U.S. patent application number 11/360165 was filed with the patent office on 2007-08-23 for thermal and intrinsic stress compensated micromirror apparatus and method.
This patent application is currently assigned to Rockwell Scientific Licensing, LLC. Invention is credited to Robert L. III Borwick, Jeffrey F. DeNatale, Philip A. Stupar, Chialun Tsai.
Application Number | 20070195439 11/360165 |
Document ID | / |
Family ID | 38427930 |
Filed Date | 2007-08-23 |
United States Patent
Application |
20070195439 |
Kind Code |
A1 |
DeNatale; Jeffrey F. ; et
al. |
August 23, 2007 |
THERMAL AND INTRINSIC STRESS COMPENSATED MICROMIRROR APPARATUS AND
METHOD
Abstract
A micromirror apparatus includes a device layer having a recess,
a multilayer thin-film dielectric reflector coupled to and
structurally supported by the device layer on the opposite side of
the device layer from said recess, and a stress compensator seated
in the recess, with the stress compensator operable to resist
device layer bending moments resulting from intrinsic and thermal
mismatch stresses between the multilayer thin-film dielectric
reflector and the device layer.
Inventors: |
DeNatale; Jeffrey F.;
(Thousand Oaks, CA) ; Stupar; Philip A.; (Oxnard,
CA) ; Tsai; Chialun; (Thousand Oaks, CA) ;
Borwick; Robert L. III; (Thousand Oaks, CA) |
Correspondence
Address: |
KOPPEL, PATRICK & HEYBL
555 ST. CHARLES DRIVE
SUITE 107
THOUSANDS OAKS
CA
91360
US
|
Assignee: |
Rockwell Scientific Licensing,
LLC
|
Family ID: |
38427930 |
Appl. No.: |
11/360165 |
Filed: |
February 22, 2006 |
Current U.S.
Class: |
359/871 |
Current CPC
Class: |
G02B 7/181 20130101;
G02B 26/0841 20130101 |
Class at
Publication: |
359/871 |
International
Class: |
G02B 7/182 20060101
G02B007/182 |
Claims
1. A micromirror apparatus, comprising: a device layer having a
recess; a multilayer thin-film dielectric reflector coupled to and
structurally supported by said device layer on the opposite side of
said device layer from said recess; a stress compensator seated in
said recess, said stress compensator operable to resist device
layer bending moments resulting from intrinsic and thermal mismatch
stresses between said multilayer thin-film dielectric reflector and
said device layer.
2. The apparatus according to claim 1, wherein said stress
compensator comprises a second multilayer thin-film dielectric
reflector.
3. The apparatus according to claim 1, further comprising: a
flexure extending from said device layer.
4. The apparatus according to claim 3, wherein said flexure
comprises a thinned extension of said device layer.
5. The apparatus according to claim 3, further comprising: a
mechanical support connected to said flexure, said flexure enabling
movement of said device layer relative to said mechanical
support.
6. The apparatus according to claim 5, further comprising: a
support column coupled to said mechanical support; a supporting
substrate coupled to said support column, said supporting substrate
spaced adjacent to said stress compensator.
7. The apparatus according to claim 6, wherein said supporting
substrate comprises and a first electrode.
8. The apparatus according to claim 7, wherein said device layer
comprises a second electrode, so that application of a voltage
differential between said first and second electrodes results in
movement of said multilayer thin-film dielectric reflector.
9. The apparatus according to claim 6, wherein said support column
comprises gold (Au).
10. The apparatus according to claim 1, wherein said device layer
comprises an electrode.
11. The apparatus according to claim 1, wherein said device layer
comprises a material selected from the group consisting of
single-crystal silicon and polysilicon.
12. A micromirror apparatus, comprising: a device layer; first and
second multilayer thin-film dielectric reflectors carried on
opposite sides of said device layer, said second multilayer
thin-film dielectric reflector seated in said device layer and
having a common linear thermal expansion coefficient with said
first multilayer thin-film dielectric reflector to reduce warping
of said device layer in response to intrinsic and thermal mismatch
stresses between said first multilayer thin-film dielectric
reflector and said device layer.
13. The apparatus according to claim 12, further comprising a
flexure extending from said device layer.
14. The apparatus according to claim 13, further comprising: a
flexure support connected by said flexure to said device layer,
said flexure enabling movement of said device layer relative to
said flexure support.
15. The apparatus according to claim 14, further comprising: a
support substrate coupled to said flexure support, said support
substrate comprising active control circuitry to provide actuation
of said first multilayer thin-film dielectric reflector.
16. The apparatus according to claim 14, further comprising: a
support substrate coupled to said flexure support, said support
substrate comprising active control circuitry to provide
electrostatic actuation of said first multilayer thin-film
dielectric reflector.
17. The apparatus according to claim 12, wherein said support
substrate further comprises an electrode.
18. A micromirror array, comprising: a plurality of micromirror
structures, each structure comprising: a device layer; a multilayer
thin-film dielectric reflector coupled to and structurally
supported by said device layer; a stress compensator seated in the
opposite side of said device layer from said multilayer thin-film
dielectric reflector, said stress compensator operable to resist
device layer bending moments resulting from intrinsic and thermal
mismatch stresses between said micromirror and said substrate; and
respective flexures extending from said device layers.
19. The array of claim 18, wherein each structure further
comprises: respective supports connected to said flexures, said
flexures enabling movement of said device layers relative to said
supports.
20. The array of claim 18, further comprising: respective
electrodes positioned to actuate movements of corresponding device
layers in response to applied voltages between said electrodes and
their respective device layers.
21. The array of claim 18, wherein at least one of said plurality
of micromirror structures further comprises said multilayer
thin-film dielectric reflector and said stress compensator sharing
approximately equal linear thermal expansion coefficients to reduce
warping of said respective device layer.
22. The array of claim 18, further comprising: a base substrate
which provides support for each of said electrodes.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to micromirrors, and more
particularly, micromirrors used in micro electrical mechanical
systems (MEMS) devices.
[0003] 2. Description of the Related Art
[0004] Micromirrors are used in a variety of consumer and
industrial devices, including wavefront correction arrays, digital
projection displays and fiber optic switching. For example,
micromirrors in digital light processing (DLP) televisions are used
to turn light to the projection screen on and off at the pixel
level to form a projected image. In fiber optic switches,
micromirrors are used to steer light from one fiber to another for
reconfigurable signal routing. In wavefront-correction arrays,
micromirrors are translated relative to one another to correct for
wavefront distortion in a propagating optical wave.
[0005] In general, it is desirable to have a micromirror reflect
light with high efficiency and high fidelity. This imposes two
common and desirable design characteristics on the micromirrors
used in such applications: high reflectivity at the operating
wavelength and high optical figure, otherwise known as mirror
flatness. To achieve high reflectivity, reflective metal films are
often deposited onto the microfabricated MEMS mirror.
Unfortunately, intrinsic stress associated with the thin film
deposition and thermal stresses arising from differences in
coefficients of thermal expansion may compromise the mirror
flatness for such micromirror assemblies. For example, some
micromirrors incorporate deposited metal layers on a mechanical
support microfabricated from materials such as polysilicon or
single crystal silicon. Intrinsic stresses created during
deposition and subsequent coalescence of the metal layers may
result in deformation of the mirror structure. Thermal stresses
introduced by differential expansion of the reflective and support
layers, respectively, when introduced to environmental heating and
cooling, may similarly result in mirror deformation. The problem is
exacerbated as thinner structural supports are used for the mirror
surface to accomplish quicker micromirror response.
[0006] A number of solutions exist for addressing the intrinsic and
thermal mismatch stresses in micromirror assemblies that may lead
to loss of mirror flatness. To minimize thermally induced
distortion, constraints on the operating temperature of the device
may be imposed. This adds considerable system-level complexity and
associated cost. Similarly, the deformation induced by the thin
film layer stresses may be reduced by measures such as reducing the
thickness of the reflective metal film, reducing the lateral size
of the micromirror itself to reduce the bending moment caused by
the stress, or by tailoring the stresses in the metal layers used
for the micromirror surface to achieve a stress-neutral state. In
another solution, a double-layered metallization is used to deposit
the same metallization in exactly the same thickness onto both the
top and bottom surface of the mirror support, so that the
metallization-induced stresses are balanced. (See U.S. Pat. No.
6,618,184). In yet another solution, a stress-balancing layer is
formed on a side of the mirror support opposite to that of the
light reflective optical layer, with the stress-balancing layer
being the same material or a different material as the light
reflective optical layer. (See U.S. Pat. No. 6,639,724)
[0007] Unfortunately, for some micromirror applications, such as
high-intensity projectors or those subject to illumination by
moderate-to high-energy lasers, the thin metal reflective layers
may not have sufficient optical durability. The ability to use
thicker metal reflective layers would improve the robustness and
reliability of the micromirrors relative to those using thin metal
layers. The thicker metal layers would, however, impose greater
stress-induced deformation to the mirror relative to the thin
layers. Similarly, micromirrors used in these high-intensity
applications would benefit from the lower energy absorption (higher
reflectivity) provided by non-metallic, multilayer thin-film
dielectric mirrors. These multilayer dielectric reflectors may be
quite thick, however, and may similarly exacerbate the
stress-induced deformation of the micromirror. In those
applications, reducing the thickness of the micromirror surface to
reduce stress-induced deflection of the entire assembly is not
possible without degrading the mirror's performance in the
wavelength band of interest. Also, further reduction in reflecting
area of the micromirror to reduce warping introduces manufacturing
challenges for the typically thick, multi-layer dielectric
mirrors.
[0008] A need exists, therefore, for a structure and method to
reduce the deformation of micromirrors incorporating thick or
complex optical coatings such as dielectric reflectors induced by
intrinsic and thermal stresses without requiring a reduction in
reflecting area of such micromirrors.
SUMMARY OF THE INVENTION
[0009] A micromirror apparatus is disclosed for use in micro
electrical mechanical (MEMS) devices. It has a device layer having
a recess, a multilayer thin-film dielectric reflector coupled to
and structurally supported by the device layer on the opposite side
of the device layer from said recess, and a stress compensator
seated in the recess, with the stress compensator operable to
resist device layer bending moments resulting from intrinsic and
thermal mismatch stresses between the multilayer thin-film
dielectric reflector and the device layer.
[0010] A micromirror apparatus is also disclosed that has two
multilayer thin-film dielectric reflectors carried on opposite
sides of the device layer with the second reflector seated in the
device layer. Each of the reflectors shares a common linear thermal
expansion coefficient to reduce warping of the device layer in
response to intrinsic and thermal mismatch stresses between the
first reflector and the device layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The components in the figures are not necessarily to scale,
emphasis instead being placed upon illustrating the principals of
the invention. Like reference numerals designate corresponding
parts throughout the different views.
[0012] FIG. 1 is a perspective view of a stress-compensated
micromirror in accordance with one embodiment of the invention;
[0013] FIG. 2 is a top plan view of the embodiment of the invention
illustrated in FIG. 1.
[0014] FIG. 3 is a cross-section view of the embodiment of the
invention shown in FIG. 2 along the line 3-3.
[0015] FIGS. 4A-4E are cross-sectional views illustrating various
stages of fabrication of the embodiment of the invention
illustrated in FIG. 3;
[0016] FIG. 5 is a perspective view of an array of
stress-compensated micromirrors in accordance with an embodiment of
the invention; and
DETAILED DESCRIPTION OF THE INVENTION
[0017] A micromirror device is described that compensates for
intrinsic and thermal mismatch stresses without resorting to
disadvantageous reduction in the mirror's reflecting area or
reflecting surface thickness. A stress compensator is seated in the
support of a multilayer thin-film dielectric reflector (a "device
layer") on a side opposite to that of the reflector. Mismatch
stresses created between the stress compensator and the device
layer are approximately equal to those mismatch stresses created
between the multilayer thin-film dielectric reflector and the
device layer, creating opposite bending moments and resulting in
improved micromirror flatness both as-fabricated and during
subsequent thermal environmental changes.
[0018] In one embodiment of the invention illustrated in FIG. 1, a
micromirror assembly 100 has a device layer 105 providing
structural support for a highly reflective micromirror reflective
layer 110. The device layer 105 is preferably single-crystal
silicon (Si), but can be any of a variety of materials used for the
structural layers in a MEMS device, such as polysilicon, metals, or
dielectric thin films, to allow fabrication of the micromirror
assembly in standard surface or bulk micromachining processes. The
micromirror reflector layer 110 is preferably a thin film reflector
built up from multiple layers of dielectric materials. Highly
reflective multilayer interference coatings comprised of dielectric
thin films are well known, with the materials and thicknesses of
the layers selected to achieve particular optical performance
characteristics. Other materials and approaches can similarly be
adopted to achieve the highly reflective surface. These may include
thin or thick metal layers (individually or in combination, such as
Au, Ag, Au/Ag) or metal-containing compounds (such as hydrides,
nitrides, silicides, carbides, etc.) The device layer 105 provides
a substantially planar structural support for the micromirror
reflective layer 110.
[0019] Flexures 115 are connected to the device layer 105 and
ultimately to a rigid base substrate 130 (connection not shown)
(otherwise referred to as a "support substrate") to enable
mechanical movement of the micromirror relative to the base
substrate 130. These compliant flexures are typically designed to
achieve particular mechanical characteristics, such as mechanical
stiffness and resonant frequencies, commonly dictated by the
application and other elements of the complete micromirror
assembly. For the embodiment of a micromirror device illustrated in
FIG. 1, which uses single crystal Si as the structural support for
the reflective layer 100, the flexures 115 are formed from the Si
device layer 105, and may further be thinned in an etching step to
provide greater mechanical compliance than what would otherwise
exist without such thinning. The flexures 115 allow elastic
coupling of the micromirror assembly 100 to a fixed flexure support
at a distal end 117 of the flexures 115. The fixed flexure support
is preferably defined by a continuation of the etched device layer
105 (see 205, below) and flexures 115 and is itself coupled to the
base substrate 130 (connection not shown).
[0020] A stress compensator, preferably a stress compensator layer
120, is formed on a side opposite to the reflector layer 110 and is
formed partially seated in the device layer 105 to reduce the
height of that portion of stress compensator layer 120 extending
from the surface of the device layer 105. The material of the
stress compensator layer 120 is preferably substantially similar to
the material of reflector layer 110 so that a stress induced in the
device layer 105 (which would also introduce a bending moment in
such layer 105) as a result of intrinsic and/or thermal mismatch
stresses between the reflector layer 110 and device layer 105 is
opposed by approximately equal intrinsic and/or thermal mismatch
stresses between the stress compensator layer 120 and device layer
105. More particularly, the reflector layer 110 and stress
compensator layer 120 preferably have equal linear thermal
expansion coefficients to accomplish the function of balanced
thermal mismatch stresses. Similarly, the intrinsic stress
associated with the stress compensator layer 120 should be
substantially similar to that of the reflector layer.
[0021] Preferably, the stress compensator and reflector layers 120,
110 would have identical lateral dimensions, permitting identical
3D layer structures to be used for stress balancing. Differences
between the lateral dimensions of the two may be required by the
specific micromirror device design or process used, and in these
cases slight differences in compensator thickness or fabrication
process parameters may be used to accomplish a substantially
similar intrinsic stress as that generated by the reflector layer.
Similarly, for the thermal expansion stresses, the stress
compensator layer 120 may accomplish the same function if composed
of approximately the same volume fraction of the component
materials as the reflector layer but may require different
thicknesses to achieve the same thermal expansion stress as a
function of temperature. For example, although illustrated in FIG.
1 as having the same dimensions as the reflector layer 110, the
stress compensator layer 120 may have a greater thickness with a
shorter lateral length or width to accomplish comparable stress
characteristics as exists with the reflector layer 110. Similarly,
the stress compensator layer 120 may be thinner than the reflector
layer 110, but may have a greater width or length or both to
accomplish comparable stress characteristics.
[0022] Also, although labeled as a "layer", if the reflector layer
110 is a thin film reflector, the stress compensator layer 120
would preferably be formed from multiple layers of dielectric
materials similar to the reflector layer 110 to accomplish a
comparable structure, material distribution and total thickness as
exists with the reflector layer 110.
[0023] An electrode 125 sits on the base substrate 130 and is
spaced apart from and in complementary opposition to the device
layer 105. The electrode and device layer 105 function as
electrically isolated counterelectrodes. Upon application of a
voltage differential between them, the micromirror assembly 100
will mechanically deflect in an essentially vertical fashion with
respect to the electrode 125. Upon removal of the voltage
differential, the micromirror assembly 110/105/120 returns to its
resting position by virtue of release of elastic energy stored in
the compliant flexures. In the preferred embodiment, the mechanical
deflection of the micromirror is accomplished using electrostatic
actuation, although alternate actuation methods, such as thermal,
piezoelectric, electromagnetic, Lorentz force, or others may be
used without limitation within the scope of the invention. While
the specific example described above is for a vertical-motion
(piston) device, the present invention can apply equally well to a
micromirror designed for one-axis tilt, two-axis tilt, or combined
piston-tilt operation without limitation.
[0024] The plan view of FIG. 2 illustrates one implementation of
the embodiment of the invention shown in FIG. 1. Flexures 115 are
coupled on their distal ends 117 to a second portion of the device
layer 205 for fixed structural support. The reflector layer 110
sits on a central portion of the device layer 105. During
operation, the device layer 105 and reflector layer 110 are
operable to translate together with respect to the second portion
of the device layer 205 by means of compliant flexing between
proximal and distal ends 210, 117 of the flexures 115. Although the
flexures 115 are illustrated as having a rectangular cross section
extending parallel with the device layer 105, they may be formed in
a curved, perpendicular or other spatial arrangement with respect
to the device layer second portion 205.
[0025] FIG. 3 illustrates one embodiment of a cross section taken
along the line 3-3 in FIG. 2. The reflector layer 110 and stress
compensator layer 120 are seated on opposite sides of the device
layer 105. In the implementation of FIG. 3, the stress compensator
layer 120 is seated in the recess 122 in the device layer to reduce
the potential for mechanical impediment caused by the stress
compensator layer translation within the gap between the base
substrate 310 and device layer 105, the gap established in this
embodiment by spacer layers 315 and 320. Complex multilayer
coatings preferably used by the stress compensator and reflective
layers 120, 110 may be characterized by significant physical
thicknesses comparable to the dimensions of the device layer 105
and the gap distance between the device layer 105 and the electrode
125, and hence the recessing of the stress compensator layer 120
may be a critical element in preserving electromechanical
functionality of the micromirror assembly 100. In the specific
embodiment shown in FIG. 3, the recess 122 would be preferably
formed by dry etch processes, although this feature could be formed
by a variety of process methods known to those skilled in the art,
and the preferred method of forming said recess would depend on the
specific design and fabrication process chosen for the micromirror
assembly. In one embodiment of the invention, flexures 115 are not
thinned with respect to device layer 105 resulting in greater
rigidity than what would otherwise exist with thinned flexures.
[0026] In a preferred embodiment, the mechanical deflection of the
micromirror is accomplished using electrostatic actuation, although
alternate actuation methods, such as thermal, piezoelectric,
electromagnetic, Lorentz force, or others may be used without
limitation within the scope of the invention.
[0027] A substrate electrode 305 is formed preferably on, or in,
the base substrate 130 by conductive thin films, such as metals, or
by suitably doping the substrate material to sufficiently low
resistivity. In one embodiment, an insulating layer 310 sits on the
substrate electrode layer 305 (not shown in FIG. 1) to prevent
electrical shorting of the counter electrodes, should they come
into physical contact. Such electrical isolation may be
accomplished by other techniques known to those skilled in the art,
such as the use of insulating standoffs. Alternate embodiments may
have independently-addressable micromirror assemblies spaced in an
array, which require electrically isolated bottom addressing
electrodes. In such case, the bottom electrode would be
electrically isolated from the device layer using dielectric
layers. In the construction of the micromirror assembly 100,
mechanical interconnection of the movable device layer 105 and
stationary base substrate 130 is accomplished using the spacer
layers 315, 320 as bonding elements. These simultaneously serve to
establish the dimension of the gap between the movable and
stationary elements of the assembly, and can also be used to
accomplish electrical interconnection between the device layer 105
and electrical addressing connections on base substrate 130 (not
shown). Such bonding can be accomplished by a number of different
methods known to those skilled in the art. For the embodiment shown
in FIG. 3, Au--Au thermocompression bonding is preferred. These
mechanical bonding pads 315, 320 couple the device layer 105 to the
oxide layer 310 such that the stress compensator layer 120 is
positioned adjacent and opposite to the electrode 125. In the
embodiment illustrated in FIGS. 1-3, the various elements of the
micromirror assembly 100 have the approximate thickness and widths
listed in Table 1. These values are exemplary only, and are not
intended to limit the scope of the invention. TABLE-US-00001 TABLE
1 Thickness (.mu.m) Width (.mu.m) Length (.mu.m) Device layer 20
100-400 100-400 105 Micromirror 110 5-50 100-400 100-400 Flexures
115 1-20 1-50 30-200 Stress 5-20 100-400 100-400 compensator layer
120 Electrode 125 1-20 1-400 100-400 Recess 122 5-20 105-395
105-395 Base substrate 400-800 100-400 100-400
[0028] While the specific embodiments described in FIGS. 1-3
incorporated electrostatic actuation to accomplish the mechanical
displacement of the micromirror assembly, the present invention
could equivalently employ alternate actuation mechanism within the
scope of the invention. Such approaches may include, without
limitation, thermal, piezoelectric, electromagnetic, Lorentz Force
actuation as is known by those skilled in the art.
[0029] The use of multiple layers of dielectric thin films to
create transmissive or reflective optical devices such as that
preferably used by the reflective surface 110 is well known, and
described in references such as: "Thin-Film Optical Filters, Third
Edition", by H. Angus Macleod. IoP, 2001, or "Optical Interference
Coatings," by Norbert Kaiser and H. K. Pulker, Editors. Springer,
2003.
[0030] Selection of the materials, thicknesses, and stacking
sequences of these layers, provides great design flexibility in
tailoring the optical response characteristics (for example
transmission or reflection as a function of wavelength) of the
device. One well-known multilayer dielectric stack configuration
uses alternating layers of material with high optical index of
refraction and low index of refraction, each at a thickness of
one-quarter wavelength optical thickness at the desired operating
wavelength. This layer structure will create a high optical
reflectance at the design wavelength. In this structure, a
quarter-wave optical thickness of high index material is denoted H
and a quarterwave optical thickness of low index material is
denoted L. The multilayer device structure of this device, referred
to as a quarterwave stack, may be described by the notation:
Incident medium/(HL).sup.NH/substrate (1) Where (HL).sup.N denotes
N sequential pairs of quarterwave layers of the high and low-index
materials. These devices are described in the references noted
above. A key consideration in the implementation of these devices
is that the level of the reflectance at the design wavelength
increases with the number of sequential pairs, N.
[0031] Another type of optical thin film device is the gradient
index, or rugate, filter. In these devices, sinusoidal variations
in optical index of refraction as a function of thickness are
created in the thin film structure. Fabrication processes such as
controlled co-deposition of high-and low-index material or
sequential deposition of digital approximations can be used to
accomplish the desired index profiles. These devices are
characterized by high levels of reflection over a narrow wavelength
range, and are described in: [0032] 1. Rugate notch filters find
use in laser-based applications, George Minott, Robert Sprague
Boris Shnapir Way, Laser Focus World September, 2004. [0033] 2.
Rugate Filter Design: The Modified Fourier Transform Technique, B.
G. Bovard, Appl. Opt. 29, 24 (1990). [0034] 3. Fourier Synthesis of
Multilayer Filters, E. Delano, J. Opt. Soc. Am. 57, 1529, (1967).
[0035] 4. Using Apodization Functions to Reduce Sidelobes in Rugate
Filters, W. H. Southwell and R. L. Hall, Appl. Opt. 28, 5091
(1989).
[0036] The level of reflectance of the rugate filter at the design
wavelength will depend in part on the number of periods of the
sinusoidal variation in index with thickness.
[0037] In both examples (quarterwave stack and rugate filter), the
level of reflectance at the design wavelength will depend on the
number of periodic cycles of index of refraction. Thus, to achieve
higher levels of optical reflectivity one must use larger numbers
of dielectric film layers, which results in greater total
thicknesses for the thin film stack.
[0038] FIGS. 4A-4D illustrate various stages in the assembly and
manufacture of the embodiment illustrated in FIG. 3. In FIG. 4A,
the electrode 125 is deposited on top of the base substrate 130 for
later alignment with its movable counter electrode (device layer
105 not shown). The mechanical bonding pads 320, which are Au in
the preferred embodiment, sit on the oxide layer 310 in preparation
for the next processing step. The various electrode 125 and metal
bonding pads 315, 320 are preferably formed using a combination of
process technologies such as dry etching, wet etching, and resist
liftoff, as well known to those skilled in the art, to define a
electrode intermediate assembly 400A. In FIG. 4B, a silicon on
insulator (SOI) wafer is processed using the above-referenced
techniques to introduce the stress compensator layer 120 into the
device layer recess 122. The micromirror bonding pads 320 are
coupled on the device layer 105 of the SOI wafer. A handling
substrate 405B of the SOI wafer is available to move the
micromirror intermediate assembly 400B from step to step during
processing. In FIG. 4C, the micromirror intermediate assembly 400B
is bonded to the electrode intermediate assembly 400A at the
bonding pads 320, 315, respectively. An underfill material 405C is
introduced between the intermediate assemblies 400A, 400B to
provide structural support between them for subsequent processing
steps. In the preferred embodiment, this underfill material is an
organic epoxy, which is later removed using an oxygen plasma
etching process. Other materials may be suitable for the underfill,
provided their introduction, curing, and removal is compatible with
the device fabrication process. The wafer handle 405B is removed
(indicated by dashed lines) by a mechanical and dry-etch process
down to the insulator oxide 410B. The insulator oxide 410B
(indicated by dashed lines) is also removed with a standard wet
etch process to strip it down to the device layer 105. As
illustrated in FIG. 4D, the reflector layer 110 is deposited and
patterned and the device layer 105 is patterned with a resist 400D
to enable etching of the flexures. FIG. 4E illustrates the flexures
115 formed in the device layer 105 to allow movement of the
micromirror 110 after removal of the organic epoxy 405C. The epoxy
405C is subsequently removed using an oxygen-plasma process. The
abovementioned fabrication process is exemplary only, illustrating
a preferred embodiment of the fabrication sequence. It is not
intended to be limiting in the scope or application of the
invention.
[0039] FIG. 5 illustrates an embodiment of a micromirror array 500
that uses the base substrate 130 with multiple and electrically
independent electrodes 505 in complementary opposition to a
plurality of respective micromirrors 510. Each micromirror 510 has
a stress compensator (not known) separated from it by a device
layer. Flexures 520 are coupled between second portions 525 of each
respective device layer to allow piston-like movement of the
micromirrors 510 in relation to the base substrate 130. Support
columns 530, preferably formed from Au, provide mechanical
interconnection of the micromirror array 500 to the supporting
substrate 130. Alternate bonding processes may be used as known by
those skilled in the art.
[0040] The supporting substrate 130 has been shown as a purely
mechanical element in the embodiment described above. In an
alternate embodiment, the supporting substrate may contain active
electronic circuitry used to provide the electrical drive signals
needed to actuate the individual mirror elements. For the case of
electrostatically-actuated micromirrors, the circuitry would
provide a varying voltage signal to control mirror deflection. In
these embodiments, the materials and processes used for the
micromirror-to-substrate bonding step would be selected to maintain
compatibility with the restrictions of the circuit wafer.
[0041] While various implementations of the application have been
described, it will be apparent to those of ordinary skill in the
art that many more embodiments and implementations are possible
that are within the scope of this invention.
* * * * *