Download Thin-Film Piezoelectric Unimorph Actuator-Based Deformable Mirror With a Transferred Silicon Membrane

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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 5, OCTOBER 2006
Thin-Film Piezoelectric Unimorph Actuator-Based
Deformable Mirror With a Transferred
Silicon Membrane
Eui-Hyeok (EH) Yang, Senior Member, IEEE, Yoshikazu Hishinuma, Jian-Gong Cheng,
Susan Trolier-McKinstry, Senior Member, IEEE, Eric Bloemhof, and B. Martin Levine
Abstract—This paper describes a proof-of-concept deformable
mirror (DM) technology, with a continuous single-crystal silicon
membrane reflecting surface, based on PbZr0:52 Ti0:48 O3 (PZT)
unimorph membrane microactuators. A potential application for
a terrestrial planet finder adaptive nuller is also discussed. The
DM comprises a continuous, large-aperture, silicon membrane
“transferred” onto a 20 20 piezoelectric unimorph actuator
array. The actuator array was prepared on an electroded silicon
substrate using chemical-solution-deposited 2-thick PZT
films working in a d31 mode. The substrate was subsequently
bulk-micromachined to create membrane structures with residual
silicon acting as the passive layer in the actuator structure. A
mathematical model simulated the membrane microactuator performance and aided in the optimization of membrane thicknesses
and electrode geometries. Excellent agreement was obtained
between the model and the experimental results. The resulting
piezoelectric unimorph actuators with patterned PZT films produced large strokes at low voltages. A PZT unimorph actuator,
2.5 mm in diameter with optimized PZT/silicon thickness and
design showed a deflection of 5.7
at 20 V. A DM structure
with a 20-thick silicon membrane mirror (50 mm 50 mm
area) supported by 400 PZT unimorph actuators was successfully
fabricated and optically characterized. The measured maximum
mirror deflection at 30 V was approximately 1
. An assembled
DM showed an operating frequency bandwidth of 30 kHz and an
influence function of approximately 30%.
[1738]
m
m
m
m
Index Terms—Deformable mirror, large-area mirror, PZT thinfilm, unimorph actuator.
I. INTRODUCTION
EQUIREMENTS on optical instrumentation for future astronomical observations from the ground and in space will
place rigorous demands on wavefront quality. As an example,
the Terrestrial Planet Finder (TPF) Mission [1] has sought to
R
Manuscript received January 1, 2006; revised February 20, 2006. This work
was supported in part by the National Aeronautics and Space Administration.
The work of J.-G. Cheng was conducted while he was with the Materials Research Institute, Pennsylvania State University, University Park, PA 16802 USA.
Subject Editor N. F. de Rooij.
E.-H. Yang is with Stevens Institute of Technology, Hoboken, NJ (e-mail:
[email protected]).
E. Bloemhof and B. M. Levine are with the Jet Propulsion Laboratory,
Pasadena, CA 91109 USA.
Y. Hishinuma is with the Fuji Photo Film Company, Ltd., Kanagawa 2588538, Japan.
J.-G. Cheng is with the Shanghai Institute of Microsystem and Information
Technology, Chinese Academy of Sciences, Shanghai 200050, China.
S. Trolier-McKinstry is with the Materials Research Institute, Pennsylvania
State University, University Park, PA 16802 USA.
Digital Object Identifier 10.1109/JMEMS.2006.880208
Fig. 1. Large-area continuous membrane DM concept. The mirror membrane
is backed by an array of piezoelectric unimorph microactuators. The advantage
of this approach is that the small strains obtainable from a piezoelectric material
at modest voltages are translated into relatively large displacements.
image Earth-like planets orbiting in “habitable zones” around
nearby stars and to characterize their atmospheres, if any, in the
visible to infrared (IR) bands. The telescope is a two-mirror,
off-axis Cassegrain. The coronagraph inside the thermal instrument enclosure requires wavefront correction over the full aperture of the primary mirror and a deformable mirror (DM) with
a large number of actuators and low stroke, located at a conjugate surface. This task requires imaging with wavefront fidelity equivalent to the phenomenally high dynamic range of
–
, depending on the approach taken. Future groundbased observatories with 30–100-m apertures also seek to detect and characterize Earth-like exoplanets in addition to other
very demanding observations [2]. Large-stroke actuators will be
needed on the primary mirror in order to form an image. There
may also be a DM in a conjugate plane to enable diffraction-limited imaging. Similarly, missions such as the Single Aperture
Far-Infrared (SAFIR) Observatory [3] and associated interferometric missions [4] demand extreme wavefront quality at cryoK. The SAFIR telescope is conceived
genic temperatures of
as a larger and colder version of the segmented James Webb
Space Telescope point-design. Wavefront correction will be on
each of the segments instead of the conjugate plane. Because the
wavelength range of this telescope is between 20
and 1 mm,
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YANG et al.: THIN-FILM PIEZOELECTRIC UNIMORPH ACTUATOR-BASED DM
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Fig. 2. Unimorph actuator concept. An electric field applied perpendicular to the membrane-mounted piezoelectric thin film induces a contraction in the lateral
direction, converted by the membrane geometry to a large out-of-plane deflection. The vertical deflection acts on the portion of the mirror membrane mounted over
the microactuator. Piezoelectric unimorph actuators will provide: 1) large wavefront correction, with a highly localized influence function; 2) scalability, potentially
up to 10 actuators; 3) fast response and low power (30 s=cycle, 4 nF); and 4) functionality over a wide temperature range.
the actuators will need to have much larger stroke; however,
there will be fewer of them than would be required on a visible
wavelength telescope, for instance. Laser communication is another area of great current interest, and it too levies challenging
requirements, not only on the wavefront quality but also on the
speed at which the wavefront is corrected [5], [6]. For many
of these applications, the availability of high-performance DMs
will play a critical enabling role. Space-based planet finders like
over
TPF require residual wavefront errors of less than
the full aperture [7]. Although SAFIR has no formal requirement for DM performance, its long wavelength range makes
it likely that adaptive correction actuators operating at cryogenic temperatures will be integrated into the primary or secondary mirrors rather than a conjugate surface. This minimizes
the number of optical surfaces in the beam train, thus reducing
thermal background.
Given the variety of applications relevant to astronomical
observations and space operations, one sees there is no single
ideal DM architecture. Electrostrictive lead magnesium niobate
(PMN) actuators have achieved a surface stability of 1
and a surface figure of
[8]. However, although these
technologies are in widespread use, they have only limited acstroke at
actuator
tuator stroke (approximately 0.5
density, for PMN-based mirrors). Micromachined continuous
membrane DMs have been fabricated [9]–[13]; however,
such devices are based on electrostatic actuation and, conse. Currently, the
quently, have limited mirror stroke
large-aperture technology development being pursued under
the Gossamer program has yet to demonstrate the potential for
diffraction-limited wavefront quality over large apertures. We
believe that our large-actuator-stroke device is well suited to
correct the large wavefront errors associated with space-based
telescope apertures. The unique approach of combining uni(PZT) actuator technology with
morph
mirror-quality membrane transfer technology [8] provides
flexible capabilities in fabricating DM structures. Therefore,
a class of DM architectures can be adapted to meet specific
operating requirements. Part of the motivation for developing
both the PZT unimorph DM and membrane transfer techniques
is precisely because they are so adaptable. Our DM concept
is shown in Fig. 1. For instance, the stroke of the DM and
the density of actuators can be traded, that is, large stroke is
obtained with larger actuator separation using our fabrication
technology. With this in mind, our devices are fabricated
with a very high-fidelity process for the transfer of the silicon
face-sheet, which is grown on a separate wafer for superior
optical quality. In this paper, we present the development
results of thin-film PZT fabrication process, the PZT unimorph
actuators, a proof-of-concept continuous membrane DM using
the PZT actuators, and the membrane transfer process. We also
discuss potential future application for a TPF adaptive nuller.
II. PZT UNIMORPH ACTUATOR-BASED DM
The DM described in this paper comprises a continuous membrane mirror supported by an array of PZT unimorph actuators.
The unimorph actuation principle is illustrated in Fig. 2. The
actuation principle details are as follows: an electric field applied perpendicular to the membrane-mounted PZT thin film
induces a contraction in the lateral direction, converted by the
membrane geometry to a large out-of-plane deflection. The vertical deflection acts on the mirror membrane that is mounted
over the microactuator. Compared with the bulk piezoelectric or
electrostrictive stack actuators widely employed in commercial
DMs, thin films require far lower voltages and less power to produce the same mirror deflection. To avoid stress concentration
which might induce cracking in the actuator, circular diaphragm
elements were chosen. In order to optimize the geometry of the
unimorph actuator structure, a mathematical model was developed using an energy minimization method [14]. In this model,
the total energy of the unimorph membrane under deflection is
calculated using a deflection profile predicted by thin plate deflection theory. Subsequently, the total energy, consisting of the
elastic energy due to the bending of the membrane, the potential energy stored by the film stresses, and the work done by the
piezoelectric actuation, is minimized with respect to a Lagrange
multiplier. The detailed explanation of the mathematical modeling and the method of energy minimization are available in
[15]. The energy minimization calculation was performed for
both continuous and patterned piezoelectric films.
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III. FABRICATION AND CHARACTERIZATION
Here, the fabrication and characterization processes are
described. The membrane transfer process [8] is used to
transfer a silicon membrane onto the PZT unimorph actuator array. Section III-A describes the PZT film development
process including its fabrication and analysis. The actuator
array is prepared on an electroded substrate using the chemical-solution-deposited PZT films. Section III-B describes the
fabrication and characterization processes for the unimorph
actuator arrays using the PZT film technology described in
Section III-A. Section III-C describes the wafer-scale transfer
process of single-crystal silicon mirror membranes onto the
fabricated actuator array described in Section III-B, as well
as the results of surface figure characterization for the mirror
membrane.
A. Thin Film PZT: Deposition and Characterization
For this study, thin PZT films were prepared using a chemical
solution deposition process. The procedure has been described
in detail elsewhere [16]. Briefly, the procedure used was a
modification of that first introduced by Budd et al. [17] for
processing of PZT films
thick. Lead acetate trihydrate, zirconium -propoxide, and titanium isopropoxide were
used as the precursors, while 2-methoxyethanol (2-MOE) was
the primary solvent. After dissolution, the lead precursor was
dehydrated under vacuum. A mixture of zirconium -propoxide
and titanium iso-propoxide in 2-MOE at room temperature
was added and the entire solution was refluxed for 2 h under
Ar at 120 . After a second vacuum distillation, 2-MOE was
added. The solution was then modified with acetylacetone (20
vol%) and acetic acid (5 vol%). The final solution was 0.70M
with 20mol% Pb in excess of stoichiometry to compensate
for high Pb volatility. The substrates used in this study were
/Si wafers (Nova
commercially manufactured Pt(111)/Ti/
Electronic Materials, Inc., Richardson, TX) for electrical characterization and platinum-coated silicon on insulator (SOI)
wafers for device fabrication. Prior to deposition of the PZT
for 60 seconds
film, substrates were pre-annealed at 500
to clean the surface. Solutions were dispensed through a syWhatman filter (Aldrich Chemical Co.,
ringe with a 0.1Milwaukee, WI) and the substrate was spun at 1500 rpm for
30 s using a photoresist spinner (Headway Research, Inc.,
Garland, TX). The majority of the organics were removed in
two pyrolysis steps, each 60 s in duration, on a hot plate at
and 500 , respectively. During the second treatment,
350
a 1-mm-thick aluminum plate was positioned parallel to and
about 3.5 mm above the film surface to enhance heating. A
crystallization anneal was performed after deposition of each
layer in a Heatpulse 610 rapid thermal processing unit (AG
for 60 s. Each sequence
Associates, Sunnyvale, CA) at 700
produced a layer approximately 0.20
in thickness, and the
sequence was repeated to obtain the desired film thickness.
radiation was
X-ray diffraction with Ni-filtered Cu
performed to characterize the orientation and crystal structure
of the PZT films (Scintag, Inc., Sunnyvale, CA). Microstructural features of the films were examined using an S-3500N
scanning electron microscope (SEM) (Hitachi LtD., Tokyo,
Fig. 3. SEM micrographs of (a) whole thickness and (b) part of a cross section
of a PZT thick film.
Japan). To enable electrical characterization, Pt/PZT/Pt capacitors were fabricated by sputtering Pt top electrodes through
a shadow mask. The electrodes were circular with an area
. Before measurement, the samples were
of 1.3
for 1 min to improve the quality of the
annealed at 500
film/top electrode interface. To expose the bottom electrode,
a two-step wet etching process for PZT thick films was used
[18]. The film thickness was measured using an Alpha-Step
surface profilometer. The dielectric constant and loss tangent of
the PZT films were measured using a HP4192A LF impedance
analyzer (Hewlett-Packard, Palo Alto, CA) at 100 kHz and an
. Polarization-electric field hysoscillation voltage of 30
teresis behavior of the PZT films was measured using an RT66A
standardized ferroelectric test system (Radiant Technologies,
Albuquerque, NM) in the virtual ground testing mode. Fig. 3
shows the SEM micrograph cross section of a 7- -thick PZT
film. As seen in the cross-sectional micrographs, a boundary
including some porosity is apparent between each crystallized
layer, and layers appear to be one grain thick. Because each
layer was crystallized separately, it is likely that the top surface
YANG et al.: THIN-FILM PIEZOELECTRIC UNIMORPH ACTUATOR-BASED DM
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Fig. 4. XRD patterns for PZT films with different thickness. The peaks are
indexed in terms of a pseudocubic perovskite structure.
Fig. 6. Flowchart for the preparation of doped PZT films.
m
Fig. 5. Polarization-Electric field hysteresis loop for a 1- -thick PZT film.
Good saturation is apparent.
of each layer provided nucleation sites for the layer above it.
The grain size is relatively uniform in plane and is about 150
nm. XRD measurements showed that the films were phase-pure
perovskite (i.e., no pyrochlore phase was identified), and had
a gradual change in the preferred orientation with increasing
, the
film thickness (Fig. 4). When the film was thin
PZT films had (111) preferred orientation on (111) oriented
platinum coated substrates, indicating a predominance of
nucleation from the bottom electrode. With increasing film
thickness, the films became more and more randomly oriented
as the influence of the Pt/PZT interface was reduced [19].
Fig. 5 shows a polarization-electric field hysteresis loop of a
1- -thick PZT film. The polarization versus applied electric
field loop shows a good square shape. Remanent polarization
and coercive field
values extracted from the P-E
and 51 kV/cm, respectively.
hysteresis loop are 36
The relative permittivity and tangent loss for a 1- -thick PZT
film are about 1040 and 2.5% at room temperature, respectively. These values are characteristic of a high-quality film. For
Fig. 7. X-ray diffraction patterns for Mg- and Sr-doped PZT thin films. Doping
has little influence on either the film crystallinity or orientation.
applications of DMs, there is interest in the low-temperature
properties of the piezoelectric film. In particular, since the
ultimate application of the mirrors will be in space, operation
at cryogenic temperatures is important. For this reason, several
dopants for the PZT films were investigated to determine if
superior low-temperature performance could be achieved.
Due to the divergence in the permittivity near a phase
transition, the piezoelectric properties increase as the transition is approached, providing that a well-poled state can
be maintained. Thus, lower transition temperatures should
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Fig. 8. Permittivity versus temperature curves for (a) undoped and (b) 16mol% Mg—doped PZT 52/48 films showing that Mg successfully lowers the Curie
temperature.
enable higher piezoelectric responses at room temperature,
ceramics and films
and below. Undoped
have transition temperatures between 350
and 400
. To
decrease the Curie temperature, Mg and Sr doping of PZT
films was undertaken. However, it is important to note that the
morphotropic phase boundary (MPB) in PZT can also shift as
dopants are incorporated [20]. Because there is a significant
increase in the piezoelectric response at MPB due to the higher
polarizabilities, the Zr:Ti ratios in the doped PZT films were
adjusted for a given dopant concentration in order to optimize
the piezoelectric response. Fig. 6 shows a schematic of the route
developed for doped PZT films. Mg ethoxide and strontium
acetate were chosen as the Mg and Sr precursors, respectively.
The Mg ethoxide had good solubility in 2-methoxyethanol and
could be added to the solution after dehydration of the lead
precursor. To improve the solubility of the strontium precursor,
it was dissolved in acetic acid and was added to the PZT
solution at the same time as the acetylacetone modification.
Other processing details follow those outlined above. Typically,
0.4-M solutions were made using this approach. The solutions
were batched assuming that the dopant would replace Pb on
the A-site of the perovskite lattice. All solutions were prepared
with 20mol% excess PbO to minimize second phase formation
during crystallization. Several doping levels were investigated
in order to progressively lower the phase transition temperature. It was found that in general the doped PZT are more
susceptible to cracking than undoped PZT thin films prepared
with a comparable route. The cracking was particularly severe
in the Mg-doped films. To minimize cracking, many of the
doped PZT films are comparatively thin relative to the undoped
PZT films. Both Mg- and Sr-doped films were single phase
perovskite, with some degree of (111) orientation, as seen in
Fig. 7. The lattice parameters are somewhat different from
those of the undoped films, suggesting that at least some of the
dopant is incorporated into the lattice.
On films with low crack densities, the electrical properties
were measured. The transition temperature was reduced about
relative to that for undoped PZT, as seen in Fig. 8.
100
Fig. 9. Permittivity versus temperature curves for 16 mole% Sr-doped PZT
films with different Zr:Ti ratios: (a) a sample on the tetragonal side of the MPB
and (b) a film with a composition close to the bulk MPB.
The Sr-doped PZT films also showed reduced transition temperatures relative to undoped PZT. Fig. 9 shows the permittivity–temperature curves for films with 16mol% Sr-doped PZT
YANG et al.: THIN-FILM PIEZOELECTRIC UNIMORPH ACTUATOR-BASED DM
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Fig. 10. Hysteresis loops for Sr-doped PZT films. The doped films show significantly smaller remanent polarizations than undoped PZT.
films. The morphotropic phase boundary for this Sr-doping level
is shifted to a Zr:Ti ratio of 57/43. This film had a transition
, more than 120
lower than the untemperature of 223
doped film. All of the Sr-doped films show low dielectric loss
values. Clear hysteresis loops were observed for the films (see
Fig. 10). As expected, the decrease in transition temperature resulted in lower remanent polarization values than those seen for
undoped films. The remanent polarization was also decreased
by the tilting of the hysteresis loops, probably due to the tensile stress in the films associated with differences in the film
and substrate thermal expansion coefficients. The room temperature permittivity of the films did not increase in all of the doped
films, suggesting either the presence of a low permittivity interfacial layer whose influence is more predominant in thinner
films and/or the presence of a small amount of second phase material (e.g., unincorporated dopant) that was not detected in the
X-ray diffraction patterns.
values of both undoped and doped PZT films were
The
measured as a function of temperature using the wafer flexure
method [21]. Films were poled and allowed to age for 1 day
prior to the measurements to minimize the effect of aging on
determination of temperature dependence. Because the samples
Fig. 11. Temperature dependence of the piezoelectric response for (a) undoped
PZT 52/48 films of different thickness and (b) 16mol% Sr-doped films on
(square) and off (triangle) the morphotropic phase boundary.
were poled at room temperature, without hot poling or ultravicoefficients decreased relative to their
olet exposure, the
coeffias-poled values due to the initial aging. Remanent
cients of
are achievable at
. Fig. 11 shows
a comparison of the temperature dependence of the piezoelectric
response for a number of different films. In these undoped PZT
52/48 films, the piezoelectric response was somewhat higher for
thicker films. It is also apparent that for the Sr-doped samples,
the piezoelectric response is larger near the bulk morphotropic
phase boundary composition (Sr/Zr/Ti 16/57/43). This suggests that in films, as in bulk, doped PZT, Sr addition stabilized
the tetragonal phase [22]. It also demonstrates that it is important, in the doped films to readjust the Zr:Ti ratio to the new
MPB. It was also determined that the properties of the doped
films are not as high as the undoped PZT. This is probably
due to a combination of factors, including: 1) the fact that the
doped films were thinner; 2) fine, hairline cracks were observed
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Fig. 12. Dependence of deflection on silicon membrane thickness for both continuous and patterned PZT films. There is a significant increase in the deflection
for the patterned PZT actuator. The data points represent an average of ten separate measurements on two different pixels within an array. The is the electrode
size relative to the unimorph membrane.
in some areas of the doped films; and 3) the lowered remanent
polarization in the doped films. All of these factors reduce the
in-plane piezoelectric response. The first two difficulties could
probably be eliminated with improvements in the film pyrolysis conditions. Due to the superior properties, DM devices were
made with undoped PZT 52/48 films.
B. PZT Unimorph Actuator Arrays
A major part of the PZT unimorph actuator technology has
already been developed. The detailed fabrication process of the
unimorph actuators is described in a previous paper [23]. In this
section, the top-level fabrication sequence is given as follows.
The fabrication of unimorph actuators started using a thermally
oxidized SOI wafer. Ti/Pt layers were subsequently evaporated
on the front surface of the wafer. Then, PZT films were prepared
using a chemical-solution deposition process as described in the
previous section. Undoped PZT films were used in all of the
actuators reported here. Cr/Pt/Au layers were evaporated onto
the PZT layer, and patterned to form top electrodes and contact
pads. A wet etching process for the PZT thick films was used
to expose the bottom electrode. Finally, backside cavities were
formed by deep reactive ion etching (DRIE) until the buried silicon dioxide was exposed. After this layer was removed in a
buffered HF solution, further RIE was performed to thin the silicon membrane as needed. A Kapton tape-based masking approach was used to cover some areas of the wafer for selective
etching of the cavities, thus allowing several different silicon
membrane thicknesses on a single wafer.
A WYKO RST Plus Optical Profiler was used to analyze the
deflections of the actuator membranes. We observed a significant increase in the deflection when the PZT film surrounding
the actuator was removed, as shown in Fig. 12. Three different
Fig. 13. Actuator stroke as a function of voltage for three different types of
actuators. Significant improvement in stroke is achieved using unimorphs with
patterned PZT with optimized geometry.
unimorph actuator structures have been designed, fabricated,
modeled, and tested. In Fig. 12, Case A shows a schematic of
a typical unimorph actuator previously developed (described in
detail in [23]). It consists of a patterned top electrode (Au) on top
of a PZT layer. This stack was prepared on a bottom electrode
(Pt) deposited on the silicon layer (membrane). The size of the
top electrode and the thicknesses of the PZT and silicon layers
were optimized using based on modeling, which employs the
energy minimization approach [15]. Case B shows a schematic
of a unimorph actuator with a “patterned” PZT layer. The size
of the PZT pattern was optimized using the modeling method
previously developed [14]. Case C presents a schematic of an
actuator with patterned PZT and patterned silicon layers. Modeling was not performed for this case, since the modeling code
YANG et al.: THIN-FILM PIEZOELECTRIC UNIMORPH ACTUATOR-BASED DM
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Fig. 14. Process schematic for fabrication and transfer of a mirror membrane.
was not optimized for patterned (noncontinuous) silicon structures. Finite-element model (FEM) analysis may be required
in order to accurately assess the actuator behavior for Case C,
but this would be beyond the scope of the work described in
this paper. The measured deflection for an optimized actuator is
at 20 V (for an actuator with a 2.5-mm diameter, PZT/Si
- -thick, PZT patterned over 60% of the membrane diameter, and silicon membrane patterned). This deflection increase is believed to be due to a decrease in the residual
stress in the actuator.
5.7
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In order to maximize the deflection, unimorph actuators with
different silicon membrane thicknesses were characterized. Actuators with different membrane thicknesses were obtained on
the same wafer by selectively etching the silicon membrane to
different final thicknesses using the Kapton tape-masked backside etching process [23]. Fig. 13 compares the measured actuator strokes with the modeled values. The actuation results
from using 2.5-mm-diameter membrane actuators with: 1) continuous PZT film (i.e., Case A) and 2) patterned PZT film (i.e.,
Case B) are shown as a function of membrane thickness. The
experimental results are superimposed over their respective predicted deflection curves from the simulation model. As predicted by our model, the maximum deflections were obtained at
intermediate silicon-membrane thicknesses of approximately:
with the continuous PZT film and 2) 10
with
1) 15
the patterned PZT film.
C. Deformable Mirror
A proof-of-concept DM structure was fabricated using typical
unimorph actuators (Case A in Fig. 12), and its actuation performance was characterized using the WYKO RST Plus Optical
Profiler. For fabrication of the mirror membrane, we utilized
the membrane transfer process [8] developed at JPL in order
to transfer the mirror membrane onto the fabricated actuator arrays. The membrane transfer process is briefly described as follows. The mirror membrane transfer process involves transfer
of the single-crystal silicon layer from the SOI wafer following
the metallization, bonding, and etching processes. SOI carrier
wafers were used for the fabrication of the single-crystal silicon
mirror membrane. The thickness of the transferred membrane
is determined by the thickness of the SOI top silicon layer. A
20- -thick single-crystal silicon mirror membrane was transferred onto the actuator wafer. The actuator wafer and the SOI
carrier wafer were prepared [see Fig. 14(a)]. Cr/Pt/Au metal
layers were deposited and patterned to form bonding pad arrays on both the carrier and actuator wafers. A 1- -thick In
layer and a 100-Å-thick Au layer were subsequently deposited
and patterned on both the carrier and actuator wafers using a
lift-off process [see Fig. 14(b)]. The SOI carrier wafer was subsequently bonded to the substrate wafer [see Fig. 14(c)]. The
backside etching was conducted in a 25 wt% solution of tetramuntil the buried
ethylammonium hydroxide (TMAH) at 80
oxide was exposed. A specially designed Teflon fixture was used
to protect both the backside of the bonded substrate wafer as
well as the bonded interface [see Fig. 14(d)]. The exposed oxide
was removed by using 49% HF droplets [see Fig. 14(e)]. An
plasma etch was incorporated, as necessary, to selectively etch
the transferred membrane in order to release membrane structures [see Fig. 14(f)].
Fig. 15 contains a cross-sectional view of the DM structure
and photographs of a fabricated DM with a 20- -thick silicon
membrane mirror (50 mm 50 mm area) supported by 400 PZT
unimorph actuators. Fig. 16 shows the local surface deformation
of a DM created by the underlying actuators (i.e., typical unimorph: Case A). The measured maximum mirror deflection at
. This deflection for a DM shows
30 V is approximately 1
that the stroke of the mirror membrane is approximately 40% of
the stroke of actuator alone. The measured influence function
2
Fig. 15. Fabricated deformable mirror with 20 20 piezoelectric unimorph actuator array. (a) Cross-sectional schematic. (b), (c) Photographs of the actuator
arrays and the DM.
(interactuator coupling) was approximately 30%. The full-scale
measurement (up and down) of the mirror and actuator combination was not performed due to a lack of reference area on
the mirror membrane. Hence, the measurements on the mirror
membrane were made in the “differential mode” only [23]. The
full-scale optical measurement protocol as well as the software
and driver electronics for a DM device have to be established
in the future in order to fully characterize the DM performance.
For the actuator alone, the measurements were made with respect to reference electrodes. The stroke reduction can be minimized by varying the mechanical compliance (by optimizing
the PZT/ actuator membrane/ mirror membrane thickness ratio).
The frequency responses for the unimorph actuator, with and
without the mounted mirror membrane, were obtained using a
laser-doppler vibrometer (shown in Fig. 17). The resonance frequency of a 2-mm-diameter, 2- -thick PZT/15- -thick silicon membrane, and 60% electrode actuator was measured at
63 kHz, which far exceeds the bandwidth requirement for most
[23]. The bandwidth of the DM in this paper
DMs
far exceeds the bandwidth requirement for most DMs that are
applicable to several space and Earth science missions being
envisioned by NASA. The future development plan includes
demonstration of the DM technology by: 1) flattening the mirror
under a Michelson interferometer setup; 2) measuring the gains
of all actuators; 3) measuring the temporal frequency response
of several actuators; and 4) measuring the influence function of
all actuators.
IV. APPLICATIONS: PRESENT AND FUTURE
Large ground-based observatories will need a combination
of integrated wavefront correction with correction at conjugate
pupils to produce diffraction-limited imagery. The use of such
control systems may greatly ease the tolerances on the primary
mirror, which are generally a major cost driver. At high frequencies, as are relevant for ground-based telescopes exposed
to atmospheric turbulence, correction of the primary mirror is
referred to as adaptive optics; at quasi-static frequencies of perhaps 1 Hz and below, as are typically required by space-based
YANG et al.: THIN-FILM PIEZOELECTRIC UNIMORPH ACTUATOR-BASED DM
1223
Fig. 16. Deflection of a DM with a single actuator activated. From this profile, the measured influence function (crosstalk between pixels) is approximately 25%.
Fig. 17. Measured frequency response of the piezoelectric unimorph actuator
alone and with a mounted mirror membrane. These plots demonstrate that the
DM is capable of wide-bandwidth operation.
missions, such corrections are called active optics. The DM
technology reported here is appropriate to a wide range of image
correction applications, active or adaptive, on the ground or in
space. The achievement of large-actuator stroke is particularly
important for projects targeting observations at longer wavelengths and/or in strong turbulence. Scalability to large formats
generally results in reduction of fitting errors and achievement
of high correction (Strehl ratio), and it has additional benefits
for imaging related to suppressing speckle noise by reducing
the intensity of typical remnant speckles. Furthermore, it can be
important to produce physically large DMs, because extreme reduction of scale from the pupil to the reimaged pupil can create
excessive optical aberrations.
The coronagraphic version of the TPF project that has been
studied at JPL required adaptive correction of a coronagraphic
imager in a mode in which individual speckles, due to either
phase or amplitude imperfections across the aperture, are
targeted and removed. Interferometric planet finders require
nulling interferometry in which the two arms of a Michelson
interferometer are fed with beams that are very precisely
matched in intensity and phase, in each polarization and at
each wavelength over a broad band, so that a deep null results
when one arm is given an extra achromatic phase shift of
rad. The required intensity match is on the order of 0.1%; the
required phase match is about 1 mrad. To ease the requirements
on the optical components, JPL is developing an “adaptive
nuller” device, based on a DM, that quasi-statically adjusts the
intensity and phase in each of about a dozen or so wavelength
channels [24]. The adaptive nuller is a scheme for balancing
the two arms of a nulling interferometer, in both amplitude and
phase, independently in each linear polarization, in each of
perhaps a few dozen spectral channels. A device like this can
substantially ease the requirement for matching optics in the
two arms of the interferometer. The heart of the device is a DM
that applies the required piston to adjust phase and, in the orthogonal axis, tilt to adjust amplitude of the incident beam that
has been spectrally dispersed and separated by polarizations.
Optimized versions of the prototype DM described above will
be well suited to the adaptive nulling task, particularly due to
their low influence function, which allows clean, independent
correction of nearby spectral channels. This example illustrates
how new applications result when the capabilities of DMs become available and known to optical designers. It is likely that
many currently unforeseen applications will be developed when
the technology is more widely available.
V. CONCLUSION
We have successfully fabricated, assembled, and characterized a large-aperture DM composed of a continuous
single-crystal silicon membrane supported by PZT unimorph
actuator arrays. A mathematical model based on an energy
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 5, OCTOBER 2006
minimization method was used to assist the optimization of
membrane thickness and electrode sizes. Excellent agreement
with experiment was obtained. Improved PZT unimorph actuators with patterned PZT film designed with optimized PZT/Si
thicknesses can produce large strokes at low voltages. DMs
consisting of 20- -thick single-crystal silicon membranes
20 actuator arrays were fabricated and
supported by 20
optically characterized. Application in an adaptive nulling
task has been discussed. Our devices have a low influence
function of 30%, which is desirable for closed-loop adaptive
optics applications. The PZT actuators have sufficiently high
piezoelectric coefficients that use of the mirrors at cryogenic
temperatures should be possible at modest applied voltages. In
all of the key parameters, the unimorph MEMS DMs described
here can be optimized for delivery of high-quality image
correction. Improvements in the fabrication process for better
optical quality mirror membranes and optimization of the DMs
for larger strokes are underway. Optimized devices should be
well suited to the adaptive nulling task, particularly in their low
influence function, which allows clean independent correction
of nearby spectral channels.
ACKNOWLEDGMENT
Some of the research described in this paper was carried out
at the Jet Propulsion Laboratory, California Institute of Technology. The research described here was performed under the
Director’s Research and Development Fund at JPL.
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Eui-Hyeok (EH) Yang (S’90–A’95–M’03–SM’06)
received the B.S., M.S., and Ph.D. degrees from
the Department of Control and Instrumentation
Engineering, Ajou University, Korea, in 1990, 1992,
and 1996, respectively.
Presently, he is an Associate Professor in the Mechanical Engineering Department, Stevens Institute
of Technology, Hoboken, NJ. He joined the Fujita
MEMS Research Group at the Institute of Industrial
Science, University of Tokyo, Japan, as a Visiting
Postdoctoral Researcher in 1996. He received a
research fellowship from the Japan Society for the Promotion of Science from
1996 to 1998. In 1999, he joined NASA’s Jet Propulsion Laboratory (JPL),
Pasadena, CA, where he initiated the development of MEMS actuator-based
adaptive optical devices. He was then a Senior Member of the Engineering Staff
and the task manager for several technology development projects at JPL in the
area of micro- and nanotechnologies. He initiated and led the development of
MEMS-based deformable mirrors and actuators for future large aperture telescopes, and also led the development of MEMS-based piezoelectric valves for
future microspacecraft applications. He participated in the technical evaluation
YANG et al.: THIN-FILM PIEZOELECTRIC UNIMORPH ACTUATOR-BASED DM
of MEMS mirror array technologies developed for the Multi Object Spectrometer (MOS) project for the James Webb Space Telescope (JWST). He was a
technical monitor for a NASA SBIR project. He was a Research Adviser for
National Research Council (NRC) in the area of piezoelectric microactuators
for active-mirror technologies. He has been successful in winning extremely
competitive major research grants which represents an exceptional achievement
and productivity within NASA. He has extensive experience in microactuator,
deformable mirror, and optical membrane fabrication. His current research
interests include all aspects of microsensors/actuators, microfluidics, adaptive
optics, micro/nano energy conversion, and nano-manufacturing technologies.
He has published over 90 papers in the field of MEMS, and has six patents
issued or pending.
Dr. Yang is a member of the Technical Program Committee (TPC) of the
IEEE Sensors Conference. He is Topic Organizer of the Micro and Nano Devices Topic, within the MEMS Division, of the ASME International Mechanical
Engineering Congress and Exposition. He has been serving as a referee for several archival journals, international conferences and proposals. In recognition
of his excellence in advancing the use of MEMS-based actuators for space applications, he received the Lew Allen Award for Excellence for 2003 at JPL.
Yoshikazu Hishinuma received the B.S. degree in
applied and engineering physics from Cornell University, Ithaca, NY, in 1997, and the M.S. and Ph.D.
degrees in applied physics from Stanford University,
Stanford, CA, in 1999 and 2002, respectively. His
doctoral work concentrated on experimental and theoretical studies on refrigeration effect of low work
function materials during electron tunneling at room
temperature.
From 2002 to 2005, he was with the Microdevices
Laboratory, Jet Propulsion Laboratory, Pasadena, CA
where his research focused on the development of micromachined deformable
mirrors. Currently, he is with Fuji Photo Film Company, Ltd., Kanagawa, Japan,
where he continues to work on various microscale devices. His research interests
include microfabrication technologies, MEMS actuator designs, and sensors for
small-signal detections.
Jian-Gong Cheng received the M.S. degree in
condensed-matter physics from Shandong University, Jinan, China, in 1997, and the Ph.D. degree
in microelectronics and solid-state electronics from
Shanghai Institute of Technical Physics, Chinese
Academy of Sciences, Shanghai, China, in 2000.
He was a Senior Research Fellow with the Corporate Technology of Siemens AG, Munich, Germany,
from 2001 to 2002. He was with the Material Research Laboratory of Pennsylvania State University
as a Postdoctoral Fellow from 2003 to 2005. Since
April 2005, he has been a Professor with the Shanghai Institute of Microsystem
and Information Technology, Chinese Academy Sciences, Shanghai, China. His
research covers ferroelectric materials and their applications as pyroelectrics in
infrared detectors, dielectrics in capacitors, and as piezoelectric transducers for
biosensors. He is pursuing his research interests in developing novel biosensors.
Susan Trolier-McKinstry (M’92–SM’01) received
the B.S., M.S., and Ph.D. degrees from the Pennsylvania State University (Penn State), University Park,
all in ceramic science.
After graduation, she joined the faculty at Penn
State, where she is currently a Professor of ceramic
science and engineering and Director of the W.
M. Keck Smart Materials Integration Laboratory.
Her main research interests include dielectric and
piezoelectric thin films, the development of texture
in bulk ceramic piezoelectrics, and spectroscopic
1225
ellipsometry. She has held visiting appointments with the Hitachi Central
Research Laboratory, the Army Research Laboratory, and the École Polytechnique Federale de Lausanne.
Prof. Trolier-McKinstry is a Fellow of the American Ceramic Society and a
member of the Materials Research Society. She is past-president of both Keramos and the Ceramics Education Council and is co-chair of the committee
revising the IEEE Standard on Ferroelectricity. She has served as Vice-President for Ferroelectrics of the IEEE UFFC and is now President-Elect of the
society. She was the recipient of the Robert Coble Award of the American Ceramic Society, the Wilson Award for Outstanding Teaching in the College of
Earth and Mineral Sciences, the Materials Research Laboratory Outstanding
Faculty Award, and a National Science Foundation Career grant.
Eric Bloemhof received the Ph.D. degree in physics
from the University of California, Berkeley, in the
Quantum Electronics and Astrophysics group of
Prof. C. Townes.
He was a Center Postdoctoral Fellow with the
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, and continued work at the Smithsonian
Astrophysical Observatory in the area of submillimeter instrumentation, including superconducting
tunnel-junction mixers and receivers. He was Chief
Physicist for the Palomar Observatory, California Institute of Technology, where he was involved primarily on the PALAO adaptive
optics system being built at the Jet Propulsion Laboratory (JPL) for the 5-m
Hale telescope. In 2001, he joined JPL to work on various instrumentation connected with space-borne interferometers, including the Space Interferometry
Mission (SIM) and the interferometric Terrestrial Planet Finder (TPF-I). He
currently leads the collector optics group for SIM, and the mask development
effort for JPL’s innovative four-quadrant- phase-mask coronagraph that has
demonstrated deep starlight suppression on the Hale telescope. Recent research
has concentrated on the behavior of speckles in highly corrected imagers and
coronagraphs and inventing techniques for suppressing speckle noise and
enhancing companion-detection contrast with an eye toward planet searches.
Dr. Bloemhof is a member of the American Physical Society, the American
Astronomical Society, and URSI.
B. Martin Levine received the B.S. degree from
the Rochester Institute of Technology in 1972, the
Master’s degree in statistics from the Colorado State
University in 1976, and the Ph.D. degree in optics
from the University of Rochester, NY, in 1986.
He has 20 years experience in the design and
construction of adaptive optics systems working
as a consultant for the U.S. Air Force and also at
Adaptive Optics Associates. Currently, he holds the
positions of Deputy Leader, Interferometry Center of
Excellence, and also is the Manager of the Advance
Telescopes Technologies and Concepts Office at the Jet Propulsion Laboratory,
Pasadena, CA, where he is working on developing advanced concepts for future
space missions.
Dr. Levine is a member of the Optical Society of America, SPIE-The Optical
Engineering Society, and the American Astronomical Society.