror over
the flexible actuator membranes.
4. DISCUSSION
The optical profile measurement results show that the fabricated actuator membrane has a non-uniform surface
topography and the profile of each actuator varies over its position in the array. The membrane is constrained by the
corrugated profile and deflection of a membrane affects the shape of adjacent pixels. A modified actuator design is under
development to fabricate actuators array with a uniform membrane profile. Each pixel is mechanically and electrically
isolated in the 2' generation design. The 4 .tm stroke design will be incorporated to provide the enabling capability for
astronomical adaptive optics (e.g. segmented space telescopes).
Proc. SPIE VoL 4091 87
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Fig. 5 Deflection ofthe membrane with the applied voltages.
A bulk-micromachining approach is being taken to overcome the limitations of the surface micromachining embodied in
the MUMPS process thus providing greater actuator stroke, surface quality, and non-planar topography. We are exploring
Indium bonding as a potential solution by forming a more compliant interface, which is an interim solution to demonstrate
the device using a silicon direct bonding approach in the fmal design to avoid possible fracture and/or distortion due to
thermal mismatch at cryogenic temperature.
Potential applications for the JPLMEMS-DM technology in addition to space telescopes include:
(a) Reduced transmitter power requirements for long distance space-to-space or space-to-ground optical communications
links
(b) Very large, non-optical (e.g. A cost-effective array of high bandwidth communication stations in geosynchronous orbit.),
space based apertures fabricated using Silicon telescope technology
(c) Imaging the human retina at high resolution as a biometric signature for security purposes using a very compact, portable
device. Although the turbulence structure of the human eye requires only 5-10 Hz closed-loop bandwidth correction,
very large numbers of actuators are required for diffraction-limited operation.
5. CONCLUSION
A new concept for building a continuous membrane SCS MEMS-DM has been proposed. The actuator membrane has
been fabricated by membrane transfer technology developed at JPL. The surface profile and static deflection of actuator
membranes have been measured. The transfer process of the SCS mirror membrane onto the existing actuator membrane is
under development. If our approach to build a continuous membrane SCS MEMS-DM is successful, it will be a
breakthrough for the development of large aperture, light-weight, space telescopes at a low cost (based on the power laws of
cost-savings vs. decreasing optical quality). This technology will reduce the volume and mass of a pixel by more than two-
orders-of-magnitude, which will promote the design of smaller optical systems by a factor roughly proportional to the pixel
size, while delivering a high quality, compact energy efficient system. This will also allow over a 10-fold increase in light-
gathering capability over NGST for astronomical capabilities at similar cost, and potentially, a 10-fold increase in theoretical
resolution of down looking surveillance telescopes over the HST-class observatories operating at visible wavelengths.
REFERENCES
1. M. A. Ealey and J. F. Washeba, "Continuous face sheet low voltage deformable mirrors," Optical Eng., (29) p.1 191, Oct.
1990
88 Proc. SPIE Vol. 4091
Applied Voltage (V)
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Design and fabrication of electrostatic actuators with corrugated membranes for MEMS deformable mirror in space

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