EP0331724A1 - Relais optique a cristaux liquides a diode schottky unique et procede de production - Google Patents

Relais optique a cristaux liquides a diode schottky unique et procede de production

Info

Publication number
EP0331724A1
EP0331724A1 EP88909160A EP88909160A EP0331724A1 EP 0331724 A1 EP0331724 A1 EP 0331724A1 EP 88909160 A EP88909160 A EP 88909160A EP 88909160 A EP88909160 A EP 88909160A EP 0331724 A1 EP0331724 A1 EP 0331724A1
Authority
EP
European Patent Office
Prior art keywords
liquid crystal
light valve
layer
schottky
photoconductor
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP88909160A
Other languages
German (de)
English (en)
Inventor
Uzi Efron
Paul O. Braatz
Jan Grinberg
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Raytheon Co
Original Assignee
Hughes Aircraft Co
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Hughes Aircraft Co filed Critical Hughes Aircraft Co
Publication of EP0331724A1 publication Critical patent/EP0331724A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/135—Liquid crystal cells structurally associated with a photoconducting or a ferro-electric layer, the properties of which can be optically or electrically varied
    • G02F1/1354—Liquid crystal cells structurally associated with a photoconducting or a ferro-electric layer, the properties of which can be optically or electrically varied having a particular photoconducting structure or material

Definitions

  • This invention relates to liquid crystal light val ⁇ ves, and more particularly to light valves which are .based upon a depletion of majority carriers in a photoconductor layer, and methods of operating the same.
  • Light valves generally employing liquid crystals as an electro-optic medium, are used to spatially modulate a readout beam in accordance with an input signal pattern applied to the light valve. They can be used to greatly amplify the input pattern by controlling a readout beam of much greater intensity, to convert spatially modulated incoherent input radiation to a coherent readout laser beam with a similar spatial modulation, for optical data processing, wavelength conversion, or for other purposes that involve the conversion of an input signal pattern to a corresponding spatial modulation on a separate readout beam.
  • FIG. 1 A simplified block diagram of a typical light valve system is illustrated in FIG. 1.
  • An input beam 2 is developed from a source such as the screen of a cathode ray tube 4 and imaged through lens 6 onto the input side of a light valve 8.
  • a readout beam 10 is generated by a laser 12, and directed onto the readout side of the light valve by a polarizing beam splitter 14.
  • the input beam 2 establishes a spatial polarization of a liquid crystal layer within the light valve 8, and this layer controls the reflection of the readout bre m from the light valve.
  • Certain portions of the readout, beam are incident upon locations in the liquid crystal layer where the liquid crystal molecules have been rotated in response to the voltage generated by the input radiation, and these portions are retro-reflected back through beam splitter 14 to emerge as an output beam 16.
  • the liquid crystal in the light valve modulates the spatial intensity of the readout beam into a corresponding but amplified intensity pattern of the input beam.
  • the • main parameters of light valves are the input sensitivity, output, and resolution modulation (contrast ratio) , as well as output uniformity and frame rate. While high contrast, moderate brightness and color capa ⁇ bility are required for command and control displays, very high brightness and resolution, as well as fast response, are required for flight-simulation applications. Optical data processing applications require low wavefront dis- tortion (output uniformity) and high diffraction effi ⁇ ciency. In addition, for real-time portable scene corre ⁇ lators, high frame rate, wide spectral range, small size, and low power consumption are also required. Most of these requirements are met by a cadmium sulfide liquid crystal light valve developed by Hughes Aircraft Company. This device is described in articles by J.
  • Grinberg A. Jacobson, W. P. Bleha, L. Miller, L. Fraas, D. Boswell and G. Myer, "A New Real-Time Non-Coherent to Coherent Light Image Converter - The Hybrid Field Effect Liquid Crystal Light Valve", Optical Engineering 14, 217 (1975), and J. Grinberg, W. P. Bleha, A. Jacobson, A. M. Lackner, G. Myer, L. Miller, J. Margeru , L. Fraas and D. Boswell, "Photoactivated Birefringent Light-Crystal Light Valve for Color Symbology Display", IEEE Transactions Electronic Devices ED-22, 775 (1975).
  • the main drawback of the CdS-based light valve has been its slow response time.
  • a second generation, sili ⁇ con-based liquid crystal light valve has been developed which retains the advantages of the CdS-based li.ght valve and has a considerably faster response time.
  • the silicon- based device employs a metal-oxide-semiconductor (MOS) structure, and is described in an article by U. Efron, J. Grinberg, P. 0. Braatz, M. J. Little, P. G. Reif and R. N. Schwartz, "The Silicon-Liquid Crystal Light Valve", Journal of Applied Physics 57(4) 1356-68 (1985). This article .also summarizes some of the prior light valve efforts.
  • MOS metal-oxide-semiconductor
  • FIG. 2 The internal construction of an MOS light valve is shown in FIG. 2.
  • An input image beam on the right hand side of the device is identified by reference numeral 18, while a readout beam 20 is directed onto, and reflected from, the left hand side of the device.
  • a layer of high resistivity silicon photoconductor 22 has a thin p + back contact layer 24 formed on its readout side. This back contact provides a high sheet conductivity to present a very small load at any point in the device's cross-section where carriers are generated.
  • An Si ⁇ 2 oxide layer 26 is provided on the input side of back contact 24, with a fiber optic plate 28 adhered to the oxide layer by means of an optical cement 30.
  • a DC-biased n-type diode guard ring 32 is implanted at the opposite edge of the silicon photoconductor wafer 22 from back contact 24 to prevent peripheral minority carrier injection into the active region of the device.
  • An Si0 gate insulator layer 34 is formed on the readout side of the silicon photoconductor wafer 22. Isolated potential wells are created at the Si/Si0 2 interface by means of an n-type microdiode array 36. This prevents the lateral spread of signal electrons residing at the interface.
  • a unified thin film dielectric mirror 38 is located on the readout side of the oxide layer 34 to provide broad-band reflectivity, as well as optical isolation to block the high intensity readout beam from the photocon ⁇ ductor.
  • a thin film of fast response liquid crystal 40 is employed as the light modulating electro-optic layer on the readout side of mirror 38.
  • a front glass plate 42 is coated with an indium tin oxide (ITO) counter-electrode 44 adjacent the liquid crystal.
  • the front of glass plate 40 is coated with an anti-reflection coating 46, and the whole structure is assembled within an airtight anodized aluminum holder.
  • Silicon photoconductor 22 is coupled with oxide layer 34 and transparent metallic electrode coating 44 to form an MOS structure.
  • the combination of the insulating liquid crystal, oxide and ' mirror act as the insulating gate of the MOS structure.
  • an alternating voltage source 48 is connected on one side to back contact 24 by means of an aluminum back contact pad 50, and on its opposite side to counter-electrode 44.
  • the voltage across the two elec ⁇ trodes causes the MOS structure to operate in alternate depletion (active) and accumulation (inactive) phases.
  • the depletion phase the high resistivity silicon photo- conductive layer 22 is depleted and electron-hole pairs generated by input light beam 18 are swept by the electric field in the photoconductor, thereby producing a signal current that activates the liquid crystal.
  • the electric field existing in the depletion region acts to sweep the signal charges from the input side to the readout side, and thus preserve the spatial resolution of the input image.
  • the polarized readout beam 20 enters the readout side of the light valve through glass layer 42, passes through the liquid crystal layer, and is retro-reflected by dielectric mirror 38 back through the liquid crystal. Since the conductivity of each pixel in photoconductive layer 22 varies with the intensity of input beam 18 at that pixel, a voltage divider effect results which varies the voltage across the corresponding pixel of the liquid crystal in accordance with the spatial intensity of the input light.
  • the liquid crystals at any location will orient themselves in accordance with the impressed voltage, and the liquid crystal orientation relative to the readout light polarization at any particu ⁇ lar location will determine the amount of readout light that will be- reflected back off the light valve at that location.
  • the spatial intensity pattern of the input light is transferred to a spatial liquid crystal orientation pattern in the liquid crystal layer, which in turn controls the spatial reflectivity of the light valve to the readout beam.
  • Active light valve operation takes ' place only during the depletion phases. It is necessary to reverse the polarity of the applied voltage and thereby intersperse shorter accumulation periods between the depletion periods to prevent any appreciable DC current through the liquid crystal. This is because the liquid crystal tends to decompose under a DC current. Since the photoconductor layer 22 is photosensitive, a dielectric mirror/light blocking layer 38 is required that will prevent the high intensity readout light from generating spatially unresolved carriers in the photocon ⁇ ductor that would otherwise swamp the signal charge.
  • the dielectric mirror/light blocking layer 38 must attenuate the readout beam by a factor of about 10 6 or larger, so that the number of carriers accumulated during the active phase due to light leakage through the dielectric mirror/light blocking layer does not approach or exceed the signal charge. It is quite difficult to fabricate a dielectric mirror with this capability. Although an attenuation of 10 7 has been achieved, some applications require greater attenuations, for which adequate dielectric mirrors are not presently available.
  • a metal matrix mirror is illustrated in FIG. 3.
  • a matrix of reflective islands 52 is • formed on an insulative layer 54 such as Si ⁇ 2 « The islands 52 are separated from each other by insulating channels so as to avoid short- circuits across the face of the mirror.
  • the dimensions of the individual islands 52 are determined from a minimum size .- ' for adequate reflection, on the order of 5-20 mic ⁇ rons, and the resolution or pixel element size for which the light valve is designed.
  • the thickness of the islands depends upon the specific reflective material employed. There is a basic requirement that the free electron den ⁇ sity of ' the reflective material be sufficient to interact with the readout radiation and scatter it back out of the material.
  • Metals such as aluminum or silver or metal/- semiconductor compounds such as platinum-silicide may be used.
  • the MOS light valve described above has several limitations. While the photoconductor is initially deeply depleted, the depletion region gradually collapses (over the order of tens of milliseconds) because of thermal generation effects which deplete the majority carriers. Eventually the voltage drop shifts to the oxide from the photoconductor. Also, the process for applying the Si ⁇ 2 layer requires high temperatures in the order of 1000° C. At these temperatures it is difficult to keep the light valve substrate perfectly flat. Any curvature or waviness in the substrate will distort the readout from the valve. Another disadvantage is that a certain amount of sheet conductivity has been noted at the Si0 2 /Si0 2 interface. This effect degrades both the resolution and the dynamic range of the device.
  • a metal matrix mirror is preferable to a dielectric mirror because of its lower impedance, its use has been limited principally to the infrared region. In the visible region the readout light leaks through the channels between the metal islands, causing activation at the underlying photoconduc ⁇ tor.
  • Another type of light valve which is at least poten ⁇ tially capable of even better performance than the MOS light valve is referred to as the double Schottky diode light valve. It is disclosed in a co-pending patent application entitled “Double-Schottky Diode Liquid Crystal Light Valve” by Paul 0. Braatz and Uzi Efron, two of the present inventors. The application was filed on July 25, 1985 under Serial No. 758,917, and is assigned to Hughes Aircraft Company, the assignee of the present invention. This device is illustrated in FIG. 4. It consists of a photoconductor substrate 58 with Schottky diodes formed on either side.
  • the metal pads 60 of a metal matrix mirror form a pattern of Schottky contacts with the photoconductor, while on the input side a metal electrode 62 contacts the phase of the photoconductor to form another Schottky diode.
  • a face plate 64 is attached to the input side of electrode 62 by an optical cement 66.
  • the liquid crystal layer 68, counter-electrode 70 and glass counter-electrode substrate 72 are similar to the MOS device described above. Alignment layers 74 and 76 are provided on either side of the liquid crystal, which is confined by spacers 78.
  • the double-Schottky diode light valve is operated with a balanced AC voltage drive 80 applied across the back electrode 62 and counter-electrode 70.
  • one or the other of the Schottky diodes will be reverse biased at substantially all times, depending on the phase of the voltage source 80 at any given time. This causes the photoconductor 58 to maintain a state of substantially continuous depletion.
  • the device avoids the inactive .accumulation periods necessary with the MOS light valve, and inherently balances the net current through the liquid crystal to zero.
  • the purpose of the present invention is to provide a liquid crystal light valve that retains the benefits of the double-Schottky diode light valve, avoids the depletion region collapse, high temperature fabrication and sheet conductivity prob- lems of the MOS light valve, and yet is more practical to fabricate than a double-Schottky diode device.
  • a Schottky diode light valve in which a Schottky contact is made with the readout side of the photoconductor by means of a metal matrix mirror, while the back contact is established by a doped semiconductor electrode.
  • the device can be operated in an AC mode with relatively long depletion periods and shorter inactive periods, or the liquid crystal can be doped with current carrying ions for DC operation with continuous depletion.
  • a dielectric mirror can be provided behind the metal matrix mirror, or the insulating channels of the metal matrix mirror can be coated with an opaque material. The device avoids the depletion region collapse which charac ⁇ terizes MOS light valves, and is easier to successfully fabricate than either MOS or double-Schottky devices.
  • FIG. 1, described above, is a block diagram of a conventional light valve system
  • FIG. 2, described above, is a sectional view of a prior art MOS liquid crystal light valve
  • FIG. 3, described above, is a plan view of a metal matrix mirror structure
  • FIG. 4, described above, is a sectional view of a double-Schottky liquid crystal light valve;
  • FIG. 5 is a sectional view of one embodiment of the present invention.
  • FIG. 6 is a sectional view of the light valve of FIG. 5 with the addition of a dielectric mirror
  • FIG. 7(a) and 7(b) are graphs of the applied voltage and the liquid crystal current, respectively, during AC operation.
  • FIG. 5 One embodiment of the light valve of the present invention is shown in FIG. 5. It is similar to the double-Schottky light valve of FIG. 4 in certain respects, and common elements are identified by the same reference numerals. It includes a liquid crystal layer 68 confined by alignment layers 74 and 76 and by spacers 78, a counter-electrode 70 on glass substrate 72, a semiconduc ⁇ tor photoconductor layer 58 which is preferably of silicon but may also be gallium arsenide, indium arsenide or indium phosphide, a metal matrix mirror 60 between the liquid crystal layer and photoconductor, and an input face plate 64.
  • a liquid crystal layer 68 confined by alignment layers 74 and 76 and by spacers 78
  • a counter-electrode 70 on glass substrate 72 a semiconduc ⁇ tor photoconductor layer 58 which is preferably of silicon but may also be gallium arsenide, indium arsenide or indium phosphide
  • metal matrix mirror pads 60 form a series of Schottky contacts on one side of the photoconductor layer 58.
  • a back contact electrode 82 formed from heavily n-doped silicon is provided on the other side of the photoconductor.
  • An alternating voltage from source 84 is applied across electrodes 70 and 82 to operate the device.
  • the device can be operated in a DC mode with a DC voltage source 86 by doping the liquid crystal with current carry ⁇ ing ions. See for example, U.S. Patent No. 4,066,589, "Dopants For Dynamic Scattering Liquid Crystals" issued 1978 to H. S. Li .
  • light leakage through the channels 50 can be blocked by means of a commercially available opaque, high resistivity blocking material 88, such as GaAs or CdTe, inset in the readout end of each channel.
  • the described single-Schottky light valve has several distinct advantages over MOS devices. Minority carriers in the photoconductor are attracted to the junction with the metal matrix mirror and can flow across the junction into the mirror, thereby preventing a collapse of the depletion region.
  • the avoidance of an insulating oxide layer, which is fabricated at about 1000" C for Si0 2 allows for a much cooler fabrication with maximum tempera ⁇ tures of about 300°-400* C. This results in a flatter device and an accordingly improved readout.
  • the sheet conductivity exhibited by MOS devices at the Si0/Si0 2 interface is eliminated, thereby improving resolution and dynamic range. Also, the use of lattice-damaging process ⁇ ing steps such as ion implantation and thermal oxide growth is avoided.
  • FIG. 6 Another embodiment of the invention is shown in FIG. 6. This embodiment is essentially similar to that of FIG. 5, except a dielectric mirror 90 is provided on the read ⁇ out side of the metal matrix mirror 60 to prevent leakage of visible readout light into the photoconductor 58. This eliminates the use of the channel blocking material 88 of FIG. 5. For operation in non-visible regions such as infrared, both blocking mechanisms can be omitted since leakage of the readout radiation into the photoconductor will not interfere with the operation of the device.
  • the asymmetric AC operation of the device is illu ⁇ strated in FIGs. 7(a) and 7(b). The voltage applied by source 84 across the electrodes, shown in FIG.
  • the resulting current pattern through the device is illustrated in FIG. 7(b).
  • a relatively low level current pulse 96 which lasts for a relatively long period of time, with interven ⁇ ing shorter but correspondingly higher magnitude current pulses 98 in the opposite direction during forward bias- ing.
  • the forward bias current pulses are necessary to balance the net current through the liquid crystal to approximately zero. This is necessary because the liquid crystal will decompose under a DC current. If the liquid crystal is doped with current carrying ions as mentioned above, the ions will carry the DC current through the liquid crystal and the light valve can be opera'ted with a DC drive voltage as mentioned above.
  • a high resistivity n-type silicon wafer is first double-side lapped and chemo-mechanically thinned down to a thickness of about 250 microns.
  • a typical resistivity is in the range of 0.5-3 K ohms-cm.
  • An n-type doping layer is then formed at the back of the wafer by either implanting or diffusing a suitable donor impurity, such as phosphorous or arsenic, to yield a sheet resistivity less than 10 ohms per square.
  • the wafer is then glued with optical cement or electro-bonded to a glass substrate, which may be a single glass or a quartz plate.
  • a fiber optic face plate can be used for display and adaptive optics applications.
  • the mounted wafer is next thinned down using chemo- mechanical lapping to a thickness of between 30 and 60 microns, and its surface * polished with a high optical quality polish.
  • a low temperature insulator such as CVD oxide, plasma oxide, photo-oxide or low temperature sili- con nitride is next deposited over the wafer surface.
  • the insulating material may be chosen to have a high light absorption coefficient, such as CdTe.
  • the next step is the formation of the metal matrix mirror.
  • a grid pattern is formed on the substrate by photolithography, in which the grid (channel) lines have a typical width of 2-5 microns and a periodicity of 10-20 microns.
  • the inner regions of the grid are then etched away, exposing the n-silicon substrate in these regions.
  • the insulator material is left in place in the channel (grid) lines.
  • a Schottky metal is then evaporated into the areas where the grid has been etched away. Any metal such as Pt with a high work function for n-type materials can be used.
  • the photoresist in the grid lines is then lifted to remove the metal from the grid lines, completing the metal matrix array.
  • a dielectric mirror tuned to the spectral width of the desired readout beam is deposited on the Schottky matrix array to prevent readout beam penetration through the grid lines.
  • two alternate methods can be employed. First, an insulat ⁇ ing low-temperature oxide can be formed on the counter- electrode 70. Secondly, an external capacitor can be used. Alignment layers are next ' formed on both the sub- strate and the counter-electrode using medium and shallow angle deposition. Finally, the light valve is assembled in a standard holder assembly and the liquid crystal introduced.
  • the described light valve has significant operational advantages over MOS light valves, and is easier to fabri ⁇ cate than double-Schottky devices. While particular embodiments have been shown and described, numerous varia ⁇ tions and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited on in terms of the appended claims.

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Liquid Crystal (AREA)
  • Mathematical Physics (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)

Abstract

Cristal liquide à diode Schottky unique, dans lequel une série de contacts Schottky sont formés sur un côté d'un support photoconducteur par un miroir à matrice métallique, une électrode de contact à semi-conducteur dopé étant formé sur l'autre côté du support. Ce relais optique offre à l'utilisation plusieurs avantages par rapport aux dispositifs MOS, et il est plus simple à réaliser que les optiques à double diode Schottky. Ce relais peut être alimenté soit en un mode à courant alternatif soit, par dopage des ions du cristal liquide, en un mode à courant continu.
EP88909160A 1987-09-14 1988-08-30 Relais optique a cristaux liquides a diode schottky unique et procede de production Withdrawn EP0331724A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US9580687A 1987-09-14 1987-09-14
US95806 1987-09-14

Publications (1)

Publication Number Publication Date
EP0331724A1 true EP0331724A1 (fr) 1989-09-13

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Family Applications (1)

Application Number Title Priority Date Filing Date
EP88909160A Withdrawn EP0331724A1 (fr) 1987-09-14 1988-08-30 Relais optique a cristaux liquides a diode schottky unique et procede de production

Country Status (4)

Country Link
EP (1) EP0331724A1 (fr)
JP (1) JPH02501774A (fr)
IL (1) IL87416A0 (fr)
WO (1) WO1989002613A1 (fr)

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH01179015A (ja) * 1987-12-31 1989-07-17 Hamamatsu Photonics Kk ライトバルブ装置
EP0402944A3 (fr) * 1989-06-16 1992-05-27 Seiko Instruments Inc. Valve de lumière à cristal liquide adressée optiquement
US5210628A (en) * 1991-10-21 1993-05-11 Hughes-Jvc Technology Corporation Liquid crystal light valve with photosensitive layer having irregular diode pattern
US5612800A (en) * 1994-11-02 1997-03-18 Hughes Aircraft Company LCLV having photoconductive pedestals each having a cross-sectional area no greater than 5 percent of the area of its respective reflective pad

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4191454A (en) * 1977-06-20 1980-03-04 Hughes Aircraft Company Continuous silicon MOS AC light valve substrate
IL79186A (en) * 1985-07-25 1991-08-16 Hughes Aircraft Co Reflective matrix mirror visible to infrated converter light valve
US4842376A (en) * 1985-07-25 1989-06-27 Hughes Aircraft Company Double-schottky diode liquid crystal light valve

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO8902613A1 *

Also Published As

Publication number Publication date
WO1989002613A1 (fr) 1989-03-23
IL87416A0 (en) 1989-01-31
JPH02501774A (ja) 1990-06-14

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