U.S. patent application number 11/152028 was filed with the patent office on 2008-06-05 for micromirror systems with concealed multi-piece hinge structures.
This patent application is currently assigned to Exajoule LLC. Invention is credited to Christopher M. Aubuchon.
Application Number | 20080130090 11/152028 |
Document ID | / |
Family ID | 32868905 |
Filed Date | 2008-06-05 |
United States Patent
Application |
20080130090 |
Kind Code |
A1 |
Aubuchon; Christopher M. |
June 5, 2008 |
Micromirror systems with concealed multi-piece hinge structures
Abstract
Micromirror systems with concealed multi-piece hinge structures
are provided for reflective applications. Generally, light is
reflected by these structures adapted for three-dimensional tilt as
well as up-and-down or out-of-plane actuation. Devices can be
produced utilizing the various optional features described herein
to provide miniaturized, highly controllable solutions for use in
optical switching, projection and other applications, especially
optical applications.
Inventors: |
Aubuchon; Christopher M.;
(Palo Alto, CA) |
Correspondence
Address: |
CHRISTOPHER M. AUBUCHON
1066 Metro Circle
Palo Alto
CA
94303
US
|
Assignee: |
Exajoule LLC
|
Family ID: |
32868905 |
Appl. No.: |
11/152028 |
Filed: |
June 13, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10374574 |
Feb 24, 2003 |
6906848 |
|
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11152028 |
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Current U.S.
Class: |
359/291 |
Current CPC
Class: |
G02B 26/0858 20130101;
G02B 26/0866 20130101; G02B 26/085 20130101; G02B 26/0841
20130101 |
Class at
Publication: |
359/291 |
International
Class: |
G02B 26/00 20060101
G02B026/00 |
Claims
1. A micromirror device comprising: a substrate with electrical
components including address circuitry; a micromirror; and a
support structure underlying said micromirror and interconnecting
said substrate and said micromirror, said support structure
including at plurality of deflection members, each deflection
member mounted to said substrate and said micromirror, and
configured to permit rotation of said micromirror about multiple
axes of rotation and drawing said micromirror toward said
substrate.
2. The device of claim 1, wherein two deflection members are
provided and mounted to two opposite ends of said micromirror.
3. The device of claim 2, wherein said two opposite ends of said
micromirror are opposite corners.
4. The device of claim 2, wherein said two opposite ends of said
micromirror are opposite sides.
5. The device of claim 1, wherein three deflection members are
provided and mounted to ends of said micromirror inopposite of one
another.
6. The device of claim 5, wherein said ends of said micromirror are
corners.
7. The device of claim 5, wherein said ends of said micromirror are
sides.
8. The device of claim 7, wherein said three deflection members are
mounted adjacent to corners of said micromirror.
9. The device of claim 1, wherein said deflection members are
mounted to said micromirror at substantially equally spaced
intervals.
10. The device of claim 1, wherein said deflection members are
mounted to said mirror in a pattern with bilateral symmetry.
11. The device of claim 1, wherein said deflection members are
mounted to said mirror in a pattern with three-way symmetry.
12. The device of claim 1, wherein said micromirror is
substantially quadrilateral.
13. The device of claim 12, wherein said micromirror is
substantially square.
14. The device of claim 1, wherein said micromirror is
substantially hexagonal.
15. The device of claim 1, wherein said deflection members are
mounted to said substrate at a common location.
16. The device of claim 1, wherein said deflection members are
mounted to said substrate at discrete locations.
17. The device of claim 1, wherein said deflection members comprise
at least one portion having a component in one direction and
another portion having a component in another direction.
18. The device of claim 17, wherein said components are provided in
a plane.
19. The device of claim 17, wherein said portions are provided by
straight sections.
20. The device of claim 19, wherein said portions are provided by
curved sections.
21. The device of claim 17, wherein said components are provided in
two different directions.
22. The device of claim 17, wherein said components are provided in
three different directions.
23. The device of claim 1, wherein said micromirror has a diameter
of less than or equal to about 1 mm.
24. The device of claim 23, wherein said micromirror has a diameter
of less than about 10 microns.
25. The device of claim 1, wherein said electrical components
further comprise electrodes adapted to apply attractive forces to
said micromirror.
26. The device of claim 25, wherein at least one of said electrodes
is configured with a plurality of portions at different levels, so
that portions further from a center of rotation of said micromirror
are at a greater distance from the micromirror than portions closer
to the center of rotation.
27. The device of claim 26, wherein each of said electrodes
comprises a stepped configuration.
28. The device of claim 26, wherein said electrode portions of each
said electrode are continuous with one another.
29. The device of claim 26, wherein said portions are discrete
members.
30. The device of claim 26, wherein each said electrode comprises a
continuous angled member.
31. The device of claim 26, wherein said portions of each said
electrode form an electrode array, and wherein at least one of said
portions of at least one of said electrodes is addressable
independently of the other of said portions.
32. The device of claim 31, wherein each said portion is
independently addressable.
33. A micromirror array comprising, a plurality of devices as
described in claim 1.
34. The array of claim 33, wherein a common mounting portion to
said substrate is provided for adjacent deformable members.
35. An optical switching mechanism, comprising: a first array of
optical reflectors adapted to receive and reflect optical signals
from at least one optical input source; and a second array of
optical reflectors adapted to receive optical signals reflected
from said first array of optical reflectors and reflect the optical
signals toward at least one optical output; wherein at least one of
said optical reflectors comprising an assembly of micromirror
devices according to claim 1.
36. The optical switching mechanism of claim 35, wherein each said
micromirror device of said at least one assembly of micromirror
devices is adapted for independent three dimensional
orientation.
37. The optical switching mechanism of claim 35, wherein each said
optical reflector comprises an assembly of micromirror devices.
38. The optical switching mechanism of claim 37, wherein each said
micromirror device is adapted for independent three dimensional
orientation.
39. The optical switching mechanism of claim 35, wherein each said
assembly of micromirror devices forms a smart surface.
40. A method of optical switching, comprising: providing an optical
switching mechanism as described in claim 35; directing light
through said optical switching mechanism; and switching light
between a plurality of channels.
41. The method of claim 40, further comprising shaping a reflected
wavefront of light with a plurality of micromirror devices
according to claim 1.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to the field of
spatial light modulators that can modify or correct an optical
wavefront. More particularly, the invention relates to micro
electro-mechanical systems (MEMS) in the form of micromirror
devices used in adaptive optics, optical switching applications, or
other light manipulation applications such as displays.
BACKGROUND OF THE INVENTION
[0002] MEMS devices are small structures, typically fabricated on a
semiconductor wafer using processing techniques including optical
lithography, metal sputtering or chemical vapor deposition, and
plasma etching or other etching techniques that have been 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 amount of commercial success.
[0003] MEMS micromirror devices are being used in a variety of
applications, including optical display systems, optical
cross-connects for switching of optical data signals and adaptive
optics for phase and other types of correction. One type of display
device that has been used with a great deal of success is the Texas
Instruments DLP.TM.. In this system, many mirrors are operated
individually in a bistable, digital fashion to create a projected
display. Although current commercial technology has been limited to
about 1.3 million pixels in the mirror array, greater mirror
densities and higher yields should improve this in the future as
the technology progresses.
[0004] Arrays of multi-axis tilting mirrors can also be found in
other applications, such as beam steering, printing, scanning, and
projection, among many. Most current arrays of micromirrors can be
separated into two categories: relatively large single mirrors that
steer a single beam, or arrays of smaller mirrors, where many
mirrors aim each light beam.
[0005] Larger mirrors can offer some advantages when steering a
smaller number of discrete light beams in terms of providing an
unbroken, nominally flat surface with high reflectivity. However,
if the beams are too large for the mirrors, or if they are
misaligned, the reflected beam is clipped and has less intensity.
These types of arrays are less suitable for reflecting larger,
continuous light such as an optical image. Generally, the support
structures between actuating mirror elements leave too much space
and thus create noticeable holes in the reflected image. Arrays of
smaller mirrors also have drawbacks. Many current designs may only
move in one axis, which limits some of their potential
applications. Others that can move in a multi-axis fashion also
often have relatively large gaps from one mirror to the next that
affect the quality of the reflected beam or image. Micromirrors set
in an array such as this must have some gaps between them to allow
full movement of each mirror, but it is advantageous to decrease
the size of the gaps as much as possible. In addition, many designs
have support structures that are small, yet are part of the visible
surface. These can also contribute to the spacing between mirrors.
Supports and hinges that are hidden behind the mirror surface would
improve the overall reflective surface area.
[0006] A particularly important application for multi-axis tilting
micromirrors is in the field of optical switching. A typical
optical cross-connect for an optical networking switch includes a
switching matrix having two arrays or clusters of MEMS
micromirrors. The first array of micromirrors is arranged so that
micromirrors in the first array receive optical input signals from
one or more input sources, such as optical fiber input(s) and the
second array of micromirrors is arranged so that micromirrors in
the second array receive optical signals reflected from
micromirrors in the first array and direct the signals as optical
output signals to one or more optical outputs.
[0007] The micromirrors in each array are capable of being
adjusted, steered or tilted, so that a micromirror in the first
array is capable of directing a reflected optical signal to a
micromirror in the second array selected from a plurality of the
micromirrors in the second array. Similarly, the micromirrors in
the second array can be adjusted, steered or tilted so as to align
with a micromirror in the first array selected from a plurality of
the micromirrors in the first array. Thus, by appropriate
orientation of the micromirrors by adjustment, steering or tilting,
a first micromirror in the first array can be set to deliver an
optical signal to a first, second, or third, etc. micromirror of
the second array, as desired, and so forth, thereby providing the
switching capability of the cross-connect.
[0008] The performance of optical cross-connects that use such
arrangements of MEMS micromirrors depends upon a number of factors,
including how well the micromirrors in the first array are
optically aligned with the micromirrors in the second array,
changes in temperature, voltage drifts, and performance of the
mirror surfaces of the micromirrors, which are affected by the
shape or flatness of the mirror surface. Even under the best
circumstances, when the micromirrors in the first and second arrays
are accurately aligned and the other factors mentioned above are
minimized, current cross-connects often lose 60% to 70% (about 4-5
dB losses) of the light passing through the system.
[0009] Although factors such as lost reflection of infrared
wavelengths from the mirror surfaces and poor coupling of fiber to
lenses play a role in these losses, light scattering and other
imperfections in the surfaces of mirrors are also significant
factors. There is a current need for improvements in optical
switching devices that will reduce the amount of losses in light
outputted by such devices when compared with the amount of light
inputted thereto.
[0010] Further improvements in optical switching devices, as well
as in micromirror devices in general would be desirable as regards
power consumption. The utilization of large mirrors relative to the
size of the light beam can involve rapidly switching high voltages.
One avenue for micromirror device improvement lies in continued
miniaturization of the devices. In terms of performance, smaller
sizes can improve power efficiency since smaller distances between
parts and lower mass parts will improve energy consumption. In
terms of manufacturing, continued miniaturization of mirror
elements offers greater yields for a wafer of a given size.
[0011] One other common application of micromirror devices is for
adaptive optics and phase correction. Although many types of mirror
arrays correct for tip and tilt such as those discussed for optical
switches, often correction of phase distortion is more desired.
Even though a static correcting mirror shape has its uses, phase
distortion is generally dynamic, and thus the mirror surface must
be constantly updated. A system such as this generally consists of
two parts, which are a wavefront detector and a deformable mirror.
A portion of the light being measured in question is split off and
directed to a wavefront detector such as a Schack-Hartman sensor
which measures tilts of the beam at various spatial positions
within the beam, or a similar sensor. Distortions in the light beam
can be detected, and feedback correction signals are then sent to a
deformable mirror surface to be updated in real time.
[0012] A number of designs for the deformable mirror using MEMS
have been presented in the last several years. One popular design
is that of a single flexible mirrored surface, with many individual
actuators that deform the entire surface at each point. Another
design is of multiple small mirrors, each operating in a manner
similar to a piston, with each individual mirror actuating
perpendicularly to the plane of the mirror. One enhancement to this
application can be seen in the present invention. Flexible members
supporting micromirrors with free ends can allow movement of an
entire mirror surface in the vertical direction as well as allowing
for tip and tilt. While the overall range of the device limits
motion ranges for each of the various types of motion, different
types of motion do not interfere with each other, and more than one
type of compensation could be done simultaneously.
[0013] 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, the advances offered by
aspects of the present invention represent a departure from
structural approaches represented by current micromirror
designs.
SUMMARY OF THE INVENTION
[0014] The present invention involves micromirror structures,
optionally used in adaptive optics or optical switches. 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, support members
or portions thereof. Support members are provided to hold a
mirror/micromirror above the hinge and the electrode features used
to actuate it.
[0015] The invention involves supporting each micromirror element
above its respective hinge portions at or along the sides or
corners of the mirror. Deformable hinge members are provided for
each mirror that are themselves supported above the substrate by
one or more features. The location of supports between the hinge
portions and a mirror may vary. Preferred placement locations
include opposite corners or sides of the mirrors, and alternating
(every-other) corner or side locations. Generally, mirrors will
have a polygonal plan in which the shapes are closely-packed (e.g.,
triangles, hexagons, and quadrilaterals such as squares,
rectangles, trapezoids, parallelograms, and rhombi).
[0016] In operation, the micromirrors are preferably operated in an
analog fashion, although operation in a digital fashion is
contemplated. The micromirrors are supported by hinge structures
which allow torsion motion and flexure or cantilever motion.
Individual hinges will most commonly have a bent shape with
substantially straight section or have a curvilinear profile,
either of which facilitates both of these types of motions. The
structures of the hinges and supports are designed to move in a
continuous, controllable fashion simultaneously in at least two
separate axes. Certain designs also allow for movement of the
mirror perpendicularly to the plane of the mirror in addition to
tilt. The bent shape of the hinges which twist as well as move in a
cantilever fashion give enough flexibility of motion to let the
mirror tilt and move perpendicularly at the same time.
[0017] By utilizing side-support features according to an aspect of
the present invention, it is possible to produce certain mirror
face embodiments that are unbroken by light-scattering or
non-reflective features. This approach to mirror and hinge support
or attachment described helps maximize available reflective surface
area. Further details regarding this approach including other
useful design characteristics are described much more extensively
in co-pending, commonly owned application Ser. Nos. 10/269,796;
10/269,763 and 10/269,478, each filed Oct. 11, 2002 and each of
which is incorporated by reference herein in its entirety.
[0018] For instance, in connection with such a side-supported
mirror approach, manufacturing techniques are taught in which
support precursor regions that are ultimately removed are
temporarily located where space is to be opened upon releasing the
individual micromirror elements of an array. As such, the space
required for effectively depositing/forming support structures is
not wasted but falls within space that must be left open anyway in
order to allow mirror actuation. In other variations of the
invention, more traditional columnar mirror supports formed within
"vias" are provided. However, these are still located near the
perimeter of a given mirror.
[0019] 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
[0020] FIG. 1 is a perspective view showing prior art of a generic
torsion bar mirror and gimbal assembly.
[0021] FIG. 2 shows a top view of prior art showing a single mirror
using flexure hinges as part of an optical switching array.
[0022] FIG. 3A shows a perspective view of the present invention in
its preferred embodiment; FIG. 3B shows the same views as FIG. 3A
but with the mirror removed;
[0023] FIG. 3C shows the same element from as a side view.
[0024] FIG. 4A shows a perspective view of a different
configuration of the present invention; FIGS. 4B and 4C are similar
to those in FIG. 3.
[0025] FIGS. 5A-5G show a process for making the device shown in
FIG. 3
[0026] FIG. 6A is a perspective view of a sample 7.times.7 array of
mirrors shown in FIG. 3 with some of the mirror tops removed; FIG.
6B shows a smaller portion of a similar array of FIG. 4 in its
preferred configuration.
[0027] FIGS. 7A-7E are perspective views showing a larger array of
FIG. 6B used as a programmable "Fresnel mirror."
[0028] FIG. 8 is a top view of a simple linear 3 by 3 optical
switch showing the mirror elements and light beams.
[0029] FIG. 9 is a close up view of a single light path of FIG. 8,
including optical fiber coupling using a single mirror switching
device.
[0030] FIG. 10 is a close up view of a single light path of FIG. 8,
including optical fiber coupling using a multiple mirror switching
device.
[0031] FIG. 11 is a perspective view of a three-dimensional 4 by 4
optical switch showing the mirror elements and light beams.
[0032] FIG. 12 is a perspective view of the same switch as shown in
FIG. 11, but with two mirrors replaced by multi-mirror arrays.
[0033] FIGS. 13A and 13B show prior art for a MEMS-based
phase-corrective adaptive optics technique; FIGS. 13C and 13D show
such activity as enabled by the present invention.
[0034] FIGS. 14A-14C show the same views of a variant element of
FIG. 3 using a different flexure design with via electrodes.
[0035] FIGS. 15A-15R shows a number of advantageous variants in
hinge and electrode design in simplified form, wherein
substantially straight hinge sections are employed; FIGS. 15A'-15R'
shows a number of corresponding variants to those in FIGS. 15A-15R
in which substantially curved hinge sections are employed.
[0036] FIGS. 16A-16C show several types of segmented electrode
designs with varying heights.
DEFINITIONS
[0037] The phrase "beam steering" refers to operation of one or
more micromirror devices in analog mode by charging address
electrode(s) to a voltage corresponding to a desired deflection of
the mirror to direct or "steer" the light reflected off the mirror
in the intended direction.
[0038] The term "diameter," is defined herein to mean 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.
[0039] The phrase "dim space" or "dead space" refers to areas or
spaces in the reflective surface(s) of a micromirror or micromirror
assembly which are not reflective or are poorly reflective.
[0040] The term "hinge" refers to a deflectable member or
deflectable member segments together (e.g., as formed by/in a
single layer of material); the hinge may be elastically deformed in
torsion, bending (tension and compression), or in some combination
thereof.
DETAILED DESCRIPTION OF THE INVENTION
[0041] In describing the invention in greater detail than provided
in the Summary above, applicable technology is first described.
Following this is a detailed description of exemplary micromirror
devices and assemblies according to the present invention, as well
as an exemplary process of production. Application of the invention
as a programmable lens surface is also discussed. This discussion
is followed by description of a known optical switching matrix and
its function. Finally, the applicability of the micromirrors of the
present invention to optical switch technology, as well as to other
fields of adaptive optics such as phase-correction is discussed,
along with several additional variants of the invention.
[0042] 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, as such 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. It is also to be understood that
the terminology used herein is for the purpose of describing
particular embodiments only, and is not intended to be limiting,
since the scope of the present invention will be limited only by
the appended claims.
[0043] Methods recited herein may be carried out in any order of
the recited events which is logically possible, unless the recited
language clearly indicates otherwise. Where a range of values is
provided, it is understood that each intervening value, to the
tenth of the unit of the lower limit unless the context clearly
dictates otherwise, between the upper and lower limits of that
range is also specifically disclosed. Each smaller range between
any stated value or intervening value in a stated range and any
other stated or intervening value in that stated range is
encompassed within the invention. The upper and lower limits of
these smaller ranges may independently be included or excluded in
the range, and each range where either, neither or both limits are
included in the smaller ranges is also encompassed within the
invention, subject to any specifically excluded limit in the stated
range. Where the stated range includes one or both of the limits,
ranges excluding either or both of those included limits are also
included in the invention.
[0044] 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.
[0045] Unless defined otherwise, 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. Although
any methods and materials similar or equivalent to those described
herein can be used in the practice or testing of the present
invention, the preferred methods and materials are now described.
All publications mentioned herein are incorporated herein by
reference to disclose and describe the methods and/or materials in
connection with which the publications are cited.
[0046] It must be noted that as used herein and in the appended
claims, the singular forms "a", "and", and "the" include plural
referents unless the context clearly dictates otherwise. Thus, for
example, reference to "a micromirror" includes a plurality of such
micromirrors and reference to "the input" includes reference to one
or more inputs and equivalents thereof known to those skilled in
the art, and so forth.
[0047] 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 publication by virtue of prior invention.
Further, the dates of publication provided may be different from
the actual publication dates which may need to be independently
confirmed.
[0048] Turning now to FIG. 1, a common type of multi-axis mirror is
shown in simplified form; this type of design should be well known
to those who have ordinary skill in the art. This is a
mirror/gimbal device, and works by electrostatic attraction of the
mirror to four individually controlled electrodes placed underneath
the mirror surface. Restoring force to bring the mirror back to a
central position and stabilize it at many intermediate positions is
provided by torsion bars. FIG. 1 shows a single multi-axis mirror
20, consisting of a mirror surface 22 connected via two torsion
bars 24 to a frame acting as a gimbal 26 that allow rotation around
the Y axis. The frame 26 is in turn connected by two more torsion
bars 28 to a stationary base 30. The frame and mirror assembly may
then rotate around the X axis. Four electrodes 33-36, set in an
array 32 can then be charged to various voltages to allow analog
control of the position of the mirror in both axes. For instance,
if electrode 33 is charged to a given voltage while electrodes 34,
35, and 36 remain uncharged, the mirror will rotate in both axes
toward that electrode. By careful control of the voltages of each
electrode, nearly any angle within the range of the device can be
set on one particular mirror, as the two axes of motion are able to
rotate independently.
[0049] This style of micromirror is most often used to direct or
steer a single beam of light, and is thus relatively large, that is
on the order of 1.times.1 mm. Gimbal 26 only moves in one
dimension, and the base 30 does not move at all, which makes these
portions of the mirror unsuitable for control. In the figure, the
gaps between mirror, gimbal, and base have been exaggerated for
clarity, but still must exist to allow free movement of the
constituent parts. In an array of mirror assemblies 20, a large
percentage of the total surface of the matrix is unusable.
[0050] FIG. 2 shows a different style of known micromirror 38 shown
here in a simplified form. In this design, a mirror 40 is attached
to a stationary base (not shown) via four posts 42 by rotationally
symmetric flexure bars 44 (in this case, four-fold rotational
symmetry). Here the primary restoring force is experienced in terms
of cantilever-action as opposed to torsion, but the primary axes of
movement are the same as the mirror shown in FIG. 1. The mirror is
mounted over an electrode array 32 similar to that shown in FIG. 1.
Once again, this type of mirror is designed to aim a single beam of
light, so there is a significant amount of dead space between
different mirrors used for the flexure bars and other parts of the
stationary frame. Even for aiming discrete beams however, it would
be advantageous for either of the systems shown in FIG. 1 or 2 for
this dead space to be substantially reduced or eliminated. When
packed in an array, mirrors such as this can direct hundreds of
beams at once. With less dead space between mirrors, a greater
number of beams could be switched with the same device, as the
controllable angle for each mirror is limited.
[0051] FIG. 3A shows a perspective view of a single micromirror
device 46, which will be referred to for purposes of describing
techniques for making the devices according to the present
invention. 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 the Background section
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. 6,198,180 to
Garcia, entitled Micromechanisms with Floating Pivot"; 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.
[0052] Regarding the features of the present invention, FIG. 3A
shows a micromirror device 46 according to the present invention.
FIG. 3B shows the micromirror device 46 minus its mirror 48. FIG.
3C shows the same device pictured in FIG. 3B in side view. The
mirror surface 48 shown in FIG. 3A is uninterrupted, thereby
providing superior reflection characteristics. While the "potential
face" of the mirror surface 48 (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) reflection losses
occurring from "dim" or "dead" space resulting from gaps 50 and the
spaces between the individual micromirror devices 46 in an assembly
of devices 46 is more than compensated for by the ability to orient
the individual devices to direct light that would otherwise be lost
due to scatter, misalignment, etc., as discussed above. Further, as
described below, the "dim" or "dead" space resulting from gaps or
openings 50 may be minimized or even eliminated according to an
aspect of the present invention with careful design of the mirror
supports 52.
[0053] As an alternative to the mirror supports 52 shown, columnar
supports or posts (not shown) may be utilized which may be created
by filling in vias produced in sacrificial material. Supports may
have a wall at the edge of mirror (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 the supports 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. Further details about these support variations can
be found in co-pending, commonly owned application Ser. No.
10/269,796, entitled "Micromirror Systems with Side-Supported
Mirrors and Concealed Flexure Members," incorporated by reference,
in its entirety, above (e.g., see FIG. 9A' in that
application).
[0054] However constructed, in the variation of the invention shown
in FIGS. 3A-3B, the location of the mirror supports are placed
equidistant from one another, forming (in a sense) an equilateral
triangle. Accordingly, their location is symmetric along three
mirror planes coincident with mirror 48 (i.e., they offer three-way
symmetry). Another way of looking at the arrangement is that the
supports are present on alternating corners. As such, opposites of
the mirror have no support counterpart (supports are inopposite one
another).
[0055] In certain other variations of the invention pictured (e.g.,
in FIGS. 4A-4B), each of a pair of supports is actually positioned
opposite one another and across the body of mirror 48.
Advantageously, the support positioning produces bilateral
symmetry. Equidistant placement about the mirror face is again
preferred. In any case, equal spacing or providing some measure of
symmetry offers greater predictability in control for various modes
of mirror actuation (i.e., it produces substantially equivalent
behavior). For the same reasons(s) a measure of rotational symmetry
in hinge structure is often desired--as is apparent in the figures
embracing aspects of the present invention.
[0056] Moving on with further details of the invention, one such
aspect concerns the manner in which mirror 48 is attached to its
hinge. Supports 52 on several sides of mirror element 48 secure it
to hinge portions 54 of an overall hinge 57. Hinge portions 58 are
attached to substrate 60 by a central hinge joint 63 that underlies
a portion (the center, in the example shown in FIG. 3B) of the
mirror element 48. Each hinge 57 (formed by portions 54, 55 and 58)
is provided in a single piece of metal, constrained in the center
to act as three separate flexure pieces or deformable member
portions. A base 72 of each support 52 may directly connect each
hinge portion 54. Alternatively, an intermediate layer or nub 74 of
material (e.g., serving as a bonding interface) may be employed. In
any case, the hinge elements are hidden or concealed beneath the
mirror.
[0057] Hinges 57 are elevated above substrate 60 to permit torsion
and cantilevered flexure about portions 54 and/or 58 as the mirror
element 48 is tilted or rotated about various axes. The two
portions 54 and 58 of each hinge are connected at a bend 55 that
allows greater movement and flexibility. Although other types of
central support are possible, hinge joint 63 is a via-type support
structure, preferably open underneath the hinge center 67, except
for a central post 69. The distance the hinges are set above the
surface of substrate 60 may be as little as about 0.1 micron, or
less.
[0058] As to the separation between the hinge portions and the
underside of mirror 48, this may--likewise--be as little as about
0.1 micron, or less. Hence, the mirror of each micromirror device
or element to be provided in an array or otherwise may be located
as little as about 0.2 micron, or less, above the surface of
substrate 60.
[0059] Avoidance of a central yoke to hold the mirror structure (as
in known devices produced by Texas Instruments) 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, electrostatic,
thermo-mechanical or piezo-based approaches.
[0060] 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 76. The electrodes may be configured
with a plurality of portions 78 and 80 (or more) at different
levels. Whether provided in a series of steps by continuous or
contiguous members (as shown with a support portion 82 between each
stage 78/80), 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. In this configuration, all three electrodes 76 are of
identical shape. Similar electrode shape and area simplify the
process of actuating a multi-axis mirror system, but other
configurations of electrode shapes and sizes, including asymmetric
variants are possible. In this case, there is a plurality of base
portions 80 for each electrode for added structural stability,
although a singular base portion 80 may be preferred.
[0061] The electrode configuration shown with higher portions
closer to the center of the mirror or overall device and lower
portions more distant therefrom provides clearance for the mirror
as it is tilted at an angle. Furthermore, the configuration
provides for sequential attraction of mirror 48. 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
downward (i.e., towards the upper electrode portion), the influence
of the electrode lower portion(s) increases as the distance between
the lower portion(s) and the mirror decreases. 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 80.
This and other variants of possible electrode shapes are explored
more fully in application Ser. No. 10/269,763, entitled
"Micromirror Systems with Electrodes Configured for Sequential
Mirror Attraction," which is incorporated by reference herein in
its entirety.
[0062] Device 46 is actuatable to move in a plurality of axes. One
difference of the present invention is one of scale.
Advantageously, the scale of the present invention may be made much
smaller than currently available micromirrors, with the diameter of
each mirror element being on the order of magnitude of 10-20
microns, or more. An array of hundreds or thousands of these
micromirrors may be used to replace a single mirror element as
shown in FIGS. 1 and 2. A greater number of micromirrors for each
portion of the incoming wavefront provides for much more
customization of the properties of the outgoing wavefront.
Miniaturizing the individual mirror size carries a number of other
benefits that will be detailed later. Nevertheless, it is
contemplated that the size of the mirror diameter may be increased
to as high as about 1 mm or more, with dimensions of the other
components of the device having increased dimensions in proportion
thereto. This would allow a single mirror element of the present
invention to effectively control an entire light beam.
[0063] In the configuration shown in FIG. 3, the hinges are
supported on the substrate at the center of the device. FIG. 4
shows another highly advantageous design, but with the hinges being
supported on the outside of the device. The features of this mirror
assembly 47 are displayed similarly to that of FIG. 3. Many of the
same features are present, but are connected in a different
fashion. In this variation, the base and mirror are square,
although a hexagonal or other shape would also work with this type
of hinge structure.
[0064] As in FIG. 3, FIG. 4 shows a mirror 48 connected to a hinge
portion 54. The other end 58 of each hinge portion 57 is connected
to a hinge joint 62. Similar to FIG. 3, the hinge portions 54 and
58 are connected by a bent portion 55. As compared to device 46 in
FIG. 3, in device 47, hinges 57 are physically separated.
[0065] Hinge joints 62 are bridge-type support structures,
preferably open underneath the hinge center 64, which is attached
to a spanning segment 66 between vertical support segments 68. Feet
70 may additionally be provided to stabilize the support structure.
Yet another option is to produce support segments 68 at an angle
relative to the surface of the substrate (i.e., having both
vertical and horizontal components). Features of the electrodes 76
are essentially equivalent to those as presented in FIG. 3.
[0066] Each of the designs presented in FIGS. 3 and 4 are connected
differently, but they both offer much of the same function. Both
are designed to tilt in a plurality of axes simultaneously in a
manner similar to that shown in U.S. patent application Ser. No.
Attorney Docket No. EXAJ-003, entitled, "Multi-tilt Micromirror
Systems with Concealed Hinge Structures," filed on even date
herewith and incorporated by reference. Yet, because the hinges
have been designed to allow both torsion and cantilever, they are
also flexible enough to easily permit movement of the mirror in the
direction perpendicular to that of the plane of the mirror surface.
By providing for multi-axis tilt and vertical motion (relative to
the substrate) the devices according to the present invention allow
for correction of both tilt and phase as discussed below.
[0067] Further differences between the center-supported and
end-supported hinge structures in aspects of the invention (e.g.,
as in FIG. 3 vs. FIG. 4) concern deflection angels as may be
obtained for each given configuration. Namely, use of a central
support clear regions otherwise occupied by hinge supporting
structure. Accordingly, greater deflection angels can be achieved
for central-supported devices than end-supported devices of the
same thickness (mirror height).
[0068] The manner in which a micromirror device 46 according to the
present invention may be produced is illustrated in FIGS. 5A-G. Of
course, the process steps employed will vary depending on which
inventive features are actually employed in a given variation of
the invention, as would be readily apparent to those of ordinary
skill in the art.
[0069] In FIG. 5A, a sacrificial layer of material 84 is set upon
substrate 60. It is patterned with a first mask 86 to define a
substrate-level portion 88 upon etching. In FIG. 5B, a hinge metal
layer 90 is deposited over the entire surface including a portion
of the sacrificial layer. A second mask 92 is utilized in defining
a passivation layer (not shown) over the region(s) of layer 90
serving as a hinge precursor region 94. Metal layer 90 fills in
vias 96 provided in substrate 60 to form a connection 98 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 60.
[0070] As shown in FIG. 5C, a thicker layer of conductive material
100 is deposited over the hinge material. This layer builds-up the
electrodes 76 and nubs 74, and hinge support 63 for hinge portions
57. Layer 100 also further fills in via 96 and connecting structure
98. A third mask 102 is employed to define a protective layer (not
shown) over the region of layer 100 serving as electrode
precursor(s) 104, nub precursors 106, and hinge support precursor
108.
[0071] In FIG. 5D, layers 90 and 100 are shown selectively etched
to reveal hinges 57, hinge joint 63, and electrode portions 76. In
FIG. 5E, one can see another sacrificial layer 110 which then
covers these structures. A fourth mask 112 is used to pattern
sacrificial layer 110 to form mirror support precursor regions 114
upon etching the sacrificial layer.
[0072] FIG. 5F shows sacrificial layer 110 as it is selectively
etched, and then coated with a layer 116 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 118 is used in order to define a
passivation layer over mirror precursor regions 120 to be retained,
but not the adjacent borders 122, which are removed to form spaces
between adjacent micromirrors 46.
[0073] FIG. 5G shows a micromirror element 46 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 various 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.
[0074] FIG. 6A shows an exemplary array assembly 124 of
micromirrors 46 in a seven across arrangement. Part of the tops of
the surface of the mirrors 48 have been removed to show the
underlying structures. This type of mirror array, comprised of
potentially millions of individually controlled mirrors, can be
used for a number of applications. Not only can the micromirrors 46
of assembly 124 be positioned to mimic a tilting of micromirror
device 20 about its tilt axes (or some combination of both) and
thus represent a flat reflective surface, but the micromirrors 46
can be independently positioned to form a "smart surface" (i.e.,
one that can be adapted to modify the shape of a reflected
wavefront, in response to a given wavefront that is made incident
thereon). For this reason, assembly 124 can be described as a
"Fresnel mirror" or a "programmable lens". FIG. 6A can also be seen
or regarded as only a small section of a much larger array having a
million such mirrors or more. Of course, array size will vary upon
choice of application and the number of mirrors reference is in no
way intended to be limiting.
[0075] Array 124 as shown in FIG. 6A has some advantages because of
its simplicity in layout. Additionally, addressing for this layout
would be consistent for each mirror element 46. A similar array of
mirrors can be made from device 47 shown in FIG. 4, with each
mirror element being set in the same orientation in a square
array.
[0076] However, it may be preferred to use a different layout of
mirror elements 47 as shown in FIG. 6B. Here, a smaller 2 by 2
section of a slightly different array 126 is displayed. In this
configuration, the devices 47 (shown without their mirror top as in
FIG. 4B) are unchanged, but the orientation of the elements 47
varies. Every other mirror element 47 is rotated by 90.degree. in a
checkerboard pattern. The main advantage is seen in the position of
the hinge supports 62, for which each support 62 meets at a corner
with three other supports 62 to make one large support 128 common
to the adjacent deformable hinge portions. Large support 128
reduces the number of discrete features to be manufactured, and
offer greater structurally stability than individually manufactured
supports 62. In addition, because the structure shown in FIG. 6B is
a combined structure (as opposed to four free standing elements),
the overall dimensions of the support can be made smaller, which
can lead to longer hinges and more mobility of the hinge.
[0077] Micromirror arrays 124 or 126 provide great versatility in
their ability to form surfaces for performing wavefront
correction/shaping, and by their ability to be oriented to form
other than flat surfaces, including configurations which form
effective programmable lenses. Examples of such configurations are
shown schematically in FIG. 7A-E. FIG. 7A displays an array similar
to the same array assembly 126 as seen in FIG. 6B. For illustrative
purposes, a lowercase "e" 130 is used as a sample object to be
reflected off mirror assembly 126. The image 132 results from the
particular configuration of the micromirrors. A dotted line 134
portrays where the "e" is reflected on the mirror surface, and the
path of light follows an arrow 136. In FIG. 7A, the array is set
completely flat, so there is little change in the image other than
simple reflection. For comparison to later figures, the image 132
is set against a reference set of crosshairs 138.
[0078] For simplicity, FIGS. 7B-E will all use the same object 130
as seen in FIG. 7A, which will be omitted in these figures. Only
the final image 132 will be shown. FIG. 7B shows mirror array 140,
which is the same array as 126, but with all of the mirrors tilted
in the same direction. The image 142 size is unchanged, but its
position has moved. Thus the array 140 acts here as a tilting flat
mirror. FIG. 7C displays an array 144, once again the same array
126, but this time individual elements 47 are tilted to approximate
the surface of a focusing mirror, as can be seen in the resulting
image 146, shown having a reduced size.
[0079] FIG. 7D shows a similar array 148 that is now set with
mirror elements 47 approximating a defocusing mirror, which again
can been seen in a correspondingly larger image 150. In FIG. 7E,
array 152 displays both tilt and focus on the same set of mirrors,
with the results showing up in image 154. Because each mirror
element can be set independently of each other, potentially several
types of correction could be implemented simultaneously. Although
tilt and focus can be addressed on the same set of mirrors, there
are some important limitations. Depending on the dimensions of the
components, tilt angles of individual mirrors might be only as much
as 10.degree. in each axis. However, tilt angles of some designs
may be as large as 20.degree. or more. If a set of mirrors is
tilted acting as a flat mirror so that all of the mirrors are near
or at their maximum angle, it will not be able to further focus or
defocus the bean. In addition, tilting of curved surfaces can lead
to astigmatism in the reflected beam, although this can be
corrected by careful shaping of the programmable lens at the
specified angle.
[0080] Although they are not shown here, many other types of
wavefront shaping are possible. Various types of lenses other than
spherical can be created, such as cylindrical or parabolic mirrors.
Distortions in the incoming beam due to imperfections in earlier
surfaces can be detected and corrected. Any number of other
surfaces can be created subject to the angular limitations of the
micromirrors.
[0081] In practical use, such a small subset 126 (or 124) of a much
larger mirror array would not be used for shaping beams or images,
but this size is used here for illustrative purposes. The mirrors
in a somewhat larger portion of the array shaped in this manner
would not be seen as clearly to have a curvature because the
differences from one mirror to the next would be slight. One
advantage to the independently addressable nature of these mirrors
is that the array as a whole can be divided into many fields, each
with their own properties. One simple way to accomplish this is to
use a defined grid, where the center of each subsection of the grid
is set to be the center of a programmable Fresnel mirror, each with
its own focal length and tilt angle. The present invention is not
limited to this manner of controlling the micromirrors. Any
arbitrary number or shape of field may be independently
controlled.
[0082] Another important application of the present invention is in
the area of switches for optical networking. Looking at FIG. 8, a
schematic representation of a switching matrix 154 is shown. For
simplicity of explanation and drawing, only a linear matrix 154 is
shown, although two-dimensional matrices are also commonly used.
The matrix of FIG. 8 includes two arrays 156, 166 of steerable
micromirrors (158, 160, 162 and 168, 170, 172, respectively). Each
of the micromirrors are "steerable" by their operation in analog
mode, which involve actuation of micromirrors 158, 160, 162, 168,
170, 172 to a voltage corresponding to a desired deflection of the
mirror surface of the micromirror, as is known to those of ordinary
skill in the art.
[0083] In this way, each of the three micromirrors 158, 160, 162
can be tilted about an axis of rotation so as to direct an optical
signal received from its input channel (163, 164, 165 respectively)
toward any one of micromirrors 168, 170, 172 that corresponds to
the output channel 173, 174, 175 that is desired to be outputted
to. Input and output channels are often optical fibers with
focusing optics. For example, micromirror 158 can be oriented to
reflect an input signal received from input channel 163 to
micromirror 168, which, in such instance, is also tilted to
optically couple the output from micromirror 158 with optical
output fiber 173. Alternatively, the voltage can be varied to the
electrode(s) of micromirrors 158 and 172 so that they are optically
coupled with one another in which case the optical input from
optical input fiber 163 is outputted to optical output fiber 175,
and so forth. U.S. Pat. No. 6,389,190 describes a switching matrix
of the type shown in FIG. 8 in detail, including an example of MEMS
micromirrors that may be employed in the construction of such a
switching matrix.
[0084] As noted in the Background section of the present
application, currently available optical switching mechanisms
experience a significant loss of the optical signals passing from
an input to the output thereof, i.e. on the order of a 60% to 70%
loss. Significant contributions to these losses are due to scatter
of light from the light signal as it passes from an input optical
fiber to first and second mirrors (in input and output mirror
arrays) and finally to an optical output fiber. Aberrations in the
surfaces of the micromirrors on the input and output arrays can
contribute to the scattering of light as well as to deformation of
the light beam to the extent that a portion of the light beam can
become misaligned with the output optical fiber by the time that
the input optical signal (light beam) has been reflected off an
input side micromirror and an output side micromirror. Misalignment
of the fibers to the mirrors can be a significant source of loss,
in addition to loss coupling into the output fiber due to
mismatched beam sizes.
[0085] Another source of error can be created if the tilt axes of
the input and output mirrors are not parallel in an arrangement
such as shown in FIG. 8, although this primarily affects only
arrays of mirrors that only have a single axis of movement. In such
a case, opposing input and output mirror surfaces of input and
output side micromirrors, which are intended to be aligned to
optically couple an input optical fiber and an output optical
fiber, can never be aligned in parallel, which results in a skewing
of the optical beam such that a portion of the output signal does
not enter the output optical fiber. Similar negative effects can
occur due to thermal and other environmental effects, as well as
aging of the components.
[0086] The present invention can be applied to optical switching
configurations to correct for, and thereby substantially eliminate
sources of losses due to misalignment of mirrors, imperfections in
mirror surfaces and other physical causes of light loss through a
switching mechanism. FIG. 9 is a schematic, two-dimensional
representation of a switching arrangement which shows only one
micromirror device (158, 168 respectively) from each of the input
and output arrays, for simplicity of discussion. An optical input
signal is incident upon micromirror device 158 as delivered by
input channel 163. Seven light rays 177-183 representing parts of a
single light beam are schematically represented in FIG. 9 as being
incident on the mirror surface of micromirror device 158, from
which they are then reflected toward micromirror device 168 and are
then directed to output channel 173, where 158a and 168a represent
the axis of rotation of these mirrors. In this example, light rays
177 and 183 become misaligned with the mirror surface of
micromirror device 168 due to expansion of the beam hitting the
mirrors. Similarly, light ray 179 is misdirected and is not
delivered to output channel, due to light scattering which may be
caused by a lack of flatness in the mirror surface of micromirror
device 158 or other malformation which cause misalignment of the
light reflecting off of a portion of the mirror.
[0087] Note that although the micromirror devices 158 and 168 are
adjustable by tilting about axes 158a, 168a, that any further
adjustment of the micromirror devices 158, 168 in FIG. 9 would not
result in a reduction of light loss. For example, if micromirror
158 were tilted by a slight rotation in the clockwise direction,
then ray 183 might be directed onto the surface of micromirror 168,
but at the same time, ray 178 might then miss the reflective
surface of micromirror 168, and certainly ray 177 would still not
be reflected off of micromirror 168. Similarly, while rotation of
micromirror 168 in the counter clockwise direction might direct ray
177 into the optical output channel 173, it would also misdirect
ray 178 at the same time, so as to no longer be channeled into the
output channel 173.
[0088] By replacing each of the micromirror devices 158, 168 with a
plurality of smaller and more controllable micromirror devices 46
as shown in FIG. 10 according to the present invention, a
micromirror assembly (184, 186, respectively) is provided to
replace each of the micromirror devices 158, 168 of FIG. 9. Each
micromirror assembly is made up of a plurality (seven, in the
non-limiting schematic example shown) of micromirror devices 46
(labeled here as 46-1 through 46-14), each of which is
independently controllable for movement of a mirror surface about
two axes of rotation independently, or about an unlimited number of
axes of rotation defined by the resultant vectors occurring when
rotating to varying degrees about both axes of rotation. In the
example shown, the micromirror device 46-1 has been rotated or
tilted in the clockwise direction so as to align ray 177 with the
corresponding micromirror (46-8) on assembly 186, so that ray 177
is now properly channeled to the optical output channel 173. It is
noted that the rotation of micromirror 46-1 is shown in an
exaggerated fashion for purposes of explanation, and that the
actual rotation of the mirror would be much less, so as to be
essentially imperceptible, while having the effect of "flattening"
the reflective surface in this location for reflecting the ray 177
in the correct direction. Similarly, micromirror 46-7 is tilted or
rotated somewhat counterclockwise, so as to accurately direct light
ray 183 to be incident upon micromirror device 46-14, which in turn
directs the ray 183 into the optical output channel 173. The
micromirror device 46-3 in assembly 184 has been tilted or rotated
in the counterclockwise direction to redirect ray 179 into the
proper position on assembly 186 to permit proper direction into the
output channel.
[0089] In this particular example seen in FIGS. 9 and 10, the light
beam from optical input channel 163 is shown slightly expanding to
illustrate details of the present invention. In an actual system,
the light beam would be carefully focused to keep the spatial
profile of the beam as small as possible throughout the length of
the optical switch. By keeping the beam size small, the size of the
individual mirrors and the spacing between them can be made smaller
as well, which allows for a high density switching matrix. This is
desirable, since MEMS micromirror designs generally have a limited
usable angular range of movement, so a greater density leads
directly to a higher port count in the switch. Changing the beam
size in this manner has several adverse consequences however. Even
if all of the rays are focused so that they hit the focusing lens
at the output channel, the outgoing angle of each ray (177-183) may
not be best for good coupling efficiency into the fiber. Adjustment
of both of the switching mirrors for best coupling does not
eliminate much of the loss. To accommodate for relatively large
spacing between the mirrors, often a long distance between mirrors
is needed. Maintaining small beam spot sizes throughout a longer
distance requires very good uniformity of focusing elements on the
incoming beams. These constraints can make it difficult to create a
working device with a high port count when using larger flat
mirrors.
[0090] By providing a plurality of independently adjustable
micromirrors 46 to form each micromirror assembly (such as 184 and
186), input and output arrays of these assemblies can then be
constructed in making an optical switching or cross-connect
apparatus which is capable of reducing the amount of light loss, as
compared with those currently available. The ability to
independently adjust devices 46, in effect, gives the assemblies
the ability to optically adapt to the wavefront of the optical
signal that is being received, and to manipulate that wavefront to
maximize the amount of the signal that is ultimately received at
the output end of the switching device. The reflective surfaces of
the micromirrors 46 in assemblies 184 and 186, as has been
previously mentioned, can thus be used as programmable Fresnel
mirrors to focus the light received and reflected so as to minimize
losses. It is much easier to keep light focused properly over a
short distance as opposed to a long distance. Using a plurality of
mirrors allows any focusing element after an input fiber to be
optimized for only a short distance to the first mirror array. The
first mirror array can focus properly to the second mirror array,
and the second mirror array will then focus properly for good
coupling into the output fiber.
[0091] Although a small amount of light is lost due to the spacing
between the micromirror devices 46 in each array, this loss is
small compared to the amount of light that would ordinarily lost by
a system such as shown in FIG. 9, but which is recaptured by proper
orientation of the micromirrors 46 as discussed with regard to FIG.
10. Although the example of FIG. 10 has been described with regard
to a one dimensional assembly of micromirrors 46 to form the
assemblies 184, 186 (i.e., a 1.times.7 array of the micromirrors 46
in each assembly), it is to be noted that the present invention is
in no way limited to such arrangement, as such arrangement has been
described only for purposes of simplicity. In fact, the more likely
assembly is to include a two-dimensional array of micromirrors 46
to form each assembly, even with a switching setup as shown in FIG.
8. For example, a micromirror device 20 can be replaced by a
10.times.10 or 100.times.100 array of micromirrors 46 to form an
assembly such as the array 124 shown in FIG. 6. Because the
micromirrors 46 have the ability to tilt or rotate about two axes
(or combinations thereof, as noted above) it is preferable to form
a two-dimensional array of the micromirrors in each assembly, for
three-dimensional redirecting, or focusing, of incident light
signals.
[0092] FIG. 11 is a schematic representation of a three-dimensional
cross-connect arrangement 190. In this arrangement, the micromirror
devices can be three-dimensionally positioned so as to optically
connect any micromirror in the optical input array 192 with any
micromirror in the optical output array 198. Thus, for example, by
appropriate position of micromirror 194 and a corresponding
micromirror in the optical output array 198, the optical input 204
can be directed to be outputted to any of optical outputs 206-209
by appropriate optical connection of micromirror 194 with
respective ones of optical output micromirrors 200-203. Each of
input side micromirrors 195-197 has similar possibilities, with the
appropriate corresponding adjustment of the output
micromirrors.
[0093] FIG. 12 is a three-dimensional cross-connect 210 similar to
the cross-connect 190 in FIG. 11, but with two of the mirror
elements 194 and 196 being replaced by exemplary 5.times.5 arrays
214 and 216. Replacing the single mirror elements has a number of
advantages already addressed, but which may be more apparent as
seen in this figure. In this example, the two arrays are set to act
as two different focusing elements for the beams being switched. If
input beam 204 has slightly changed focusing characteristics than
input beam 218, the array elements can be adjusted independently to
maximize the coupling efficiency. Most often, the input beams of an
optical switch are focused coming out of an optical fiber by either
a lens array or an individual lens on each fiber. Uniformity of
focal lengths for these lenses can be very difficult to achieve,
yet small changes in the consistency of these lenses can have a
large effect on the focusing of the beam, and thus on the
efficiency of coupling light back into the output fiber. Using a
small array of mirrors to act as an additional lens element as a
part of the optical path allows for slight changes in the input
beam without sacrificing power throughput into the output. In an
actual system based on these designs, all of the rest of the input
and output mirrors 195, 197, 200-203 would be represented by a
similar array of micromirrors.
[0094] Another advantage of the multimirror array is that of
positioning. In a single mirror system as shown in FIG. 11, each
input fiber must be precisely aligned to the center of its
corresponding mirror along with all the other input fibers. Small
irregularities may make the input beam clip on the edge of the
mirror or miss it altogether. Individual adjustments of each fiber
and/or focusing components is commonly not possible, so much of the
time, the entire array of input fibers and lenses are aimed to find
the best compromise position, even though throughput on many of the
channels suffers as a result. Achieving exact uniformity is no
simple task. One possible solution for this problem is to make the
size of each individual mirror in the arrays 192 and 198 larger.
While this reduces the problem of losing power from the beam at the
edges of each mirror, it does have the drawback of lowering the
density of the mirror array as a whole, which as has already been
discussed, will lower the total port count of the switch. In FIG.
12, a single mirror is replaced by an array of mirrors. In
actuality, each single mirror would be replaced by an array of
mirrors with no particular boundaries between the original surfaces
of the single micromirrors, but which was displayed in this fashion
for clarity. In FIG. 12, between array 214 and 216 would appear a
number of identical mirrors; in aggregate, all of the mirrors would
make a single large array. If the center of a single fiber was
tilted incorrectly by a small amount to change the position of its
input beam on the mirror array, this new location could be chosen
as the center of its steering set of mirrors with no extra loss to
the beam falling off an edge. Of course, uniformity of the input
fibers and beams cannot be so poor as to overlap parts of more than
one beam at any one place on the array as a whole, but small
changes that would adversely affect performance on single-mirror
systems will not harm throughput here. In addition, the lack of
boundaries for the array domain of a single light beam allows a
tighter spacing of the beams, which as has been mentioned
previously, would allow for a higher density switch with greater
port count.
[0095] FIG. 6A showed an assembly of micromirror devices 46 that
comprise the present invention which form a micromirror assembly
124 effective to replace micromirror device 194 (or any of the
other micromirror devices 195-197 and 200-203) in FIG. 11, although
this size of array is not limiting. A typical micromirror device
currently used in the optical switching field employs a mirror
surface having width and length dimensions of about 1 mm.times.1
mm. Using present techniques, micromirror devices 46 or 47
according to the present invention may be made to have a mirror
surface as small as about 8 microns by 8 microns. Thus, an assembly
of 15,000 micromirrors (i.e., a 125.times.125 array) could be
provided to replace a single micromirror device in the prior art.
As noted previously, the present invention is not limited to this
size of micromirror, as micromirrors having dimensions larger than
8 microns by 8 microns can certainly be made, such that an assembly
having a plurality, but a lesser number than one hundred
micromirror devices 46 could be assembled to replace a single known
micromirror device. Also, it is contemplated that even smaller
micromirror devices 46 will be possible as the state of the
manufacturing arts progresses.
[0096] Another important application of the mirror devices in the
present invention is one commonly used in adaptive optics, which is
that of phase-correction. This has been increasingly utilized in
various fields such as astronomy or confocal microscopy to improve
the quality of their imaging techniques, in some cases drastically.
The primary component of a phase-correcting adaptive optics system
is a deformable mirror surface, which was discussed in general in
the Background section but will be shown here in more detail. The
deformable surface is generally changed in real time to correct for
phase distortions between various portions of the image.
[0097] FIG. 13A shows a portion of a common type of adaptive optics
mirror used in the prior art in a simplified form. The mirror
assembly 240 is composed of a single flexible mirror surface 242
over a substrate 246. The mirror surface 242 is held over the
substrate 246 by a sparse array of linear actuator elements 244,
six of which are shown here. Actuator elements 244 might work using
a variety of methods, such as electrostatic attraction,
piezoelectric effects, thermomechanical, or others. Spacing of
actuators 244 is normally chosen to maximize control of the mirror
surface but minimize complexity. Because the mirror surface
connects all of the actuators into one large system, moving the
mirror to a desired shape can involve complicated interactions
between various elements 244. Often detailed models are needed in
order to adjust the mirror surface 242 to a desired shape. One
advantage to this type of system is that the reflective surface is
single and unbroken.
[0098] FIG. 13B displays a similar device 248, but in this case,
individual mirrors 250 replace the large mirror surface 242. As can
be seen here, the heights of the mirrors can be adjusted to mimic
most any shape capable of by device 240. In comparison, this mirror
surface has a number of small gaps in it, which may scatter
incoming light, especially if the gaps are too large. This is
compensated for by the flexibility of the device and the
straightforward control, as each mirror's position does not affect
the one next to it. For ease of comparison, the two devices shown
in FIGS. 13A and 13B are the same scale, although this is not
typically the case. In most cases, the multimirror device 248
requires many more micromirrors and a higher density of actuators
to achieve the same shape; power consumption of this device can be
higher.
[0099] A device that can be built with the present invention bears
some resemblance to array 248 in FIG. 13B. FIG. 13C shows an array
of individual elements 46 that can be actuated in an up/down
fashion. If all of the electrodes of a particular mirror element
such as 46 are driven at the same voltage, equal force will be
applied to the entire mirror in the vertical direction
perpendicular to the plane of the mirror. Because the design of the
hinges gives great flexibility, the assembly 46 should have dynamic
range down to the stopping point of the mirror. However, as shown
in FIG. 13D, with mirror elements according to the present
invention, additional features may be offered. Namely, one of the
advantages of the inventor's designs is the ability to actuate
mirrors in multiple axes at once. Unequal driving voltages on the
various electrodes will both tilt the mirror and drive it
vertically simultaneously.
[0100] Thus, both tilt-correction and/or phase-correction of
incident light can be implemented on the same surface. This not
only can greatly simplify the control electronics needed to drive
the system as a whole, but also may have other optical benefits as
well. In some types of systems, spatial distortions can be created
in a light beam when the tip/tilt and phase-corrective surfaces are
separated in space. By having both corrections performed by the
same surface, some artifacts such as parallax errors can be
eliminated
[0101] Note also that scale of the micromirrors can play an
important role here. In the micromirrors 46 as described earlier,
the preferred size of each mirror is relatively small, down to the
order of 8-10 microns in diameter. Smaller sizes allow for lower
power consumption and greater angle for a given mirror height. This
may not be best for phase-correction applications; many of these
applications need movement of the mirror surface to be several
wavelengths of the light in question. In most cases, this is no
more than several microns. The smallest scale of the mirrors does
not allow for this range of movement. Creating the micromirrors
somewhat larger or with different thicknesses of components will
allow for a greater range, but may limit the angle permitted for
tilting. It is likely that for a mirror surface that allows both
tilt and vertical movement, the scale chosen will have to be a
compromise between these two extremes to give a device useable for
both types of movement, at least for some applications. For other
applications, little transverse movement of the mirror is
necessary, and even the smallest mirrors will work well.
[0102] A number of other variants to the design of the present
invention are possible other than the one shown in FIG. 3. FIGS.
14A-C provide details of a micromirror 220 with a hexagonal-shaped
mirror top supported at several side positions. Its construction
and appearance closely resemble the micromirror elements 46 shown
in FIGS. 3A-C, and views for each part of the figure are the same.
In this variant of the device, the hexagonally shaped mirror 48 is
attached to the hinge structure 54 at its side supports 52, another
hinge portion 58 is attached to the substrate 60 with a similar
hinge joint 63.
[0103] As for hinge structure, a difference to be observed is that
each hinge structure 57 includes an additional bar/section 61
between the other two portions of the hinge (54 and 58). Longer
hinge sections as thus provided offer more compliance than that of
hinge 57 in FIG. 3, and may allow for easier actuation of the
mirror. A potential drawback in this design, however, is that the
additional area taken up by the larger hinge impinges on area
previously taken up by electrode 76. Yet, the greater movement
allowed by the longer hinge structure should make up for this
deficiency. As noted above, the selection of certain features of
the invention for their relative benefits may depend on the
application sought.
[0104] Another difference in this design as compared to that in
FIGS. 3A-3B is shown in the electrode design. In FIGS. 14A-114C,
instead of a staggered multi-level electrode 76, a single stepped
electrode 226 is shown. Using a via-based support column 228, the
entire electrode surface 230 is raised close to the mirror. This
configuration allows for the maximum electrostatic attraction.
[0105] Although a via electrode may be preferred in that less
voltage should be needed to achieve the same degree of
electrostatic attraction, for manufacturability, a staggered
electrode may be preferred. A via-type electrode may potentially
block the mirror from reaching its maximum angle of tilt in certain
directions for some electrode/hinge layouts. One other electrode
variant, not pictured here, is that of using a flat single
electrode low on the substrate 60. Although this design would have
the least electrostatic attraction, and thus be more difficult to
actuate, it would be very simple to manufacture. Of course, any of
the electrode designs described with regard to the example in FIG.
14B is not limited to the micromirror shown in FIG. 14, but may be
applied to any of the other designs described herein and vice
versa.
[0106] Note that regardless of the choice of electrode
configuration in any of the invention's variants, the hinges,
mirror, and electrode have been designed not to interfere with each
other. Because the hinge and electrode layers are constructed at
the same time, there is no overlap between them except at the
junctions. At the mirror's maximum angular extension in both axes,
the mirror touches down only on the substrate 60 or on the hinge
supports 62. While in normal operation, the mirror and attached
metal structure are charged up to a bias voltage, and the
addressing electrodes are charged to a different voltage, there is
no danger that the structures will short out and potentially damage
or disable one or many other mirrors.
[0107] A number of other configurations of hinges and electrodes
are possible using the same conceptual framework than have been
taught here. A schematic view of some of the connectivity possible
is shown in FIG. 15. The schematic mirror layout 252 in the upper
left hand corner of FIG. 15A corresponds to the design presented in
FIGS. 3A-3C; that of FIG. 15 N corresponds to the design of FIGS.
4A-4C is in; FIG. 15C corresponds to FIGS. 14A-14C.
[0108] In each mirror layout, a dark circle 256 corresponds to a
generalized connection to the substrate, dark lines 254 correspond
to hinge material, and open circles 258 correspond to connections
of hinge material to the mirror. The listing of possible
configurations in each of FIG. 15 is by no means intended to be
exhaustive, and is only a sample of the possibilities covered
herein. In addition, although for simplicity only hexagonal and
square shapes are shown, many other shapes are possible that neatly
tile a plane. The designs shown, however may be most preferred for
the superior capabilities they offer as determined by the present
inventor.
[0109] Still further, it is noted that the variations of the
invention schematically pictured in FIGS. 15A-15R are represented
by counterparts in FIGS. 15A'-15R'. These "counterparts" that
employ substantially curved hinge sections. In both sets of
figures, the hinges share the common characteristic of having
components or portions that are directed along different paths
(i.e., that extend in different directions). For stress relief
purposes, it may in fact be the case that hinges/deformable members
with a more or less curvilinear profile are employed in the
invention.
[0110] Still further variation possible in connection with the
present invention has to do with multiple electrodes replacing any
of the electrodes discussed previously, whether staggered, via, or
flat. Some of these possible designs are shown in FIGS. 16A-C. FIG.
16A shows substantially the same element as is seen in FIG. 4B, but
with the hinge substructure removed so that only the substrate 60
and the electrodes 232 are left. In this case, each of the
electrodes 76 (e.g., from the embodiment of FIG. 4B) has been
replaced by an electrode array 232. Although various types of
electrodes have been proposed with different shapes and height
configurations, the electrode array 232 is different than
previously mentioned ones because each element of the array 232 can
be addressed independently.
[0111] FIG. 16B shows a different type of electrode array 234 used
in place of single electrodes 76. Here, not only the shape of each
subelectrode is varied, but also the height, to vary the field
strength of each element in the array 234. FIG. 6C displays yet
another potential variation, where electrode arrays 236 are broken
up further in two dimensions. Single element electrodes 76 require
fine control of voltage to enable analog movement of the mirror for
continuous positioning. Using a multi-element electrode, control
circuitry for each mirror element 46 may be considerably less
complex. Potentially, with sufficient numbers of addressable
electrodes underneath a mirror, near analog control of a mirror may
be possible using digital addressing.
[0112] Other configurations of electrodes 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. Notably, the height or relative
spacing of selected items may impact the size and/or orientation of
components such as the electrode regions. That is, electrode shape
and height may require customization to avoid interference in
meeting desired deflection ranges of the micromirror.
[0113] In addition, it is noted that 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,
height and/or length. Such larger structures may find applications
outside the fields mentioned here. 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 optical
switching arrangements, but further applications involving adaptive
optics may apply.
CLAIMS
[0114] Whatever the case may be, 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.
* * * * *