United States Patent (19)
`Hornbeck
`
`(11) Patent Number:
`(45) Date of Patent:
`
`5,061,049
`Oct. 29, 1991
`
`54, SPATIAL LIGHT MODULATOR AND
`METHOD
`
`(75)
`
`Inventor: Larry J. Hornbeck, Van Alstyne,
`Tex.
`
`73
`
`Assignee: Texas Instruments Incorporated,
`Dallas, Tex.
`
`21)
`
`Appl. No.: 582,804
`
`22)
`
`Filed:
`
`Sep. 13, 1990
`
`I63)
`
`Related U.S. Application Data
`Continuation of Ser. No. 355,049, May 15, 1989, aban
`doned, which is a continuation of Ser. No. 168,724,
`Mar. 16, 1988, abandoned, which is a continuation-in
`part of Ser. No. 159,466, Feb. 19, 1988, abandoned,
`which is a continuation of Ser. No. 636,180, Jul. 31,
`1984, abandoned, and Ser. No. 43,740, Apr. 29, 1987,
`abandoned, which is a continuation-in-part of Ser. No.
`792,947, Oct. 30, 1985, Pat. No. 4,662,746, and Ser. No.
`129,353, Nov.30, 1987, abandoned, which is a continu
`ation of Ser. No. 877,654, Jun. 23, 1986, abandoned,
`which is a continuation-in-part of Ser. No. 646,399,
`Aug. 31, 1984, Pat. No. 4,596,992.
`
`51 Int, Cl. .......................... G02F 1/27; B44C 1/22;
`H04N 5/74: GO2B 26/08
`U.S. Cl. ............................." 359/224; 358/206;
`(52)
`359/213; 359/298; 359/847
`58 Field of Search ......................................... 350/6.9
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`2,993,403 7/1961 Harries ................................. 350/6.5
`... 315/373
`3,896,338 7/1975 Nathanson et al.
`... 350/6.6
`4,317,611 3/1982 Petersen ............
`... 350/360
`4,441,791 4/1984 Hornbeck ......
`... 350/486
`4,592,628 6/1986 Altman et al.
`... 350/360
`4,596,992 6/1986 Hornbeck ......
`... 350/360
`4,638,309 1/1987 Ott .................
`332/7.51
`4,662,746 5/1987 Hornbeck ..
`... 332/7.51
`4,698,602 10/1987 Armitage ...........
`... 332/7.51
`4,710,732 12/1987 Hornbeck ..........
`... 350/6.6
`4,793,699 12/1988 Tokuhara ......
`4,831,614 5/1989 Dueriget al. ....................... 369/101
`FOREIGN PATENT DOCUMENTS
`0232413 10/1986 Japan .................................... 350/6.5
`0021115 1/1987 Japan.
`... 350/6.5
`0035321 2/1987 Japan.
`... 350/486
`0035322 2/1987 Japan.
`... 350/6.5
`01.00417 5/1988 Japan ...........
`... 350/6.5
`1441840 3/1974 United Kingdom ................ 350/487
`Primary Examiner-Nelson Moskowitz
`Attorney, Agent, or Firm-James C. Kesterson; James T.
`Comfort; Melvin Sharp
`57)
`ABSTRACT
`An electrostatically deflectable beam spatial light mod
`ulator with the beams (30), address electrodes (42,46),
`and landing electrodes (40, 41) to provide soft-landing
`of the beams on the landing electrodes (40, 41) which
`gives uniform large-angle deflection plus high reliabil
`ity.
`
`28 Claims, 34 Drawing Sheets
`
`
`
`PCNA Ex. 1039
`U.S. Patent No. 9,955,551
`
`

`

`U.S. Patent
`U.S. Patent
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`Oct. 29, 1991
`Oct. 29, 1991
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`U.S. Patent
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`Oct. 29, 1991
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`U.S. Patent
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`Sheet 3 of 34
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`U.S. Patent
`U.S. Patent
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`Oct. 29, 1991
`Oct. 29, 1991
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`Sheet 4 of 34
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`Oct. 29, 1991
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`U.S. Patent
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`Oct. 29, 1991
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`

`Sheet 7 of 34
`Sheet 7 of 34
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`U.S. Patent
`U.S. Patent
`
`
`
`Oct. 29, 1991
`Oct. 29, 1991
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`5,061,049
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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 8 of 34
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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 9 of 34
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`5,061,049
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`

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`U.S. Patent
`U.S. Patent
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`Oct. 29, 1991
`Oct. 29, 1991
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`Sheet 10 of 34
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`U.S. Patent
`U.S. Patent
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`Oct. 29, 1991
`Oct. 29, 1991
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`Sheet 11 of 34
`Sheet 11 of 34
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`

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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 12 of 34
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`

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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 13 of 34
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`

`

`U.S. Patent
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`Oct. 29, 1991
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`Sheet 14 of 34
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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 15 of 34
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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 16 of 34
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`5,061,049
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`

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`U.S. Patent
`U.S. Patent
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`Oct. 29, 1991
`Oct. 29, 1991
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`Sheet 17 of 34
`Sheet 17 of 34
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`U.S. Patent
`U.S. Patent
`
`Oct. 29,1991.
`Oct. 29, 1991
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`—
`
`Sheet 18 of 34
`Sheet 18 of 34
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`

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`U.S. Patent
`U.S. Patent
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`Oct. 29, 1991
`Oct. 29, 1991
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`5,061,049
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`Sheet 19 of 34
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`5,061,049
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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 20 of 34
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`5,061,049
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`

`U.S. Patent
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`Oct. 29, 1991
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`Sheet 21 of 34
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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 22 of 34
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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 23 of 34
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`U.S. Patent
`U.S. Patent
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`Oct. 29, 1991
`Oct. 29, 1991
`
`5,061,049
`5,061,049
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`U.S. Patent
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`Oct. 29, 1991.
`
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`U.S. Patent
`U.S. Patent
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`Oct. 29, 1991
`Oct. 29, 1991
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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 27 of 34
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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 28 of 34
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`5,061,049
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`U.S. Patent
`U.S. Patent
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`Oct. 29, 1991
`Oct. 29, 1991
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`U.S. Patent
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`Oct. 29, 1991
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`U.S. Patent
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`Oct. 29, 1991
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`Sheet 31 of 34
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`U.S. Patent
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`U.S. Patent
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`Oct. 29, 1991
`Oct. 29, 1991
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`U.S. Patent
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`

`1.
`
`SPATIAL LIGHT MODULATOR AND METHOD
`
`5
`
`10
`
`15
`
`RELATED APPLICATIONS
`This application is a continuation of application Ser.
`No. 07/355,049, filed May 15, 1989 now abandoned
`which is a continuation of Ser. No. 07/168,724, filed
`Mar. 16, 1988, abandoned; which is a continuation in
`part of Ser. No. 07/159,466, filed Feb. 19, 1988, aban
`doned; which is a continuation of Ser. No. 06/636,180,
`filed Jul. 31, 1984, abandoned; and a continuation of
`Ser. No. 07/043,740, filed Apr. 29, 1987, abandoned;
`which is a continuation in part of Ser. No. 06/792,947,
`filed Oct. 30, 1985, now U.S. Pat. No. 4,662,746; and a
`continuation in part of Ser. No. 07/129,353 filed Nov.
`30, 1987, abandoned; which is a continuation of Ser.
`No. 06/877,654 filed June 23, 1986, abandoned; which
`is a continuation in part of Ser. No. 06/646,399 filed
`Aug. 31, 1984, now U.S. Pat. No. 4,596,992.
`20
`BACKGROUND OF THE INVENTION
`The present invention relates to spatial light modula
`tors (light valves), and, more particularly, to spatial
`light modulators with pixels formed of electronically
`addressable deflectable beams.
`25
`Spatial light modulators (SLM) are transducers that
`modulate incident light in a spatial pattern correspond
`ing to an electrical or optical input. The incident light
`may be modulated in its phase, intensity, polarization, or
`direction, and the light modulation may achieved by a
`variety of materials exhibiting various electrooptic or
`magnetoopotic effects and by materials that modulate
`light by surface deformation. SLMs have found numer
`ous applications in the areas of optical information pro
`cessing, projection displays, and electrostatic printing.
`35
`See references cited in L. Hornbeck, 128X 128 Deform
`able Mirror Device, 30 IEEE Tran. Elec. Dev. 539
`(1983).
`A well known SLM used for large bright electronic
`displays is the Eidophor, a system which uses an elec
`trostatically dimpled oil film as the active optical ele
`ment. See, E. Baumann, The Fischer large-screen pro
`jection system (Eidophor), 20 J. SMPTE 351 (1953). In
`this system a continuous oil film is scanned in raster
`fashion with an electron beam that is modulated so as to
`create a spatially periodic distribution of deposited
`charge within each resolvable pixel area on the oil film.
`This charge distribution results in the creation of a
`phase grating within each pixel by virtue of the electro
`static attraction between the oil film surface and the
`supporting substrate, which is maintained at constant
`potential. This attractive force causes the surface of the
`film to deform by an amount proportional to the quan
`tity of deposited charge. The modulated oil film is illu
`minated with spatially coherent light from a xenon arc
`lamp. Light incident to modulated pixels on the oil film
`is diffracted by the local phase gratings into a discrete
`set of regularly spaced orders which are made to fall on
`a schlieren stop consisting of a periodic array of alter
`nating clear and opaque bars by part of the optical sys
`60
`tem. The spacing of the schlieren stop bars is chosen to
`match the spacing of the diffracted signal orders at the
`stop plane so that high optical throughput efficiency is
`achieved. Light that is incident to unmodulated regions
`of the light valve is blocked from reaching the projec
`65
`tion lens by the opaque bars of the schlieren stop. Im
`ages formed of unmodulated areas on the light valve by
`the schlieren imaging system on the projection screen
`are therefore dark, while the phase perturbations intro
`
`5,061,049
`2
`duced by the modulated electron beam are converted
`into bright spots of light at the screen by the schlieren
`projector. In spite of numerous technical difficulties
`associated with oil polymerization by electron bon
`bardment and organic vapor contamination of the cath
`ode, this type of oil-film system has been successfully
`developed to the point that it is the almost universally
`used system for a total light requirement of thousands of
`lumens at the screen. However, such systems are expen
`sive, bulky, and have short-lived components.
`A number of non-oil-film SLMs have also been devel
`oped and include deflectable element types, rotation of
`plane of polarization types, and light scattering types.
`These SLM types employ various effects such as defor
`mation of reflective layers of metal, elastomer, or elas
`tomer-photoconductor, and polarization and scattering
`of ferroelectrics, PLZT ceramics, and liquid crystals.
`For example, R. Sprague et al, Linear total internal
`reflection spatial light modulator for laser printing, 299
`Proc. SPIE 68 (1981) and W. Turner and R. Sprague,
`Integrated total internal reflection (TIR) spatial light
`modulator for laser printing, 299 Proc. SPIE 76 (1982)
`and U.S. Pat. No. 4,380,373 describe a system for non
`impact printing on a photosensitive medium in which
`laser light is formed into a line of illumination and
`passed through a linear array of light modulators and
`then imaged onto the photosensitive medium. The arra
`is implemented as a total internal reflection spatial light
`modulator with the electrodes and drive electronics
`fabricated on an integrated drive element which is
`placed against the total internal reflection surface of an
`electrooptic crystal such as lithium niobate. The local
`ized change in index of refraction produced by the
`fringing field between each two electrodes is read out
`with schlieren readout optics which image the TIR
`interface onto the photosensitive medium. This is a one
`dimensional image, and the photosensitive medium is
`rotated on a drum beneath the image of the linear array
`to generate the two dimensional image (e.g., a page of
`text) for printing applications. However, the SLM
`(light valve) is highly susceptible to fabrication prob
`lems due to its hybrid nature. The fringing field
`strength, and hence the amount of light diffracted from
`modulated pixels, is sensitive to changes in the air gap
`thickness between the address electrodes and the elec
`trooptic crystal surface of less than one tenth micron.
`Thus, even very small particles trapped between the
`crystal and electrode structure could cause illumination
`nonuniformity problems at the photosensitive medium.
`The system optical response for pixels located at the
`boundary between modulated and unmodulated areas of
`the light valve is also significantly lower than the re
`sponse for pixels near the middle of a modulated region
`due to the nature of the addressing technique. A com
`mercially available printer based on this technology has
`not been introduced to date.
`M. Little et al., CCD-Addressed Liquid Crystal
`Light Valve, Proc. SID Symp. 250 (April 1982) de
`scribes a SLM with a CCD area array on the front side
`of a silicon chip and a liquid crystal array on the back
`side of the chip. Charge is input into the CCD until a
`complete frame of analog charge data has been loaded;
`the charge is then dumped to the backside of the chip
`where it modulates the liquid crystal. This device suf
`fers from severe fixed pattern noise as well as resolution
`degradation due to the charge spreading from the front
`to-back transfer.
`
`30
`
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`trapped particles, and the lateral extent of such tents is
`Another SLM type which may be fabricated in both
`much larger than the size of the particle itself, and these
`one and two dimensional arrays is the deformable mir
`tents would in turn be imaged as bright spots by a
`ror. Deformable mirrors may be subdivided into three
`schlieren imaging system.
`classes: elastomers, membranes, and cantilever beams.
`A cantilever beam deformable mirror is a microme
`In the elastomer approach a metallized elastomer is
`chanical array of deformable cantilever beams which
`addressed by a spatially varying voltage that produces
`can be electrostatically and individually deformed by
`surface deformation through compression of the elasto
`some address means to modulate incident light in a
`mer. Because of the address voltage requirements in the
`linear or areal pattern. Used in conjunction with the
`order of one or two hundred volts, the elastomer is not
`proper projection optics, a cantilever beam deformable
`a good candidate for integration with a high-density
`mirror can be employed for displays, optical informa
`silicon address circuit. See, generally, A. Lakatos and
`tion processing, and electrophotographic printing. An
`R. Bergen, TV projection display using an amorphous
`early version with metal cantilever beams fabricated on
`Se-type RUTICON light valve, 24 IEEE Tran. Elec.
`glass by vacuum evaporation appears in U.S. Pat. No.
`Dev. 930 (1977).
`3,600,798. This device has fabrication problems which
`Membrane deformable mirrors come in a variety of
`15
`include the alignment of the front and back glass sub
`types. One type is essentially a substitute for the oil film
`strates arising from the device's nonintegrated architec
`of the Eidophor system discussed above. In this system
`a thin reflective membrane is mounted to the faceplate
`ture.
`A cantilever bean deformable mirror device is de
`of a cathode ray tube (CRT) by means of a support grid
`scribed in R. Thomas et al, The Mirror-Matrix Tube: A
`structure. Addressing is by a raster scanned electron
`20
`Novel Light Valve for Projection Displays, 22 IEEE
`beam as with the Eidophor. The charge deposited on
`Tran. Elec. Dev. 765 (1975) and U.S. Pat. Nos.
`the glass faceplate of the CRT by the electron beam
`3,886,310 and 3,896,338. This device is fabricated as
`electrostatically attracts the membrane which is held at
`follows: a thermal silicon dioxide layer is grown on a
`a constant voltage. This attractive force causes the
`silicon on sapphire substrate; the oxide is patterned in a
`membrane to sag into the well formed by the grid struc
`cloverleaf array of four cantilever beans joined in the
`ture, thereby forming a miniature spherical mirror at
`middle. The silicon is isotropically wet etched until the
`each modulated pixel location. The light diffracted
`oxide is undercut, leaving within each pixel four oxide
`from this type of modulated pixel is concentrated into a
`cantilever beams supported by a central silicon support
`relatively narrow cone that is rotationally symmetric
`post. The cloverleaf array is then metallized with alumi
`about the specularly reflected beam. This type of light
`30
`num for reflectivity. The aluminum which is deposited
`valve is thus used with a schlieren stop that consists of
`on the sapphire substrate forms a reference grid elec
`a single central obscuration positioned and sized so as to
`trode which is held at a DC bias. The device is ad
`block the image of the light source that is formed by the
`dressed by a scanning electron beam which deposits a
`optical system after specular reflection from unmodu
`charge pattern on the cloverleaf beams causing the
`lated areas of the light valve. Modulated pixels give rise
`35
`beams to be deformed by electrostatic attraction
`to a circular patch of light at the schlieren stop plane
`towards the reference grid. Erasure is achieved by neg
`that is larger than the central obscuration, but centered
`atively biasing a closely spaced external grid and flood
`on it. The stop efficency, or fraction of the modulated
`ing the device with low-energy electrons. A schlieren
`pixel energy that clears the schlieren stop, is generally
`projector is used to convert the beam deformation into
`somewhat lower for projectors based on deformable
`40
`brightness variations at the projection screen. A signifi
`membranes than it is for the oil film Eidophor projector.
`cant feature of this device is the cloverleaf geometry
`Further, such membrane deformable mirror systems
`have at least two major problems. High voltages are
`which leads to beam deflection in a direction rotated
`forty-five degrees from the openings between the
`required for addressing the relatively stiff reflective
`beams; this permits use of a simple cross shaped schlie
`membrane, and slight misalignments between the elec
`45
`ren stop to block out the fixed diffraction background
`tron beam raster and the pixel support grid structure
`signal without attenuating the modulated diffraction
`lead to addressing problems. Such misalignments would
`signal. The device was fabricated with a pixel density of
`cause image blurring and nonuniformity in display
`five hundred pixels per inch with beams deflectable up
`brightness.
`to four degrees. The optics employed a 150 watt xenon
`Another type of membrane deformable mirror is
`arc lamp, reflective schlieren optics and a 2.5 by 3.5 foot
`described in L. Hornbeck, 30 IEEE Tran. Elec. Dev.
`screen with a gain of five. Four hundred TV lines of
`539 (1983) and U.S. Pat. No. 4,441,791 and is a hybrid
`resolution were demonstrated with a screen brightness
`integrated circuit consisting of an array of metallized
`of thirty-five foot-lumens, a contrast ratio of fifteen to
`polymer mirrors bonded to a silicon address circuit.
`one, and a beam diffraction efficiency of forty-eight
`The underlying analog address circuit, which is sepa
`55
`percent. Write times of less than 1/30 second were
`rated by an air gap from the mirror elements, causes the
`array of mirrors to be displaced in selected pixels by
`achieved and erase times were as short as 1/10 of the
`write time. However, the device has problems, includ
`electrostatic attraction. The resultant two-dimensional
`ing degradation of resolution from scanning errors,
`displacement pattern yields a corresponding phase mod
`poor manufacturing yield, and no advantage over con
`ulation pattern for reflected light. This pattern may be
`60
`ventional projection cathode ray tubes. That is, the
`converted into analog intensity variations by schlieren
`Scan-to-scan positioning accuracy is not high enough to
`projection techniques or used as the input transducer
`reproducibly write on individual pixels. The resulting
`for an optical information processor. However, the
`membrane deformable mirror has manufacturability
`loss of resolution forces at least a four fold increase in
`the number of pixels required to maintain the same
`problems due to the susceptibility to defects that result
`resolution compared to comparably written phosphor.
`when even small, micron sized particles are trapped
`Also, the device yield is limited by the lack of an etch
`between the membrane and the underlying support
`stop for the cloverleaf support post, the wet etching of
`structure. The membrane would form a tent over these
`
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`duced substantially together with the diffraction effi
`the beams leading to beam breakage, and the need to
`ciency. This means more lamp power is required for the
`evaporate normally tensile aluminum in a state of zero
`same screen brightness. Because the address circuitry
`stress on the oxide beams. Further, the device offers no
`requires additional area, the pixel size is increased far
`apparent cost or performance advantage over conven
`beyond the flap area with a resulting decrease in achiev
`tional projection CRTs.
`able resolution. The wet etching required to form the
`Cantilever beam deformable mirrors integrated on
`silicon with addressing circuitry, thus eliminating the
`wells leads to low electrical and mechanical yield; in
`deed, wet cleanups, such as after dicing into chips, de
`electron beam addressing with its high voltage circuitry
`stroy flaps and diving boards because during the spin
`and vacuum envelopes of the previously described can
`rinse/dry cycle the water trapped under the beam
`tilever device, appear in K. Petersen, Micromechanical
`breaks the beam as it is spun from the surface. If the
`Light Modulator Array Fabricated on Silicon, 31 Appl.
`water is instead evaporated from the surface it leaves
`Phys. Lett. 521 (1977) and U.S. Pat. No. 4,229,732. The
`behind surface residues which can increase surface leak
`first of these references describes a 16 by 1 array of
`age currents contributing to erratic device operation.
`diving board-shaped cantilever beams fabricated as
`Also, the addressing circuitry being on the silicon sur
`follows: an epitaxial layer of (100)-oriented silicon (ei
`face is exposed to the incident light to be modulated and
`ther p or n) of thickness of about 12 microns is grown on
`a p-substrate (or buried layer); the epilayer is oxidized
`creates unwanted diffraction effects from the transistor
`gates plus lowers the contrast ratio. In addition, light
`to a thickness of about 0.5 micron and covered with a
`leakage into the address structure produces photogene
`Cr-Au film of thickness about 500 A. The Cr-Au is
`rated charge and reduces storage time. Lastly, the ox
`etched away to form contact pads and address lines and
`ide/metal flap has the insulating side facing the well and
`to define the diving board metallization. The oxide is
`will charge up due to the intense electric fields which
`etched away in a comb pattern around the metallization
`exist across the well; this produces a residual (“burn
`in a second masking step. Finally, the silicon itself is
`in”) image. The AC drive required to eliminate this
`etched in a solution of ethylenediamine and pyrocate
`residual image problem cannot be supplied by the
`chol at 120 degrees C. If the proper orientation of the
`NMOS drive circuitry described. Further, if the flap is
`mask with respect to the crystalline axes is maintained,
`deflected past the maximum stable deflection, then it
`the metal-coated oxide diving boards will be undercut
`will collapse and stick to the bottom of the well. Thus,
`by the etch and freed from the silicon. Since the etch is
`voltages over the collapse voltage must be absolutely
`anisotropic, further lateral etching will be stopped by
`the (111) planes defining the rectangular envelope of the
`avoided.
`A variation of the cantilever beam approach appears
`comb pattern. In addition, the etchant is inhibited by
`in K. Petersen, Silicon Torsional Scanning Mirror, 24
`p--material, so the depth of the well beneath the diving
`IBM J. Res. Devp. 631 (1980) and M. Cadman et al,
`boards is defined by the thickness of the epilayer. When
`New Micromechanical Display Using Thin Metallic
`a dc voltage is applied between the substrate and the
`Films, 4 IEEE Elec. Dev. Lett. 3 (1983). This approach
`diving board metallization, the thin oxide diving board
`forms metallic-coated silicon flaps or metallic flaps
`will be electrostatically deflected downward into the
`which are connected to the surrounding reflective sur
`etched well. Diving boards of length 106 microns and
`face at two hinges and operate by twisting the flaps
`width 25 microns showed a threshold voltage of about
`along the axes formed by the hinges. The flaps are not
`66 volts.
`formed monolithically with the underlying addressing
`The second reference (Hartstein and Petersen, U.S.
`substrate, but are glued to it in a manner analogous to
`Pat. No. 4,229,732) describes devices fabricated in a
`manner similar to the diving board device (a buried
`the deformable membrane devices mentioned above.
`Cade, U.S. Pat. No. 4,356,730 combines aspects of the
`p-layer as an etch stop for forming the wells under
`foregoing and has a silicon substrate with metal coated
`neath metallized silicon dioxide cantilever beams) but
`silicon dioxide cantilever diving boards, corner-hinged
`has a different architecture; namely, the cantilever
`45
`flaps, and torsion hinged flaps. The addressing elec
`beams are in the shape of square flaps hinged at one
`trodes (two per diving board or flap for x-y addressing
`corner, the flaps form a two dimensional array instead
`as in a memory array) are on the surface, and the diving
`of the one dimensional row of diving boards, and the
`board or flap may be operated as a switch or memory
`wells underneath the flaps are not connected so that
`bit and collapsed to the bottom of the pit etched in the
`addressing lines for the flaps may be formed on the top
`50
`silicon substrate by application of a threshold voltage.
`surface of the silicon between the rows and columns of
`The diving board or flap can then be held at the bottom
`flaps. Of course, the corner hinging of the flaps derives
`of the pit by a smaller standby voltage.
`from the cloverleaf architecture of U.S. Pat. Nos.
`The cantilever beam references discussed above sug
`3,886,310 and 3,896,338, but the full cloverleaf architec
`gest that schlieren projection optical systems be used.
`ture could not be used because this would preclude the
`with the cantilever beam devices. But such systems
`surface addressing lines since cloverleaf flaps are hinged
`have limitations in terms of attainable optical perfor
`to a central post isolated from the silicon surface. Fur
`mance. First, the aperture diameter of the imaging lens
`ther, these devices have problems including poor reso
`must be larger than is necessary to pass the signal en
`lution and low efficiency due to density limitations and
`ergy alone. Hence the speed of the lens must be rela
`the small fractional active area, low manufacturing
`tively high (or, equivalently, its f-number must be rela
`yield, degradation of contrast ratio due to diffraction
`tively low) to pass all the signal energy around the
`effects from the adtdress circuitry, and residual image
`central schlieren stop obscuration. In addition, the sig
`due to the charging effects of the oxide flap. More par
`nal passes through the outer portion of the lens pupil in
`ticularly, the addressing circuitry is squeezed around
`this imaging configuration. Rays of light emanating
`the active area (flaps) because no option exists for plac
`65
`from any given point on the SLM and passing through
`ing the address circuitry under the active area due to
`the wells being formed by etching away the epilayer
`the outermost areas of an imager lens pupil are the most
`difficult ones to bring to a well-corrected focus during
`down to the p-i-etch stop. Thus the active area is re
`
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`the optical design of any imaging lens. When the outer
`BRIEF DESCRIPTION OF THE DRAWINGS
`rays are brought under good control, the rays passing
`FIGS. 1A-C illustrate in perspective, cross sectional
`through the center of the imager lens are automatically
`elevation, and plan views a first preferred embodiment
`well-corrected. Hence, a greater level of optical design
`pixel;
`complexity is required of the imagining lens. Second,
`FIG. 2 illustrates deflection of the pixel beam of the
`the field angle over which the imaging lens can form
`first preferred embodiment;
`well-corrected images of off-axis pixels on a cantilever
`FIGS. 3A-B show the dependence of beam deflec
`beam SLM is also restricted. Any lens design task in
`tion on applied voltage for a simplified version of the
`volves a compromise between the speed of the lens and
`first preferred embodiment pixel and a plan view of the
`the field angle it can cover with good image quality.
`simplified version pixel;
`Fast lenses tend to work over small fields, while wide
`FIG. 4 is a cross sectional elevation schematic view
`angle lenses tend to be relatively slow. Since the Schlie
`of the simplified version pixel defining analysis terms;
`ren imager must be well-corrected over its entire aper
`FIG. 5 illustrates the torques on the beam in the sim
`ture, and since this aperture is larger in diameter than is
`plified version pixel;
`required to pass the image forming light, the field angle
`FIG. 6 illustrates the beam deflection as a function of
`that can be covered by the lens is smaller than it could
`control voltage for the simiplified version pixel;
`be if a different imaging configuration could be devised
`FIG. 7 is a schematic cross sectional elevation view
`in which the signal was passed through the center of an
`of the first preferred embodiment;
`unobscured, smaller diameter lens. Lastly, for an imager
`FIG. 8 illustrates the torques on the beam in the first
`lens having a given finite speed, the use of the schlieren
`preferred embodiment;
`stop configuration also limits the size of the light Sour