U.S. patent application number 10/952237 was filed with the patent office on 2005-07-21 for micromirror systems with open support structures.
Invention is credited to Aubuchon, Christopher M..
Application Number | 20050157373 10/952237 |
Document ID | / |
Family ID | 32068792 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050157373 |
Kind Code |
A1 |
Aubuchon, Christopher M. |
July 21, 2005 |
Micromirror systems with open support structures
Abstract
Micromirror devices, especially for use in digital projection
are disclosed. Other applications are contemplated as well. The
devices employ a superstructure that includes a mirror supported
over a hinge set above a substructure. Various improvements to the
superstructure over known micromirror devices are provided. The
features described are applicable to improve manufacturability,
enable further miniaturization of the elements and/or to increase
relative light return. Devices can be produced utilizing the
various optional features described herein, possibly offering cost
savings, lower power consumption, and higher resolution.
Inventors: |
Aubuchon, Christopher M.;
(Palo Alto, CA) |
Correspondence
Address: |
CHRISTOPHER M. AUBUCHON
1066 Metro Circle
Palo Alto
CA
94303
US
|
Family ID: |
32068792 |
Appl. No.: |
10/952237 |
Filed: |
September 27, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10952237 |
Sep 27, 2004 |
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10269478 |
Oct 11, 2002 |
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6798560 |
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Current U.S.
Class: |
359/291 |
Current CPC
Class: |
G02B 26/06 20130101;
G02B 26/0841 20130101 |
Class at
Publication: |
359/291 |
International
Class: |
G02B 026/00 |
Claims
1. A micromirror device comprising: a substrate with electrical
components including address circuitry, and an array of
micromechanical light modulator elements, each micromechanical
light modulator element comprising a mirror portion above said
substrate, a plurality of hinge portions below said mirror portion
and a plurality of electrode portions, wherein at least one portion
of each micromechanical light modulating element is held above said
substrate by an open support.
2. A micromirror device comprising: a substrate with electrical
components including address circuitry, and an array of
micromechanical light modulator elements, each micromechanical
light modulator element comprising a mirror portion above said
substrate, a plurality of hinge portions below said mirror portion
and a plurality of electrode portions, wherein at least one portion
of each micromechanical light modulating element is held above said
substrate by a support located at a border region.
3. The device of claim 1 or 2, wherein each mirror portion is held
above said substrate by a pair of discrete supports.
4. The device of claim 3, wherein said supports comprises a
plurality of vertical sections.
5. The device of claim 4, wherein said plurality of vertical
sections meet at a common base affixed to one of said hinge
portions.
6. The device of claim 1 or 2, wherein said hinge portions are held
above said substrate by at least one support.
7. The device of claim 6, wherein said hinge portions are attached
to a connector segment spanning adjacent supports.
8. The device of claim 1 or 2, wherein at least one of said
electrode portions is held above said substrate by at least one
support.
9. The device of claim 8, wherein only one support is provided for
each electrode portion, thereby supporting each said portion in a
cantilever fashion.
10. A method of manufacturing support structures in a micromirror
device comprising, a substrate with electrical components including
address circuitry, and an array of micromechanical light modulator
elements, each micromechanical light modulator element comprising a
mirror portion above said substrate, a plurality of hinge portions
below said mirror portion and a plurality of electrode portions,
said method comprising: producing a precursor structure comprising
two columns and an upper surface, removing portions of said columns
and said upper surface, leaving only a spanning segment of said
precursor upper surface located between said precursor columns, and
support portions of said columns attached to said spanning
segment.
11. The method of claim 10, wherein said spanning segment comprises
an electrode portion.
12. The method of claim 10, wherein said spanning segment comprises
a mirror portion.
13. The method of claim 10, wherein said spanning segment is
attached to at least one hinge portion.
14. A method of manufacturing electrodes in a micromirror device,
comprising, a substrate with electrical components including
address circuitry, and an array of micromechanical light modulator
elements, each micromechanical light modulator element comprising a
mirror portion above said substrate, a plurality of hinge portions
below said mirror and a plurality of electrode portions, said
method comprising: providing a sacrificial layer of mater and on a
first surface of said substrate, depositing electrode material over
said first surface and a second surface of said sacrificial layer,
and removing said sacrificial layer, leaving at least one electrode
with an upper portion supported by a support portion above an
electrode lower portion on said substrate.
15. The method of claim 14, wherein said support portion is
substantially vertical.
16. The method of claim 14, wherein said electrode at least one
electrode comprises two levels.
17. The method of claim 14, wherein said upper portion is angled
with respect to said substrate first surface.
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to the field of
spatial light modulators that can form optical images by the
modulation of incident light. The invention may involve micro
electromechanical systems (MEMS) in the form of micromirror device
arrays for use in optical display, adaptive optics and/or switching
applications. Optionally, the invention also comprises individual
or isolated micromirror elements.
BACKGROUND
[0002] Generally, MEMS devices are small structures, typically
fabricated on a semiconductor wafer using processing techniques
including optical lithography, metal sputtering, plasma oxide
deposition, and plasma etching developed for the fabrication of
integrated circuits. Micromirror devices are a type of MEMS device.
Other types of MEMS devices include accelerometers, pressure and
flow sensors, fuel injectors, inkjet ports, and gears and
motors--to name a few. Micromirror devices have already met with a
great deal of commercial success.
[0003] Micromirror devices are primarily used in optical display
systems. The large demand for micromirror-based display systems is
a result of the superior image quality the systems can provide.
Commercial and home-theater segments drive this facet of market
demand. Other market segments are characterized by cost concerns
more than image quality concerns. Since these devices are produced
in bulk on semiconductor wafers, they take advantage of the same
wafer processing economies of scale that characterize the
semiconductor industry, thus making the sale of these devices
competitive at all price points.
[0004] In display systems, the micromirror device is a light
modulator that often uses digital image data to modulate a beam of
light by selectively reflecting portions of the beam of light to a
display screen. While analog modes of operation are possible, many
micromirror devices are operated in a digital bistable mode of
operation.
[0005] The unique properties of current and future
micromirror-based display systems will allow them to capture market
share for applications including theatre and conference room
projectors, institutional projectors, home theater, standard
television and high definition displays from various lesser-quality
solutions including liquid crystal display (LCD) and cathode ray
tube (CRT) type systems. Micromirror-based display systems now
offer compact, high resolution and high brightness alternatives to
other existing technology.
[0006] Presently, such systems are further characterized by:
all-digital display (mirror control is completely digital except
for the possible A/D conversion necessary at the source);
progressive display (removing interlace display artifacts such as
flicker--sometimes necessitating an interlace to progressive scan
conversion); fixed display resolution (the number of mirrors on the
device defines the mirror array resolution; combined with the 1:1
aspect ratio of the on-screen pixels, the fixed ratio presently
requires re-sampling of various input video formats to fit onto the
micromirror array); digital color creation (spectral
characteristics of color filters and lamp(s) are coupled to digital
color processing in the system); and digital display transfer
characteristics (micromirror device displays exhibit a linear
relationship between the gray scale value used to modulate the
mirrors and the corresponding light intensity, thus a "de-gamma"
process is performed as part of the video processing prior to
display).
[0007] MEMS display devices have evolved rapidly over the past ten
to fifteen years. Early devices used a deformable reflective
membrane that was electrostatically attracted to an underlying
address electrode. When address voltage was applied, the membrane
would dimple toward the address electrode. Schlieren optics was
used to illuminate the membrane and create an image from the light
scattered by the dimpled portions of the membrane. The images
formed by Schlieren systems were very dim and had low contrast
ratios, making them unsuitable for most image display
applications.
[0008] Later generation micromirror devices used flaps or
cantilever beams of silicon or aluminum, coupled with dark-field
optics to create images having improved contrast ratios. These
devices typically used a single metal layer to form the reflective
layer of the device. This single metal layer bent downward over the
length of the flap or cantilever when attracted by the underlying
address electrode, creating a curved surface. Incident light was
scattered by this surface thereby lowering the contrast ratio of
images formed with flap or cantilever beam devices.
[0009] Devices utilizing a mirror supported by adjacent torsion bar
sections were then developed to improve the image contrast ratio by
concentrating the deformation on a relatively small portion of the
reflecting surface. These devices used a thin metal layer to form a
torsion bar, which is often referred to as the hinge, and a thicker
metal layer to form a rigid member. The thicker member typically
has a mirror-like surface. The rigid mirror remains flat while the
torsion hinges deform, minimizing the amount of light scattered by
the device and improving its contrast ratio. Though improved, the
support structure of these devices was in the optical path, and
therefore contributed to an unacceptable amount of scattered
light.
[0010] The more successful micromirror configurations have
incorporated a "hidden-hinge" or concealed torsion/flexure
member(s) to further improve the image contrast ratio by using an
elevated mirror to block most of the light from reaching the device
support structures. Because the mirror support structures that
allow it to rotate are underneath the mirror instead of around the
perimeter of the mirror, more of the surface area of the device is
available to reflect light corresponding to the pixel image. Since
much of the light striking a concealed-flexure micromirror device
reaches an active pixel surface and is either used to form an image
pixel or reflected away from the image to a light trap, the
contrast ratio of such a device is much higher than the contrast
ratio of other known devices.
[0011] Some of this progression is published on the world wide web
site of Texas Instruments. Further review and technical details as
may be employed (including in the present invention) are presented
in MEMS and MOEMS Technology and Applications, by P. Rai-Choudhury,
169-208 (SPIE Press, 2000).
[0012] Despite such advances in design, several aspects of known
micromirror devices may be further improved. First, general
considerations of manufacturability, which play directly into cost,
may be improved. For instance, increasing the yield of devices (in
the form of pixels that pass functional criteria) from a given
processed wafer offers both improvement in product quality and cost
savings. In addition, less complicated manufacturing procedures,
including a process requiring fewer masks or steps for production
of micromirror devices would be desirable.
[0013] Still further, performance aspects of existing micromirror
devices can be improved. One such aspect concerns increasing the
percentage of light return from the micromirrors. Another involves
the angular displacement that can be realized in deflecting a given
mirror. The overall deflection ability or total angular resolution
can be particularly important in terms of optical switching
applications as well as in the contrast ratio of image
production.
[0014] Yet another performance aspect in which improvement is
possible concerns power consumption. Micromirror devices currently
in production for SVGA applications include over half a million
active mirrors, SXGA applications require over one point three
million active mirrors. Since powering so many elements has a
cumulative effect, addressing power consumption issues will be of
increasing importance in the future as the number of pixels
employed in image creation continues to increase.
[0015] Yet another avenue for micromirror device improvement lies
in continued miniaturization of the devices. In terms of
performance, this can improve power consumption since, smaller
distances between parts and lower mass parts will improve energy
consumption and increase display system resolution by providing a
micromirror device with greater mirror density given overall
package size constraints. In terms of manufacturing, continued
miniaturization of mirror elements can offer a greater number of
micromirror systems for a wafer of a given size.
[0016] Various aspects of the present invention offer improvement
in terms of one or more of the considerations noted above. Of
course, certain features may be offered in one variation of the
invention, but not another. In any case, features offered by
aspects of the present invention represent a departure from
structural approaches represented by the Texas Instruments DMD.TM..
The inventive features represent an altogether distinct
evolutionary branch of "hidden-hinge" or concealed-flexure
micromirror device development, rather than mere sequential
refinement of features as may be noted in the development of the
Texas Instruments DMD.TM. element described in detail below. The
divergent approaches marked by aspects of the present invention
offer a competitive edge to the present invention to benefit
consumers in any of a number of ways.
SUMMARY OF THE INVENTION
[0017] The present invention involves micromirror structures,
optionally used in display systems. Micromirror array devices
according to the present invention generally comprise a
superstructure disposed over a substructure including addressing
features. Features of the superstructure set upon and above the
substrate include electrodes, hinges, micromirrors or portions
thereof. Variations of the invention concern support members
provided to support such items.
[0018] According to the present invention, "open" support
structures are employed variously to separate a given element from
the feature to which it is secured. By an open support, it is meant
that the structure does not have a closed periphery as do known
support structures, e.g., as in the Texas Instruments DMD.TM.
columnar support posts formed within "vias" having a substantially
square cross section.
[0019] Support members according to the invention may be
single-sided or multi-sided/faceted. They may support a given
structure in a cantilevered manner or at points across from each
other. Mirror and hinge elements are preferably supported along
opposite sides, i.e., by placing supports across from each other.
Electrodes are preferably configured to include a cantilever
section. Other variations within the scope of the invention are,
however, possible.
[0020] Laying down material over a stepped or angled sacrificial
material during manufacture may produce cantilever-style
electrodes. When the sacrificial material is removed the structures
remains. To produce other open support structure configurations
according to the present invention, at least two column or via-type
support precursors are provided with a spanning portion there
between. Then, any sacrificial material employed as a depositing
surface for the spanning portion is removed along with portions of
the column not providing support for the spanning portion, thus
creating one or more "open" supports.
[0021] The present invention includes any of these improvements
described either individually, or in combination. Systems employing
micromirror devices including the improved superstructure form
aspects of the invention, as does methodology associated with the
use and manufacture of apparatus according to the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIGS. 1-8H represent information known in the art, in which
FIGS. 6 and 8A-8H represent aspects of a known micromirror device.
The features shown in the other figures may be used in the present
invention. FIGS. 9A-15H show features particular to the present
invention. FIGS. 11A and 11B compare micromirror devices according
the present invention against the device shown in the referenced
figures. Certain aspects of the figures diagrammatically represent
the present invention, while others are indicative of preferred
relations. Regardless, variation of the invention from what is
shown in the figures is contemplated.
[0023] FIGS. 1A and 1B are side views illustrating bi-stable
micromirror operation.
[0024] FIG. 2 is a perspective-combined view illustrating the
projection of three pixels utilizing a portion of a micromirror
device display system.
[0025] FIG. 3 is a perspective view illustrating grayscale image
production for a single line of mirrors in a micromirror device
utilizing pulse width modulation (PWM).
[0026] FIG. 4 is a perspective view of an exemplary color
micromirror projection system.
[0027] FIG. 5A is a perspective view of a micromirror device based
projector; FIG. 5B is a perspective view of a micromirror device
based projection television.
[0028] FIG. 6 is an exploded perspective view of a DMD.TM.
element.
[0029] FIG. 7 is a circuit diagram showing a manner of addressing a
micromirror device array.
[0030] FIGS. 8A-8H are perspective views showing the micromirror
elements of FIG. 6 at various stages of production.
[0031] FIG. 9A shows a perspective view of a micromirror element
according to the present invention; FIG. 9B shows the element in
FIG. 9A without a mirror; FIG. 9C shows the element of FIG. 9A from
the side. FIGS. 9A'-9C' show the same views of another variation of
the invention employing a single-stage electrode, with an alternate
mirror support approach. FIGS. 9A"-9C" show the same views of a
further variation of the present invention that employs a hexagonal
mirror.
[0032] FIGS. 10A-10G are perspective views showing the micromirror
element(s) of FIGS. 9A-9C at various stages of production.
[0033] FIG. 11A is a top view comparing the DMD.TM. of FIG. 6 with
the micromirror device of FIG. 8; FIG. 11B is a perspective view of
arrays of elements as shown in FIG. 11A.
[0034] FIGS. 12A-12C show different mirror support configurations
according to the present invention.
[0035] FIGS. 13A and 13B show optional manners of producing support
portions with and without a base, respectively.
[0036] FIGS. 14A-14C show different mirror configurations in an
intermediate stage of production.
[0037] FIGS. 15A-15H are side views of various electrode
configurations employing a variety of levels, shapes and support
approaches.
DETAILED DESCRIPTION
[0038] In describing the invention in greater detail than provided
in the Summary above, applicable technology is first described.
This discussion is followed by description of a known micromirror
device and its manner of production. Then a variation of a
micromirror device according to the present invention is disclosed,
as well as a preferred manner of production. Next, comparative
views of the known and inventive micromirror devices are described.
Finally, additional optional aspects of the present invention are
described, including various optional support, micromirror and
electrode configurations.
[0039] Before the present invention is described in such detail,
however, it is to be understood that this invention is not limited
to particular variations set forth and may, of course, vary.
Various changes may be made to the invention described and
equivalents may be substituted without departing from the true
spirit and scope of the invention. In addition, many modifications
may be made to adapt a particular situation, material, shape of
design, composition of matter, process, process act(s) or step(s),
to the objective(s), spirit or scope of the present invention. All
such modifications are intended to be within the scope of the
claims made herein.
[0040] Methods recited herein may be carried out in any order of
the recited events which are logically possible, as well as the
recited order of events. Furthermore, where a range of values is
provided, it is understood that every intervening value, between
the upper and lower limit of that range and any other stated or
intervening value in that stated range is encompassed within the
invention. Also, it is contemplated that any optional feature of
the inventive variations described may be set forth and claimed
independently, or in combination with any one or more of the
features described herein.
[0041] All existing subject matter mentioned herein (e.g.,
publications, patents, patent applications and hardware) is
incorporated by reference herein in its entirety except insofar as
the subject matter may conflict with that of the present invention
(in which case what is present herein shall prevail). The
referenced items are provided solely for their disclosure prior to
the filing date of the present application. Nothing herein is to be
construed as an admission that the present invention is not
entitled to antedate such material by virtue of prior
invention.
[0042] Reference to a singular item, includes the possibility that
there are plural of the same items present. More specifically, as
used herein and in the appended claims, the singular forms "a,",
"and," "said" and "the" include plural referents unless the context
clearly dictates otherwise. It is further noted that the claims may
be drafted to exclude any optional element. As such, this statement
is intended to serve as antecedent basis for use of such exclusive
terminology as "solely," "only, "or "lacking" and the like in
connection with the recitation of claim elements, or use of a
"negative" limitation.
[0043] Unless defined otherwise below, all technical and scientific
terms used herein have the same meaning as commonly understood by
one of ordinary skill in the art to which this invention belongs.
Still, certain elements may be defined herein for the sake of
clarity, possibly providing an alternate meaning.
[0044] Turning now to FIGS. 1A and 1B, bistable operation of a
micromechanical light modulator 2 is shown. The device comprises a
mirror portion 4, a hinge portion 6 and electrode portions 8 set
upon a substrate 10.
[0045] In FIG. 1A, the mirror is shown rotated or flexed about a
hinge portion 6 in a clockwise direction from a horizontal
position. The hinge is configured to provide a mechanical restoring
force in returning from mirror rotation. Mirror rotation occurs as
a result of electrostatic attraction between at least the mirror
portion 4 and an electrode portion 8 of the device located above a
substrate 10 which carries each of the elements.
[0046] Thus attracted, the mirror is pinned at a stable, minimum
potential energy state. FIG. 1B shows the mirror deflected to a
second minimum potential energy state opposite a second electrode.
Operation of a micromirror device mirror between two such
full-angle states represents what is referred to as "bistable"
operation. Such operation is employed in a digital mode.
[0047] Digital operation sometimes involves employing a relatively
large address voltage to ensure the mirror is fully deflected.
Address electrodes are driven by underlying logic circuitry. A bias
voltage, usually a positive voltage, is typically applied to the
mirror metal layer to control the voltage difference between the
address electrodes and the mirrors. Setting the mirror bias voltage
above what is termed the "threshold voltage" of the device ensures
the mirror will fully deflect toward the address electrode, even in
the absence of an address voltage. Where a large bias voltage is
employed, lower address voltages may be used since the address
voltages need only cross a meta-stable point to enter an opposite
bi-stable minimum potential energy state.
[0048] Micromirror devices may also be operated in analog mode.
Sometimes referred to as "beam steering," this operation involves
charging address electrode(s) to a voltage corresponding to the
desired deflection of the mirror. Light striking the micromirror
device is reflected by the mirror at an angle determined by the
deflection of the mirror. A ray of light reflected by an individual
mirror is directed to fall outside the aperture of a projection
lens, partially within the aperture, or completely within the
aperture of the lens, depending on the voltage applied to the
address electrode(s). The reflected light is focused by the lens
system onto an image plane. Each individual mirror pixel
corresponds to a pixel on the image plane. As the ray of reflected
light is moved from completely within the aperture to completely
outside the aperture, the image location corresponding to the
mirror dims, creating continuous brightness levels.
[0049] Note also, that both digital and analog micromirror device
operation is applicable in the context of such devices used for
optical switching applications. That is to say, micromirror devices
(especially those produced according to the present invention) lend
themselves to directing light from one path to another to optically
connect and disconnect pathways as desired.
[0050] Yet, for the sake of discussion in introducing aspects of
the invention in contrast to known designs, FIG. 2 illustrates an
approach to producing images in a digital mode of micromirror
device operation. Incident light from a light source 12 striking a
mirror 2 rotated toward the light source is reflected to pass
through a lens 14 and be displayed as a corresponding bright pixel
16 on a screen or the like (turned upward relative to the other
components shown for ease of viewing). In contrast, mirrors rotated
away from the light source reflect light away from the projection
lens into a light trap 18 leaving a corresponding dark pixel 20 at
the projection image surface. Mirrors rotated to produce a bright
pixel may be regarded as "on," while those positioned to leave a
pixel dark may be regarded as
[0051] FIG. 3 illustrates a manner in which intermediate pixel
brightness may be obtained. Digital mode micromirrors employ pulse
width modulation techniques to rapidly rotate a mirror on and off
to vary the quantity of light reaching the image plane. The human
eye integrates the light pulses and the brain perceives a
flicker-free intermediate brightness level. In FIG. 3, an active
row of micromechanical light modulator elements 2 are depicted,
forming a portion of a larger array 22. Directional markers 24
indicate the location of corresponding pixels within a projected
pixel row 26 opposite a lens 14. A full-intensity bright pixel 16
is displayed by constant application of light rays 28. A dark pixel
20 is provided by leaving the corresponding reflective element 2
"off" so that essentially no light reaches the projection target.
Pixels of intermediate intensity 30 are provided by application of
intermediate lighted intervals by turning "on" and "off" the
corresponding micromirror element 2.
[0052] FIG. 4 shows a digital projection subsystem 32 in which the
digital operation principle(s) discussed above are applied to
project a cogent image on a screen 34. The subsystem includes a
light source 12 and a projection lens 14 as well as a board or
module 36 including a processor 38, memory 40 and micromirror array
42 comprising light modulating elements 2. The micromirror device
shown is "packaged" in that the MEMS portion micromirror array 22
element of the device is set within a housing 44 sealed by a window
46.
[0053] These components alone, perhaps with intermediate optics to
shape the light emanating from source 12, would be sufficient to
present a gray-scale or "black and white" image. Additional
components in the form of a color filter or "color wheel" 48 and
optics for use therewith including a condensing lens 50 and a
shaping lens 52 to focus and restore a columnar light beam through
colored sections of the lens as its rotates are provided. Through
coordinated rotation on the wheel and actuation of the micromirror
elements 2, full color synthesis is possible.
[0054] Full-color images are generated by sequentially forming
three single-color images. This process in concert with the former
discussion of analog or digital methods of grayscaling gives many
levels of shading of each color. The viewer perceives a single,
full color image from the sum of the three single-color grayscaled
images.
[0055] In addition to color wheel approaches, others are known. In
accordance with known techniques, dedicated one-color or filtered
light sources may be provided instead of a color wheel, especially
utilizing a plurality of micromirror devices. Alternately, a color
wheel may continue to be utilized with a plurality of micromirror
devices in conjunction with a color separating prism (not shown).
Still further, a plurality of micromirror devices may be provided
and used in conjunction with a light source, no color wheel, but
with color filtering prisms.
[0056] The choice of optics may vary. Providing additional light
sources and/or additional micromirror arrays allows for image
creation through superposition offering the potential for greater
brightness and resolution. Simply providing dedicated light sources
for a single micromirror array may improve brightness as well. One
limitation to current micromirror device implemented solutions
involves brightness levels. Since there is a practical limit to the
brightness of a single source, one solution to this malady is to
utilize multiple light sources. Factors of greater cost/system
complexity will typically be weighed in determining whether to
implement these improvements in a given system.
[0057] Regardless of the ultimate configuration selected, one of
two media formats is preferably employed with the micromirror
devices, though others are possible. These are illustrated in FIGS.
5A and 5B. The first figure depicts a projector 54. The projector
shown is suitable for the typical consumer home-theater. Other
devices that may incorporate systems according to the present
invention may be suitable for larger venues (i.e., staging events
and cinema presentations), being configured for high light output
and waste heat generation. The second figure depicts a projection
television 56. The television pictured is a rear projection system,
though other styles (e.g., front projection) may be employed.
[0058] Whatever the case, such systems may be specifically designed
for or designed around micromirror devices according to the present
invention. Alternately, it is contemplated that a packaged "light
engine" according to the present invention could be substituted
into existing systems (with or without further modification or
substituting the entire module 36) to upgrade performance.
[0059] To appreciate the performance advantages available through
various aspects of the present invention, it is important to first
appreciate the structure of the above-referenced Texas Instruments
devices that are believed to define the state of the art at the
time of filing. FIG. 6 shows a single mirror element 2 of an array
in an exploded perspective view.
[0060] Several levels of structure are expressed. The bottom level
is a semiconductor substrate 58 with electrode addressing circuitry
60 provided thereon. The manner in which such circuitry is
addressed (whether as provided in the referenced micromirror
devices or those according to the present invention) is illustrated
in FIG. 7. For the various rows and columns of micromirror elements
2, addressing architecture is shown that incorporates N addressing
inputs 62 for every 2N rows and 1 data input 64 for every 16
columns. Such substrate material in various configurations, with a
passivation layer including vias to provide connectivity at
selected locations/spacing is commercially available.
[0061] Returning to FIG. 6, the physical alignment of
superstructure components above the address circuitry is such that,
upon selection, address voltage is applied to the electrodes of the
device. The bias voltage discussed above is applied to the mirror
by way of intermediate structures connected to a bias/reset bus 66
provided upon substrate 58.
[0062] Hinge supports 68 are set above the bias bus, and supported
above bus 66 by substantially square, columnar via-based supports
70. (The final alignment of these components and others is
indicated in dashed lines.) The support posts are produced by
deposition within a hole provided within a sacrificial layer of
material in an intermediate stage of device production.
Accordingly, they are not solid, but rather hollow until the solid
base portion 72, with a closed outer wall or periphery. The hinge
supports are attached to hinge segments or portions 74 which are
in-turn attached to a yoke 76. The corners of the yoke are provided
with spring tips 78. The spring tips provide bumpers to cushion or
moderate contact between the yoke and bias bus upon full mirror
actuation, rather than having to precisely control voltages or rely
on other interfering contact. While potentially useful, it is
contemplated that micromirror devices according to the present
invention may or may not make use such features.
[0063] Above the yoke, micromirror element 2 includes a mirror 80.
The mirror is connected to the yoke by way of a via-type support 70
like those provided for the hinge supports, leaving a hole 118 in
the mirror face. By way of the connecting structures, each of the
mirror, yoke, hinges and hinge supports are charged to the bias
voltage of bus 66.
[0064] To actuate the device, a voltage is applied to the
electrodes 82 and 84 that electrostatically attract both the mirror
and yoke, respectively. The electrodes are set at two levels. The
higher-up outer electrode portions 82 are electrically connected to
the lower electrode portions 84 by way of another connecting
columnar via 70. This combination of elements is placed in
electrical contact with the addressing circuitry by a filled-in via
86 in the base of each electrode portion 84. The upper electrodes
are positioned to attract the mirror, whereas the lower electrodes
are positioned to attract the yoke.
[0065] The manner of producing the superstructure of micromirror
device 2 is represented in FIGS. 8A-8H. The stages shown are
indicative of action taken after intermediate masking steps between
material deposition (sacrificial material or structural material)
and sacrificial material removal. To most clearly portray the
structure being produced, the perspective view shown takes the
device across the sectional line shown in FIG. 6 and tilts the
structure.
[0066] In FIG. 8A, a portion of bus 66 and a lower electrode 84 are
shown, formed by a conductive material. These are provided by
material deposited over substrate 10, with the overlaid material
strategically etched away. The raised portions will have been
covered by a protection layer, configured using a first mask 88
(diagrammatically pictured). The substrate comprises the addressing
circuitry covered by a passivation layer, the layer having holes
strategically placed to provide access vias to the underlying
circuitry. The vias are filled-in to provide electrical connections
86 between the substrate and electrodes as noted above with respect
to FIG. 6.
[0067] FIG. 8B shows a layer of sacrificial material 90, deposited
over the structure in FIG. 8A. Via column holes 92 are provided,
again by selectively etching the material in connection with a
second mask 94.
[0068] In FIG. 8C another layer of conductive material 96 suitable
for use in producing hinge sections 74 and spring tips 78 is
laid-down. Following this, a third mask 98 is employed in setting a
protective layer such as an oxide (not shown) over the regions of
layer 96 serving as hinge precursors 100, and spring tip precursors
79.
[0069] In FIG. 8D, another layer of conductive material 102 is
deposited thereon. A fourth mask 104 is utilized to form a
protective layer (not shown) over the regions of layer 102 serving
as hinge support precursors 106, a beam or yoke precursor 108 and
upper electrode precursor(s) 110.
[0070] Both the hinge metal layer 96 and yoke/electrode metal layer
102 fill via holes 92, providing columnar support portions 70. The
portions of the material layers not protected during processes
involving the third and fourth masks are selectively etched as
shown in FIG. 8E to define hinge supports 68, hinges 74, yoke 76
and upper electrode portions 82.
[0071] FIG. 8F shows the micromirror device in another intermediate
stage of production with another layer of sacrificial material 112.
This layer is deposited over the structure in FIG. 8E. It includes
a via column hole 96, patterned utilizing a fifth mask 114. When a
mirror material layer 116 is deposited over sacrificial layer 112
as shown in FIG. 8G, via hole 96 is partially filled in, providing
support column 70, but leaving a hole or opening 118 in what is to
become the "face" of the mirror element. Following a deposited
metal oxide layer (not shown), a sixth and final mask 120 is used
to pattern and define a mirror precursor region 122 and adjacent
borders indicated by dashed lines, the latter being removed to form
spaces between adjacent mirrors 80 in a complete micromirror array.
Finally, FIG. 8H shows the micromirror element 2 as completed, with
all sacrificial material removed to release the structure.
[0072] The details of the materials employed, intermediate
preparation steps and further constructional details associated
with the methodology described are known by those with skill in the
art, within the scope of reasonable experimentation by the same
and/or may be appreciated by reference to background noted above or
the following U.S. patents: U.S. Pat. No. 5,083,857 to Hornbeck,
entitled "Multi-level Deformable Mirror Device"; U.S. Pat. No.
5,096,279 to Hornbeck, et al., entitled "Spatial Light Modulator
and Method"; U.S. Pat. No. 5,212,582 to Nelson, entitled
"Electrostatically Controlled Beam Steering Device and Method";
U.S. Pat. No. 5,535,047 to Hornbeck, entitled "Active Yoke Hidden
Hinge Digital Micromirror Device"; U.S. Pat. No. 5,583,688 to
Hornbeck, entitled "Multi-level Digital Micromirror Device"; U.S.
Pat. No. 5,600,383 to Hornbeck, entitled "Multi-level Deformable
Mirror Device with Torsion Hinges Placed in a layer Different From
the Torsion Beam Layer"; U.S. Pat. No. 5,835,256 to Huibers,
entitled "Reflective spatial Light Modulator with Encapsulated
Micro-Mechanical Element"; U.S. Pat. No. 6,028,689 to Michalicek,
et al., entitled "Multi-Motion Micromirror"; U.S. Pat. No.
6,028,690 to Carter, et al., entitled "Reduced Micromirror Mirror
Gaps for Improved Contrast Ratio"; U.S. Pat. No. 6,323,982 to
Hornbeck, entitled "Yield Superstructure for Digital Micromirror
Device"; U.S. Pat. No. 6,337,760 to Huibers, entitled:
"Encapsulated Multi-Directional Light Beam Steering Device"; U.S.
Pat. No. 6,348,907 to Wood, entitled "Display Apparatus with
Digital Micromirror Device"; U.S. Pat. No. 6,356,378 to Huibers,
entitled "Double Substrate Reflective Spatial Light Modulator";
U.S. Pat. No. 6,369,931 to Funk, et al, entitled "Method for
Manufacturing a Micromechanical Device"; U.S. Pat. No. 6,388,661 to
Richards, entitled "Monochrome and Color Digital Display System and
Methods"; U.S. Pat. No. 6,396,619 to Huibers, et al., entitled
"Deflectable Spatial Light Modulator Having Stopping Mechanisms".
In any case, micromirror devices according to the present invention
may be produced and/or operated according to the same details or
otherwise.
[0073] Regarding the features of the present invention, FIG. 9A
shows a preferred micromirror element 124 per the invention. The
variation of the invention shown includes each of the optional
features that may be employed, though not all such features need be
provided in a given product. FIG. 9B shows the micromirror device
124 in FIG. 9A minus its mirror. FIG. 9C shows the same from the
side.
[0074] Optional features of the invention that may be employed
together or individually break down into three basic groups. A
first group concerns supporting a mirror portion 126 at its sides;
a second group concerns providing electrodes 128 adapted for
sequential attraction of the mirror; and a third group concerns
supporting various components including the mirror, electrode
portions and/or hinge portions 130 with open support structures.
These features are addressed variously in the following
description.
[0075] The mirror shown in FIG. 9A has an uninterrupted "face" in
that its reflective surface is unbroken as compared to device 2 of
FIGS. 6 and 8. While the "potential face" or "prospective face" of
the mirror (indicated by solid and dashed lines together) may be
somewhat larger than the actual face of the mirror (the area
indicated by solid lines alone), "dim" or "dead" space 132
resulting, generally, in light scattering may be reduced. As
described below, such space may be minimized or even eliminated
according to an aspect of the present invention.
[0076] First, general features of element 124 under the mirror are
described. One such aspect concerns the manner in which mirror 126
is attached to its hinge. Supports 134 on opposite sides of mirror
element 126 secure it to hinge portions 130. The hinge portions may
comprise individual segments, or may be part of a unitary
structure. In any case, the hinge defined is attached to substrate
136 by a bridge-type support 138. The support is preferably open
underneath the hinge center 140, which is attached to a spanning
segment 142 between vertical support segments 144. Feet 146 may
additionally be provided to stabilize the support structure. Yet
another option is to produce support segments 144 at an angle
relative to the surface of the substrate (i.e., having both
vertical and horizontal components).
[0077] Likewise, support 134 may be set at an angle with respect to
the substrate. Yet, it is more preferable that support(s) be
provided orthogonally as shown. A base 148 of each support 134 may
directly connect each hinge portion 130. However, it may be
preferred that an intermediate layer or nub 150 of material (e.g.,
serving as a bonding interface) is employed.
[0078] In any case, the device is configured so that the hinge is
set some distance (as little as about 0.1 micron, or less) above
the surface of substrate 136 and mirror 126 is set some distance
(as little as about 0.1 micron, or less) above the hinges (as
little as about 0.2 micron, or less, above the surface of substrate
136). Avoidance of a yoke allows creation of very low profile
micromirror devices by the invention that are still able to attain
high deflection angles (typically about +/-10 deg., even upwards of
about +/-15 deg., to about +/-20 deg. or more). Of course,
mirror/micromirror devices according to the present invention may
be advantageously manufactured on a larger scale (even using MEMS
techniques)--possibly utilizing other actuation techniques,
including electromagnetic, electromechanical, thermo-mechanical or
piezo-based approaches--especially for non-projection
technology.
[0079] An aspect of the invention that facilitates provision of
adequate electrostatic attraction in response to hinge restoring
forces that increase with angular deflection has to do with the
configuration of electrodes 128. The electrodes may be configured
with a plurality of portions 152 and 154 (or more) at different
levels. Whether provided in a series of steps by continuous members
(as shown with a support portion 156 between each stage 152/154),
by steps formed with discrete members or a continuous angled
member, the electrodes are configured so that portions further from
the center or point of rotation of the mirror are at a lower
level.
[0080] The electrode configuration shown with higher portions
closer to the center and lower portions more distant provides
clearance for the mirror as it is tilted at an angle. Furthermore,
the configuration provides for sequential attraction of mirror 126.
When the mirror is angled away from a set of electrodes, the upper
electrode portion is the first to exert significant attractive
electrostatic force on the mirror (in light of the inverse squared
relationship between electrostatic attraction and distance between
objects). As the upper electrode portion(s) effectively attract the
mirror drawing inward, the influence of the electrode lower
portion(s) increase. Further aiding attraction of the mirror to its
full angular displacement is the increased mechanical advantage or
lever arm offered at more remote regions of the mirror interacting
with lower electrode portion 152.
[0081] The manner in which a micromirror device 124 according to
the present invention may be produced is illustrated in FIGS.
10A-10G. Of course, the process steps employed will vary depending
on which inventive features are actually employed in a given
variation of the invention. But again, a most preferred approach is
shown.
[0082] In FIG. 10A, a sacrificial layer of material 158 is set upon
substrate 136. It is patterned with a first mask 210 to define
openings 160 and a substrate-level portion 162 upon etching. In
FIG. 10B, a hinge metal layer 164 is deposited over the entire
surface including a portion of the sacrificial layer. A second mask
166 is utilized in defining a passivation layer (not shown) over
the region(s) of layer 164 serving as a hinge precursor region 168.
Metal layer 164 fills in via 206 provided in substrate 136 to form
a connection 208 between underlying address circuitry beneath an
oxide layer of the substrate. The same approach to addressing and
substrate construction may be employed as described above, or
another manner of electrical control of device superstructure
produced may be utilized. This holds true with respect to
connectivity between the device elements as well as the
configuration of substrate 136.
[0083] As shown in FIG. 10C, a thicker layer of conductive material
170 is deposited over the hinge material. This layer builds-up the
electrodes 128 and further fills openings 160, defining a support
precursor region 172 for hinge portions 130. Layer 170 also further
fills in via 206 and connecting structure 208. A third mask 174 is
employed to define a protective layer (not shown) over the region
of layer 170 serving as electrode precursor(s) 176.
[0084] In FIG. 10D, layers 164 and 170 are shown selectively etched
to reveal hinge 130, support spanner 142, and electrode portions
152 and 154. As shown in FIG. 1E, these structures are then covered
by another sacrificial layer 178. A fourth mask 180 is used to
pattern sacrificial layer 178 to form support precursor regions 182
upon etching the sacrificial layer.
[0085] FIG. 10F shows sacrificial layer 178 as it is selectively
etched, and then coated with a layer 184 of conductive material
suitable to serve as a mirror (or a substrate that may be
subsequently coated with a highly reflective metal or dielectric
material). A fifth mask 186 is used in order to define a
passivation layer over mirror precursor regions 188 to be retained,
but not the adjacent borders 190, which are removed to form spaces
between adjacent micromirrors 126.
[0086] FIG. 10G shows a micromirror element 124 according to
aspects of the invention after all sacrificial materials have been
removed. As discussed above, the mirror is supported at or along
its opposite sides or edges by supports attached to a hinge, which
is in turn supported above the device substrate. In addition to
being placed at opposite sides/portions of the mirror, the support
members may be characterized as being "open" in nature. Progressive
or dual-stage electrodes are shown as well.
[0087] It is further noteworthy that a micromirror device produced
according to the methodology described merely requires 5
masks--i.e., as constructed on a pre-fabricated substrate. In
contrast, the Texas Instruments DMD.TM. is produced using 6 masks
under the same conditions. Thus, the methodology according to the
present invention is highly advantageous from both fabrication cost
and device yield standpoints.
[0088] Still, a micromirror device according to the present can be
produced with the same pixel dimensions as known devices. In doing
so, a device according to the present invention will offer a
performance benefits at least in terms of light return. Reasons for
this advance are discussed below.
[0089] Before such discussion, it is helpful to first consider a
side-by-side comparison of micromirror elements as provided in
FIGS. 11A and 11B. A Texas Instruments DMD.TM. element 2 is shown
from above on the left with a micromirror device 124 according to
the present invention next to it. The size differences between the
two are immediately apparent. Using present techniques, micromirror
devices according to certain aspects of the present invention may
be made smaller than the referenced devices by between about 25%
and about 65% or more (i.e., devices according to the present
invention may be about 75% to about 35% of the size of known
devices) due the absence of a yoke layer in order to allow for a
smaller sacrificial layer gap--while still employing a plurality of
electrode levels.
[0090] Reduction of the support footprint (pixel size) allows for a
smaller mirror with the same hinge length. The reduced sacrificial
layer gap allows for overall thinner structure, which reduces the
horizontal pivoting space necessary to deflect a mirror, thus
reducing the gap necessary between adjacent mirrors.
[0091] Generally, mirrors elements employed in the present
invention can be made smaller than DMD-sized mirrors that have
roughly a 19 micron diameter. Mirrors/pixel elements according to
the present invention may advantageously be produced at less than
about 10 microns in diameter. By "diameter," what is meant is the
distance across any long axis that may be defined; stated
otherwise, the diameter will correspond to that of any circle in
which the structure can be circumscribed.
[0092] Where electrodes are provided only directly opposite (flat
against) the substrate or multiple-stage electrodes are not
employed, even smaller mirrors may be produced. In such instances,
mirrors used in the present invention may be as small as 6 microns
in diameter in view of present manufacturing techniques. A mirror
so-sized may represent a 69% reduction in diameter from known
DMD.TM. mirror size (i.e., the inventive mirror element will be
about 31% the diameter of known mirrors). As techniques develop,
even smaller sized mirrors may be possible, regardless of electrode
configuration.
[0093] FIGS. 9A'-9C' show components of a device 124 according to
the present invention constructed using a single-level set of
electrodes 128. The configuration shown may be produced using a
modified version of the five-mask process described above. The
differences in production methodology will be readily apparent to
one with skill in the art. Generally, it will be preferred to
maximize the size of the electrodes given space constraints and in
view of clearance considerations as in other variations of the
invention.
[0094] In addition, FIGS. 9A'-9C' show components of a device 124
constructed using another means or approach to mirror support. The
support configuration shown may also be produced in connection with
a modified version of the five-mask process described above,
wherein differences in production methodology will be readily
apparent to one with skill in the art. Basically, in this variation
of the invention, columnar supports or posts 212 are utilized which
may be created by filling in vias produced in sacrificial material.
As in other variations of the invention pictured, each of the pair
of supports is positioned opposite one another and across the body
of mirror 126. Supports 212 are shown to have a wall 214 at the
edge of mirror 126 (each may have four walls or more or may define
curved surfaces--depending on the original via shape that is
filled-in to create the structure). Yet, the supports may be inset
from the side/corner or edge of a mirror (depending on the style of
micromirror device chosen) to which they are closest. However, it
may be preferred to position supports 212 in such a way as to
maximize hinge or torsion member length in view of the mirror
style/format selected (i.e., square with corner support positions,
hexagonal with corner supported positions, hexagonal with side
support positions, etc.). In which case, the base of each support
(or an intermediate structure) will be positioned at the end of any
hinge portions. However configured, supports 212 will generally be
positioned outside of the hinge support member 138 or members.
[0095] FIGS. 9A"-9C" provide details of a hexagonal-shaped mirror
supported at opposite corner positions. Its construction and
appearance closely resemble the micromirror elements 124 shown in
FIGS. 9A-9C. However, the hexagonal mirror format offers certain
advantages in use. For one, they can be closely packed in a manner
like a honeycomb, where sequential rows (or columns) overlap. Such
overlap provides the ability in image creation to mimic higher
resolution output where there is overlap. The principles of such
operation are well documented and may be understood in reference to
U.S. Pat. No. 6,232,936 to Gove, et al., entitled "DMD Architecture
to Improve Horizontal Resolution". Further potential advantages
associated with the mirror format shown in FIGS. 9A"-9C" are
presented below.
[0096] Especially with respect to that shown in FIGS. 9A-9C and
9A"-9C" another immediately apparent distinction between the Texas
Instruments device and those shown in the reference figures
concerns what may be regarded as "dead" or "dim" space that is
substantially non-reflective or poorly reflective relative to the
mirror face(s). A large central hole 118 is present in mirror face
80 of the former structures. As shown in FIG. 2, this actually
results in a central dark or missing region in each pixel image. By
way of comparison, each mirror 126 in FIGS. 9A and 9A" is inviolate
at the center. Any dim or dead space 132 associated with the
prospective mirror face only involves the space above support base
portions 148.
[0097] As alluded to above, however, depending on support
configuration, this space may be minimized or even eliminated.
Different support configurations are shown in FIGS. 12A, 12B and
12C. FIG. 12A show mirror sections 192 from above, the base 148 of
each support member and wall portions 194 defining vertical
sections(s) in connection with square mirrors. FIG. 12B shows
configurations advantageously employed with hexagonal mirrors as
indicated by identical reference numerals. As shown in FIG. 12C,
base 148 may even be altogether eliminated, especially in mirror
side-mount configurations. Here, a hexagonal mirror is portrayed in
which support wall(s) 134 attach directly to the underlying
structure without the addition of an extended base portion 148.
Supports 134 are depicted in broken line because (as apparent in
FIG. 9A) some thickness of the wall resides below the surface of
mirror 126 as viewed from above.
[0098] The manner in which producing support regions with no base
is depicted in FIGS. 13A and 13B. In FIG. 13A a support precursor
196 is shown. It is etched-out as indicated by dashed lines 198 in
accordance with the discussion above, removing region 200. The
resulting, separated structures include support 134 and base 148
regions, with mirror regions 126 above. In FIG. 13B, the support
precursor region is so small that removal of region 200 leaves no
discrete base(s) 148, but only base surfaces 202 (attached to
underlying structure).
[0099] In view of the different manner of supporting mirrors as
offered by aspects of the present invention, it is possible to
achieve a situation where between about 88% and about 100% of the
prospective mirror face is utilized, and therefore comprises
reflective surface. The limit for the known devices described
herein is below 88%.
[0100] Though not offering these particular advantages, the
variation of the invention shown in FIG. 9A' offers advantages
relative to the Texas Instruments approach that includes a large,
central hole 118 in each pixel. The dead or dim zones associated
with mirror holes 216 as provided in mirror faces according to the
present invention are spread apart from each other and of a
combined area that is less than the Texas Instruments column
support. Also, it is believed that this delocalization of such
space will make its effects less apparent to a viewer.
Decentralization of dim or dead space in the pixel may further
diminish the ability of a viewer to pick-out the features upon
close inspection.
[0101] However the supports are configured, as may be observed in
FIGS. 11A and 11B, each micromirror element is surrounded by a
border 188. This gap or border provides clearance for the mirrors
as they tilt back and forth in an array. In the active regions of
any micromirror array, this dead space cannot be eliminated. It
can, however, be reduced by providing lower-profile micromirror
assemblies. Highly-elevated mirrors as in the Texas Instruments
DMD.TM. that are set above a yoke 76 and greatly separated from the
underlying hinge and/or substrate require more lateral space in
which to accomplish such angular deflection as desired than lower
profile structures as may be achieved with the present invention.
The ability to produce low-profile micromirror devices according to
aspects of the present invention enables reducing overall gap or
border space to less than in known micromirror devices, where gap
space is believed to represent about 11.4% of the area in the
active array region.
[0102] In certain instances reduction in gap size may be more
significant than increasing use of prospective mirror face. For
example, where shorter supports 134 are provided (or via hole 118
is more filled-in), partial light return can be expected. In which
case, the zones are more "dim" than "dead" as to reflection.
[0103] Nevertheless, the array 22 comprising Texas Instruments
micromirror devices as described is not capable of producing the
resolution of array 191 using micromirror devices as may be
produced according to the present invention. In roughly the same
space, array 191 packs 100 light modulator elements as compared to
36 in array 22. The result of this difference nearly triples of the
number of pixels that may be projected.
[0104] The increased pixel density allows for finer detail
construction of an image. Furthermore, dim or dead zones are more
diffuse--and smaller (by way of smaller gaps 188 and/or spaces 132
versus holes 118). Each factor contributes to making their effect
less notable, just as they are more difficult to discern in FIG.
11B. The fact that the overall dead space is less, leads to overall
greater image brightness versus known devices. The distribution of
the dead space over a greater number of regions leads to greater
apparent image quality. The human eye is highly attuned to pattern
recognition. The dispersal of the "dead" or "dim" areas, reducing
their concentration, counters this ability.
[0105] Provision of such a dramatically increased number of mirrors
may, however, require certain accommodations. Considering that
mirrors in a DLP.TM. system are controlled by loading data into the
memory cell below the mirror, a data stream configured to actuate a
lesser number of mirrors with different addressing will typically
not be suitable for running another array. Accommodation for such
differences as presented may be provided by means of
hardware/software. Equipment exists that can take a given input
signal at a particular resolution and either up- or down-convert
the signal to a resolution that is compatible with the device at
hand.
[0106] In producing mirror arrays according to the present
invention, as discussed above, mirror precursor regions are
provided. These are patterned in such a way as to provide for
supports. Mirror precursor region sections 206 are shown for three
different mirror types in FIGS. 14A-14C. Dashed lines are presented
to indicate the location where individual mirror elements 126 will
reside upon separation. The solid lines indicate pits or holes 132,
portions of the edges will form support sections 134 (and possibly
portions of the bottom forming bases 148 as well). What may be
observed is that spaces 132 reside partly in the spaces 188 to be
provided between each mirror element. This positioning, in effect,
allows certain "theft" of space in producing the support
structures. The reason for such a characterization stems from
process limitations requiring that any hole in photoresist of a
given depth must have a certain aspect ratio or size/diameter to be
properly filled-in upon metal deposition. However, by locating open
regions during manufacture in areas that must ultimately be left
open anyway, losses of reflective space are minimized.
[0107] Regarding the various mirror configurations shown, each
presents certain noteworthy advantages that may be realized to
varying degrees depending on other material factors in array
construction. These are described in turn in terms of their
potential relative merits.
[0108] As to the square mirrors utilizing corner mounts, this
configuration accommodates the longest hinge length for the
smallest pixel area. Especially where very small mirrors/pixels are
concerned, longer hinge length can be very useful. Since for a
given hinge cross-section, stiffness decreases and overall
torsional displacement capability increases with length, it will be
possible to achieve relatively larger mirror deflection using such
a design. Additionally (or alternately), the additional hinge
length available allows for producing the smallest pixel size
possible--at least with respect to such other mirror and connector
configurations shown and discussed herein.
[0109] With the hexagonal mirrors using corner mounting points a
larger relative mirror area versus hinge length can be achieved.
Such a configuration provides for generating greater electrostatic
forces. According, reduced voltages may be applied to deflect each
mirror. Reducing voltages allows a beneficial reduction in overall
device power requirements.
[0110] Regarding the hexagonal mirrors employing side mounts, this
configuration accommodates a longer mirror axis perpendicular to
the hinge and mirror area versus hinge length. Depending on other
factors, especially hinge construction and electrode configuration,
the increased lever-arm offered by the overhanging mirror portion
at the corner of the mirror (as compared to the hex mirror/hinge
configurations where opposite edges are parallel to the hinge) may
offer greater electrostatic attraction, especially toward the
extremes of mirror actuation where restoring forces from the hinge
are greatest. As such, this may offer relative advantages in power
consumption and/or maximum mirror deflection.
[0111] Further optional advantages in the invention may be realized
utilizing different electrode configurations. The plan or top view
of electrodes may, of course, be altered or optimized for a given
situation. Moreover, FIGS. 15A-15H present side view of various
potential electrode configurations. Each figure shows an electrode
including a plurality of levels. In the variation in FIG. 15A two
levels 152 and 154 are shown. Progressively more levels 218 are
shown in FIGS. 15B-15D. In FIG. 15E, a continuum of levels is
presented in the form of a substantially uniform or angled
electrode 204. Whereas the continuum of levels in FIG. 1 SE
provides a simply angled surface, in FIG. 15F, an electrode with a
measure of curvature is provided. A curved section 220, may be
useful in tailoring electrostatic attractions between an electrode
and mirror (or electrode and any intermediate structure such as a
yoke as in the Texas Instruments design) in order to match or
otherwise account for nonlinearities in restoring force provided by
flexure members. The curve shown is merely exemplary. However
configured, curved and angled electrode formats may be produced
utilizing advanced photolithography techniques (e.g., grayscale
masking) known to those with skill in the art.
[0112] Further variation of electrode structure that is
contemplated concerns providing the various electrode levels by
discrete, but electrically connected, members, rather than in a
continuous fashion. FIGS. 15G and 15H provide examples of such
approaches. In FIG. 15G, level steps 222 are provided, optionally
supported by a column 224 with a central via 228, a cantilever
design 226, or any combination of electrode designs described
herein. In FIG. 15H, level steps 222 and angled steps 230 are
provided. Any such electrodes may be addressed individually or
electrically interconnected.
[0113] Such structures may be provided by the technique(s)
described above or otherwise. For example, one method involves
deposition of multiple layers that build up the tiers.
Alternatively, from a single deposition of conductive material,
stepped electrodes can also be created using an individual mask per
tier. Each mask allows selective etching to define the separate
tiers of the whole electrode. Lastly, the Sandia developed
SUMMiT.TM. technique involves a combination of these and other
techniques.
[0114] Determining optimal curvature (and plan view), angle or
electrode level(s)--relative to substrate 136--may be determined
using known empirical and/or statistical modeling or analysis
techniques. The design of such aspects of the invention may account
for relationship between desired hinge/torsion bar deflection and
associated stresses, together with electrostatic attractions.
Certain configurations may be contemplated that have electrostatic
actuation advantages for given mirror and/or deflection
characteristics. Electrode shapes, in any of three dimensions, may
be determined via mathematical models accounting for theoretical
attractions and/or computer simulation or otherwise. For bistable
operation, the electrode shapes and nature of the models may be
relatively simple. Where the intent is to provide micromirror
devices suited for control analog or beam steering techniques, more
complex relationships between mirror angular displacement, related
forcing and electrode attraction may be required.
[0115] In addition to such variation as possible in the present
invention as described directly and incorporated herein, other
electrode configurations and overall mirror and related hinge
connection configurations are within the scope of the present
invention. In the embodiments of the invention shown and such
others as may be envisioned, it can be appreciated that variation
may also be presented, for example, with respect to the vertical
spacing of elements.
[0116] Notably, the height or relative spacing of selected items
may impact the size and/or orientation of components such as the
electrode regions. Namely, electrode shape and height may require
customization to avoid interference in meeting desired deflection
ranges of the micromirror.
[0117] In any event, numerous variations and possible micromirror
device configurations and related systems can be made utilizing the
various optional features disclosed herein. These variations each
present certain respective advantages as suitable for a given
application. Some of these advantages and applications have been
described merely by way of example. Such discussion is not intended
to limit the scope of the present invention. Indeed, certain
variations of the invention covered hereby may not even present
such advantages presented above by way of example. Further, the
invention may comprise, individually, micromirror devices or
element as described herein, just as it may encompass arrays of
such structures. The applicability may depend on the intended use,
many of which (but not all possible uses) have been mentioned.
[0118] In addition, it is noted that the features described herein
in connection with MEMS processing may be applied on a relatively
large scale. That is to say, as used herein the term "micromirror"
may be applicable to mirror structures upwards of 1 mm in
diameter/length/width. Such larger structures may find applications
outside the field of known projector or monitors. In all, it is to
be appreciated that devices made according to the present invention
may be employed not only in the context discussed referring to
displays and image projection. Further applications may involve
optical switching, adaptive optics, communications, light-shaping,
photocopiers, micro-displays (such as used in mobile electronics),
etc.
[0119] The breadth of the present invention is to be limited only
by the literal or equitable scope of the following claims. Efforts
have been made to express known equivalent structures and/or
features as may be applicable. That any such item or items may not
be expressed herein is not intended to exclude coverage of the same
in any way. Accordingly, I claim:
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