U.S. patent application number 10/705213 was filed with the patent office on 2004-08-19 for actuator apparatus and method for improved deflection characteristics.
Invention is credited to Helmbrecht, Michael Albert, Knollenberg, Clifford F..
Application Number | 20040160118 10/705213 |
Document ID | / |
Family ID | 32854278 |
Filed Date | 2004-08-19 |
United States Patent
Application |
20040160118 |
Kind Code |
A1 |
Knollenberg, Clifford F. ;
et al. |
August 19, 2004 |
Actuator apparatus and method for improved deflection
characteristics
Abstract
A micromachined actuator including a body or platform mounted to
a suspension system anchored to a substrate. In one embodiment, the
suspension system is comprised of a set of one or more spring
flexures connecting the actuator body to the substrate with strain
relief provided via connecting torsional elements. In another
embodiment, the suspension system includes a first set of one or
more spring flexures each with one end anchored to a largely rigid
intermediate frame and the other end attached to the body. A second
set of one or more flexures is attached between the intermediate
frame and the substrate. A third actuator embodiment maximizes
force electrode area to minimize voltage required for electrostatic
actuation. A fourth embodiment provides electrical interconnect to
an actuator or an actuator array using polysilicon with silicon
nitride isolation. Actuators may be fabricated by combining the key
features of all four embodiments or actuators may be fabricated
using any combination of two or three of the embodiments.
Inventors: |
Knollenberg, Clifford F.;
(El Cerrito, CA) ; Helmbrecht, Michael Albert;
(Lafayette, CA) |
Correspondence
Address: |
STATTLER JOHANSEN & ADELI
P O BOX 51860
PALO ALTO
CA
94303
|
Family ID: |
32854278 |
Appl. No.: |
10/705213 |
Filed: |
November 7, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60425049 |
Nov 8, 2002 |
|
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|
60425051 |
Nov 8, 2002 |
|
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Current U.S.
Class: |
303/113.1 |
Current CPC
Class: |
B81B 2203/053 20130101;
G02B 26/06 20130101; H01H 59/0009 20130101; B81B 2203/056 20130101;
H01H 2059/0081 20130101; B81B 3/0037 20130101; G02B 26/0825
20130101; B81B 2203/0163 20130101 |
Class at
Publication: |
303/113.1 |
International
Class: |
B60T 008/34 |
Claims
We claim:
1. A micromachined actuator on a substrate, comprising: a
suspension system anchored to the substrate; and a body mounted to
the suspension system; wherein the suspension system comprises a
set of one or more spring flexures connecting the actuator body to
the substrate with strain relief provided via connecting torsional
elements.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/425,049 entitled Reduced Rotation MEMS
Deformable Mirror Apparatus and Method, and U.S. Provisional Patent
Application No. 60/425,051 entitled Deformable Mirror Method and
Apparatus Including Bimorph Flexures and Integrated Drive, both
filed Nov. 8, 2003.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to micro-fabricated actuators, and
more particularly relates to improving long-stoke deflection
characteristics.
[0004] 2. Description of the Related Art
[0005] The advent of micromachining has enabled the economic
fabrication of tiny precision micro-actuators and micromachines
using techniques first pioneered in the semiconductor industry.
Micro-fabricated actuators with long stoke are used in a diverse
range of applications including adaptive optics, disk drives,
fluidic valves, video displays, and micro-positioning.
[0006] Microfabricated actuators are often comprised of an
actuation means acting on a body or platform mounted to a substrate
via a flexible suspension. The suspension allows the actuator to
move while providing a restoring force that is a function of
deflection. The restoring force allows precise actuator positioning
at equilibrium points where the restoring force counter balances
the applied actuation force. The design requirements to ensure good
deflection characteristics for the actuator are manifold. The
suspension must be rigid enough so actuator natural frequency is
above the minimum needed for fast dynamic response. In addition,
the suspension must have enough rigidity to ensure robust
mechanical shock and vibration survival. On the other hand, the
suspension must be flexible to allow full scale deflection below
the maximum actuation force. As the actuator is deflected, the
suspension should not warp the body nor cause any excessive
extraneous motion that is not in the desired direction of
actuation. Finally, the suspension must be as compact as possible
to fit within a small footprint to reduce device area and hence
cost. A compact suspension is even more critical for tightly packed
arrays of actuators such as optical cross connects or deformable
mirror arrays.
[0007] Some devices require that a micromachined actuator move
substantially perpendicularly to the substrate in a piston motion
or move in a piston motion as well as rotating about the axes
substantially parallel to the substrate in a tip/tilt fashion.
Several designs have been invented in an attempt to provide
acceptable deflection characteristics for such devices, however all
previous solutions have serious drawbacks.
[0008] One field where micro-actuators are prevalent is adaptive
optics. Adaptive optics ("AO") refers to optical systems that adapt
to compensate for disadvantageous optical effects introduced by a
medium between an object and an image formed of that object. Horace
W. Babcock proposed the concept of adaptive optics in 1953, in the
context of mirrors capable of being selectively deformed to correct
an aberrated wavefront. As shown in the prior art FIG. 1, a typical
application adjusts the wavefront of incoming light 105 using a
deformable mirror 100 formed by an array of actuators so that the
outgoing light wavefront 110 has reduced aberrations. Numerous
actuators in the form of mirrors are tightly packed to form a
deformable mirror surface that locally alters light path length.
The full system to correct light wavefront aberration is shown in
FIG. 2. The light to be corrected 200 enters the device 205,
reflects off the deformable mirror 210 and is divided using a beam
splitter 220. One portion of the split light enters a wavefront
sensor 230 that detects aberrations. A wavefront reconstructor 235
and mirror controller are used to shape the deformable mirror to
remove light wavefront aberrations. The second portion of light
from the beam splitter 220 enters the science camera 225. The
correction performed using the deformable mirror improves the image
resolution of the science camera. See John W. Hardy, Adaptive
optics for astronomical telescopes, Oxford series in optical and
imaging sciences 16, Oxford University Press, New York, 1998.
Adaptive optics has a wide range of uses including correcting
telescopes for atmosphere turbulence, correcting ophthalmic images
for eye cornea distortions, and focusing laser.
[0009] Helmbrecht, in Micromirror Arrays for Adaptive Optics, PhD.
Thesis, University of California, Berkeley (2002), discloses a
segmented deformable mirror for use in AO applications that
exhibits high fill-factor and offers the potential for high mirror
stroke.
[0010] Early MEMS resonators and actuators, for example those
pictured in U.S. Pat. No. 5,025,346 Tang (1991), attempted to
achieve good deflection characteristics over large motions by using
folded beam structures exhibiting strain relief. In U.S. Pat. No.
6,091,050, Carr disclosed a similar folded beam technique using two
long bimorph flexures connected at one end forming a U-shaped
suspension. The first bimorph is anchored to the substrate with the
other end attached to the second bimorph. The second bimorph folds
back parallel to the first bimorph and attaches to an actuator
body. However, the folded suspension as documented has considerable
limitations that make it impractical in practice.
[0011] In summary, the prior art does not provide good deflection
characteristics for actuators moving substantially perpendicular to
a substrate.
SUMMARY OF THE INVENTION
[0012] It is therefore an object of the present invention to
provide an actuator that can be operated over large deflections
with minimal actuation force.
[0013] A further object of the invention is to provide an actuator
that requires lower actuation voltage to achieve deflection.
[0014] Another object of the invention is to provide an actuator
that that does not exhibit undesirable rotations or displacements
during actuated deflection.
[0015] A further object of the invention is to provide an actuator
that does not impact adjacent actuators in a tightly packed
array.
[0016] Yet another object of the invention is to provide an
actuator that does not significantly warp or misshape the actuator
body during large deflections.
[0017] Another object of the invention is to provide an actuator
that is space efficient to assure a small device area and the
ability to tightly pack large arrays of actuators.
[0018] A further object of the invention is to provide an actuator
that maximizes electrode area beneath the actuator body in the case
of electrostatic actuation.
[0019] Another object of the invention is to provide an actuator
that minimizes actuator body exposed to the actuation force of
adjacent actuators in the case of tightly packed actuator
arrays.
[0020] A further object of the invention is to provide an actuator
that does not have exposed high voltage interconnect to reduce to
reduce the potential for shorting due to particles or process
coatings.
[0021] Another object of the invention is to provide an actuator
suspension that exhibits less spring softening during electrostatic
actuation to reduce the problem of snap-in.
[0022] Still further objects and advantages will become apparent
from a consideration of the ensuing description and drawings.
[0023] The present invention, roughly described, pertains to a
micromachined actuator on a substrate. The actuator comprises a
suspension system anchored to the substrate and a body mounted to
the suspension system. In one aspect, the suspension system
comprises a set of one or more spring flexures connecting the
actuator body to the substrate with strain relief provided via
connecting torsional elements.
[0024] In one embodiment of the present invention, the suspension
system is comprised of at least one spring flexure connecting the
actuator body to the substrate and having strain relief provided
via torsionally weak attachments or torsional springs.
[0025] In the second embodiment of the present invention, the
suspension system includes a first set of one or more spring
flexures each with one end anchored to a largely rigid intermediate
frame and the other end attached to the actuator body. A second set
of one or more of flexures is attached between to the intermediate
frame and the substrate.
[0026] In the third embodiment of the invention the electrostatic
forcing electrodes are shaped to reduce drive voltage.
[0027] In a fourth embodiment of the invention is a buried
interconnect comprised of polysilicon conductive lines isolated
with silicon nitride to provide voltage to the force electrodes and
mirror suspensions.
[0028] In another embodiment, all four of the above embodiments
above may be combined such that all flexure connections to the
frame and body of the second embodiment are joined using the
torsional springs used in the first embodiment. Furthermore,
electrode area is maximized and polysilicon/nitride interconnect
are used.
[0029] In still another embodiment, the first, second and third
embodiments are combined and prior art polysilicon/oxide
interconnect or integrated circuitry is used to control actuation
electrode voltages.
[0030] Numerous other embodiments are possible by combining any
three or two of the embodiments to form new embodiments.
[0031] These and other objects and advantages of the present
invention will appear more clearly from the following description
in which the preferred embodiment of the invention has been set
forth in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 shows a prior art example of how an array of
actuators forming a deformable mirror may adjust light.
[0033] FIG. 2 shows a prior art example of how an array of
actuators forming a deformable mirror may be used in an optical
system.
[0034] FIG. 3 shows a partially explored perspective view of an
actuator in accordance with the first embodiment.
[0035] FIG. 4 illustrates a single flexure joined to an actuator
body using a simple torsional spring according to the present
invention.
[0036] FIG. 5 illustrates a serpentine flexure joined to an
actuator body using a serpentine torsional spring according to the
present invention.
[0037] FIG. 6 shows a top plan view of the second embodiment of the
present invention having an intermediate frame.
[0038] FIG. 7 shows a side view of the second embodiment of the
present invention having an intermediate frame.
[0039] FIG. 8 illustrates electrode shaping to reduce electrostatic
drive voltage.
[0040] FIG. 9 illustrates the fabrication layers used to fabricate
the invention with multiple polysilicon layers and nitride or oxide
isolation for interconnect, beams, and electrodes.
[0041] FIG. 10 illustrates the fabrication layers used to fabricate
the invention with a simple polysilicon layer for interconnect and
electrodes.
[0042] FIG. 11 illustrates the fabrication of actuators by
assembling micromachined components with integrated circuits.
[0043] FIG. 12 shows a simple bimorph cantilever to highlight
changes in angle and location of the free end.
[0044] FIG. 13 shows a table of torsional spring rigidity based on
beam dimensions and material properties of silicon.
DETAILED DESCRIPTION
[0045] A new micromachined actuator has been developed to provide
improved deflection characteristics for piston motion largely
perpendicular to a substrate and piston plus rotation about axes
substantially parallel to said substrate. The actuator body or
platform is mounted to a suspension system anchored to said
substrate. An actuation force acting largely on the body will cause
it to move. During actuation, the body will move to an equilibrium
displacement dictated by the flexibility of the suspension
system.
[0046] One embodiment of the present invention includes an actuator
having bimorph flexures that connect (electrically and
mechanically) to the actuator and elevate it above the substrate.
Although this actuator could be used for many applications, the
invention will be described with respect to its application in a
deformable mirror. A partially exploded perspective drawing of a
single actuator is shown in FIG. 3 in accordance with the present
invention. In the exemplary arrangement of FIG. 3, an actuator
segment 300, which is shown as a hexagon but could also be an
equilateral triangle, square, or any other shape, is disposed over
a platform 305 to which the actuator segment 300 is bonded. The
platform 305 may be mechanically and electrically attached to one
or more elevating bimorph flexures 310. In the exemplary
arrangement of FIG. 3 there are three such flexures 310 per
hexagonal actuator segment. In an alternative embodiment, the
platform 305 is not required and the actuator segment 300 is
attached directly to the flexures 310. In an exemplary arrangement
for the implementation shown in FIG. 3, the flexure is on the order
of two hundred microns in length and the actuator segment is on the
order of 210 microns on a side. Residual stress differences or
differences in the coefficient of expansion between the polysilicon
layer 306 and the silicon nitride layer 308 forming the bimorph
flexures will cause curvature of the flexures thereby lifting the
actuator segment to achieve an elevation on the order of twenty
microns above the substrate.
[0047] In an exemplary arrangement, the anchor points 303 are
torsionally strong to adhere the entire structure to the underlying
substrate 320. The set of attachments 307 to the actuator platform
are torsionally weak, so as to relieve angular strain caused by the
difference between the flexure 310 angle and the angle of the
platform 305. While other structures and arrangements of
torsionally strong and torsionally weak attachments are possible to
achieve the same or a similar effect, the above described exemplary
arrangement is simple, robust and effective. Other possibilities
include reversing the ends of the flexure to which the torsionally
weak and strong attachments are made or even placing a torsionally
weak element in the center of the flexure with both ends strongly
attached. For purposes of the present invention, in one aspect,
torsionally weak attachments have average angles of twist per unit
moment (.theta./Nm) of greater than about 7.00E+06, and torsionally
strong attachment have angles of twist per unit moment (.theta./Nm)
of less than about 2.5E+06. Other angles of twist and ranges
thereof are within the scope of the present invention.
[0048] FIG. 4 shows a simple torsional bar as the weak torsion
element. The strength can be adjusted by increasing length,
decreasing thickness normal to the page, or decreasing width
vertical to the page. Generally the thickness is set by process and
flexure design constrains, so length and width are the key
variables. A more area efficient design for the torsional
attachment is shown in FIG. 5 where the beam is folded to form a
serpentine. Indeed, a wide range of torsionally weak attachments
may be used, and the examples described herein should not be
considered limiting.
[0049] Referring again to FIG. 3, the actuator platform 305 can be
actuated in a piston motion largely perpendicular to the substrate
as well as a tilting rotation about the axes largely parallel to
the substrate. To achieve actuation, parallel plate drive
electrodes 315, one for each flexure in the arrangement of FIGS. 3,
actuate the hexagonal actuator segment by means of electrostatic
actuation. Typically the actuator platform and mirror segment are
held at a constant voltage potential by electrical connect through
the conductive flexures. Placing an equal voltage on all three
electrodes will provide purely piston translation, while applying
different voltages on each electrode will provide both piston
translation and rotation. The present invention provides an
actuator that operates with less than 100V, while maintaining large
stroke (displacement). Three actuators provide the ability to
control the three displacement variables; however, obviously fewer
or more electrodes could be used depending on the applications.
Furthermore, the use of electrostatic actuator is used as a an
example, but other actuation means including thermal, magnetic,
piezoelectric, pressure, mechanical, or any combination could be
utilized.
[0050] Turning next to FIG. 6, a second embodiment of the current
invention is presented for reduction of rotation due to both
elevation during manufacture and actuation is illustrated
schematically. A first set of bimorph flexures 601 is affixed to
the substrate 600 by anchor points 603. Next, the bimorph flexures
601 are connected to an intermediate frame 605 via an attachment
portion 607. Third, a second set of bimorph flexures 609 is
connected to the intermediate frame 605 via attachment portions
610. Fourth, the second set of flexures 609 is connected to the
actuator platform 612 or directly to the actuator segment itself
(not shown) via attachment portions 611.
[0051] The elevation/deflection of the bimorph flexures causes
their length projected on to the plane of the substrate to
contract. Deflection of the first set of bimorph flexures 601
induces the intermediate frame 605 to rotate clockwise. Likewise,
the deflection of the second set of bimorph flexures 609 induces a
counterclockwise rotation between the intermediate frame and the
actuator platform. This rotation is counter to the rotation of the
first set of bimorphs flexures 601. If the contractions are
designed to be equivalent, the actuator segment electrode 612 does
not rotate during elevation or actuation. FIG. 7 shows a side view
of the second embodiment to detail the elevation of the
intermediate frame and the actuator platform.
[0052] As with the first embodiment, actuation is achieved using
three electrodes 615 beneath the platform 611 to impart piston as
well as tilting rotation to the actuator. One of the three
electrodes can be seen in the side view of FIG. 7. A silicon mirror
segment 613 is attached to the top of the actuator to provide a
flat, reflective surface.
[0053] Another embodiment of the present invention minimizes the
voltage necessary for actuation through the shaping of the
actuation electrodes. For any actuator using electrostatic forcing,
minimizing the voltage required for actuation is desirable as high
voltage circuits are both expensive and complicated. From the
equation governing electrostatic attraction between two parallel
plate capacitors, those skilled in the art will recognize that for
a fixed actuation force (Fe), decreasing the applied voltage (V)
requires a decrease in the electrode gap (g), or an increase in the
electrode area (A). Since the gap (g) is fixed by the stroke
required of the actuator, the electrode area (A) must be maximized
to overcome the voltage decrease.
[0054] However, prior art actuator electrodes have been made to be
symmetric at the expense of total electrode area. Specifically, the
electrodes have been symmetric with respect to the line between the
electrode center of force and the center of the actuator device.
The symmetry produces tip and tilt motions that are likewise
symmetric and presumably easier to calibrate and control. On the
other hand, this has increased the voltage requirements for
actuation.
[0055] In a feature of the present invention, an asymmetric
electrode is contemplated which reduces the voltage necessary to
deflect the actuator. For the drive electrodes, increasing the area
is trivial as they can simply be made larger until they encroach on
the electrodes of the neighboring actuator or lie beneath regions
of the flexure that do not have enough clearance to protect against
snap-in. An adequate gap (>0.2 microns) must generally be left
between electrodes to prevent shorting. As an example, the
electrodes of first and second embodiment may be diamond shaped
using prior art, but will have a skewed non-diamond shape if
maximized using this feature of the invention. FIG. 8 illustrates
an asymmetric shaped electrode 315 that minimizes voltage without
encroaching on adjacent actuator electrodes or endangering flexure
snap-in for the first embodiment.
[0056] A fourth embodiment, shown in FIG. 9 of the present
invention provides interconnect to the forcing electrodes of an
actuator or an array of actuators. The invention allows
interconnect lines to be routed beneath exposed forcing electrodes
315 while providing compatibility with the processing required for
actuators. One or more interconnect lines made of polysilicon 905
are isolated from the substrate 320, other interconnect lines, and
the actuator structure by silicon nitride layers 903 and 907. Each
polysilicon electrical interconnect is encased in silicon nitride
except at via locations 919 that allow electrical connection to
polysilicon and actuator structural layers above.
[0057] In another embodiment, all four of the above embodiments
above are combined to maximum benefit. Referring to FIG. 6, the
anchoring to the substrate of the first set of flexures 601 and the
attachment points 610 of the second set of flexures 609 attached to
the intermediate frame 605 are rigid. Conversely, the attachment
points 607 of the first set of flexures 601 to the intermediate
frame 605 and the attachment points 611 of the second set of
flexures attached to the actuator platform 612 are both relatively
weak in torsion. This provides strain relief between the bimorph
flexures and both the intermediate frame and actuator platform.
While other structures and arrangements of torsionally strong and
torsionally weak attachments are possible to achieve the same or a
similar effect, the above described exemplary arrangement is used.
In addition, the electrode area is maximized as in embodiment three
and polysilicon/nitride interconnect are used as in the embodiment
of FIG. 9.
[0058] In still another embodiment, the first, second, and third
embodiments are combined. Numerous other embodiments are possible
by combining any three or two of the embodiments contained
herein.
[0059] The actuator embodiments described above can be fabricated
using many micromachining means. The structures may be fashioned
from polysilicon, single crystal silicon, germanium, metals,
conductive polymers, and many other options. In addition, the
actuator may be coupled to various electrical interconnection
layers or electrical circuitry to provide drive voltages. The
actuators may be fabricated directly atop interconnect and
circuitry, or may be fabricated separately and then assembled. An
exemplary fabrication method described herein builds a polysilicon
actuator on top of a polysilicon interconnect isolated with
nitride. Then a single crystal silicon hexagon mirror surface is
attached to the actuator using an assembly process.
[0060] The fabrication process proceeds by first forming the
electrical interconnection on a silicon wafer. The present
invention offers an improved design for connecting the drive
circuitry to the actuator forcing electrodes. In the present
invention, as shown in FIG. 9, the electrical connections to the
actuator segment electrodes are covered with an isolation layer
that is compatible with the subsequent actuator segment
fabrication. This is achieved by first depositing over a substrate
320 a silicon nitride (SixNy) passivation/isolation layer 903,
followed by an in-situ doped polysilicon layer 905 on top of the
silicon wafer (using low pressure chemical vapor deposition). The
polysilicon is then photolithographically patterned using a plasma
etch to define the electrical connection lines. Thus the
polysilicon may be thought of as a wiring layer. A second SiN
passivation layer 907 is then deposited by LPCVD to encase the
conductive polysilicon lines in an electrically isolating medium.
In one embodiment, a low-stress, non-stoichiometric silicon nitride
layer is used, however in theory, any isolating, HF resistant thin
film would work. The SiN layer 907 is then photolithographically
patterned and etched to form vias 919 to the electrically
conductive polysilicon lines 905. A second polysilicon layer is
then deposited and patterned to form the actuation electrodes 315
and provide an electrical connection between the bimorph flexures
and the wiring layer 905. Next, a sacrificial silicon oxide is
uniformly deposited over the entire wafer (not shown).
Photolithography and subsequent etching form holes through the
sacrificial oxide that allow electrical connection to the flexures
described next. The interconnection polysilicon layer are typically
0.25 to 4 micrometers in thickness.
[0061] The mechanical actuator fabrication can now continue with
polysilicon deposition and photolithographic patterning of the
first bimorph flexure layer 911 and actuator platform 305 (both
typically formed from the same polysilicon layer). The vias 919 in
the SiN layer together with the polysilicon connections 905 provide
the electrical connection between the drive electronics and
actuation electrodes. At this point, the second layer 913 of the
bimorph flexures is deposited and patterned such that it covers
only the portions of the flexures that are to exhibit curvature to
elevate the platform. The second bimorph material may be metal or
other materials but in one embodiment is silicon nitride. The
deposition of both layers of the bimorph is engineered to ensure
the differences in residual stress or differences in thermal
expansion create the desired curvature in the finished device.
Structural polysilicon and silicon nitride layers are typically
0.25 to 4 micrometers in thickness.
[0062] Next, a single crystal silicon mirror segment 300 is
attached to the underlying polysilicon actuator platform. This
segment is fabricated by deep reactive ion etching (DRIE) the top
layer of a silicon-on-insulator (SOI) wafer. The segment may be on
the order of 5-30 micrometers in thickness. The single crystal
segment is attached using solder bump formation and flip-chip
bonding well known in the art. After bonding, the SOI handle wafer
is removed and HF release removes all exposed silicon oxide thereby
allowing the segment to be elevated by the bimorph flexures. At
this point, optional optical coatings such as aluminum, gold, or
dielectrics may be applied to the mirror top surface.
[0063] Many other fabrication methods are available for building
the present invention. Two examples are a process that does not
have buried interconnect as shown in FIG. 10 and a fully integrated
process shown in FIG. 11. In the first case, the polysilicon
interconnect with silicon nitride isolation is not undertaken.
Instead the process begins with a silicon nitride layer on a
silicon wafer and continues with the electrode 315 polysilicon
layer. The process of FIG. 11 assembles mechanical element onto a
circuit substrate 1120 to form the actuator and drive electronics.
The mechanical element is fastened to the circuit element by a
bonding layer 1110. A barrier layer 1125 protects the circuits from
the etching process used to release the actuator from a handle
wafer as commonly used in the art.
[0064] Alternative embodiments may be created by forming a large
cavity beneath the actuator rather then elevating the actuator
using bimorph flexures. In this case, the actuation electrodes
would be positioned at the bottom of a cavity, perhaps 20
micrometers deep. The actuator would be mounted to the top rim of
the cavity by the flexures without the second bimorph layer being
applied. In the absence of actuation voltage, the actuator would
remain largely level with the rim of the cavity. When voltage is
applied to the actuation electrodes, the actuator platform would
deflect down into the cavity. This method would require a very
thick, perhaps more than 20 micrometer, sacrificial layer or could
be fabricated by assembly. Most of the advantages described for the
bimorph embodiment would also apply to this cavity embodiment.
[0065] The operation of each of the embodiments is similar. In each
case, bimorph flexures curve upward away from the substrate thereby
elevating the actuator platform and mirror segment. The flexures
provide a flexible suspension upon which the actuator platform can
be moved in translation substantially perpendicular to the
substrate or tilted in rotation about the two axes substantially
parallel to the substrate. The actuation is accomplished using
three high voltage electrodes placed beneath the actuator
platform.
[0066] FIG. 12 illustrates key features of the bimorph flexures
that will aid in understanding the advantages of this invention. An
isolated bimorph with one end anchored and the other end free is
depicted. Note that the free end is not parallel to the substrate
but rather at an angle. In contrast, FIG. 3 shows that both ends of
the bimorph flexures 310 to be parallel to the substrate 320 in the
complete actuator. Hence, there must be a moment on the end of the
beam to force the angle from the free position. This moment can
warp the platform and the mirror segment if the strain is not
relieved. Furthermore, the moment acts against the elevation force
of the flexures thereby reducing achievable height from the
substrate. Finally, this is a spring hardening geometry, so the
suspension is nonlinear and softens with higher actuation voltage
thus invoking earlier snap-in during electrostatic actuation. All
of these drawbacks may be circumvented by the present invention in
the first embodiment. Namely, the torsionally weak attachment
points or torsional springs 307 on the flexures allow the flexure
ends to have an angle not parallel to the substrate. This strain
relief reduced platform warpage, increases stoke height, and
reduces spring softening during actuation.
[0067] The stiffness of the attachment portion is a function of the
material and beam dimensions including height 1301 (h), length 1303
(i), and film thickness (t) as defined in FIG. 13. In an exemplary
arrangement, the attachment portions are made of polysilicon. As
discussed above, if the stiffness of the attachment portion is too
great, it will decrease the overall deflection/elevation of the
bimorph flexures and cause unwanted bowing of the actuator segment
electrode. Therefore, the present invention provides attachment
portion designs with lower stiffness. For purposes of the following
discussion, the properties of the attachment are characterized in
terms of the angle of twist per unit moment (.theta./Nm), which is
the inverse of the torsional rigidity (Nm2/.theta.) over the
attachment height (h). These characteristics may be better
appreciated from the table of FIG. 13, which provides examples of
selected values.
[0068] In one arrangement, it is desirable for the angle of twist
per unit moment for the attachment to be greater than about
7.0.times.106 .theta./Nm.
[0069] The thickness (t) of the attachment is the most physically
constrained, and it is difficult to make a polysilicon attachment
less than about 0.25 .mu.m and greater than about 4 .mu.m. Below
0.15 .mu.m, polysilicon attachments simply are not strong enough to
support the actuator segment during actuation. Above 4 .mu.m, the
fabrication process is rendered too complex, time-consuming and
expensive. The table of FIG. 13 shows some acceptable values for i
and h, given a t value of 1.10 microns, and the corresponding
acceptable angle of twist per unit moment. Within the range of 0.25
.mu.m to 4 .mu.m for t, embodiments of the present invention for i
have to be at least 2 .mu.m but not greater than 20 .mu.m, and h
have to be at least 2 .mu.m but not greater than 40 .mu.m.
[0070] Another important point notable in FIG. 12 is that the
horizontal distance from the anchor point to the free end shrinks
during elevation. When this happens with the first embodiment of
the invention of FIG. 3, the platform will rotate clockwise (as
viewed when looking down at the substrate) due to the change in
projected horizontal distance of the flexures 310. In many
applications with tight spacing requirements such as deformable
mirror arrays or optical crossconnect switches, this rotation can
be detrimental as adjacent actuators could touch. Note that
flipping some flexures to extend clockwise and others
counterclockwise causes spring hardening and does not remove the
rotation for odd numbers of flexures. The second embodiment negates
this draw back by using two sets of flexures and an intermediate
frame. The example in FIG. 6 shows the flexures are designed so the
intermediate frame 605 rotates clockwise while the actuator
platform 612 rotates counterclockwise. If designed properly, these
rotations cancel out leaving the mirror without rotation. Combining
the advantages of the first and second embodiment provides the
maximum benefit. In all cases, the suspension are compact to reduce
device area and maximize electrostatic actuation electrode
area.
[0071] The present invention has several features to reduce
actuation voltage requirements. In the first and second
embodiments, because the attachment points are weak, less bimorph
force is needed to elevate the actuator platform to the required
stroke height. This reduces the level of electrostatic force needed
to deflect the bimorph springs downward. The rather compact flexure
suspension designs allow greater area beneath the platform to be
used for actuation electrodes. Electrostatic force increases with
increased area, so the voltage required decreases. This is taken
fully advantage of in the third embodiment by using electrodes that
maximize area regardless of asymmetry. Another advantage is the
novel interconnect of embodiment four that uses polysilicon signal
lines and silicon nitride isolation to allow routing of drive lines
in a layer beneath the forcing electrodes. Hence, electrode area is
not compromised by interconnect and actuators are not exposed to
the control voltages of adjacent mirrors. Finally, the buried
interconnect reduces the chance that particles and fabrications
coatings may cause shorts in the electrical lines.
[0072] The present invention overcomes many of the drawback
plaguing prior art. The design provides a simple, manufacturable,
and economical method for fabricating micromachined actuators with
large stroke and solid robustness. New suspension designs increase
the stroke height of a bimorph suspension and reduce the actuation
voltage required to achieve the full stroke. The effects of spring
softening are also reduced for more stable electrostatic actuation.
The novel suspensions reduce actuator platform deformation and
alleviate unwanted rotation of the actuator about the axis
substantially perpendicular to the substrate. Negating the rotation
is particularly important for applications with tightly packed
actuators. The need for compactness to reduce device area and allow
tight packing is also fulfilled. The compact suspensions and new
silicon/silicon nitride interconnect allow more area for
electrostatic force electrodes thereby reducing actuation voltage.
This is further improved by using non-symmetrical forcing
electrodes that maximize electrode area. The buried interconnect
ensures reduced cross coupling between mirrors because drive lines
are not expose to adjacent mirrors. Finally, the possibility of
shorting due to particles and process coating is reduced by the use
of buried interconnect.
[0073] It will be appreciated by those skilled in the art that
exemplary embodiments of the invention have been described and are
illustrated in the accompanying drawings. While the invention has
been described in conjunction with these specific embodiments, it
will be understood that the invention is not be limited to these
embodiments but instead covers alternatives, modifications, and
equivalents as are within the spirit and scope of the invention.
For example the actuator body may be square, triangular, rhombic,
or any other shape; more or fewer than three flexures may be used;
various fabrication materials may be used; various fabrication
process sets may be used; the weak and strong torsional elements
may be in different locations along or at the ends of the flexures;
torsionally weak elements my be straight, serpentine, or other
forms; actuation may be electrostatic, magnetic, piezoelectric,
thermal, via pressure, and mechanical means.
[0074] While numerous specific details have been set forth in order
to provide a thorough understanding of the present invention,
numerous aspects of the present invention may be practiced with
only some of these details. In addition, certain process operations
and related details which are known in the art have not been
described in detail in order not to unnecessarily obscure the
present invention.
[0075] The foregoing detailed description of the invention has been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise
form disclosed. Many modifications and variations are possible in
light of the above teaching. The described embodiments were chosen
in order to best explain the principles of the invention and its
practical application to thereby enable others skilled in the art
to best utilize the invention in various embodiments and with
various modifications as are suited to the particular use
contemplated. It is intended that the scope of the invention be
defined by the claims appended hereto.
* * * * *