JPH02501774A - Single Schottky diode liquid crystal light valve and method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Single Schottky Diode Liquid Crystal Light Valve and Method Background of the Invention 1. field of invention This invention relates to a liquid crystal light valve, in particular a light valve based on the reduction of majority carriers within a photoconductor layer. Concerning valves and how they operate.

2. Explanation of related technology Generally, light valves that use liquid crystal as an electronic/optical medium are applied to light valves. used to spatially modulate the readout beam according to the input signal pattern . They control the incidence pattern by controlling a readout beam of high intensity. very large amplification and spatially modulated incoherent incident radiation. into a coherent readout laser beam through similar spatial modulation. However, for optical data processing, there is no need for wavelength conversion or separate readout signals. for other purposes, including transforming the input signal pattern for corresponding spatial modulation on the can be used for

A simplified block diagram of a typical light valve system is shown in FIG. Enter The radiation beam 2 is generated from a source such as the screen of a cathode ray tube 4 and is input to a light valve 8. The image is projected through the lens 6 onto the emission side. Readout beam 10 on the other side of the light valve is generated by a laser 12 and a polarizing beam splitter onto the readout steel of the light valve. Directed by 14. The incident beam 2 is spatially polarized in the liquid crystal layer within the light valve 8. , and this layer controls the reflection of the read beam from the light valve.

A portion of the readout beam is rotated by the liquid crystal molecules in response to the voltage generated by the input radiation. The light is incident on a location within the liquid crystal layer that is being rotated.

These portions are then combined with the output beam 16 passing through the beam splitter 14. This is the back reflected light expressed as . In this example, the liquid crystal inside the light valve reads The spatial intensity of the beam is modulated into a corresponding amplified intensity pattern of the incident beam.

The main characteristics of a light valve are input sensitivity, output, and resolution modulation (contrast ratio). , as well as output uniformity and frame rate. Meanwhile, high contrast, moderate brightness , color capability is required for command and control displays. super bright and minute Higher resolution and even faster response are required for flight simulation applications. be done. Optical data processing applications have almost no wavefront distortion (output uniformity). ) and require high diffraction efficiency. In addition, real-time portal scene correlation equipment High frame rates, wide spectral coverage, small dimensions, and low power consumption are also required for It is said that Most of these necessities were provided by Hughes Aircraft Company. filled by a cadmium sulfide liquid crystal light valve developed by. this device J, Grinberg, A, Jacobson, W, P, BIeha. , L. Miller, L. Fraas.

Reference “A New Real-T” by D. Bosvell and G. Myer IIIle Non-Coherent to Coherent Light Io+age Converter-Th’eHybrid Field Ef’f’ect Liquid Crystal Light Valve "0ptical Engineering 14 p. 217 (1975) and and J. Grinberg.

A, P, Bieha, A, Jacobson, A, M, Lackner, G , Myer, L., Miller.

References by J. Margerum, L. Fraas, D. Boswell ″Photoactivated Birefringent Light-C rystal LightValve for Co1or Syibolog y　Display　’　IEEE TransactionsElectro nic Devices ED-22775 page (1975).

The main drawback of CdS-based light valves is slow response time. 2nd generation Silicon-based liquid crystal light valves retain the advantages of CdS-based light valves. Developed to have fairly fast response times. Silicon-based devices are gold A genus oxide semiconductor (MOS) structure is used, and the literature (U, Efron, J, G rinberg, P, 0. Braatz, M.J., Little, P.G. .

“The’ 5il11con-” by Re1f, R,N, 5chvartz Liquid Crystal Light Valve”Journal of ' Applied Physics 57 (4) 135B-68 pages, 198 5 years). This document also summarizes some of the results of the research on light valves. I have a contract.

The internal structure of the MOS light valve is shown in FIG. Incoming image view on the right hand side of the device. The system is shown at reference numeral 18. On the other hand, the readout beam 20 is directed towards the left hand side of the device. and reflected on the left-hand side of the device. The layer of high resistivity silicon photoconductor 22 is thin. A p+ back contact layer 24 is formed on its read side.

This back contact is large at any point in the cross-section of the equipment where carriers are generated. Gives high sheet conductivity to give small loads. 5i02 oxide layer 26 is , with an optical fiber plate 28 attached to the oxide layer by an optical cement 30. It is provided on the input side of the back contact 24. DC bias n-type diode protection ring 32 to prevent peripheral minority carriers from being injected into the effective range of the device. The silicon photoconductor wafer 22 is implanted at the edge opposite the back contact 24. . A 5i02 gate insulating layer 34 is formed on the readout side of the silicon photoconductor wafer 22. has been completed. The isolated potential well is connected by an n-type microdiode array 36. is formed at the St/5i02 interface. This is the side of the signal electrons present at the interface. prevent the spread of

A uniform thin film dielectric mirror 38 provides broadband reflectance and also provides high Readout oxide layer 34 to provide optical isolation to block the intense readout beam. Exists on the output side. The thin film of the liquid crystal 40 with fast response is used for reading out the mirror 38. Used as an electro-optical layer to modulate the light on the side. The front glass plate 42 is It is covered with an indium tin oxide (ITO) counter electrode 44 adjacent to the crystal. glass plate 4 The front face of the 0 is covered by an anti-reflective coating 46 and the entire structure is made of airtight anodized aluminum. It is assembled inside the um holder.

The silicon lead conductor 22 includes an oxide layer 34 and a thin metal conductor to form a MOS structure. It is coupled to the polar sheath 44 . The combination of insulating liquid crystal, oxide, and mirror has a MOS structure. acts as an insulator gate.

In operation, AC voltage source 48 is connected on one side by aluminum back contact pad 50. The other side is connected to the back contact 24, and the opposite side is connected to the counter electrode 44. two The voltage across the electrodes is divided into depletion (active) and accumulation (inactive) phases in the MOS structure. make it work in turn. In the depletion layer, the high resistivity silicon photoconductive layer 22 is in a depletion state. The electron-hole pairs generated by the incident light beam 8 are photoconductive. Driven by an electric field throughout the body, it is removed, thus producing a signal current that activates the liquid crystal. to be accomplished. The electric field in the depletion region causes the signal charge to advance from the input side to the readout side, so Maintain the spatial resolution of the incident image.

The polarized readout beam 20 enters the light valve through the glass layer 42 and passes through the liquid crystal layer. It is reflected back by dielectric mirror 38 and travels back through the liquid crystal. Photoconductive layer 2 Since the conductivity of each pixel in 2 varies depending on the intensity of the beam 18 incident on that pixel, The voltage divider effect, which changes the voltage across corresponding pixels of the liquid crystal, affects the spatial intensity of the incident light. Therefore arises. As is well known, liquid crystals are subject to the voltage applied to them no matter where they are placed. They themselves are oriented in a certain direction. Readout light polarization in any unique location A liquid crystal oriented in a certain direction with respect to the readout reflects the light bulb back at that location. Determine the amount of light. Therefore, the spatial intensity pattern of the incident light is determined by the readout within the liquid crystal layer. Transfer the spatial liquid crystal orientation butter to control the spatial reflectance of the light valve for the beam. be done.

Active light valve operation occurs only during the depletion layer. Reverse the polarity of the supplied voltage , thereby short deposition between depletion periods to prevent some DC current through the liquid crystal. It is necessary to vary the period. Because liquid crystal decomposes under DC current This is because there is a tendency.

Since the photoconductor layer 22 is highly photosensitized, the dielectric mirror/light blocking layer 38 is is the spatially unresolved carrier generated in the photoconductor that removes the signal charge. It is necessary to prevent high intensity readout light from being transmitted. Typical dielectric mirror /The light-blocking layer 38 is designed so that the leakage of light passing through the dielectric mirror/light-blocking layer approaches the signal charge. A large number of carriers are deposited between the active phases to prevent The read beam must be attenuated by a factor of about 106 or more to Must be. It is quite difficult to fabricate a dielectric mirror with this ability. Ru. Even if 10 degrees of attenuation is achieved, some applications require large attenuation. do. Therefore, sufficient dielectric mirrors are not currently available.

Recently developed metallic matrices as possible alternative substrates for dielectric mirrors Smirror provides excellent electrical and optical properties for valve operation in the infrared region It has been shown that This mirror type is described in my U.S. Patent Application Section 759, No. 004 (“Reflective Matrix Mirror Visib Let.

Infrared Convertor Light Valve' inventor P, O, Braatz).

A metal matrix mirror is shown in FIG. Matric of reflective island 52 The substrate is formed on an insulating layer 54 such as 5i02. The island 52 is located on the surface of the mirror. Each is separated from each other by an insulating channel to avoid short circuits . The dimensions of the individual islands 52 range from 5 to 20 microns for sufficient reflected light. Determined from the smallest dimension of the order, the resolution i.e. for forming the light valve. Determined by the dimensions of the pixel element. The thickness of the islands depends on the particular reflective material used. Exists. The free electron density in the reflective material is sufficient to interact with the readout radiation. It is basically necessary that the It is essential. Metals like aluminum or silver or platinum silicide /semiconductor compounds can be used.

The MO8 light valve has several limitations. The photoconductor is initially extremely depleted. However, the depletion layer gradually collapses as heat generation causes a decrease in majority carriers. break (on the order of 10 milliseconds or more). Finally, the voltage drop decreases from the photoconductor to the oxide. Move to. Also, the method for supplying the 5i02 layer requires high temperatures on the order of 1000°C. Requires. It is difficult to maintain a perfectly flat light valve substrate at these temperatures. be. Any curvature or ripples in the substrate will distort the readout from the valve. one more The disadvantage is that a certain value of the conductivity of the sheet is shown at the 5i02/5i02 interface That's true. This result reduces both the resolution and dynamic range of the device.

Additionally, metal matrix mirrors have lower impedance than dielectric mirrors. is preferred over dielectric mirrors, but its use is primarily limited to the infrared region. visible In the region the readout light leaks through the channel between the metal islands and the underlying photoconductor. causes activation.

Another type of light valve that has at least better characteristics than the MO5 light valve is: It is a double Schottky diode light bulb. That's Paul O, Braa The applicant's patent “Doub” was created by two inventors, tz and Uzi Ef’ron. le-8chottky Diode Liquid Crystal Llgh The application is filed under U.S. Patent Application No. 758,917. Filed on July 25, 1985, by Hughes Air, the applicant of the present invention. Transferred to Craft Company. This device is shown in FIG. That's on the other hand A photoconductor substrate 58 comprising a Schottky diode formed on steel. Ru. On the readout side, the metal pad 60 of the metal matrix mirror is connected to the photoconductor and shot. Form a pattern of key contacts. On the other hand, the metal electrode 62 on the incident side is connected to another Schottky. Contact the photoconductor phase to form a diode. The face plate 64 , is attached to the entrance side of the electrode 62 by optical cement 66.

The liquid crystal layer 68, the counter electrode 70, and the glass counter electrode base 72 are connected to the MOS device. It is the same as the position. The aligned layers 74 and 76 are confined by spacers 78. provided on each side of the liquid crystal.

For the operation of MOS devices with relatively long depletion periods and relatively short deposition periods, A bull Schottky diode light valve is provided between the back electrode 62 and the counter electrode 70. It is operated by a balanced AC voltage drive 80 that supplies. In operation, Schottky One or the other of the diodes can be reverse biased at virtually any time, and at any given time depends on the phase of voltage source 80. As a result, photoconductor 58 is substantially continuously depleted. maintain the condition.

The device thus avoids the inert deposition cycle required by MOS light valves. , which inherently balances the real current through the liquid crystal to zero. It is M OS 9th place The sheet conductor on the surface of the photoconductor is manufactured at a lower temperature than the Does not show electric rate. However, metal back contacts 62 are very difficult to manufacture. , which prevents the full realization of the potential of the double Schottky device.

[Summary of the invention] In view of the above problems, the object of the present invention is to provide a double Schottky diode light valve. Maintaining the advantages of depletion layer decay and high temperature fabrication of MOS light valves and sheet conductivity avoids problems and is more practical to manufacture than double Schottky diode devices. The purpose of the present invention is to provide a liquid crystal light valve that is useful for various purposes.

The purpose of these is that the Schottky contact is connected to the photoconductor by a metal matrix mirror. The back contact consists of a doped semiconductor electrode. This is achieved by a single Schottky diode light valve. this device has a relatively long depletion period and a shorter inactivity period and can operate in AC mode. The liquid crystal is powered by a current carrying ions for DC operation due to continuous depletion. can be processed. For operation in the visible region, dielectric mirrors are fitted with metal matrix An insulated channel behind the mirror or in a metal matrix mirror can be coated with an opaque material. This device characterizes MOS light valves Easier to manufacture than MOS devices and double Schottky devices. It is.

These and other features and advantages of the invention are described in the detailed description of the preferred embodiment below. It will be clear to those skilled in the art from the description and accompanying drawings.

[Brief explanation of the drawing] FIG. 1 is a block diagram of the above-mentioned conventional light valve device.

FIG. 2 is a sectional view of the MO5 liquid crystal light bulb of the prior art.

FIG. 3 is a plan view of the metal matrix mirror structure.

FIG. 4 is a sectional view of the double Schottky liquid crystal light valve.

FIG. 5 is a cross-sectional view of one embodiment of the present invention.

FIG. 6 is a sectional view of the light valve shown in FIG. 5 with a dielectric mirror added thereto.

Figures 7A and 7B show the respective graphs of the supplied voltage and liquid crystal current during AC operation. It is f.

[Detailed description of preferred embodiments] One embodiment of the light valve of the present invention is shown in FIG. It is in some respects It is similar to the double Schottky light valve in Figure 4, and common elements have the same reference numbers. expressed. It is enclosed by aligned layers 74 and 76 and spacers 78. The liquid crystal 68 and the counter electrode 70 on the glass substrate 72 are preferably made of silicon. Semiconductor light may be gallium arsenide or indium arsenide or indium phosphide a conductor layer 58 and a metal matrix mirror 60 between the liquid crystal layer and the photoconductor; Includes a faceplate 64.

Similar to the double Schottky device, the metal matrix mirror bud 60 A series of Schottky contacts are formed on the steel of one of the electrical layers 58. However, the light On the other steel conductor is a battery made of heavily n-doped silicon. A contact electrode 82 is provided. AC voltage from power supply 84 is used to operate the device. It is supplied through electrodes 70 and 82.

Instead, the device uses DC It can be operated in DC mode by voltage source 86. For example, in 1978, H. , S., Lii, U.S. Patent No. 4.08B, 589″Dopants For Dynamic Scattering Liquid Crystal For operation in the visible light range, the leakage of light through channel 50 is Commercially available opaque G or CdT in the readout end of each channel end This is blocked by the insertion of a high resistivity blocking material 88 such as e.

The described single Schottky light valve has several obvious limitations for MOS devices. It has many advantages. The minority carriers of the photoconductor are bonded to the metal matrix mirror. can flow through the junction to the mirror, thereby reducing the depletion region prevent the collapse of The treatment at about 1000 °C for 5i02 of the insulating oxide layer is the highest temperature This is avoided by manufacturing at lower temperatures of about 300°C to 400°C. this yields a flatter device and thus improved readout. By MoS device The conductivity of the sheet shown at the S i O / S i 02 interface is removed, and its As a result, resolution and dynamic range are improved. Also, ion implantation and thermal acid Avoids the use of manufacturing processes that cause lattice damage, such as oxide growth .

Another embodiment of the invention is shown in FIG. In this embodiment, the photoconductor 5 Readout of metal matrix mirror 60 to prevent leakage of visible readout light into 8 It is basically the same as the one in Fig. 5 except for the dielectric mirror 90 provided on the steel. . This obviates the use of channel blocking material 88 of FIG. Non-infrared For operation in the visible range, both blocking mechanisms prevent readout radiation from leaking into the photoconductor. can be omitted since it does not interfere with the operation of the device. .

Asymmetric AC operation of the device is shown in FIGS. 7A and 7B. power supply 84 The voltage shown in Figure 7A applied between the electrodes by It consists of a relatively long reverse bias pulse of 20 volts in seconds, and the photoconductor and metal matrix of the order of 0.1 millisecond through the Schottky contact formed by the mirror It alternates with much shorter forward bias pulses 94. Schottky barrier order Since the forward bias impedance is negligible compared to the reverse bias, the bias The liquid crystal/mirror state has an effective Schottky impedance (in depletion state). This produces a charge transfer compensation assuming a 10:1 ratio to the mirror impedance.

The current pattern produced through the device is shown in Figure 7B. in reverse bias , lasting a relatively long time and having a short or high magnitude in the opposite direction during forward bias A relatively low level current pulse 96 terminating in a corresponding current pulse 98 of be. The forward bias current pulse balances the net current through the liquid crystal to zero. required to do. This is necessary because liquid crystals decompose under DC current. be. If the liquid crystal is doped with current carrier ions as mentioned above , the ions carry a DC current through the liquid crystal, and the light valve is driven by C as described above. It can be operated by voltage.

In the fabrication of an exemplary light valve device, a high resistivity n-type silicon wafer is initially wrapped on both sides and chemically mechanically thinned to a thickness of about 250 microns. typical The specific resistivity ranges from 0.5 to 3 kilohm-cm. n type do The doped layer is then implanted or expanded with a suitable donor impurity such as phosphorous or arsenic. to yield a sheet resistance of less than 10 ohms/square by either Formed on the back of the sea urchin. The wafer is then bonded by optical cement or electrical bonding. The substrate is bonded to a glass substrate, which may be a single glass or quartz plate. Instead, light Fiber faceplates can be used for display and adaptive optical applications It is.

The mounted wafer is then 30 to 60 microns thick using a chemical mechanical wrap. The surface is polished with a high quality optical brightener. C low temperature silicon nitride such as VD oxide, plasma oxide, photo oxide or low temperature A temperature insulator is then deposited on the wafer surface. Instead, the insulating material is C It can be chosen to have a high optical absorption coefficient, such as dTe.

The next step is the formation of the metal matrix mirror. The grid pattern is photore The grid lines are typically 2 to 5 mm wide and are formed on a substrate by a thograph process. It has a periodicity of 10 to 20 microns. Is that the area inside the grid? The n-silicon substrate is etched away in those areas to expose the n-silicon substrate. Insulating material The charges are left in place at the channel (grid) lines. schottky metal it is deposited on the areas of the grid that are etched away from the surface. High for n-type material Any metal with a high work function can be used, such as Pt. grid lie The photoresist is then refreshed to remove metal from the grid lines. The metal matrix array is completed by removing the metal matrix.

In the case of non-absorbing grid channels such as oxide channels, the required readout A dielectric mirror tuned to the spectral width of the beam penetrates the grid lines. is deposited on the Schottky metal matrix array to block the readout beam. It will be done.

Two alternative methods can be used to minimize the DC current flowing through the light valve. can be used. First, an insulating low temperature oxide may be formed on the counter electrode 70. Ru. An external capacitor can then be used. The alignment layer is then followed by intermediate and shallow angle It is formed on both the substrate and the counter electrode by a deposition method.

Finally, the light bulb is assembled into a standard collection holder and the liquid crystal is introduced. Ru.

The light valve has significant operational advantages over the MO8 light valve and has double shot easier to manufacture than tokey devices.

Although specific embodiments have been described and illustrated, many variations and alternative embodiments may occur. will be discovered by those skilled in the art. Accordingly, the invention is defined by the scope of the appended claims. It is limited only.

h62. -Jousen Kokuchi FIG, 4° 〉 international search report international search report PCT/US8B/C3100

Claims (13)

[Claims] 1. has an input side and a readout side, a liquid crystal layer; The entrance side of a liquid crystal layer for receiving input radiation and generating majority carriers in response to it. a doped semiconductor photoconductor layer above; The photoconductor layer and the matrix of Schottky contacts are located between the liquid crystal and the photoconductor layer. A metal matrix to form and a dope to form an electrical contact with the incident side of the photoconductor layer. a semiconductor electrode made of a single Schottky light valve with a counter electrode on the readout side of the liquid crystal layer; 2. The unit of claim 1, wherein the metal matrix comprises a metal matrix mirror. Schottky light bulb. 3. Dielectric mirror that blocks the transmission of visible light from the liquid crystal layer to the metal matrix mirror 3. The single Schottky light valve of claim 2, further comprising: a. 4. The metal matrix mirror is an electrically conductive material that is at least partially transparent to visible light. A matrix of metal islands electrically separated from each other by channels of gas-insulating material on said channel to further prevent the transmission of visible light through it. 3. The single Schottky light valve of claim 2 including an opaque material deposited on the opaque material. 5. The photoconductor layer is made of silicon, gallium arsenide, indium arsenide and indium phosphide. A single Schottky light bulb made of a material selected from the group consisting of Bu. 6. The liquid crystal is activated by current carrier ions to maintain a DC current through the liquid crystal layer. 2. The single Schottky light valve of claim 1, wherein the single Schottky light valve is doped with: 7. a doped semiconductor body for absorbing a beam of incident radiation; The absorption produces majority carriers generated by light, One side of the substrate to maintain the substrate in a substantially depleted state of majority carriers with reverse bias. a Schottky diode device placed on one side; A light valve optical substrate including a doped semiconductor electrode on the other side of the substrate. 8. 7. The Schottky diode device includes a metal matrix mirror. The described light valve optical substrate. 9. the semiconductor electrode is formed from the same material as the substrate doped with the same polarity; 8. The light valve optical substrate according to claim 7. 10. The semiconductor substrate may include silicon, gallium arsenide, indium arsenide, and phosphide. 8. The light valve of claim 7, comprising a material selected from the group consisting of: optical substrate. 11. The liquid crystal layer, the doped semiconductor photoconductor layer, and the light between the liquid crystal and photoconductor layer Schottky diode device in contact with the conductor layer and Schottky of the photoconductor layer a doped semiconductor layer in electrical contact with the diode device on the opposite side; and a liquid crystal light valve having a photoconductor layer and a counter electrode provided on the opposite side of the liquid crystal layer. Offer to, Activate the light valve within the period of intermittent reverse bias of the Schottky diode device. In order to sufficiently deplete the majority carriers in the photoconductive layer, there is an intersection between the electrode and the counter electrode. Apply a current voltage and set the system sufficiently to substantially balance the current flowing through the liquid crystal layer. The substantially short period of the forward bias of the Schottky diode device is replaced by the long period. A method of operating a liquid crystal light valve, including interposing it between the two. 12. The ratio of the duration of the reverse bias and forward bias periods is Schottky reverse bias. Spin impedance and light valve impedance between Schottky diode device and counter electrode 12. The method of claim 11, wherein the ratio between the -dance and the dance is approximately equal. 13. Liquid crystal layer doped with current carrier ions and doped semiconductor light a conductor layer and a Schottky die in contact with the photoconductor layer between the liquid crystal layer and the photoconductor layer; electrically connect the diode device to the opposite side of the photoconductor layer from the Schottky diode device. a doped semiconductor layer in contact with the liquid crystal layer, and a liquid crystal layer provided on the opposite side of the photoconductive layer. a liquid crystal light valve having a counter electrode; Applying a substantial DC voltage between the electrode for light valve operation and the counter electrode, the shock Method of operating a liquid crystal light valve that substantially continuously reverse biases a toky diode device .