U.S. patent application number 17/564330 was filed with the patent office on 2022-04-21 for mems device for large angle beamsteering.
The applicant listed for this patent is Government of the United States, as represented by the Secretary of the Air Force, Government of the United States, as represented by the Secretary of the Air Force. Invention is credited to Harris J. Hall, LaVern A. Starman, John PK Walton.
Application Number | 20220119244 17/564330 |
Document ID | / |
Family ID | |
Filed Date | 2022-04-21 |
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United States Patent
Application |
20220119244 |
Kind Code |
A1 |
Starman; LaVern A. ; et
al. |
April 21, 2022 |
MEMS Device for Large Angle Beamsteering
Abstract
An actuator element of a MEMS device is provided, which is
fabricated using surface micromachining on a substrate. An
insulating layer having a first portion contacts the substrate
while a second portion is separated from the substrate by a gap. A
metallic layer contacts the insulating layer having a first portion
contacting the first portion of the insulating layer and a second
portion contacting the second portion of the insulating layer. The
second portion of the metallic layer is prestressed. Alternately,
the actuator element includes a first insulating layer separated
from the substrate by a gap. A metallic layer has a first portion
contacting the substrate and a second portion contacting the
insulating layer. A second insulating layer contacts a portion of
the second portion of the metallic layer opposite the first
insulating layer, where the second insulating layer is
prestressed.
Inventors: |
Starman; LaVern A.; (Dayton,
OH) ; Walton; John PK; (Troy, OH) ; Hall;
Harris J.; (Yellow Springs, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Government of the United States, as represented by the Secretary of
the Air Force |
Wright-Patterson AFB |
OH |
US |
|
|
Appl. No.: |
17/564330 |
Filed: |
December 29, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16052018 |
Aug 1, 2018 |
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17564330 |
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62540177 |
Aug 2, 2017 |
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62587734 |
Nov 17, 2017 |
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62589610 |
Nov 22, 2017 |
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62667647 |
May 7, 2018 |
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62702595 |
Jul 24, 2018 |
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International
Class: |
B81B 3/00 20060101
B81B003/00; G02B 26/08 20060101 G02B026/08 |
Claims
1. A MEMS device on a substrate, comprising: a platform; an
actuator assembly composed of a plurality of actuator elements, the
actuator assembly connected to the platform; wherein actuation of
the plurality of the actuator elements in the actuator assembly
causes motion in the platform.
2. The MEMS device of claim 1, wherein each actuator element of the
plurality of actuator elements has a first end and a second end,
wherein the first end of a first actuator element of the plurality
of actuator elements is connected to the substrate and the second
end of a last actuator element of the plurality of actuator
elements is connected to the platform, and wherein the first ends
of the remaining plurality of actuator elements are connected to
the second ends of other actuator elements between the first and
last actuator elements of the plurality of actuator elements to
form a chain.
3. The MEMS device of claim 1, wherein the plurality of actuator
elements in the actuator element chain is arranged in a serpentine
configuration.
4. The MEMS device of claim 1, wherein the plurality of actuator
elements in the actuator element chain is arranged in a center
contact configuration.
5. The MEMS device of claim 1, further comprising: a micromirror
bonded to the platform.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 16/052,018, entitled "MEMS Device for Large Angle
Beamsteering," filed Aug. 1, 2018, which claims the benefit of and
priority to U.S. Provisional Application Ser. No. 62/540,177,
entitled "Post-Processing Techniques on MEMS Foundry Fabricated
Devices for Large Angle Beamsteering," filed on Aug. 2, 2017, and
U.S. Provisional Application Ser. No. 62/587,734, entitled
"Segmented Control of Electrostatically Actuated Bi-Morph Beams,"
filed on Nov. 17, 2017, and U.S. Provisional Application Ser. No.
62/589,610, entitled "Using Surface Micromaching to Create Large
Tip, Tilt, and Piston MEMS Beamsteering Structures," filed on Nov.
22, 2017, and U.S. Provisional Application Ser. No. 62/667,647,
entitled "Torsional Structures to Enable Large Angle Deflections,"
filed on May 7, 2018, and U.S. Provisional Application Ser. No.
62/702,595, entitled "Torsional Springs to Enable Large Angle
Tip/Tilt Beamsteering using MEMS," filed on Jul. 24, 2018, the
entireties of which are incorporated by reference herein.
RIGHTS OF THE GOVERNMENT
[0002] The invention described herein may be manufactured and used
by or for the Government of the United States for all governmental
purposes without the payment of any royalty.
BACKGROUND OF THE INVENTION
Field of the Invention
[0003] The present invention generally relates to MEMS devices and,
more particularly, MEMS devices capable of large angle
deflections.
Description of the Related Art
[0004] Within the past decade, numerous researches have invested
time in the development of micro-electro-mechanical systems (MEMS)
micromirror structures which have the ability to deflect at large
angles (greater than 20 degrees). These large tip/tilt micromirrors
are ideal for many applications to include microscopy, biomedical
endoscopy, laser communication, wavelength selectivity, optical
tuning, scene generation and various other medical
instrumentations. Although many of these research efforts exhibit
large tip/tilt angles, they generally do not include a piston
motion for optical correction requirements or exhibit high
fill-factors for large area optical scanning applications. There
currently are no MEMS large angle beamsteering approaches which
exhibit large tip/tilt and piston motion while exhibiting a
fill-factor greater than 90%, which may be fabricated using surface
micromachining. Current state of the art electrostatic designs have
a maximum tip or tilt angle of .+-.28 degrees for a single element
but generally do not possess both capabilities. Electro thermal
designs have a maximum tip or tilt angle of .+-.40 degrees for a
single element but also generally do not possess both capabilities.
No approach with a tip or tilt angle of greater than 10 degrees are
available which has a high fill-factor. Most if not all designs
with a high fill-factor have tilt angles of less than 5
degrees.
[0005] Accordingly, there is a need in the art for MEMS
micromirrors for large angle beamsteering for numerous broadband
steering and imaging applications.
SUMMARY OF THE INVENTION
[0006] Micro-Electro-Mechanical Systems (MEMS) micromirrors have
been employed in a wide range of optical applications for about two
decades. However, scanning micromirrors are far less numerous,
generally exhibit low scanning angles (less than 20.degree.) and
typically in only one direction. Embodiments of the invention
provide large angle, out-of-plane bimorph MEMS micromirrors
fabricated in foundry processes as well as in-house. Through
modeling and simulation, several techniques are possible to meet
the required large out-of-plane deflections needed for large angle
beamsteering. Both a serpentine and center anchored multi-beam
approach have been designed, modeled, fabricated, and tested to
observe deflection and overall functionality of the structures.
These structures exhibit high, out-of-plane deformations as either
a MEMS electrostatic and electrothermal actuators, which can then
be integrated with an SOI or some other fabricated micromirror
array to enable broadband steering and imaging applications. The
arrays are able to exhibit tip, tilt, and piston motion due to the
individual actuation design schemes which are utilized in each
micromirror structure while maintaining a high fill-factor and is
scalable to large aperture and array sizes. The design methodology
capitalizes on the inherent residual stresses in bimorph structures
which possess different coefficients of thermal expansions (CTE).
Through precise material selection, and design control (i.e.
structure length, material thickness, material CTE, deposition
temperature, and material layer composition), this inherent
residual stress will be used to create the upward deflections
required for these surface micromachined structures to enable large
angle micromirror movements.
[0007] Embodiments of the invention provide a MEMS device on a
substrate, which includes a platform to which a micromirror may be
attached or may be used as the fabrication point for the
micromirror. A least one actuator element chain composed of a
plurality of actuator elements is connected to the platform.
Actuation of the plurality of the actuator elements in the actuator
element chain causes motion in the platform, which can be
controlled to steer the mirror. The actuator elements of the
plurality of actuator elements form a chain, which may be arranged
in a serpentine configuration in some embodiments or with a center
contact configuration in other embodiments.
[0008] In some embodiments, the actuator elements of the MEMS
device include an insulating layer having a first contacting
portion and a second portion separated from an electrode by a gap.
A metallic layer contacts the insulating layer and has a first
portion contacting the first contacting portion of the insulating
layer and a second portion contacting the second portion of the
insulating layer, the second portion of the metallic layer is
prestressed. In some of these embodiments the second portion of the
metallic layer of the actuator element is tensilely
prestressed.
[0009] In other embodiments, the actuator elements of the MEMS
device include a first insulating layer positioned above the
substrate and separated from the substrate by a gap. A metallic
layer has a first contacting portion and a second portion
contacting the insulating layer. A second insulating layer contacts
a portion of the second portion of the metallic layer opposite the
first insulating layer, where the second insulating layer is
prestressed. In some of these embodiments, the second insulating
layer of the actuator element is compressively prestressed.
[0010] Additional objects, advantages, and novel features of the
invention will be set forth in part in the description which
follows, and in part will become apparent to those skilled in the
art upon examination of the following or may be learned by practice
of the invention. The objects and advantages of the invention may
be realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and, together with a general description of the
invention given above, and the detailed description given below,
serve to explain the invention.
[0012] FIG. 1A is MEMS structural device used for large angle
beamsteering illustrating an exemplary embodiment with a center
contact design;
[0013] FIG. 1B is MEMS structural device used for large angle
beamsteering illustrating an exemplary embodiment with a serpentine
based design with the contact at the end of the first, longest
cantilever beam, which is a modification of the serpentine design
in FIG. 2 below;
[0014] FIG. 2 illustrates and exemplary serpentine design
shown;
[0015] FIG. 3 illustrates an exemplary large angle beamsteering
micromirror design concept using the center contact design
approach;
[0016] FIG. 4A illustrates the deflection of the serpentine design
in FIG. 2 showing an upward deflection of about 60 .mu.m;
[0017] FIG. 4B illustrates the deflection of the modified
serpentine design in FIG. 1B with an upward deflection of about 145
.mu.m;
[0018] FIG. 5A illustrates a peak, upward beam deflection of 150
.mu.m for identical center contact designs with the same physical
actuator dimensions as shown in FIG. 1A with an aluminum metal
layer ;
[0019] FIG. 5B illustrates a peak, upward beam deflection of 80
.mu.m for identical center contact designs with the same physical
actuator dimensions as shown in FIG. 1A with a gold metal
layer;
[0020] FIG. 6 illustrates the total upward deflection following
release of the micromirror of FIG. 3;
[0021] FIG. 7A illustrates an overall design concept of the
actuation assembly for large out-of-plane deflections as deposited
layers prior to release;
[0022] FIG. 7B illustrates a post released structure of FIG. 7A
showing the out-of-plane upward deflection;
[0023] FIG. 7C illustrates an SEM image of bimorph cantilever beams
for an actual structure in a released configuration;
[0024] FIG. 8A illustrates PolyMUMPs foundry fabrication layers for
an exemplary device;
[0025] FIG. 8B contains a table with material layer descriptions
and thicknesses of the exemplary device in FIG. 8A;
[0026] FIGS. 9A-9C illustrate a fabrication sequence of the
post-processed actuation assembly from baseline design received
from a foundry to a released structure;
[0027] FIG. 10 shows a deformed model of an electrothermal design
as fabricated in PolyMUMPs;
[0028] FIG. 11 shows a deformed model of the electrothemal design
of FIG. 10 with a post-processed SiN layer;
[0029] FIG. 12 shows a deformed model of the post-processed
electrothermal design of FIG. 11 with a 300 C gold metal
evaporation layer replacing the PolyMUMPs gold layer;
[0030] FIG. 13A shows a deformed model of an electrostatic
serpentine design;
[0031] FIG. 13B shows scanning electron microscope images of the
electrostatic device in FIG. 13A;
[0032] FIG. 14A illustrates a torsional spring attachment which
exhibits a moderately high piston and tip/tilt spring constants,
which will make tilt of the micromirror platform difficult;
[0033] FIG. 14B illustrates the highest spring constants which make
for a very ridged and stiff structure that nearly prohibits tilting
events;
[0034] FIG. 14C illustrates a lower spring constant which enables
tilting but is not ridged enough such that it creates some of the
negative downward motion of the opposite actuator assembly;
[0035] FIG. 14D illustrates a lower spring constant for a tip/tilt
event and is rigid enough for piston actuation;
[0036] FIG. 15A includes a SEM image of a completed, unreleased
PolyMUMPs.TM. micromirror actuation assembly;
[0037] FIG. 15B includes a SEM image of the same device in FIG. 15A
following sacrificial oxide release;
[0038] FIG. 15C includes a SEM image of a portion of FIG. 15B where
the torsional spring is fully extended under an actuation
condition;
[0039] It should be understood that the appended drawings are not
necessarily to scale, presenting a somewhat simplified
representation of various features illustrative of the basic
principles of the invention. The specific design features of the
sequence of operations as disclosed herein, including, for example,
specific dimensions, orientations, locations, and shapes of various
illustrated components, will be determined in part by the
particular intended application and use environment. Certain
features of the illustrated embodiments have been enlarged or
distorted relative to others to facilitate visualization and clear
understanding. In particular, thin features may be thickened, for
example, for clarity or illustration.
DETAILED DESCRIPTION OF THE INVENTION
[0040] Embodiments of the invention address the need in the art by
enabling new and improved beamsteering systems with large
beamsteering angles and high scanning speeds while exhibiting high
fill-factor (greater than 90%) arrays, which may be scalable to
large aperture sizes as well as enable a multi-beam scanning
capability at a low bias voltage. Embodiments of the invention have
the potential to replace conventional gimbal systems on platforms
since these devices are nearly conformal, and eliminate all
macro-scale moving mechanical parts of the contemporary
scanning/detector systems. Embodiments of the invention would be
applicable to EO/IR beamsteering systems, imaging and scene
generation systems, laser communications, multi-target search and
track, among others. Conventional methods use gimbal systems which
are slow, single beam beamsteering with no multi-target
detect/track capability. Advantages of the embodiments of the
invention include elimination of most mechanical/gimbal systems on
a platform, can enable multi-beam steering, low voltage, nearly
conformal, adaptable and scalable to meet a wide range of
applications with regards to steering angles and scanning speeds.
These embodiments use MEMS to enable the large angle beamsteering,
scalable with regard to array size, flexible with regards to
material selection, wavelengths of interest, and
deflection/steering angles while being ideal for wideband
applications.
[0041] FIGS. 1A and 1B illustrate two different structural concepts
which were designed to capitalize on the inherent residual stress
in materials in addition to capitalizing on the different
coefficient of thermal expansion (CTE) of the bimorph material
layers selected. FIG. 1A shows a basic center contact design
concept with a contact pad removed for modeling purposes. This
device may be biased by individually applying a voltage potential
to each of the four actuators separately to create a tip & tilt
scenario, or all four actuators being biased simultaneously to
create a piston motion for a micromirror to enable optical phase
correction. FIG. 1B illustrates a modified serpentine design
concept in which the actuators can also be biased individually or
all simultaneously to create the tip/tilt and piston motion as
described above. These exemplary embodiments were determined for an
exemplary application where a physical beamsteering actuator was
limited to 1 mm.sup.2 or less. This choice was made to try and
maximize the initial upward deflection while also maintaining a
reasonable number of control lines for individual actuation control
as well as providing the necessary sensing lines to determine the
precise location of the mirror with respect to elevation and angle
during device operation. From either of the embodiments of FIGS. 1A
and 1B, the initial upward deflection required for tip and tilt
beamsteering as well as overall footprint for the design may be
tailored to any desired application through a number of design and
material modifications to include changing the beam lengths,
changing selected materials of the bimorph beam structure,
thickness of the beam material layers, the number of beams used for
deflection purposes, the residual stress and CTE properties of the
materials, the difference in the Young's modulus of the materials,
the deposition temperatures and composition of the material layers,
and the overlap of the various beams in the bimorph beams.
[0042] Some of the main differences in operational performance
between the exemplary embodiments in FIGS. 1A and 1B include: 1)
the center contact design exhibits a higher pull-in voltage, 2) the
structural reliability of the center contact design is greater than
the serpentine design, 3) the overall spring constant for the
center contact design is much greater than the serpentine design,
which may be advantageous in high vibration environments, and 4)
the resonant frequency of the center contact is much higher than
the serpentine design. Thus, there are a wide range of applications
that could integrate either configuration to meet a desired
application.
[0043] The exemplary embodiment of the serpentine design in FIG. 1B
is an enhanced version of another exemplary 7-beam serpentine
design as shown in FIG. 2 in which all bimorph actuator beams were
set to the same length. The serpentine design in FIG. 1B utilizes
available surface area of the wafer much better, which in turn aids
in increasing the upward deflection. Both serpentine designs in
FIGS. 1B and 2 provide low voltage pull-in for large out-of-plane
deflections while maintaining a high fill-factor once coupled with
the micromirror assembly as shown in FIG. 3 are not possible in
most MEMS actuators. A disadvantage to the design in FIG. 2 where
all beams were the same length, is that once the pull-in voltage is
reached, the entire actuator structure collapses onto the
electrode, mitigating any downward deflection control. The modified
design in FIG. 1B improves on several concepts. First, the initial
upward deflection can be increased significantly within the same
footprint from the original design as the bimorph beams are
optimally lengthened to enhance upward deflection as illustrated in
FIGS. 4A and 4B (deflecting approximately 60 .mu.m and 145 .mu.m
respectively). And, second, the actuators spring constant is varied
with respect to the bimorph beam lengths with the lowest spring
constant coming from the longest beam and steadily increases as the
bimorph beams are shortened approaching the bonding platform. Thus,
downward control of the micromirror tip, tilt, and piston
deflection is possible simply through varying the bias voltages on
the actuator electrodes.
[0044] FIGS. 5A and 5B illustrate the effect of changing the metal
layer on the performance of the center contact design of FIG. 1A
with identical physical bimorph beam dimensions. The only
difference is the metal layer chosen within the bimorph beam from
aluminum to gold. The initial upward deflection for the aluminum
beam shown in FIG. 5A is approximately 150 .mu.m while FIG. 5B
shows the gold bimorph beam with only about 80 .mu.m of deflection.
For this minor material modification, the primary reason for the
deflection difference was the difference in CTE and the difference
in Young's modulus of the materials.
[0045] FIG. 3. illustrates the center contact structural design
concept with an SOI micromirror bonded onto the bonding pad of the
actuator assembly. FIG. 3 shows the basic design concept 10 with
the cantilever style beams 12 attached to a bonding platform 14.
This bonding platform 14 is used to bond and support the
micromirror pillar 16 and mirror plate 18 to enable the deflection
and piston motion. In this design, the entire structure used for
actuation will be fabricated on a single wafer while the
micromirror pillar and mirror will be fabricated from an SOI wafer
or some other surface micromachining technique. FIG. 6 illustrates
the modeled upward deflection created by the inherent residual
stress, CTE difference, and the different Young's modulus in the
cantilever like beams to provide the initial, post-released peak
upward displacement. All remaining deflections and piston motion
will occur due to controlled, user-driven cantilever beam
deflection. The overall integration between the micromirror and the
actuation technique may be through basic adhesion bonding to the
actuation platform through the use of epoxy or metal-to-metal
fusion bonding, though other bonding methods may also be used.
[0046] Embodiments of the invention are based on different actuator
design concepts, which are tailorable to meet a wide variety of
application specifications. These designs may be fabricated using a
wide range of materials to create the large out-of-plane upward
deflections from surface micromachining principles to enable the
large angle tip/tilt and piston motion to properly steer an optical
beam. From these design concepts, large angle beamsteering can be
performed while also exhibiting a high fill-factor for optical
applications. Through some basic design changes to the center
contact design, this design may be used as either an electrostatic
or an electrothermal design. The actual selection between the
electrostatic or the electrothermal design is dependent on the
application. An electrostatic design will require higher voltages
to actuate the structure, but will benefit from switching speeds
that will be much faster than an electrothermal design. The
electrothermal design will require lower voltages and will exhibit
larger power consumption than the electrostatic design. However,
the electrothermal design will also enable larger forces to be
generated and applied to the structure for implementing the
tip/tilt and piston motions.
[0047] A large out-of-plane deflection is the first stage in
developing a large out-of-plane beamsteering technique for a
surface micromachined device. In general, the large out-of-plane
deflections may be achieved by capitalizing on the materials
inherent residual stress and Young's modulus as well as the
difference in the materials coefficient of thermal expansion (CTE)
to form a traditional bimorph design. In addition, the material
thickness, beam lengths, the number of beams, and the deposition of
the multi-layers which make up the beams all significantly
contribute to the peak out-of-plane deflection. The upward
deflections can be tailored to the application need as nearly all
metals, dielectrics, semiconductor, and polymer materials can be
used to create these devices. The only caveat to this is during the
fabrication processes, one needs to select materials that can
withstand the various etching and patterning processes. The
illustrated embodiments of an actuation system are made up of four
individually controlled bimorph actuators which enables system
tip/tilt motion to angles of .+-.45 degrees as well as provide a
piston motion if all four actuators are biased simultaneously.
These are low voltage actuation systems which operate on
electrostatics (less than 100V) to pull down the actuators to
create the required forces to tip/tilt or piston drive the overall
system. Electrothermal designs are again based on the traditional
bimorph structure but joule heating is the actuation mechanism at
less than 10 V. A preferred configuration for the overall system
would be an array of these actuation structures which exhibit a
high fill-factor greater than 90% to mitigate signal loss and
maximize beam reflection/detection.
[0048] As shown in FIGS. 1A and 1B above, to maximize the
out-of-plane deflections, different material layers are used to
create bimorph beam structures of an actuation assembly. The
actuator design concept capitalizes on the residual stress and the
coefficient of thermal expansion (CTE) differences between the
layers. An exemplary layout 20 is shown in FIG. 7A. In the
exemplary embodiment illustrated in FIG. 7A, an electrode 22 is
formed on a substrate 24. A sacrificial layer 26 is formed over the
electrode. An insulator 28 is then formed over the sacrificial
layer 26 and a portion of the substrate 24. Finally, a metal 30 is
formed over the insulator 28, where a portion 32 of the metal 30 is
under a tensile stress. When the sacrificial layer 26 is removed,
the configuration is "released" and a portion 34 of the metal layer
30 deforms due to the prestressed condition as shown in FIG. 7B.
FIG. 7C shows a scanning electron microscope (SEM) image of the
design concept as fabricated in the PolyMUMPs foundry process.
[0049] The electrostatic and electrothermal actuation systems may
be made utilizing surface micromaching in which thin material
layers are deposited and photolithography patterned on the surface
of the wafer. These actuators may be developed in a wide variety of
materials, deposition techniques, and fabrication facilities, even
to include available commercial foundries. This design concept can
be used as a key component in a wide variety of large angle beam
steering approaches for platforms and UAVs. The structures may also
be used for imaging and scene generation.
[0050] There are a wide range of alternatives to these
electrostatic and electrothermal actuators. All one needs to verify
is the residual stresses, Young's modulus, and the CTE of the
selected materials meet the application requirements. The greater
the difference in CTE values, the greater the possible deflections.
Generally, a conductive layer 30 is required to create the lower
electrode 22 and a second conductive layer as part of the actuation
platform to enable the electrostatic attraction for device
operation, which is similar to the electrothermal design with the
lower electrode 22 not being required. These metal layers could be
gold, aluminum, chromium, titanium, platinum, copper, and nickel,
among others, while the dielectric layers could be silicon dioxide,
silicon nitride, hafnium oxide, and aluminum oxide, among others.
Various polymers could also be used to create these device
structures but care would need to be taken to mitigate possible
etching during the patterning and development of the structures.
One of the key concepts in creating these structures is in the
material selection such that there is a fairly large difference in
CTE as well as the Young's modulus of the material. A higher
Young's Modulus will create a more rigid and stable structure but
there are limits as bending must occur to create the tip and
tilting of the platform from the actuators.
[0051] A MEMS commercial foundry may also be used in addition to
in-house fabrication efforts to make these large out-of-plane
structures. As illustrated in the various figures, a designer has a
wide range of options to meet there desired application goals from
the physical size of the device, to material selections, to
residual stress levels within these layers. From these options,
designers can create low angle tip/tilt/piston driven devices to
very large out-of-plane structures which enable large angle
tip/tilt and piston motion.
[0052] The above illustrated embodiments of the invention are based
on the use of the PolyMUMPs foundry fabrication as a baseline or
foundation of the overall system. From this foundry, large angle
beamsteering while exhibiting a high fill-factor is not possible.
Thus, from the baseline process, several post-processing steps may
be performed to enable the large out-of-plane upward deflections to
permit large angle beamsteering. Initial as fabricated structures
from the foundry genearlly provide a peak out-of-plane deflection
of approximately 11 .mu.m to 140 .mu.m, depending upon the design.
Performing post-processing depositions of high temperature gold and
a compressively stressed silicon nitride layer on these same
designs, the peak out-of-plane deflections increase from greater
than 200 .mu.m to over 1 mm. These post-processing methods are
viable for both electrostatic and electrothermal designs.
[0053] The electrostatic and electrothermal actuation systems are
constructed utilizing surface micromaching in which thin material
layers are deposited and photolithography patterned on the surface
of the wafer. The actuators may be developed in the PolyMUMPs
foundry process as outlined below with additional post-processing
steps to include high-temperature gold evaporation and PECVD
silicon nitride layers deposited prior to the final release. This
design concept may be used as a key component in a wide variety of
large angle beam steering approaches for platforms and UAVs. The
structures can also be used for imaging and scene generation.
[0054] The foundation of the exemplary designs use the PolyMUMPs
fabrication process which is outlined in Cowen et al.,
"PolyMUMPs.TM. Design Handbook," Revision 13, which is incorporated
by reference herein in its entirety, though other fabrication
processes may also be used. FIG. 8A illustrates a cross sectional
view of all deposition layers and the table in FIG. 8B outlines
each layer thickness and layer functionality. The surface material
layers may be deposited by low pressure chemical vapor deposition
(LPCVD). The sacrificial oxide layers, which consist of
phosphosilicate glass (PSG) for this illustrated example, serve two
purposes. First, they define the gaps between structural layers,
and second, they serve as a dopant source for the 1050 C high
temperature phosphorus diffusions, which assists in reducing the
resistivity in the polysilicon structural layers. All surface
layers may be patterned using standard photolithography techniques
and etched using Reactive Ion Etching (RIE) or other etching
methods. The final surface layer, a 0.5 .mu.m-thick gold
metallization layer with a 100 nm chrome adhesion layer is
deposited and patterned using a standard lift-off technique.
Lastly, a release etch is performed to remove the sacrificial oxide
layers freeing the structural polysilicon layers (Poly1 and Poly2).
The typical release etch is performed by immersing the die in room
temperature hydrofluoric (49%) acid for 2-3 minutes, methanol
rinses to stop the HF etch, and then a supercritical carbon dioxide
(CO.sub.2) rapid dry to minimize stiction of the actuation
assemblies. Note that for the electrothermal actuators embodiments
designed in this process, only the Poly2 and gold layers need be
used to create a foundation for further device development through
the addition of other material layers.
[0055] In order to implement the post-processing steps, which must
be completed to enable the large out-of-plane deflections, a series
of masks are needed to define the construction of the additional
material deposition patterns for the beam structures. FIGS. 9A-9C
illustrate the post-processing fabrication process. The process
begins with a baseline foundry fabrication 60 from PolyMUMPs
including a silicon dioxide layer 62, polysilicon layer 64, and
gold layer 66 on a 1 cm.sup.2 silicon die 68 as shown in FIG. 9A.
From this baseline, the PolyMUMPs gold layer 66 may be used or it
may be etched off and redeposited with a high temperature (up to
300 C) gold evaporation layer. Following this gold deposition, a
high compressive stressed silicon nitride layer 70 is deposited
using Plasma Enhanced Chemical Vapor Deposition (PECVD) at a
thickness of 1 .mu.m and then photo lithographically patterned
using AZ5214 photoresist. Following the UV exposure and
development, the silicon nitride layer 70 is etched using reactive
ion etching (RIE) and then the remaining photoresist is removed
using acetone. Following the deposition and patterning of the
silicon nitride layer, the three layer stacked material beam
structures which make up the actuation assembly is completed and
shown in FIG. 9B. Finally, the sacrificial silicon dioxide layer 62
may be removed using 49% hydrofluoric acid (see FIG. 9C) which is
then followed by a CO.sub.2 critical point dry to fully release and
dry the actuation assembly. While certain deposition and removal
methods were used with this illustrated embodiment, other
deposition and removal methods may also be used.
[0056] COMSOL.RTM. finite element modeling (FEM) software was used
to model the pre and post-processed foundry fabricated MEMS designs
to determine the out-of-plane deflections. Based on the design
constraints of the foundry process and an allotted design space
criteria for a single element (1 mm.sup.2), the PolyMUMPs foundry
does not meet the required deflections as shown in the COMSOL.RTM.
simulation shown in FIG. 10 of the structure. As seen in FIG. 10,
there is an overall peak deflection of zero microns. This is due to
the full bimorph beams lacking a bending moment component which can
force the beam tips to deflect downward, creating an elongated `S`
shaped final profile. In the current PolyMUMPs foundry fabrication,
there are no additional material layers available which can be used
to create this bending moment.
[0057] As illustrated in FIG. 10, the image illustrates the bimorph
beams upward bending which when coupled with a silicon nitride
(SiN) deposition placed on top of the gold/polysilicon stacked
beams, can indeed create the necessary bending moment to cause the
actuation assembly to deflect upward as shown in FIGS. 11 and 12.
Therefore, through the use of the PolyMUMPs structural layers as
the foundation of the actuation assembly, post processing
depositions of SiN (FIG. 11) and high temperature gold metal
evaporation (up to 300 C) (FIG. 12) may be used to achieve the
upward deflections needed for large angle beam steering. FIG. 11
illustrates the as fabricated PolyMUMPs electrothermal design with
a SiN layer deposited by plasma enhanced chemical vapor deposition
(PECVD) at a stress level of -200 MPa at 300 C shows a deflection
of .about.50 .mu.m. In FIG. 12, the PolyMUMPs gold layer was
removed and replaced with a gold layer deposited at 300 C. A -500
MPa PECVD SiN layer was then deposited at 300 C. As shown in FIG.
12, over 250 .mu.m can be achieved using these post-processing
techniques. The electrothermal PolyMUMPs design provided an
out-of-plane deflection of 162.+-.5 .mu.m using a coupled SiN layer
exhibiting a stress level of -930 MPa. All deflections are
determined using white light interferometry (IFM).
[0058] The post-processing steps outlined above were repeated for
an electrostatically actuated design utilizing a beam structure in
the form of a folded cantilever beam or serpentine layout. The
baseline electrostatic serpentine design fabricated in the
PolyMUMPs fabrication process resulted in an out-of-plane
deflection of .about.140 .mu.m as shown in the COMSOL.RTM. image in
FIG. 13A. FIG. 13B illustrates a 5.times.4 array of these
structures with a close up view shown on the right of a single
actuation assembly. The out-of-plane deflection was measured to be
.about.148 .mu.m using an IFM. This deflection does not meet the
out-of-plane deflections required for large angle beam steering;
thus, a PECVD SiN layer was deposited with the same compressive
stress level of -930 MPa as previously stated. This resulted in an
experimentally measured out-of-plane deflection of over 1 mm. This
is far too high so the PECVD deposition parameters for this design
will need to be adjusted to reduce the stress level in the nitride
layer. The electrostatically actuated center contact design has
deflections greater than 430 um.
[0059] FIGS. 14A-14D provide additional exemplary L-Edit torsional
spring design attachments, which may be used, and which exhibit
varying degrees of torsional and twisting stiffness. The
platform/spring assembly shown in FIG. 14A provides a moderately
high spring constant for both the piston and tip/tilt motions, FIG.
14B provides a high spring constant which is ideal for rigidity and
reliability of the platform assembly but also nearly mitigates any
piston or tip/tilt motions when a 2 .mu.N force is applied at Point
A and B. FIG. 14C illustrates a lower spring constant which makes
the platform motions resulting from forces applied to Points A and
B greater but still results in the opposite actuator moving
downward, reducing the peak tilt angle achievable. FIG. 14D provide
the best spring constants for this project as it allows for
significant deflection of the platform resulting from either a
force applied to Point A or B. However, the spring constants are
high enough to maintain structural integrity following either
actuation event.
[0060] The images shown in FIGS. 14A-14D show several exemplary
complete micromirror designs illustrating the torsional
spring/platform assembly design as shown in FIG. 14D with the
platform assembly boxed in FIG. 15A. FIG. 15A provides a top image
of the complete unreleased micromirror actuation design with the
actuation assembly and platform assembly integrated to form one
element of a micromirror actuation array. The structures shown in
FIGS. 15A-15C were fabricated in the PolyMUMPs.TM. process such
that the torsional spring attachment is .about.1.5 .mu.m thick with
a width of 8 .mu.m. FIG. 15B illustrates a released micromirror
actuation assembly which is deflected .about.273 .mu.m out-of-plane
with the spring attachment boxed and reimaged in FIG. 15C when the
spring is fully deflected. As can be observed in FIG. 15C, the
spring is flexible enough to not fracture during full actuation
conditions.
[0061] This realization of several exemplary torsional spring
attachments were presented, which is the critical linkage between
the micromirror actuation assembly and the platform assembly.
COMSOL.RTM. models were used to assess the viability of the various
torsional spring designs for rigidity and flexibility to perform
piston motion as well as for tip/tilt motion. The fabricated
structures were presented which clearly shows the torsional spring
does not fracture when in its fully extended position. As set forth
above, choices of materials with these geometries affect
performance and may be tuned to specific requirements and
applications.
[0062] While the present invention has been illustrated by a
description of one or more embodiments thereof and while these
embodiments have been described in considerable detail, they are
not intended to restrict or in any way limit the scope of the
appended claims to such detail. Additional advantages and
modifications will readily appear to those skilled in the art. The
invention in its broader aspects is therefore not limited to the
specific details, representative apparatus and method, and
illustrative examples shown and described. Accordingly, departures
may be made from such details without departing from the scope of
the general inventive concept.
* * * * *