United States Patent 15
`Hornbeck
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,662,746
`May 5, 1987
`
`[54] SPATIAL LIGHT MODULATOR AND
`METHOD
`
`3,896,338
`7/1975 Nathansonet al. ......eeee 358/60
`
`4,229,732 10/1980 Hartstein etal. ......
`. 358/233
`
`.. 350/360
`4,566,935
`1/1986 Hornbeck...........
`6/1986 Hornbeck .....ccsseseseeeneseeee 350/360
`4,596,992
`Larry J. Hornbeck, Van Alstyne,
`Primary Examiner—John K. Corbin
`Tee
`Assistant Examiner—Vincent J. Lemmo
`Texas Instruments Incorporated,
`Attorney, Agent, or Firm—Carlton H. Hoel; Leo N.
`Dallas, Tex.
`Heiting; Melvin Sharp
`[21] Appl. No.: 792,947
`[57]
`ABSTRACT
`[22] Filed:
`Oct. 30, 1985
`4” electrostatically deflectable beam spatiallight mod-
`[SU] Tnt. C14 oecessseceseeenee G02B 26/02; GO2B 00/00;
`ulator with the beam composedoftwolayers of alumi-
`HO1S 3/00
`num alloy and the hinge connecting the beam to the
`[52] U.S. Cl. cecsssssesccesesecssssecessseeseees 350/269; 332/7.51;
`remainder of the alloy formed in only one of the two
`350/320
`[58] Field of Search............... 350/632, 360, 487, 486,_layers; this providesa thick stiff beam and a thin compli-
`350/6.6, 269
`ant hinge. The alloy is on a spacer made of photoresist
`which in turn is on a semiconductor substrate. The
`substrate contains addressing circuitry in a preferred
`
`[75]
`
`Inventor:
`
`[73] Assignee:
`
`[56]
`
`References Cited
`
`3,600,798 B/T9TL Lee ..cccssssssersseseseerseeenees 350/320
`3,886,310
`5/1975 Guldberget al. ou. 350/360
`
`18 Claims, 23 Drawing Figures
`
`
`
`VWGO0A EX1038
`“U.S.Patent No. 9,955,551
`
`VWGoA EX1038
`U.S. Patent No. 9,955,551
`
`

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`U.S. Patent Mays, 1987
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`U.S. Patent May5, 1987
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`U.S. Patent May5, 1987
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`U.S. Patent
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`‘May’, 1987
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`U.S. Patent May5, 1987
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`Sheet50f6
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`4,662,746
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`BEAM METALIZATION AND
`ADDRESS ELECTRODE (N +14)
`
`26, 28
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`Fig. /
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`U.S. Patent May5, 1987
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`Sheet 6 of 6
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`

`SPATIAL LIGHT MODULATOR AND METHOD
`
`RELATED APPLICATIONS
`
`This application is related to applicant’s following
`copending applications: U.S. Ser. No. 635,967 filed July
`31, 1984; U.S. Ser. No. 636,180 filed July 31, 1984; and
`U.S. Ser. No. 646,399 filed Aug. 31, 1984.
`BACKGROUNDOF THE INVENTION
`
`15
`
`20
`
`25
`
`Thepresent invention relates to spatial light modula-
`tors (light valves), and, more particularly, to spatial
`light modulators with pixels formed of electronically
`addressable deflectable beams.
`Spatial light modulators (SLM)are transducers that
`modulate incidentlight in a spatial pattern correspond-
`ing to an electrical or optical input. The incidentlight
`maybe modulatedinits 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. SLMshave found numer-
`ous applications in the areas of optical information pro-
`cessing, projection displays, and electrostatic printing.
`See references cited in L. Hornbeck, 128 X 128 Deform-
`able Mirror Device, 30 IEEE Tran.Elec.Dev. 539
`(1983).
`A well known SLM usedfor 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 JLSMPTE 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 eachpixel by virtue ofthe 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 porportional
`to the
`qunatity of deposited charge. The modulated oil film is
`illuminatd 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 dis-
`crete set of regularly spaced orders which are made to
`fall on a schlieren stop consisting of a periodic array of 5,
`alternating clear and opaque bars by part ofthe optical
`system. The spacing ofthe schlieren stop bars is chosen
`to match the spacing of the diffracted signal orders at
`the stop plane so that high optical throughputefficiency
`is achieved. Light that is incident to unmodulated re-
`gions of the light valve is blocked from reaching the
`projection lens by the opaque bars ofthe schlieren stop.
`Images formed of unmodulated areas on the light valve
`by the schlieren imaging system on the projection
`screen are therefore dark, while the phase perturbations
`introduced by the molulated electron beams are con-
`verted into bright spots of light at the screen by the
`schlieren projector. In spite of numerous technical diffi-
`culties associated with oil polymerization by electron
`bombardment and organic vapor contamination of the
`cathode, this type of oil-film system has been success-
`fully developed to the point that it is the almost univer-
`sally used system for a total light requirement of thou-
`
`1
`
`4,662,746
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`2
`sands of lumens at the screen. However, such systems
`are expensive, bulky, and have short-lived components.
`A numberof non-oil-film SLMshavealso 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 througha linear array of light modulators and
`then imaged onto the photosensitive medium. The array
`is implementedas 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 reflection surface of an elec-
`trooptic crystal such as lithium niobate. The localized
`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 dimen-
`sional 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 oftext)
`for printing applications. However,
`the SLM (light
`valve) is highly susceptible to fabrication problems 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 electrooptic
`crystal surfaceof less than one tenth micron. Thus, even
`very small particles trapped between the crystal and
`electrode structure could cause illumination nonuni-
`formity problems at the photosensitive medium. The
`system optical response forpixels located at the bound-
`ary between modulated and unmodulated areas of the
`light valve is also significantly lower than the response
`for pixels near the middle of a modulated region due to
`the nature of the addressing technique. A commercially
`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 CCDareaarray 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 dumpedto 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.
`Another SLM type which may be fabricated in both
`one and two dimensionalarrays is the deformable mir-
`ror. Deformable mirrors may be subdivided into three
`classes: elastomers, membranes, and cantilever beams.
`In the elastomer approach a metallized elastomer is
`addressed by a spatially varying voltage that produces
`surface deformation through compression of the elasto-
`mer. Because of the address voltage requirements in the
`order of one or two hundred volts, the elastomeris not
`
`

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`4,662,746
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`ra 0
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`4
`3
`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 amorphor-
`early version with metal cantilever beams fabricated on
`ous-Se-type RUTICONlight valve, 24 IEEE Tran-
`glass by vacuum evaporation appears in U.S. Pat. No.
`-Elec.Dev. 930 (1977).
`3,600,798. This device has fabrication problems which
`Membrane deformable mirrors come in a variety of
`include the alignment of the front and back glass sub-
`types. One typeis essentially a substitute for the oil film
`strates arising from the device's nonintegrated architec-
`of the Eidophor system discussed above.In this system
`ture.
`a thin reflective membrane is mounted to the faceplate
`A cantilever beam deformable mirror device is de-
`of a cathode ray tube (CRT) by meansof a support grid
`scribed in R. Thomaset al. The Mirror-Matrix Tube: A
`structure. Addressing is by a raster scanned electron
`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. 3,886,310
`the glass faceplate of the CRT by the electron beam
`and 3,896,338. This device is fabricated as follows: a
`electrostatically attracts the membrane whichis held at
`thermal silicon dioxide layer is grown on a silicon on
`a constant volatge. This attractive force causes the
`sapphire substrate: the oxide is patterned in a cloverleaf
`memebrane to sag into the well formed by the grid
`array of four cantilever beams joined in the middle. The
`structure, thereby forming a miniature spherical mirror
`silicon is isotropically wet etched until the oxide is
`at each modulated pixel location. The light difracted
`undercut, leaving within each pixel four oxide cantile-
`from this type of modulated pixel is concentrated into a
`ver beams supported by a centralsilicon support post.
`relatively narrow cone that is rotationally symmetric
`The cloverleaf array is then metallized with aluminum
`about the specularly reflected beam. This type of light
`for reflectivity. The aluminum which is deposited on
`valve is thus used with a schlieren stop that consists of
`the sapphire substrate forms a reference grid electrode
`a single central obsucration positioned andsized so as to
`which is held at a DC bias. The device is addressed by
`block the imageofthe light source that is formed by the
`a scanning electron beam which deposits a charge pat-
`optical system after specular reflection from unmodu-
`tern on the cloverleaf beams causing the beams to be
`lated areasof the light valve. Modulated pixels give rise
`deformedby electrostatic attraction towardsthe refer-
`to a circular patch oflight at the schlieren stop plane
`ence grid. Erasure is achieved by negatively biasing a
`that is larger than the central obscuration, but centered
`closely spaced external grid and flooding the device
`on it. The stop efficency, or fraction of the modulated
`with low-energy electrons. A schlieren projector is
`pixel energy that clears the schlieren stop, is generally
`used to convert the beam deformation into brightness
`somewhat lower for projectors based on deformable
`variations at the projection screen. A significant feature
`membranesthanit is for the oil film Eidophor projector.
`of this device is the cloverleaf geometry which leads to
`Further, such membrane deformable mirror systems
`beam deflection in a direction rotated forty-five degrees
`have at least two major problems. High voltages are
`from the openings between the beams; this permits use
`required for addressing the relatively stiff reflective
`of a simple cross shaped schlieren stop to block out the
`membrane, and slight misalignments between the ele-
`fixed diffraction background signal without attenuating
`tron beam raster and the pixel support grid structure
`the modulated diffraction signal. The device was fabri-
`lead to addressing problems. Such misalignments would
`cated with a pixel density of five hundred pixels per
`cause image blurring and nonuniformity in display
`inch with beams deflectable up to four degrees. The
`brightness.
`optics employed a 150 watt xenon arc lamp,reflective
`Another type of membrane deformable mirror is
`schlieren optics and a 2.5 by 3.5 foot screen with a gain
`described in L. Hornbeck, 30 IEEE Tran.Elec.Dev.539
`of five. Four hundred TVlines of resolution were dem-
`(1983) and U.S. Pat. No. 4,441,791 and is a hybrid inte-
`onstrated with a screen brightness of thirty-five foot-
`grated circuit consisting of an array of metallized poly-
`mer mirrors bonded toasilicon address circuit. The
`lumens, a contrast ratio of fifteen to one, and a beam
`diffraction efficiency of forty-eight percent. Write times
`underlying analog address circuit, which is separated by
`of less than 1/30 second were achieved and erase times
`an air gap from the mirror elements, causes the array of
`were as short as 1/10 of the write time. However, the
`mirrors to be displaced in selected pixels by electro-
`static attraction. The resultant
`two-dimensional dis-
`device has problems, including degradation of resolu-
`tion from scanning errors, poor manufacturing yield,
`placementpattern yields a corresponding phase modu-
`and no advantage over conventional projection cathode
`lation pattern for reflected light. This pattern ‘may be
`ray tubes. Thatis, the scan-to-scan positioning accuracy
`converted into analog intensity variations by schlieren
`is not high enough to reproducibly write on individual
`projection techniques or used as the input transducer
`pixels. The resultinig loss of resolution forcesat least a
`for an optical
`information processor. However,
`the
`four fold increase in the numberofpixels required to
`membrane deformable mirror has manufacturability
`maintain the same resolution compared to comparably
`problemsdueto the susceptibility to defects that result
`written phosphor. Also, the device yield is limited by
`when even small, micron sized paticles are trapped
`the lack of an etch stop for the cloverleaf support post,
`between the membrane and the underlyiong support
`structure. The membrane would form a tent over these
`the wet etching of the beams leading to beam breakage,
`and the need to evaporate normally tensile aluminum in
`trapped particles, and the lateral extent of such tentsis
`a state of zero stress on the oxide beams. Further, the
`muchlarger thanthe sizeof the particle itself, and these
`device offers no apparent cost or performance advan-
`tents would in tum be imaged as bright spots by a
`tage over conventional projection CRTs.
`schlieren imaging system.
`A cantilever beam deformable mirror is a microme-
`Cantilever beam deformable mirrors integrated on
`silicon with addressing circuitry, thus eliminating the
`chanical array of deformable cantilever beams which
`electron beam addressing with its high voltage circuitry
`can beelectrostatically and individually deformed by
`and vacuum envelopesof the previously described can-
`some address means to modulate incident light
`in a
`tilever device, appear in K. Petersen, Micromechanical
`linear or areal pattern. Used in conjunction with the
`
`60
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`30
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`4,662,746
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`5
`31
`Light Modulator Array Fabricated on Silicon,
`Appl.Phy.Lett. 521 (1977) and U.S. Pat. No. 4,229,732.
`Thefirst of these references describes a 16 by 1 array of
`diving board-shaped cantilever beams fabricated as
`follows: an epitaxial layer of (100)-orientedsilicon (ei-
`ther p or n) of thickness of about 12 microns is grown on
`a p-+ substrate (or buried layer); the epilayeris oxidized
`to a thickness of about 0.5 micron and covered with a
`Cr-Au film of thickness about 500 A. The Cr-Au is
`etched away to form contact pads and addresslines and
`to define the diving board metallization. The oxide is
`etched away in a combpattern aroundthe metallization
`in a second masking step. Finally, the silicon itself is
`etched in a solution of ethylenediamine and pyrocate-
`chol at 120 degrees C. If the proper orientation of the
`mask with respect to the crystalline axes is maintained,
`the metal-coated oxide diving boards will be undercut
`by the etch and freed from thesilicon. Since the etch is
`anisotropic, further lateral etching will be stopped by
`the (111) planes defining the rectangular envelope of the
`comb pattern. In addition, the etchant is inhibited by
`p-+ material, so the depth ofthe well beneaththe diving
`boardsis defined by the thickness ofthe epilayer. When
`a de voltage is applied between the substrate and the
`diving board metallization, the thin oxide diving board
`will be electrostatically deflected downward into the
`etched well. Diving boards of length 106 microns and
`width 25 microns showed a threshold voltage of about
`66 volts.
`The second reference (U.S. Pat. No. 4,229,732) de-
`scribes devices fabricated in a manner similar to the
`diving board device (a buried p+ layer as an etch stop
`for forming the wells underneath metallized silicon
`dioxide cantilever beams) but has a different architec-
`ture; namely, the cantilever beams are in the shape of
`square flaps hinged at one corner,the flaps form a two
`dimensional array instead of the one dimensional row of
`diving boards, and the wells underneath the flaps are
`not connected so that addressing lines for the flaps may
`be formed on the top surface ofthe silicon between the
`rows and columnsofflaps. Of course, the corner hing-
`ing of the flaps derives from the cloverleaf architecture
`of U.S. Pat. Nos. 3,886,310 and 3,896,338, but the full
`cloverleaf architecture could not be used because this
`would preclude the surface addressing lines since clo-
`verleaf flaps are hinged to a central post isolated from
`the silicon surface. Further, these devices have prob-
`lems including poorresolution and low efficiency due
`to density limitations and the small fractional active
`area, low manufacturing yield, degradation of contrast
`ratio due to diffraction effects from the address cir-
`cuitry, and residual image dueto the charging effects of
`the oxide flap. More particulary, the addressing cir-
`cuitry is squeezed aroundtheactive area (flaps) because
`no option exists for placing the address circuitry under
`the active area due to the wells being formed by etching
`away the epilayer downto the p+ etch stop. Thus the
`active area is reduced substantially together with the
`diffraction efficiency. This means more lamp poweris
`required for the same screen brightness. Because the
`address circuitry requires additional area, the pixel size
`is increased far beyond the flap area with a resulting
`decrease in achievable resolution. The wet etching re-
`quired to form the wells leads to low electrical and
`mechanical yield: indeed, wet cleanups, such as after
`dicing into chips, destroy flaps and diving boards be-
`cause during the spin-rinse/dry cycle the water trapped
`under the beam breaks the beam asit is spun from the
`
`6
`surface. If the water is instead evaporated from the
`surface it
`leaves behind surface residues which can
`increase surface leakage currents contributingto erratic
`device operation. Also, the addressing circuitry being
`on the silicon surface is exposed to the incidentlight to
`be modulated and creates unwanted diffraction effects
`from the transistor gates plus lowers the contrast ratio.
`In addition,
`light
`leakage into the address structure
`produces photogenerated charge and reduces storage
`time. Lastly, the oxide metalflap has the insulating side
`facing the well and will charge up due to the intense
`electric fields which exist across the well; this produces
`a residual (burn-in) image. The AC drive required to
`eliminate this residual image problem cannot be sup-
`plied by the NMOSdrivecircuitry described. Further,
`if the flap is deflected past the maximum stable deflec-
`tion, then it will collapse and stick to the bottom ofthe
`well. Thus, voltages over the collapse voltage must be
`absolutely avoided.
`A variation of the cantilever beam approach appears
`in K. Petersen. Silicon Torsional Scanning Mirror. 24
`IBM J.Res.Devp. 631 (1980) and M. Cadmanet al. New
`Micromechanical Display Using Thin Metallic Films. 4
`IEEE Elec.Dev.Lett. 3 (1983). This approach forms
`metallic flaps which are connected to the surrounding
`reflective surface at two opposed corners and operate
`by twisting the flaps along the axes formed by the con-
`nections. The flaps are not formed monolithically with
`the underlying addressing substrate, but are glued toit
`in a manner analogous to the deformable membrane
`devices mentioned above.
`The cantilever beam references discussed aboveall
`suggest that schlieren projection optical systems be
`used with the cantilever beam devices. But such systems
`have limitations in terms of attainable optical perfor-
`mance.First, the aperture diameter of the imaging lens
`must be larger than is necessary to pass the signal en-
`ergy alone. Hence the speed of the lens must be rela-
`tively high (or, equivalently, its fnumber mustberela-
`tively low) to pass all the signal energy around the
`central schlieren stop obscuration. In addition, the sig-
`nal passes throughthe outer portion of the lens pupil in
`this imaging configuration. Rays of light emanating
`from any given point on the SLM and passing through
`the outermost areas of an imagerlens pupil are the most
`difficult ones to bring to a well-corrected focus during
`the optical design of any imaging lens. When the outer
`rays are brought under good control, the rays passing
`through the center of the imagerlens are automatically
`well-corrected. Hence, a greater level of optical design
`complexity is required of the imaging lens. Second, the
`field angle over which the imaging lens can form well-
`corrected images of an off-axis pixels on a cantilever
`beam SLMis also restricted. Any lens design task in-
`volves a compromise between the speed of the lens and
`the field angle it can cover with good image quality.
`Fast lenses tend to work over small fields, while wide
`angle lenses tendto berelatively slow. Since the schlie-
`ren imager must be well-corrected over its entire aper-
`ture, and since the apertureis larger in diameterthanis
`required to pass the image forminglight, thefield angle
`that can be covered by the lens is smaller than it could
`be if a different imaging configuration could be devised
`in which the signal was passed through the center of an
`unobscured, smaller diameter lens. Lastly, for an imager
`lens having a given finite speed, the use of the schlieren
`stop configurationalso limits the size of the light source
`that can be utilized. This in turn limits the irradiance
`
`vt 5
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`4,662,746
`
`7
`level that can be delivered to a projection screen or a
`photoreceptor at the image of a deflected pixel. This
`irradiance level, or the delivered power per unit area,
`depends on the product of the radiance of the light
`source, the transmittance of the optical system, and the
`solid angle of the cone of image forming rays oflight.
`The source radiance is determined only by the particu-
`lar lamp that is used. The optics transmittance depends
`on the stop efficiency for the particular SLM/schlieren
`stop configuration and surface transmission losses. But
`the solid angle of the image forming cone oflight is
`directly proportional to the area of the imagerlens pupil
`that is filled with signal energy. The use of a schlieren
`stop that obscures the central area of the imager lens
`pupil limits the usable pupil area and thus the image
`planeirradiance level that can be obtained for a lens of
`a given speed and a sourceofa given radiance;this is in
`addition to the fundamental irradiance limitation that
`the maximum usable coneoflight has an opening angle
`equal to the beam deflection angle.
`Thus the known cantilever beam SLMs have prob-
`lems including addressing circuitry limiting the frac-
`tional active area of the pixels, processing steps giving
`low yields, sensitivity to film stress in the beams, beam
`insulator charging effects, lack of overvoltage protec-
`tion against beam collapse, performance not compatible
`with low cost optics design, and low contrast ratio due
`to non planarized addressing circuitry on the surface.
`SUMMARYOF THE INVENTION
`
`invention provides deflectable beam
`The present
`spatial
`light modulators with pixels in the preferred
`embodiments including a spacer layer between a sub-
`strate and a reflective layer incorporating the deflect-
`able hinged beamsin which the spacer layer is a mate-
`rial that can be spun onto or conformally deposited on
`the substrate to cover any addressing circuitry,
`the
`beam is two layers of metal forstiffness but only one of
`the layers forms the hinge for compliance, and the pixel
`addressing may bein the substrate or in the reflective
`layer. The beam geometry maybe square (hingedat the
`cornerorthe side), diving boards, offset rectangular, or
`square torsion hinged. And the addressing may be con-
`figured to load only a portion of a beam.
`The problemsof the known deflectable beam spatial
`light modulators are solved by substrate addressing
`which maximizes the active area of the pixels and
`avoids low contrast due to nonplanar surfaces. The
`spacer material permits high yield processing and low
`_stress beams. Metal beamsavoid static charging and can
`provide overvoltage protection, and the two layer beam
`is stiff to avoid surface stress warping but the one layer
`hinge provides high compliance and sensitivity.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGS. 1a-1c illustrate in perspective, cross sectional
`elevation, and plan viewsa first preferred embodiment
`pixel:
`FIG.2 illustrates delfection of the pixel beam;
`FIG. 3 shows the dependence of beam deflection on
`applied voltage;
`FIGS. 4a and 40 illustrate in cross sectional and plan
`views a second preferred embodiment pixel;
`FIGS. 5a-5e illustrate in cross sectinal view the steps
`ofthe first preferred embodiment methodoffabrication;
`FIGS. 6a-6c further illustrate the last step of the
`method of fabrication.
`FIG. 7 showsalternative addressing;
`
`8
`FIGS.8a and 8billustrates in cross sectional and plan
`views a third preferred embodiment; and
`FIGS. 9a-9e show alternative beam geometries.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`— 0
`
`20
`
`25
`
`40
`
`45
`
`355
`
`60
`
`The inventive deflectable beam spatial light modula-
`tors (SLM)are typically formed of linear or area arrays
`of pixels, each pixel individually addressable and con-
`taining at
`least one deflectable reflecting cantilever
`beam:the pixels are organized in the form of monlithic
`silicon based chips. The chipsare fabricated by process-
`ing silicon wafers, dicing the wafers into chips, fol-
`lowed by processing of the individual chips. The chips
`will vary in size depending uon the application: for
`example, a 2400 by 1 linear array of pixels (which could
`be a componentof a 300 dots per inch printer) may be
`fabricated on a chip about 1300 mils by 250 mils with
`pixels about 12 microns square. The SLMsoperate by
`reflecting light off of the pixels, and the reflected light
`is modulated by varying the deflection of the deflect-
`able beams. Such SLMsare consequently also called
`deformable mirror devices (DMD)andthe deflectable
`beams are called mirror elements. The following de-
`scriptions are primarily of the individual pixels -for a
`DMD,andall of the drawings are schematicfor clarity
`of explanation. Applicant’s copending application Ser.
`No. 636,180 (hereby incorporated by reference together
`with ‘applicant’s other related applications) also dis-
`closes DMD pixels, but such pixels have problems of
`sensitivity to process induced surface stress, and the
`trade off between electromechanical
`compliance
`(which translates to sensitivity) and flatness of the mir-
`ror elements is unsatisfactory. The preferred embodi-
`ments described in the following overcome such prob-
`lems.
`A first preferred embodimentsingle pixel of a DMD
`fabricated by a first preferred embodiment method is
`illustrated in perspective view in s FIG. 1a, in cross
`sectional elevation in FIG. 14, and in plan view in FIG.
`1c. The pixel, generally denoted 20, is basically a flap
`covering a shaliow well and includessilicon substrate
`22, spacer 24, hinge layer 26, beam layer 28, flap 30
`formed in layers 26-28, and plasma etch access holes 32
`in flap 30. The portion 34 of hinge layer 26 that is not
`covered by beam layer 28 formsa hinge attaching flap
`30 to the portion of layers 26-28 supported by spacer
`24. Also, buried etch stop remnants 36 are clearly visi-
`ble in FIG. 1b and will be described in the following
`although they have no operational function. Indeed,if
`remnants 36 were removed during processing,
`then
`little operational difference would be noticed. Typical
`dimensionsfor pixel 20 would be as follows: flap 30 is a
`square with sides 12 microns long, spacer 24 is 4 mi-
`cronsthick (vertieal in FIG, 1), hinge layer is 800 A
`thick, beam layer is 3,600 A thick, holes 32 are two
`microns square, plasma etch access gap 38 (the space
`betweenflap 30 and the remainder of beam layer 28)is
`two microns wide, hinge 34 is three microns long and
`two microns wide, and remnants 36 are 1,500 A thick
`and about 1.5 microns long.
`Substrate 22 is (100) silicon with resistivity about 10
`ohm-cm and typically will have addressing circuitry
`formed on its surface, although such circuitry has been
`omitted from FIGS. 1a~1c forclarity; see FIG. 4a for a
`cross sectional! elevation view illustrating a portion of
`such circuitry. Thus in FIGS. 1a-1c substrate 22 should
`be considered to be a conductor. Spacer 24 is positive
`
`

`

`9
`photoresist which is an insulator; hinge layer 26 and
`beam layer 28 are both an aluminum,
`titanium, and
`silicon alloy (Ti:Si:Al) with 0.2% Ti and 1% Si(this
`alloy has a coefficient of thermal expansion not drasti-
`cally different from spacer 24 and thus minimizes the
`stress between the metal layers and spacer 24 generated
`during the fabrication process described in the follow-
`ing, also, the two layers 26 and 28 being the same metal
`minimizes stress): and remnants 36 are silicon dioxide.
`Note that any stress between layers in the flap or hinge
`would cause warping or curling of the flap or hinge,
`and any stress between the metal and the spacer can
`cause buckling or warping of the free portion of the
`metal over the well.
`Thearchitecture of FIGS. 1a-1c simultaneouslysatis-
`fies two criteria: (1) it is possible to make the beam metal
`as thick and the hinge metal as thin as desired without
`the problems of step coverage of the hinge metal over
`the beam metal and (2) the spacer surface under the
`beam metal is not exposed to processing side effects
`which would arise if the hinge were formedas a rectan-
`gular piece on the spacerprior to deposition of the beam
`metal.
`Pixel 20 is operated by applying a voltage between
`metal layers 26-28 and substrate 22 (actually an elec-
`trode on substrate 22 as describedin the following); flap
`30 and the exposed surface of substrate 22 form the two
`plates ofan 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 causesflap 30 to bend at hinge 34
`and be deflected towards substrate 22; see FIG.2 for an
`exaggerated view of this deflection together with an
`indication of the charges concentrated at the regions of
`smallest gap. For voltagesin the range of 20 to 30 volts,
`the deflection is in the range of 2 degrees. Of co