1. APPLIED PHYSICS LETTERS 96, 211103 2010
An all optically driven integrated deformable mirror device
V. Mathur,1,a S. R. Vangala,1 X. Qian,1 W. D. Goodhue,1 B. Haji-Saeed,2 and
J. Khoury2
1
Department of Physics and Applied Physics, Photonics Center, University of Massachusetts, Lowell,
Massachusetts 01854, USA
2
Air Force Research Laboratory/Sensors Directorate, Hanscom Air Force Base, Massachusetts 01731, USA
Received 13 March 2010; accepted 26 April 2010; published online 24 May 2010
We demonstrate a technique for actuating micromirrors vertically cascaded on wafer fused
GaAs-GaP photodiodes. Unlike traditional actuation schemes, the electrostatic drive of the
individual capacitive actuators is addressed optically in this device. Vertical mirror displacements of
up to 500 nm were observed using interferometry while addressing the photodetectors with a 5 mW
optical signal. Microlenses were used to address a 900 pixel device with patterned conductive pillars
and thin film load resistors for each actuator-detector element. This approach can enable realization
of faster and denser adaptive optics wave front corrector arrays. © 2010 American Institute of
Physics. doi:10.1063/1.3430568
Deformable micromirrors are the key-enabling compo- In this letter, we demonstrate an integrated MEMS de-
nent in a variety of optical systems such as switches in fiber vice utilizing patterned thin film resistors in parallel with
optics communication,1 image production in digital light micromirrors suspended over a photodetector array using
processors,2 and phase correctors in adaptive optics semiconductive pillars. This device allows operation in the
systems.3 Traditional actuation schemes electrostatic,4 near infrared IR regime by utilizing a wafer fusion tech-
magnetic,5 and thermal6 require integrated electrical cir- nique for fabricating GaAs photodiodes on a transparent GaP
cuitry to address each actuator individually. These generally substrate. The mirror displacement is demonstrated to be a
involve sophisticated fabrication techniques and drive up function of the incident laser power and is modulated opti-
manufacturing costs particularly in high pixel density appli- cally. We also demonstrate that the current device developed
cations. A technique utilizing one of the traditional actuation for image correction applications may also be used as an
methods but addressed optically is thus the need of the hour. optically addressable ON/OFF switch. The pixels in the array
For instance, in adaptive optics systems, Shack–Hartmann- are designed such that each mirror-detector element is in
type feedback loops require optically addressable wave front parallel with the others, thus requiring only a single electrical
correctors to achieve faster image correction. An earlier re- bias for the whole device as opposed to our earlier devices.
ported attempt7 in this regard directly utilized radiation pres- Figure 1 shows three-dimensional 3D schematic of the final
sure from focused laser beams to impart momentum to mi- device consisting of cascaded silicon nitride Si3N4 micro-
croactuators. A significant drawback of this technique is the mirrors on a wafer fused photodiode array. The photodiodes
very high optical intensity required to produce small dis- are addressed individually by focusing a control laser signal
placements. For example, a focused beam of intensity individually through a cascaded microlens array. A low stress
600 mW/ mm2 is required to rotate an actuator by half a Si3N4 piston motion spring plate mirror design developed
degree. In another reported technique, high frequency optical earlier11 with response time in microseconds, requiring low
pulses8 were utilized to excite microstructures. This tech- actuation voltages is used here. Each individual mirror is
nique finds limited applications due to the dimensional con- fabricated incorporating a patterned tantalum nitride TaN
straints it imposes on the structures to operate near their thin film resistor in the megohms range. Specially developed
structural resonance. SU-8 resist polyaniline PANI doped is then utilized to
In one of our first attempts at developing an all optically bond and electrically connect the micromirrors to the photo-
addressed microelectromechanical system MEMS diodes. Figure 2 shows a single pixel cross section of the
deformable-mirror device,9 a metalized mylar membrane was device. The cascaded microlens focuses the input control la-
suspended over an insulating photoresist grid on a photocon-
ductive substrate. Optically addressing the substrate initiated
a higher effective electrostatic drive between its front surface Phase correction
and the suspended membrane, thus actuating it. The sensitiv- Micro Mirrors
Resistors(Backside)
ity of these devices was improved10 by replacing the sub-
strate with an InGaAs photodetector array. However, to con-
trol the impedance between the suspended membrane and
detector, it was necessary to bias them with a very high fre- Gallium
Phosphide Gallium Micro lens
quency ac, or a combination of both ac and dc. It was later
Arsenide array
proposed that using semiconductive posts would allow volt- Photodetectors
age distribution across the detector and mirror using just a dc
bias.
Control laser
FIG. 1. Color online A 3D schematic of the mirror detector array ad-
a
Electronic mail: vaibhavdeniro@gmail.com. dressed via microlenses.
0003-6951/2010/96 21 /211103/3/$30.00 96, 211103-1 © 2010 American Institute of Physics
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2. 211103-2 Mathur et al. Appl. Phys. Lett. 96, 211103 2010
Phase Correction
Coated side wall Gold + (a) (b)
SU-8
SU-8
TaN Resistor Si3N4 Mirror Ohmic contact
N -
I
GaAs P
GaP
Micro lens
Laser FIG. 4. Scanning electron microscope images of the final MEMS device a
Spring plate mirrors with surrounding spiral TaN resistors b A portion of
FIG. 2. Color online Cross-section schematic of a single pixel of the
the wafer fused GaAs–GaP PIN mesa array with patterned conductive SU-8.
device.
tops were then gold coated to form highly reflective mirrors.
ser beam on the GaAs photodetectors through the transparent This metal deposition was done at an angle to ensure that the
GaP back substrate. GaP was chosen as the support substrate sidewalls of an etched opening connecting the resistor side of
due to its high optical transparency to the control laser near the mirror to the front side, was also gold coated Fig. 3 v .
IR wavelength regime. The metal coated mirror top and PIN To fabricate the detector array, a GaAs wafer with epitaxi-
diode mesa act as the upper and lower electrodes of a capaci- ally grown p-i-n structure was bonded to a GaP substrate at
tive actuator, respectively. The generated photocurrent passes 700 ° C in a custom designed furnace and fixture Fig. 3 vi .
vertically through the semiconductive pillars causing a volt- The GaAs substrate was subsequently removed by a combi-
age drop across the thin film resistor in turn actuating the nation of lapping, polishing and wet etch. The transferred
mirror. p-i-n layers were then patterned and wet etched to form the
Figures 3 i –3 x illustrate the steps involved in fabricat- GaAs p-i-n mesas. The etch was monitored closely to stop in
ing the top mirror and bottom detector array. First, a low the half micron thick GaAs P region. A five layer Ni/Ge/Au/
stress plasma enhanced chemical vapor deposition recipe12 Ni/Au metal scheme was then deposited to form ohmic con-
was used to deposit 600 nm of silicon nitride on an indium tacts on the N side of each mesa and a single surrounding
phosphide InP substrate. The TaN sheet resistivity contact on the bottom P side Fig. 3 viii . In the final step, a
1.4 k / sq was then sputtered on lithographically pat- proprietary technique for doping and patterning SU-8 2010
terned photoresist AZ1512 and lifted off to form spiral resis- Microchem Corp. with a conductive polymer PANI was
tors Fig. 3 ii . Metal contact pads were then patterned fol- used to form two micron tall pillars on the mesas Fig. 3 ix .
lowing a similar process at the resistor ends Fig. 3 iii . Figure 4 shows the scanning electron microscope images of
Spring plates were then defined lithographically Fig. 3 iv portions of the final device with 900 pixels. The individual
in the center and a reactive ion etcher used to etch the Si3N4 pixel size including the resistor was 200 microns, with ac-
down to the InP. The actuators were then released by etching tual mirror area of 66 66 m2. The existing fill factor of
away the InP selectively using HCl. The released actuator 10% Fig. 4 a can be further improved by shrinking the
resistor size by using a higher resistivity material. The peri-
odicity of the p-i-n mesas and mirrors was 250 microns, and
Si3N4 GaP
the surrounding conductive SU-8 pillars were 10 m wide
InP GaAs PIN
(vi) Wafer fusion
Fig. 4 b .
TaN GaAs After fabricating the GaAs p-i-ns their breakdown and
Metal Doped SU-8 photoresponse characteristics were measured using a 830 nm
Photoresist laser source. As the photodetectors operate in series with the
patterned load resistor, their breakdown voltage sets an upper
(i) Si3N4 PECVD deposition (vii) Lapping and polishing limit on the resulting voltage drop across the actuator. Since
a voltage drop of 10 to 15 V is desirable across these actua-
tors, the p-i-ns were designed to have reverse breakdown
voltages in excess of 20 V. Figure 5 shows the I-V charac-
(ii) TaN sputtering (viii) Wet etch and metal deposition
100.0 3mW 830nm Laser
Back illumination through microlenses
Photocurrent (Microamps)
(iii) Resistor pads metal deposition (ix) Conductive SU-8 patterning 75.0 Top illumination after wafer fusion
Top illumination before wafer fusion
50.0 Dark
(iv) Si3N4 RIE etch (x) Samples cascaded
25.0
0.0
(v) Mirror release, metal deposition
0 10 20 30
Voltage (Volts)
FIG. 5. I-V curves illustrating the photo response of optically addressed
FIG. 3. Color online Micromirror and detector array fabrication steps. GaAs PINs before and after wafer fusion.
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3. 211103-3 Mathur et al. Appl. Phys. Lett. 96, 211103 2010
830nm Laser
Frame grabber
LabVIEW
Dark Light
600
Oscilloscope
Iris
BS +
Mirror displacement (nm)
Photodiode
-
Device 400
He-Ne Laser
BS BS
200
Moving mirror
FIG. 6. Color online Interferometry and optical addressing setup for the
MEMS device beam splitter: BS .
0
teristics of the p-i-n diodes under no illumination dark , top 0 2 4 6
Optical power (mW)
illumination P-side on top before wafer fusion, and illumi-
nation through the GaP substrate P-side on bottom after FIG. 7. Interferograms showing fringe shift in shining light on the 2 M
resistor MEMS device inset . Mirror displacement vs optical intensity of
wafer fusion. A high photocurrent of about 80 A was ob- the MEMS device with 800 k resistors.
served on the first sample before wafer fusion on shining a 3
mW laser signal. The junction was found to deteriorate device, the TaN resistors were patterned to have a lower
slightly after the high temperature wafer fusion, and the pho- resistance of 800 k . The control laser intensity was var-
tocurrent dropped to 20 A when again addressed directly ied from 1–5 mW and the displacement measured in steps of
from the top. On the wafer fused test sample, the p-i-ns were quarter wavelengths = 632 nm as shown in Fig. 7. The
addressed through the GaP substrate and cascaded micro- mirror displacement was found to saturate close to 500 nm
lenses focal length 2 mm . The photocurrent further dropped corresponding to a 5 mW control laser power.
to 10 A due to additional optical losses , but was still In summary, a technique for optically addressing and
sufficient for actuation, provided a high value load resistor. actuating micromirrors using cascaded photodiodes has been
In the devices tested here, the TaN resistances were varied demonstrated. The device can be used as a simple ON/OFF
upto 2 M to ensure a voltage drop of at least 10 V across optical switch or as a phase corrector device in adaptive op-
the mirror. To monitor the mirror displacement as a function tics. The design proposed here offers several advantages over
of optical power, an inverted microscope was modified to other addressing techniques as it eliminates the need for
include standard interferometry as shown in Fig. 6. The complex circuitry, and may be adapted for several other ma-
MEMS device was then placed upside down and the micro- terial systems.
lens sample aligned carefully on top. A 830 nm Newport
LQA-830 control laser mounted on the microscope was then 1
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