United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,661,591
`
`Lin ,et a].
`
`[45] Date of Patent:
`
`*Aug. 26,} 1997
`
`USOOS661591A
`
`[54] OPTICAL SWITCH HAVING AN ANALOG
`BEAM FOR STEERING LIGHT
`
`[75]
`
`Inventors: Tsen-Hwang Lin; Philip A. Congdon;
`Gregory A. Magel, all of Dallas;
`James M. Florence, Richardson. all of
`Tex.; Robert Mark Boysel, Hopewell
`Jct, NY.
`
`[73] Assignee: Texas Instruments Incorporated,
`Dallas, Tex.
`
`[*] Notice:
`
`The term of this patent shall not extend
`beyond the expiration date of Pat. No.
`5,629,794.
`
`[21] Appl. No.: 537,179
`
`[22] Filed:
`
`Sep. 29, 1995
`
`Int. Cl.6 ..................................................... G028 26/00
`[51]
`[52] US. Cl. ........................... 359/290; 359/214; 359/850
`[58] Field of Search ..................................... 359/290, 291,
`359/212. 213, 214, 846, 847, 848, 849,
`850, 851
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS ‘
`
`Primary Examiner—Georgia Y. Epps
`Assistant Examiner—Dawn-Marie Bey
`Attorney, Agent, or Firm-—Robert C. Klinger; James C.
`Kesterson; Richard L. Donaldson
`
`[57]
`
`ABSTRACT
`
`A spatial light modulator (40,70,80,90,130) operable in the
`analog mode for light beam steering or scanning applica-
`tions. A deflectable mirror (42, 72) and which may be
`hexagonal (92, 132) is supported by a torsion hinge (44,86,
`94) ends along a torsion axis. A plurality of flexure hinges
`(48,82,106) are provided to support the ends of the mirror
`(42,72,92,132) and provide a restoration force. The combi-
`nation of the torsion hinges and the flexure hinges realizes
`a deflectable pixel that is operable in the linear range for a
`large range of address voltages. The flexure hinges also
`maintain a flat undeflected state when no address voltage is
`applied, and prevent the pixel from collapsing. The pixel
`may be reinforced, such as about its perimeter (74) to ensure
`mirror flatness and prevent warping, even during extreme
`deflections of the mirror. The pixel
`is electrostatically
`deflected by applying an address voltage to an underlying
`address electrode (60,96,98). The hexagonal mirrors (92,
`132) allow a tightly packed mirror array. and have a closely
`circular surface area so as to efliciently reflect a light beam
`of circular cross section, such as a light beam from fiber
`optics.
`
`5,489,952
`
`2/1996 Gove et a1.
`
`............................. 348/771
`
`16 Claims, 9 Drawing Sheets
`
`
`
`
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`
`
`FNC 1033
`
`

`

`US. Patent
`
`Aug. 26, 1997
`
`Sheet 1 of9
`
`5,661,591
`
`26
`
`FIG.
`
`7 A
`
`(PRIOR ART)
`
`28
`
`INCIDENT
`
`LIGHT
`
`R535?”
`
`‘
`
`FIG. 13
`(PRIOR ART)
`
`
`
`DEFLECTION
`
`COLLAPSED
`
`
`
`VOLTAGE
`
`VCOLLAPSE
`
`FIG.
`
`1C
`
`(PRIOR ART)
`
`

`

`US. Patent
`
`Aug. 26, 1997
`
`Sheet 2 of 9
`
`5,661,591
`
`’9
`
`DD
`CID
`
`DD
`
`“IUD
`
`45° FLAP
`
`.
`
`90° FLAP
`
`DIVING
`BOARDS
`
`FIG.
`
`1D
`
`FIG.
`
`1E
`
`FIG. 1F
`
`IIIEIEII a
`
`DEI
`
`45° EXTENDED
`HINGE
`
`90° EXTENDED
`HINGE
`
`FIG. 10
`
`FIG. 7H
`
`
`
`

`

`US. Patent
`
`»
`
`Aug. 26, 1997
`
`Sheet 3 of9
`
`5,661,591
`
`LIGHT
`
`STEERING
`
`RAN/Cy.
`219
`
`INCIDENT
`
`LIGHT
`
`
`
`I
`
`54
`
`56
`
`FIG. 3A
`
`DEFLECTION
`
`
`
`
`\ OPERATING RANGE
`
`ADDRESS VOLTAGE
`
`FIG. 3B
`
`70
`/
`
`
`STRUCTURE
`
`REINFORCINC
`
`

`

`US. Patent
`
`Aug. 26, 1997
`
`Sheet 4 of 9
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`5,661,591
`
`72
`
`74
`
`FIG. 5
`
`J______L
`J__._____L
`
`F_‘-'—'__1
`
`Ir-"-—--1I
`
`'74
`REINFORCING
`
`STRUCTURE
`
`

`

`US. Patent
`
`'
`
`Aug. 26, 1997
`
`Sheet 5 of 9
`
`5,661,591
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`
`
`
`
`
`
`

`

`US. Patent
`
`Aug. 26, 1997
`
`Sheet 6 of 9
`
`5,661,591
`
`
`
`

`

`
`
`
`
`
`
`
`
`
`
`
`
`

`

`US. Patent
`
`5,661,591
`
`
`
`
`
`
`
`
`
`

`

`US. Patent
`
`Aug. 26, 1997
`
`Sheet 9 of 9
`
`5,661,591
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`
`
`
`
`

`

`5,661,591
`
`2
`
`1
`OPTICAL SWITCH HAVING AN ANALOG
`
`BEAM FOR STEERING LIGHT .
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`10
`
`15
`
`25
`
`35
`
`45
`
`50
`
`55
`
`of mirrors can be modulated and directed to a projector lens
`and then focused on a display screen or a photoreceptor
`drum to form a light image. When the mirror is in one
`position, known as the “on” position, light is directed into
`the projector lens. In the other position, known as the “off"
`mirror position, light is directed to a light absorber. The
`DMD may also be monostable and operated in the analog
`mode, and finds use as a light switch, pixel steerer, optical
`shutter, scanner, and the like.
`’
`For a more detailed discussion of the DMD device and
`systems incorporating the device, cross reference is made to
`US. Pat. No. 5,061,049 to Hornbeck, entitled “Spatial Light
`Modulator and Method”; US. Pat. No. 5,079,544 to
`DeMond, et al, entitled “Standard Independent Digitized
`Video System”; and US. Pat. No. 5,105,369 to Nelson,
`entitled “Printing System Exposure Module Alignment
`Method and Apparatus of Manufacture”, each patent being
`assigned to the same assignee of the present invention, and
`the teachings of each are incorporated herein by reference.
`Gray scale of the pixels forming the image may be achieved
`by pulse width modulation techniques of the mirrors, such as
`that described in US. Pat. No. 5,278,652, entitled “DMD
`Architecture and Timing for Use in a Pulse-Width Modu-
`lated Display System”, assigned to the same assignee of the
`present invention, and the teachings of each are incorporated
`herein by reference.
`Commonly assigned US. Pat. No. 4,662,746 to Hornbeck
`entitled “Spatial Light Modulator and Method”, US. Pat.
`No. 4,710,732 to Hornbeck entitled “Spatial Light Modula-
`tor and Method”, US. Pat. No. 4.956,619 to Hornbeck
`entitled “Spatial Light Modulator”, and US. Pat. No. 5,172,
`262 to Hornbeck entitled “Spatial Light Modulator and
`Method" disclose various structures and methods of produc-
`ing micro mechanical devices. specifically, monostable
`DMD SIM’s suited for use in the analog mode, the teach-
`ings of each incorporated herein by reference.
`Commonly assigned US. Pat. No. 5,096,279 to Hornbeck
`et a1. entitled “Spatial Light Modulator and Method”, US.
`Pat. No. 5,142,405 to Hornbeck entitled “Bistable DMD
`Addressing Circuit and Method”. and US. Pat. No. 5,212,
`582 to Nelson entitled “Electrostatically Controlled Pixel
`Steering Device and Method” disclose various structures
`and methods for producing the same that are bistable and
`suited for use in the digital mode, the teachings of each
`incorporated herein by reference.
`Referning to FIGS. 1A—1H,
`these embodiments being
`disclosed in a commonly assigned US. Pat No. 5,172,262,
`there is shown a monostable DMD spatial light modulator
`that can be operated in the analog mode. One pixel, gener-
`ally denoted at 20. is basically a flap covering a shallow well
`and includes a silicon substrate 22, a spacer 24, a hinge layer
`26, a pixel layer 28, a flap 30 formed in layers 26—28, and
`plasma etch access holds 32 in flap 30. The portion 34 of
`binge layer 26 that in not covered by pixel layer 28 forms a
`hinge attaching flap 30 to the portion of layers 26—28
`supported by spacer 24. Pixel 20 is fabricated using a robust
`semiconductor process upon silicon substrate 22. Spacer 24
`may be an insulating positive photoresist or other polymer,
`hinge layer 26 and pixel layer 28 are both an aluminum,
`titanium and silicon alloy (Ti: Si: Al), although these layers
`could also comprise of titanium tungsten, or other suitable
`materials. The hinge layer 34 may be about 800 Angstroms
`thick, wherein the pixel 30 is much thicker to avoid cupping
`and warping, and may have a thickness of about 3,000
`Angstroms.
`'
`Pixel 20 is operably deflected by applying a voltage
`between mirror 30 and an underlying address electrode 36
`
`Cross reference is made to the following commonly-
`assigned co—pending patent applications, the teachings of
`which are incorporated herein by reference.
`
`
`
` Ser. No. TITLE FILING DATE
`
`
`
`08/414,831
`
`08/455,475
`
`Spatial Light Modulator with
`Supersmicture Light Shield
`08/097,824 Microminiature, Monolithic,
`Variable Electrical Device and
`Apparatus Including Same
`08/424,021 Active Yoke Hidden Hinge
`Digital Micmmirror Device
`08/171,303 Multi-Level Digital Micromirror
`Device
`08/482,477 Method and Device for Multi-
`Format Thievision
`Spatial Light Modulator Having
`an Analog Beam for Steering
`Light
`04-18-95
`08/424,021 Active Yoke Hidden Hinge
`Digital Micromirror Device
`
`03-31-95
`
`07-27-93
`
`04-18-95
`
`12-21-93
`
`06-07-95
`
`FIELD OF THE INVENTION
`
`The present invention is generally related to spatial light
`modulators for modulating incident light and suitable as an
`optical switch, and more particularly, to a device having a
`selectively deflectable mirror being supportedby at least one
`hinge for steering incident light in a direction being a
`function of the degree to which the mirror is deflected.
`
`BACKGROUND THE INVENTION
`
`Spatial light modulators (SLM‘s) have found numerous
`applications in the areas of optical information processing,
`projection displays, video and graphics monitors,
`televisions, and electrostatic printing. SLM’s have also
`found uses as optical switches, optical shutters, light valves,
`pixel steerers and so forth. SLM’s are devices that modulate
`incident light in a spatial pattern to form a light image
`corresponding to an electrical or optical input. The incident
`light may be modulated in its phase, intensity, polarization,
`and/or direction. The light modulation may be achieved by
`a variety of materials exhibiting various electro-optic or
`magneto—optic effects, and by materials that modulate light
`by surface deformation.
`The present invention relates to SLM’s of the foregoing
`type which may be used in a variety of devices, including
`light switches. light valves, pixel steerers and optical shut—
`ters.
`
`A recent innovation of Texas Instrument Incorporated of
`Dallas, Tex. is the digital micrornirror device or deformable
`mirror device, collectively known as the DMD. The DMD is
`a spatial light modulator comprising a monolithic single-
`chip integrated circuit, typically having a high density array
`of 17 micron square deflectable micrornirrors but may have
`other dimensions. These mirrors are fabricated over address
`
`circuitry including an array of memory cells and address
`electrodes. The mirrors may be bistable and be operated in
`the digital mode, the mirror being stable in one of two
`deflected positions. A source of light directed upon the
`mirror is reflected in one of two directions by the mirror.
`When used in the digital mode, incident light from the army
`
`65
`
`

`

`5,661,591
`
`3
`defined on substrate 22. Flap 30 and the exposed surface of
`electrode 36 form the two plates of an air gap capacitor. and
`the opposite charges induced on the two plates by the
`applied voltage exert electrostatic force attracting flap 30 to
`substrate 22. This attractive force causes flap 30 to bend at
`hinge 34 and be deflected toward substrate 22. Depending on
`the opposing surface area of the electrodes, the spacing
`therebetween. the difierential voltage applied, and the com—
`pliance of hinge 34. the degree of deflection of mirror 30 will
`vary. The deflection of mirror 30 is a function of the
`diiferential voltage, as graphically illustrated in FIG. 1C. As
`shown. the greater the difierential voltage. that is, the greater
`the voltage applied to mirror 30. the greater the degree of
`deflection.
`
`As also illustrated in FIG. 1C, this deflection is nonlinear.
`and is not proportional to the voltage applied. A linear
`response. which may be the ideal response. is shown by the
`dotted line generally depicted at 38. The nonlinear relation—
`ship is due to many reasons. First. the electrostatic force is
`a function of the inverse of the square of the distance
`separating the mirror 30 and address electrode 36. Secondly,
`the geometry and composition of the hinge aifects the
`compliance of hinge 34. The thickness of mirror 30 prevents
`significant warping. but the thinness of hinge 34 allows for
`large compliance. The deflection of flap 30 is a highly
`nonlinear function of the applied voltage because the restor—
`ing force generated by the bending of hinge 34 is approxi-
`mater a linear function of the deflection. but the electro-
`static force of attraction increases as the distance between
`the closest corner of flap 30 and electrode 36 decreases.
`Recall that
`the capacitance increases as the distance
`decreases so the induced charges both increase in quantity
`and get closer together. As shown in FIG. 1C. the voltage at
`which mirror 30 becomes unstable and bends all the way to
`touch and short with electrode 36 is called the collapse
`voltage. The analog operating region is that region between
`zero deflection and the collapsed situation.
`FIG.
`lD—lH illustrate equivalent alternative embodi-
`ments of the cantilever or leaf-type mirror 30 shown in FIG.
`1.
`
`When operating a spatial light modulator, such as of the
`type just discussed and referenced in FIG. 1A—1H, it may be
`desired to operated the deflectable member in the analog
`region. whereby the angle of deflection of mirror 30 is
`linearly proportional to the voltage applied. To operate the
`device as a light beam steerer, scanner. or light switch. it is
`desirable to precisely control the degree of deflection as so
`to precisely steer incident light to a receiver. such as a
`sensor. Therefore. in prior art designs, such as that shown in
`FIGS. 1A—1H. it is imperative that a repeatable process be
`followed. In the practical world. however. process tolerances
`allow for some deviation form device to device. Thus. for a
`given voltage being applied to address electrode 36. the
`deflection of mirror 30 from device to device will vary
`slightly. Consequently. characterization of the device prior
`to implementation is necessary when the device is used in
`the analog mode.
`It is desired to provide a spatial light modulator suitable
`for use as an optical switch with a deflectable pixel well
`suited to be used in the analog mode.
`
`SUI/[MARY OF THE INVENTION
`
`The present invention achieves technical advantages as a
`spatial light modulator by providing a hexagon-shaped pixel
`supported by both a torsion hinge and at least one flexure
`hinge. The torsion hinges define the tilt axis. and the flexure
`
`10
`
`15
`
`20
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`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`hinges define the flat position and provide a restoring force
`to achieve enlarged stable range of tilt angles. Through the
`combination of these hinges. the pixel is nearly monostable
`and will resist collapse. The hexagon pixel structure allows
`the pixels to be tightly arranged in an array. the hexagon
`pixel geometry also being suitable to reflect circular light
`beams such as those from fiber optics.
`The present invention comprises a spatial light modulator
`of the type which includes a generally planar hexagonal
`light-reflecting pixel. i.e. mirror. which is deflectable out of
`a first. normal position into a plurality of second positions.
`Light incident on the pixel is selectively modulated by being
`reflected from the pixel in selected directions, depending on
`the position of the pixel. The position of the pixel
`is
`dependent on a selected characteristic of an electrical signal.
`such as a voltage applied to an underlying address electrode.
`The deflection of the pixel stores potential mechanical
`energy in a pixel—supporting facility.
`this stored energy
`tending to return the pixel to the first, horizontal normal
`position. Preferably. this pixel-supporting facility includes a
`first torsion hinge connected between the pixel and the first
`stationary post and defining a torsion axis. Deflection of the
`pixel eflects its rotation about the torsional axis of the first
`hinge. At least one second flexure hinge is connected
`between the pixel and a second stationary post. Deflection of
`the pixel eflects the flexure of the second hinge. By provid-
`ing two types of hinges, deflection of the pixel is control-
`lable for a large deflection range, and approximately pro-
`portional to the electrical signal applied to the underlining
`address circuit. preferably being an electrode. The pixel is
`monostable and cannot collapse on the address electrode,
`unless of course. large address voltages are provided. The
`hexagonal geometry of the pixel is suited to reflect circular
`light beams, such as those from fiber optics, while permit-
`ting a tightly packed arrangement of pixel mirrors. Excellent
`accuracy of the pixel mirror tilt angle for light steering is
`achieved. with the second hinge providing a well-defined
`undeflected (flat) position. Both the first hinge and the
`second hinge provide a restoration force.
`The points of connection of the first and second hinges to
`the pixel are separated about the perimeter of the pixel. The
`pixel has a generally orthogonal profile. with the first hinge
`being connected to the pixel at a mid-point of a first side
`thereof. The second hinge may be connected to the first side
`of the pixel as well in one embodiment. The second hinge
`may be connected to the pixel proximate the juncture of the
`first side of pixel and a second side of the pixel. The second
`hinge may also be connected to the second side of the pixel
`including the most distal portion of the pixel. from the
`torsional axis. The torsional axis of the first hinge is gener-
`ally co-planar with the pixel, and the second hinge in its
`unflexed state is oriented so as to be generally co—planar with
`the pixel and perpendicular to the torsional axis of the first
`hinge. The second hinge in its flexed state may define a
`curved or slightly twisted surface at its corner. such that each
`end remains co-planar with the surface to which it is
`attached.
`
`the
`The characteristic of the first hinge is such that
`deflection of the pixel out of the first position is predomi-
`nantly rotational about the torsion axis. The characteristic of
`the second hinge is such that the first position of the pixel is
`predominantly determined by the second hinge. The char—
`acteristics of both the hinges are such that the pixel is
`selectively deflectable out of the first position into a plurality
`of second positions determined by the selected characteristic
`of the electrical signal. The first and second hinges are
`respectively connected to the separated first and second
`
`

`

`5,661,591
`
`5
`posts. In an alternative embodiment, the first and second
`hinges may be connected to the same post. The pixel may be
`reinforced to maintain flatness when undeflected, and when
`deflected out of the first position. Preferably, the reinforce—
`ment may comprise the perimeter of the pixel being corru-
`gated or ridged to provided rigidity and minimize interfering
`with incident light being modulated. Alternatively or
`additionally, cupping of the pixel may be minimized by
`providing radial corrugations or ridges.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGS. 1A—1H illustrate a prior art cantilever-type spatial
`light modulator. including a deflectable pixel deflectable as
`a function of an address voltage applied to an underlying
`address electrode;
`FIG. 2 is a perspective view of a spatial light modulator
`according to the present invention including a deflectable
`pixel supported both by a torsion hinge, and at least one
`flexure hinge connected proximate the corners thereof;
`FIG. 3A is a side view of the SLM of FIG. 2 with the
`torsion hinge posts removed, illustrating the pixel being
`deflected about the torsion hinge to steer incident light in a
`selected direction, the deflection of the pixel being a func-
`tion of the voltage applied to the underlying address elec-
`trode;
`FIG. 3B is a graph illustrating the linear deflection of the
`mirror as a function of the addressed voltage.
`FIG. 4 is a top View of an alternative preferred embodi-
`ment of the pixel shown in FIG. 2, whereby the perimeter of
`the pixel is reinforced to insure the pixel is flat, rigid and
`remains unflexed, even when pivoted about the torsional
`axis;
`FIG. 5 is a cross section taken along line 5—5 in FIG. 4,
`illustrating the reinforcing structure of the pixel comprising
`the pixel being corrugated about the perimeter;
`FIG. 6 is a top View of yet another alternative preferred
`embodiment wherein the torsion hinges and flexure hinges
`are supported by a common post;
`FIG. 7 is a perspective view of a spatial light modulator
`according to an alternative preferred embodiment of the
`invention including a hexagon-shaped pixel supported by a
`pair of torsion hinges, and a pair of flexure hinges;
`FIG. 8 is a top View of an array of hexagon shaped pixels
`of FIG. 7 illustrating the tightly packed arrangement of the
`pixels;
`FIG. 9 is a perspective View of a spatial light modulator
`according to another alternative preferred embodiment of
`the invention including an hexagon-shaped pixel elevated
`and overlapping underlying support superstructure;
`FIG. 10 is a partial cross section of the pixel shown in
`FIG. 9, illustrating the hexagon mirror supported by the
`underlying yoke; and
`.
`FIG. 11 is a perspective view of an array of pixels
`comprised of pixels shown in FIG. 9.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Referring to FIG. 2, there is generally shown at 40 a
`monostable spatial light modulator according to the pre-
`ferred embodiment of the present invention. SLM 40 is well
`suited to operate in the analog mode to selectively steer
`incident light in a direction being a function of an electrical
`input. The deflection of the SLM may be generally linear or
`nonlinear, as a function of the input electrical signal. The
`
`6
`pixel resists collapse on its underlying address electrode due
`to the unique combination of hinges.
`SLM 40 is a micromechanical structure formed using
`robust semiconductor processing. SLM 40 is seen to com-
`prise a generally rectangular reflective aluminum pixel 42
`being supported along its mid section by a pair of torsional
`hinges 44. Hinges 44 essentially bisect the mirror 42 and
`define a torsional axis therealong. Mirror 42 is also seen to
`be supported proximate each corner thereof by an L-shaped
`flexure-type hinge 48. Torsional hinges 44 extend from and
`are supported by a respective electrically conductive post 50,
`with the flexure hinges 48 being supported by a respective
`electrically conductive post 54. Each of posts 50 and 54 are
`supported upon a silicon substrate 56, this substrate 56 also
`supporting a pair of electrically conductive address elec-
`trodes 60. Each of address electrodes 60 are connected to
`underlining address circuitry fabricated within substrate 56
`(not shown). Hinges 44 connect to mirror 42 in a pair of
`opposed notches 58, notches 58 permitting long and com-
`pliant hinges 44. While the hinges, mirror and support posts
`are preferably comprised of electrically conductive material,
`each or all could also be comprised of electrically non-
`conductive material with the pixel having a light-reflective
`coating if desired. Hinges 48 could also have a serpentine
`shape, or be linear if desired Hinges 48 could connect to
`pixel mirror 42 or either side proximate the corner of the
`adjoining sides, and limitation to the illustrated shapes of
`hinges 48 is not to be inferred.
`Referring to FIG. 3A the angular deflection of mirror 42
`about the torsional axis defined by hinges 44 is seen to be a
`function of the voltage potential applied to one of the
`address electrodes 60. With a bias voltage being applied to
`mirror 42 via posts 54 and hinges 48, and an address
`potential being applied to one of the two address electrodes
`60, this voltage potential induces an electrostatic attraction
`between the mirror 42 and the underlying address electrode
`60, thus creating an angular deflection of mirror 42, as
`shown. Torsional hinges 44 rotate or twist with mirror 42
`and provide restoring force in the form of a mechanical
`energy. Each of the four flexure hinges 48 also provide a
`restoring force, and deform or flex, as shown, when mirror
`42 deflects about the torsional axis defined by hinges 44.
`Hinges 48 also provide restoring force in the form of
`mechanical energy, and define a normal flat or undeflected
`position when no voltage potential exists.
`By way of illustration but with no limitation to the
`following dimensions or shapes, torsional hinges 44 are
`preferably comprised of a compliant material such as
`aluminum, an aluminum alloy or titanium tungsten, each
`having a thickness of about 500 Angstroms. Each of flexure
`hinges 48 are also comprised of a compliant metal, such as
`aluminum, an aluminum alloy, or titanium tungsten, each
`having a thickness of about 500 Angstroms. The length of
`each hinge 48 is approximately 10 microns. Each hinge 48
`extends from the respective post 54 a substantial length
`towards arespective torsional hinge 44 and is perpendicular
`therewith. Each flexure hinge 48 has a 90° bend proximate
`mirror 42, and is connected to the corner of pixel 42, at the
`juncture of two adjacent sides, as shown. The short segment
`of hinge 48 insures that a majority of the flexure of hinge 48
`is along the major length, with any twisting taking place at
`the corner thereof. With the hinge 48 being about 500
`Angstroms in thickness and having a length of about 10
`microns, the flexure of these hinges permits mirror 42 to
`deflect as a function of the voltage potential provided to one
`address electrode 60. as shown in FIG. 3A. The spacing of
`mirror 42 from electrodes 60 is about 1—10 microns. This
`
`10
`
`15
`
`2O
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`6O
`
`65
`
`

`

`5,661,591
`
`7
`analog operating range is represented as angle 6. as shown
`in FIG. 3A. This corresponds to an input voltage of between
`0 and 20 volts. As pixel 42 deflects angle 6, incident light is
`steered through a range of 29. as shown in FIG. 3A. As
`expected due to optical light properties. the angular range
`that incident light can be reflected is double the angular
`deflection of the pixel 42. In the present invention, pixel 42
`can be deflected with a linear or non—linear response,
`depending on the design. up to angle being about 10°. Thus,
`the range of steering light is 20°. The response curve of
`mirror 42 as a function of address voltage is shown in FIG.
`3B.
`
`With an address voltage being applied to one address
`electrode 60 being from 0 to 20 volts, mirror 42 is deflected
`proportional
`to the address voltage. When SLM 40 is
`operated as an optical switch or light steerer, incident light
`can be precisely steered to a receiver such as an optical
`sensor or scanner. The mirror tilt angle can be achieved with
`a excellent accuracy for pixel steering. The torsion hinges 44
`define the tilt axis. whereby the flexure hinges '48 help
`achieve a controlled response and maintain mirror levelness
`when not addressed (undeflected). Both the torsion hinges
`and flexure hinges provide a mechanical restoration force
`and achieve a stable tilt range. With two flexure hinges 48
`being provided each side of the torsion axis. there is little
`possibility of a collapsed mirror, unless one of the hinges
`should break, which is not likely given the range of operable
`address voltages provided to address electrode 60. Compli-
`ance of each of flexure 48 and torsion hinges 44 is excellent.
`Referring now to FIG. 4. a top view of an alternative
`preferred embodiment of the present invention is shown as
`SLM 70, with a modified mirror 72 is shown, wherein like
`numerals refer to like elements of the first embodiment. To
`ensure that mirror 72 remains flat and does not warp. even
`in an extreme deflected state. the perimeter of mirror 72 is
`reinforced. This reinforcement preferably is achieved by
`corrugating the perimeter of the mirror. shown as a trench
`shown at 74. Referring to FIG. 5. a cross section of mirror
`42 taken along line 5—5 in FIG. 4 illustrates how mirror 42
`is corrugated about the perimeter thereof. The trench 74 in
`the metal is one reliable way to reinforce mirror 72 to
`prevent warping or cupping. even when deflected about the
`torsion axis. Other equivalent methods of reinforcing could
`include increasing the thickness of mirror 72 about the
`perimeter thereof. like a rib or ridge, and grid:
`Referring now to FIG. 6. another alternative embodiment
`of the present invention is shown. as SLM 80. wherein like
`numerals refer to like elements. Each of flexure hinges 82
`are connected to one of posts 84 from which torsion hinges
`86 extend. This embodiment requires only two support posts
`as compared to the six support posts shown in FIG. 2. Again.
`each of the flexure hinges 82 extend from the respective
`posts 84 and connect to one corner of mirror 72, proximate
`the juncture of two adjacent sides. All the hinges provide a
`restoration force to return the mirror to a flat. undeflected
`state when no address voltage is applied to address electrode
`60. This embodiment permits a higher fraction of optically
`active surfaces since SLM elements can be placed closer
`together.
`Referring now to FIG. 7. there is generally shown at 90 a
`pixel according to another alternative preferred embodiment
`of the present invention. Pixel 90 is seen to have a hexagonal
`shaped mirror 92 supported by a pair of torsion hinges 94
`over a pair of addressing electrodes 96 and 98. Each of the
`hinges 94 is supported by a support post 100 extending
`upward from a substrate 102. Also supporting mirror 98 is
`a pair of arcuately shaped flexure hinges 106. Each of
`
`8
`flexure hinges 106 is connected to the distal end of mirror 92
`at location 108, by a respective member 110. Each member
`110 perpendicularly extends from the distal end of mirror 92
`each side of the torsion axis defined by hinges 94. Each of
`hinges 106 is supported at each end by a support post 114
`also extending upwardly from substrate 102, as shown.
`Pixel 90 is similar to pixels 40 and 70 as shown in FIG.
`2 and FIG. 4, respectively. in that the pixel mirror is
`supported by both the torsion hinges and flexure hinges. The
`torsion hinges 94 define a torsion axis of rotation, whereby
`flexure hinges 106 limit the downward flexure of Inirror 92
`toward either of the addressed electrodes 96 and 98. The
`geometry of the hexagonal mirror 92 allows the pixels 90 to
`be arranged in a tightly packed arrangement. such as that
`shown in FIG. 8. The length of flexure hinges 106 are
`substantially longer than the length of torsion hinges 94 to
`provide good flexibility in the hinges 106 and facilitate
`rotation of the mirror 92 about the torsion axis. the flexure
`
`10
`
`15
`
`20
`
`hinges 106 limit the deflection of mirror 92 to prevent
`shorting of the mirror 92 to the underlying address elec-
`trodes 96 and 98.
`
`25
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`30
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`35
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`45
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`50
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`55
`
`65
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`Referring now to FIG. 8. there is shown a spatial light
`modulator pixel array generally shown at 120. As shown.
`each of the pixels 90 has a hexagon-shaped mirror 92
`oriented to be closely packed with another pixel 90. as
`shown. In addition. the hexagonal geometry of each mirror
`92 closely resembles that of a circle, and is ideally suited to
`reflect and modulate a beam of light having a circular cross
`section, such as that from fiber optic devices. Thus, the
`advantage of the hexagonal shape of mirrors 92 is two fold.
`First. it facilitates the tightly packed arrangement shown in
`FIG. 8. and secondly, has a nearly circular surface area to
`reflect a circular beam of incident light. None of the light-
`reflecting surface of mirror 92 is wasted. that is. almost the
`entire surface area of mirror 92 receives and reflects incident
`light efficiently.
`Referring now to FIG. 9. there is shown generally at 130
`a pixel according to yet another alternative preferred
`embodiment of the present invention. Pixel 130 is seen to
`include an elevated hexagonal shaped mirror 132 which
`substantially encompasses the underlying support structure.
`Referring to FIG. 10, there is shown a partial cut away view
`of Pixel 130 to illustrate the underlying yoke 134 which is
`supported by torsion hinges 94. Yoke 134 is the same size
`and profile as mirror 92 in FIG. 7. but does not necessarily
`need a reflective surface. Mirror 132 is supported upon and
`over yoke 134 by a mirror post 136. Pixel 130. in effect. has
`a hidden-hinge support superstructure, such as shown in
`FIG. 10.
`
`The advantages of this embodiment can be appreciated in
`view of FIG. 11, whereby pixels 130 of an array generally
`shown at 140 are very tightly packed, allowing for a high
`density arrangement of pixels 130 with no exposed under-
`lying superstructure. The perimeter edges of pixel mirror
`132 extend closely proximate, but spaced from. correspond-
`ing edges of the adjacent pixel mirrors 132. The pixels 130
`are arranged close enough to one another so as to substan-
`tially reduce the possibilities of any incident light from
`impinging upon and detracting from the underlying support
`structure of the pixels. such as the torsion hinges 94 or the
`flexure hinges 106 shown in FIG. 10. Again. the torsion
`hinges 94 establish the torsion axis about which the yoke
`134 and mirror 132 is rotated about. whereby the flexure
`hinges 106 restrict the rotational deflection of the yoke 134.
`and hence the mirror 132. about the torsion axis.
`For additional discussion and teaching how to fabricate
`the pixels 130. cross reference is made to commonly
`
`

`

`5,661,591
`
`9
`
`assigned co-pending patent application Ser. No. 08/424,021,
`entitled “Active Yoke Hidden Hinge digital Micromirror
`Device", whic