RADIATION DETECTOR ARRAY USING RADIATION SENSITIVE BRIDGES

Abstract: An infrared (IR) simulator is disclosed in which an array of pixels is defined on an insulative substrate by resistor bridges which contact the substrate at spaced locations and are separated from the substrate, and thereby thermally insulated therefrom, between the contact locations. Semiconductor drive circuits on the substrate enable desired current flows through the resistor bridges in response to input control signals, thereby establishing the appropriate IR radiation from each of the pixels. The drive circuits and also at least some of the electrical lead lines are preferably located under the resistor bridges. A thermal reflector below each bridge shields the drive circuit and reflects radiation to enhance the IR output. The drive circuits employ sample and hold circuits which produce a substantially flicker-free operation, whith the resistor bridges being impedance matched with their respective drive circuits. The resistor bridges may be formed by coating insulative base bridges with a resistive layer having the desired properties, and overcoating the resistive layers with a thermally emissive material. The array is preferably formed on a silicon-on-saphire (SOS) wafer. Arrays of electromagnetic radiation bridge detectors may also be formed, with the bridges having either resistor, thermocouple or Schottky junction constructions.

Description: RADIATION DETECTOR ARRAY USING RADIATION SENSITIVE BRIDGES BACKGROUND OF THE INVENTION Field of the Invention This invention relates to systems for simulating an infrared image for use in testing infrared seekers. t also relates to detector arrays for electromagnetic radi¬ ation including infrared and microwave. Description of the Related Art It would be highly desirable to be able to simulate a real-time infrared (IR - roughly 0.7-20 microns wavelength) image that is substantially free of flicker. This would provide an effective way to test IR detectors, also referred to as "seekers" and "focal plane arrays". At pre¬ sent, problems of excessive flicker impose a serious cαn- straint on IR simulation systems. A basic problem with image flicker is that it creates a false target indication, since flicker corresponds to a change in the temperature of the IR image. Unlike the human eye which integrates light flicker over a period of about 30-50 msec, IR detectors integrate flickers over periods of only about 1-5 msec. Thus, there is a significant range over which flicker (in the visible spectrum) would not be detected by the human eye but would be picked up by an IR detector if it is with¬ in the IR spectrum. Excessive flicker has been avoided heretofore with the use of a Bly cell to project a static image that has been applied to the cell. Bly cells are described in Vincent T. Bly, "Passive Visible to Infrared Transducer for Dynamic Infrared Image Simulation", Optical Engineering, Nov./Dec. 1982, Vol. 21, No. 6, pp. 1079-1082. However, the require¬ ment that this type of system be operated with a static image is a significant limitation, since a more meaningful test of IR detectors calls for the detection of images that can change in real-time. A prior attempt to produce an IR simulation system with a real-time image involved the formation of a video image by a cathode ray tube (CRT) . The CRT video image was applied as an input to a liquid crystal light valve (LCLV) , to which an IR readout beam was applied. The LCLV modulat¬ ed the IR readout beam with the video image from the CRT to produce a corresponding IR video image. See S.T. u et al, "Infrared Liquid Crystal Light Valve", Proceedings of the SPIE, Vol. 572, pages 94-101, August, 1985. This approach unfortunately was found to result in a substantial amount of flicker. The problem is that the illuminated pixels on the CRT screen decay in intensity over time prior to the next electron beam scan. This causes an undesirable intensity gradient to appear on a projected IR image from an IR-LCLV which is coupled to the CRT, and an IR detector will then detect a non-uniform image. Because the detector is generally looking for in- tensity gradients, or edges, by which its associated algo¬ rithms determine the presence of "targets", such intensity gradients are misleading. While this problem could theo¬ retically be solved by synchronizing the IR detector scan with the CRT electron beam scan, such synchronization may not be desired in many applications. Thus, although an IR- LCLV has the capability of projecting high resolution, high dynamic range, real-time simulated IR images when compared to a Bly cell, this advantage is mitigated by the CRT pixel decay. Furthermore, electrically driven matrix emitter devices have flicker if driven with simple RC-type pixel addressing circuits, since the RC decay is similar in effect to the phosphor decay of the CRT. Modifications of the basic CRT-LCLV system described above might be envisioned to reduce or eliminate flicker, but they introduce other problems. In one such modifica¬ tion, two storage CRTs are provided with shutters in front of each screen. Operation is alternated between the two CRTs by means of the shutters, so that they are alternate¬ ly applied to the LCLV. By staggering the video data frames between the two CRTs, the phosphor decay seen by the LCLV could theoretically be reduced significantly. How¬ ever, in such a system, it may be difficult to implement the very fast shutter coordination that would be necessary to substantially avoid flicker. Furthermore, storage CRTs are non-uniform, resulting in image differences and conse¬ quent flicker. Another approach would be to use a single CRT, but to increase the frame rate of the Raster scan from the conven¬ tional rate of about 30 Hz to a much higher rate, perhaps about 1,000 Hz. The CRTs of the future may provide higher bandwidths than that presently attainable, thereby making this approach more attractive. A possible approach which does not provide real-time addressable images is the use of a "flicker-free" film or slide projector like the SCANAGON ® device produced by Robert Woltz Associates, Inc. of Newport Beach, California and disclosed in Patent Nos. 4,113,367 and 4,126,386, or a comparable image projector. While the potential may exist for this limited technique, it has not been demonstrated to provide jitter-free and flicker-free images. Further¬ more, this method will not provide real-time electronical¬ ly updatable imagery. In addition to flicker-free images, it is very impor¬ tant for an IR simulation system to achieve high spatial resolution, large dynamic temperature ranges and fast res- ponse. Spatial resolutions should be at least 500x500 pix¬ els, with flicker less than 1%. For some applications frame rates should be 100 Hz or greater, and the dynamic thermal range should ideally be from near room temperature (some applications require cooled background temperatures) to l,000 β C, particularly in the 3-5 micron spectral range. This combination of dynamic range and response time re¬ quirements is difficult for commercially available liquid crystals to achieve. Furthermore, most IR simulation applications' require the simulator to be mounted on the two-axis target arm of a Carco table, which is then moved relative to the IR seeker being tested. For this purpose weight and size limitations are very important. The liquid crystal based IR simulator requires a large blackbody source, typically in excess of 100 pounds, and an expensive wire grid polarizer to be mounted with the active matrix. This reduces the mobility of the supporting Carco table arm. Liquid crystal systems are also limited in dynamic range because it is difficult to photogenerate enough charge in the silicon substrate, while maintaining spatial resolution, to rotate the liquid crystal molecules com¬ pletely. Furthermore, there is a restriction on the speed of response of liquid crystals if a reasonable thermal dy- namic range is to be maintained. The flicker problem asso¬ ciated with liquid crystal systems can be reduced by util¬ izing a flicker-free visible addressing source for the liquid crystal light valve. However, known sources of this type are limited in speed, resolution, weight and size. Patent No. 4,724,356 to Daehler discloses a resistor- based IR simulator in which an array of resistors are indi¬ vidually addressed to stimulate IR emission. The Daehler approach is also discussed in Daehler, "Infrared Display Array", SPIE Vol. 765, Imaging Sensors and Displays (1987), pages 94-101 and Burriesci et al. , "A Dynamic RAM Imaging Display Technology Utilizing Silicon Blackbody Emitters", SPIE Vol. 765, Imaging Sensors and Displays (1987), pages 112-122. Both the resistors and their respective drive transistor circuits are formed from a bulk silicon wafer. An air gap groove is formed under the resistors to help reduce thermal conduction losses to the underlying bulk silicon wafer. To facilitate the selective etching used to form the insulating air gap grooves, the top layer of the wafer is heavily doped. However, this doping makes the top layer electrically conductive, resulting in a significant impedance mismatch between the resistors and their respec¬ tive transistor drive circuits. As a result, most of the input power is lost in- the drive circuitry, rather than going into IR radiation. This reduction in the power transfer to the resistors results in a need for even more input power to the transistors, leading to a potential overheating problem. In addition to an inefficient use of input power, in the Daehler approach only a relatively small portion of the pixel area (less than 10%) is actually occupied by the radiating resistor. Much of the area that might otherwise be devoted to the resistor is occupied by the drive trans¬ istors and the insulating air gap. As a result, to achieve a given apparent temperature for the pixel as a whole the resistor itself must be heated to a significantly higher temperature. This substantially reduces the system's effective thermal dynamic range. Another problem with the Daehler approach relates to the electrical lead lines used to address the individual pixels. Relatively large silicon wafers must be used with present technology to achieve a significant number of pix¬ els. For example, a wafer of about four inches is required for a 512x512 array. This means that the electrical lead lines on the chip must also be about four inches long. This in turn leads to two significant problems. First, there is a large amount of capacitive coupling associated with the long lead lines. Parasitic capacitance between conductive substrate and metal lead lines can limit the response time. Second, defects such as pin holes can appear in the oxide insulator formed on the substrate, causing a short circuit between the line and the silicon substrate which renders the associated pixel or pixels in¬ operative. The longer the lines, the greater is the proba¬ bility of such defects. Daehler tries to resolve this problem by limiting the size of the array to 8x128 pixels, and coupling many such arrays together side-by-side to pro¬ duce an aggregate array of reasonable size. However, the coupling process itself introduces significant complexi¬ ties. There is also a need for a two-dimensional IR detector array, and for detector arrays at other electromagnetic radiation (emr) wavelengths such as microwave, which have a fast response and make efficient use of available area. Various types of IR thermal detectors are currently available. In pyroelectric detectors, a temperature change alters the dipole moment of the material, resulting in a charge difference between crystal faces. This type of de¬ tector is discussed in an article by Watton, "Ferroelec- trics for Infrared Detection and Imaging", Proceedings of the Sixth IEEE International Symposium on the Application of Ferroelectrics, June 1986, Bethlehem, Pennsylvania, pages 172-181. Another type of IR detector is the bolo¬ meter, in which the energy of absorbed radiation raises the temperature of the detecting element to change its electri- cal resistance, which change is measured as an indication of the amount of received radiation. An array of such de¬ vices is disclosed in Boninsegni et al., "Low Temperature Bolometer Array", Review of Scientific Instruments, Vol. 60, No. 4, April 1989, pages 661-665. Both the pyroelec- trie and bolometer devices exhibit a relatively slow rate of response to changes in IR level, and also require rela¬ tively large areas. The pyroelectric thermal detector also suffers from limited resolvable temperature differences, low yield and high cost processing, a necessary hybrid in- tegration and AC operation with a chopper. The limited temperature resolution, which is principally due to the inherent thermal conduction loss and large thermal mass, can be improved by thinning and reticulating the pyroelec¬ tric wafer, but this involves very difficult and expensive processing. Another approach to IR detection involves an array of single crystal Schottky junctions, as disclosed in Tsaur et al., "IR Si Schottky-Barrier Infrared Detectors With 10- / xm Cutoff Wavelength", IEEE Electron Device Letters . Vol. 9, No. 12, December 1988, pages 650-653. This device is used as a photon detector rather than a thermal detector, has a very limited spectral range, and has to be operated at cry¬ ogenic temperatures. Thermocouple devices provide another IR thermal detec- tor. In this type of device junctions are formed between dissimilar materials, with a voltage induced across the junction in response to heating. Such a device is dis¬ closed in Lahiji et al., "A Batch-Fabricated Silicon Therm¬ opile Infrared Detector", IEEE Transactions on Electron Devices, Jan. 1982, pages 14-22. In this article a series of thermocouples have hot junctions which are supported on a thin silicon membrane by integrated circuit techniques. The membrane area is coated with a layer of IR absorbing material to efficiently absorb energy from the visible to the far infrared. The sensitivity of the device is limit¬ ed because the thermocouples are formed directly on the substrate and therefore cannot heat as much as desired due to the thermal conduction losses. Also, a relatively large area is required for the detector. Bolometers and crystal detectors have also been used to sense emr at microwave frequencies. Since relatively large sensor dimensions are necessary, single detectors have been used to scan an incoming image, rather than using a detector array to sense the entire image at one time. The result has been slow frame rates. The same problem has attended the use of single scanning junction detectors in the IR require. SUMMARY OF THE INVENTION In view of the above problems, the goal of the present invention is to provide an IR simulation system which is substantially flicker-free, light in weight, has a large thermal dynamic range and fast response, is power effi- cient, devotes most of the pixel area to the radiating ele¬ ment, and avoids the capacitive coupling and insulator de¬ fect problems associated with long lead lines on a single crystal semiconductor substrate. A two-dimensional emr detector array with similar characteristics is also includ- ed. This goal is achieved in accordance with the invention by providing an IR pixel matrix in the form of an array of resistor bridges formed on an insulating substrate, such as sapphire. The resistor bridges span a portion of each pix- el cell, contact the substrate at spaced-apart contact locations, and are shaped to form a thermally insulative gap between the bridge and substrate. A semiconductor drive circuit on the substrate enables a desired amount of current flow through the bridge in response to an input control signal to heat the bridge, thereby producing a de¬ sired amount of IR radiation. Control, actuating and power lead lines extend along the substrate to deliver control and actuating signals to the drive circuits, and an elec¬ trical power signal to the resistor bridges; since the sub- strate is insulative, capacitive coupling and shorting problems are mitigated even with long lead lines. In the preferred embodiment the drive circuits are located on the substrate surface under the resistor bridges, and at least some of the lead lines also extend under the bridges. Each pixel cell includes a thermally reflective element between the resistor bridge and drive circuit to reflect IR radiation away from the drive cir¬ cuit. The thermally reflective element may comprise either a second bridge formed from a thermally reflective material which bridges the drive circuit and is spaced from both that circuit and the resistor bridge, or a thermally re¬ flective layer which overlies an insulative layer on the drive circuit. To reduce flicker to less than 1%, the drive circuit for each pixel comprises a sample and hold circuit in which a first transistor applies a control signal to a holding capacitor, and a second transistor applies a power signal to the resistor bridge in response to the capacitor signal. The electrical impedance of the resistor bridge preferably matches that of the drive circuit. The resistor bridges may be implemented in various ways. In one implementation they comprise a base bridge formed from an insulative material, with a resistive layer over the base. The resistivity and thickness of the resis¬ tive layer are selected to produce a desired resistance. The resistive layer may itself be surmounted by a coating of high thermal emissive material. The resistor bridge may also be formed from a material having an enhanced porosity which decreases its thermal conductivity. To concentrate the voltage drops towards the centers of the bridges and away from their connections with the substrate, the bridges may be tapered so that their cross-sectional areas near the center are significantly less than near their contacts with the substrate. 10 The resulting IR pixel array has a significantly better combination of thermal dynamic range, speed of res¬ ponse, resolution, weight, size, efficiency and flicker- free operation than any other known system. The invention also includes two-dimensional emr sensing arrays consisting of an array of emr detector cells formed on a substrate. Each cell includes an emr sensitive bridge structure. The shape of the bridge structure is designed to form a gener¬ ally insulative gap between the bridge and the substrate. The bridge structure has a defined characteristic which varies with the amount of received emr, and means are pro¬ vided for monitoring this characteristic for each of the cells to determine the level of emr incident on each cell. The bridge structures can be implemented by resistor bridges, thermocouple junction bridges, or Schottky junc¬ tion bridges. With resistor bridges, various bridge geo¬ metries may be selected to reduce the area of the bridge support legs relative to the center span, and thereby assist in thermally insulating the center span from the substrate. Multiple-stage amplifier readout circuits are preferably employed in connection with output voltage div¬ ider circuits to monitor the effective bridge resistances with a reduced Johnson noise. Thin layers of conductive material may be added to the support legs to electrically connect the center span to the underlying monitoring cir¬ cuits, the conductive material layers being thin enough to substantially maintain the thermal isolation of the bridge center spans from the substrate. The resistor bridges are preferably formed from an amorphous semiconductor material. With a thermocouple bridge detector array, the thermo¬ couples preferably comprise a stacked plurality of alter¬ nating semiconductor and metal layers. The junction dev¬ ices are connected in series with resistors and output vol¬ tage divider circuits, with the voltages across the output resistors monitored as a function of the emr received by the thermocouple bridges. The Schottky junction bridge embodiment employs a bridge configured as adjacent layers of a semiconductor material and a metal or doped semiconductor, meeting along a Schottky contact junction. The semiconductor is prefer¬ ably amorphous germanium or amorphous tin. The detector bridges are preferably coated with a lay¬ er of emr absorbing material to increase their sensitivity. Readout circuits for each detector cell are preferably •located on the substrate at least partially under their respective bridges to conserve substrate area. Various lead lines may also extend under the bridge structures to further conserve substrate area. A lens is employed to image an emr source onto the two-dimensional array. Addi¬ tional features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken to¬ gether with the accompanying drawings, in which: DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an IR simulator system which incorporates the present invention; FIG. 2 is a diagrammatic plan view of a portion of the pixel cell array used to create an IR image; FIG. 3 is a schematic diagram of the drive circuitry employed in each pixel cell; FIGs. 4-6 are sectional views showing different embod¬ iments of the resistor bridge which provides the IR radiat- ing element for each pixel; FIGs. 7-9 are sectional views illustrating various embodiments of the relationship between the resistor bridge of each pixel and the drive circuitry therefore; FIG. 10 is an illustrative plan view showing the dis- position of electrical lead lines with respect to the re- 12 sistor bridge for one embodiment; FIG. 11 is a sectional view of a resistor bridge emr detector and readout circuit in accordance with the inven¬ tion; FIGs. 12(a), 13(a) and 14(a) are plan views and FIGs. 12(b) , 13(b) and 14(b) are front elevation views of three different resistor bridge configurations which thermally isolate the center span of the bridge from the underlying substrate; FIG. 15-is a simplified schematic diagram of a readout circuit for a -resistor bridge emr detector; FIG. 16 is a more complete schematic diagram of the circuit shown in FIG. 15; FIG. 17 is a sectional view showing the formation of a transistor and capacitor of FIG. 16 in a single monolith¬ ic construction; FIG. 18 is a schematic diagram of another readout cir¬ cuit for a bridge resistor emr detector which reduces John¬ son noise; FIG. 19 is a sectional view of a thermocouple bridge emr detector; FIG. 20 is a simplified schematic diagram of a readout circuit for the detector of FIG. 19; FIG. 21 is a sectional view of a Schottky junction bridge emr detector; FIG. 22 is a simplified schematic diagram of a readout circuit for the emr detector of FIG. 21; and FIG. 23 is a block diagram showing the imaging of an emr source onto a detector array constructed in accordance with the invention. DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows the basic elements of an IR simulation system constructed in accordance with the invention. An IR radiating mechanism 2 is placed within a vacuum chamber 4 which has an IR window 6 at its forward end. A vacuum pump 8 evacuates the chamber in a conventional manner. The IR structure includes an insulative substrate 10, prefer¬ ably sapphire, a two-dimensional array 12 of IR emitting elements on the forward side of substrate 10, and a cooling mechanism 14 contacting the rear of the substrate. The cooling element is preferably a copper block cooled by water or freon flowing through inlet and outlet pipes 16. The IR window 6 is formed from a suitable material such germanium or zinc selinide, and is preferably provided with an anti-reflective coating to- revent reflection of emitted IR radiation back into the chamber. The individual IR emitting elements within the array 12 provide controlled emissions which are directed by a lens 18 onto an IR detector 20 being tested. The emissions from each individual IR radiating element are controlled by electrical signals applied to the array. The electrical leads into the array include a power line 22 connected to a DC power source, a series of data lines 24 which carry image information signals to the various IR elements, a series of strobe lines 26 which strobe rows of IR elements in succession in coordination with the image signal, and a ground line 28. The image data input signals may represent an actual video image, a computer simulation, or other de- sired pattern. The IR radiating array is divided into a matrix of pixel cells, each of which is individually controlled to emit a desired amount of IR radiation. The resolution of the IR image is governed by the number of pixels, which are typically provided in a 256x256, 512x512 or 1024x1024 array. A section of such an array is illustrated in FIG. 2. Each pixel cell includes a resistor bridge 30, which is the actual IR radiating element, and a drive circuit 32 for the resistor bridge. Each pixel cell is typically about 150 microns square, while the resistor bridges are about 120 microns square. The drive circuits employ two transistors, each of which is typically about 50 microns square. The aluminum electrical lead lines are typically about 25 microns wide. FIG. 2 is a simplified drawing which illustrates the functional relationship between the various elements in the array, but does not show them in their preferred physical arrangement; this will be discussed further below. One end of each resistor bridge 30 is connected to a DC power line 22, while the other end is connected to its associated drive circuit 32. Each drive circuit acts in response to an applied image signal from line 24 to control the flow of current from the power line, through the resistor bridge and drive circuit to ground. The pixel cells are arranged in a matrix of rows and columns. Electrical inputs are provided in a manner that allows each pixel to be separately programmed, yet avoids the need for separate lead lines for each pixel. Discrete signals are applied to the line 24 for each column, so that each drive circuit in a given column receives the same sig¬ nal, but the signals vary from column-to-column. Each row of pixel cells is strobed in succession via strobe lines 26, which are connected to the pixel drive circuits. The application of a strobe signal enables the operation of a drive circuit, which otherwise holds off the flow of any current through the resistor bridge. By varying the pat¬ tern of image data signals in synchronism with the progres¬ sion of the strobe signal from row-to-row, a unique signal can be applied to each pixel cell to establish the resist- or current through that cell at a unique value. The drive circuits are designed so that they can acquire an image data signal only during the brief period when the circuit is being strobed, so that the changing pattern of signals while other rows are being strobed has no effect upon the signal acquired by a particular pixel. Each drive circuit includes a sample and hold circuit which holds the acquired image data signal during the period of time required to strobe all of the other rows. With a frame rate in the order of 100 Hz, the sample and hold circuits keep the de- cay of the video signals acquired by their respective pix¬ els to less than 1%. This results in a substantially flicker-free operation. A schematic diagram of the preferred drive circuit for each pixel is given in FIG. 3. The circuit is preferably implemented with a silicon-on-sapphire (SOS) wafer in which the active circuit elements are formed from a silicon layer 12 on the sapphire substrate 10. SOS wafers are currently available in 5 inch diameters, and larger wafers will prob¬ ably be produced in the future. Since an insulating sub- strate is employed, the prior problem of shorting and capa¬ citive coupling from the lead lines is substantially elim¬ inated, thereby permitting the use of large arrays. Each sample and hold circuit includes a first FET Tl with its source-drain circuit connecting the image data input to a holding capacitor Cl. The signal held by Cl is applied to the gate of a second FET T2, and controls the amount of current flow permitted by T2. The bridge resis¬ tor 30 is connected between the DC power source and the drain or source of T2, depending upon the transistor con- nection; the opposite source-drain terminal of T2 is con¬ nected to ground. Thus, the amount of current permitted to flow through resistor 30, and accordingly the amount of IR radiation emitted by resistor 30, is controlled by the video signal held by Cl and applied to the gate of T2. A simple implementation of bridge resistor 30 is shown in FIG. 4. The resistor has a center span 34 that is ele¬ vated above the surface of sapphire substrate 10 by a pair of legs 36 at opposite ends of the center span. The verti¬ cal air gap clearance between center span 34 and the sub- strate is preferably about 2-5 microns, which is sufficient to prevent substantial thermal conduction loss from the resistor to the substrate. The method of forming such a bridge resistor is known, and is described in Patent No. 4,239,312 to Myer et al., assigned to Hughes Aircraft Co - pany, the assignee of the present invention. The bridge resistor could also be implemented with electrically con¬ ductive, metallic vertical legs and a planar center span of resistor material. In this configuration, because the legs have high electrical conductivity, there is a proportionate reduction in the voltage-induced thermal gradients, and reduction in stress. Whereas the resistors employed in the Daehler IR simu¬ lator were limited to boron doped silicon because of the bulk silicon substrate, with the present invention the re- sistor material can be selected to optimize its character¬ istics. Ideally, the resistor should have a low thermal conductivity to limit heat transfer to the substrate, an electrical conductivity which matches the resistor imped¬ ance to the impedance of the drive circuit, a low coeffi- cient of thermal expansion to prevent the resistor from ex¬ panding significantly with respect to the lower temperature substrate, and a high melting point. One suitable resistor material is polysilicon, which has a high melting tempera¬ ture, controllable resistivity, small temperature dependent resistivity, and a low coefficient of thermal expansion. Other materials which may be suitable include graphite, which has good temperature properties, high emissivity, titanium oxides, tantalum oxides, silicon oxides and cer¬ mets. To minimize thermal stress at the junction between the bridge resistor and the substrate, the bridge may be fabri¬ cated as shown in FIG. 5 so that its legs 36 are relatively thick where they contact the substrate, and progressively taper in cross-sectional area towards the center span 34. This arrangement reduces the resistance of the legs, parti- cularly in the vicinity of the substrate, and also adds mechanical support. Accordingly, the bulk of the voltage drop and the resultant high temperature will occur at the center span 34, which is desirable to both reduce mechani- cal stress and enhance IR radiation. The temperature of the bridge will be lower along its legs, particularly near the substrate, since less power is dissipated there. Further protection against heat loss may be obtained by increasing the porosity of the resistor material through electron beam deposition or other suitable technique. An increased porosity reduces the thermal conductivity of the material, making it even less susceptible. In another embodiment, illustrated in FIG. 6, the re¬ sistor bridge is formed from a base bridge 38 of insulative material such as Si0 2 , with a thin resistive layer 40 over the base bridge 38. The resistivity and thickness of the resistive layer 40 are selected to produce the desired re¬ sistance for the bridge. A very thin metal or ceramic- metal (cermet) layer, preferably on the order of about 50 Angstroms thick, would be suitable. The insulative base bridge 38 should itself be made as thin as possible, con¬ sistent with the required mechanical support, to provide fast response and low thermal conductivity. Si0 2 about 0.5- 1 micron thick would be suitable. The resistive layer 40 might be coated with a layer of high thermal emissive material 42, such as carbon black or gold black, to improve the radiative efficiency of the bridge. The thermally emissive layer may then be sealed with a sealing coat of Si0 2 about 50-100 Angstroms thick. To preserve space on the substrate and free up more area for the resistor bridges, the drive circuits 32 are preferably located under the bridges, as illustrated in FIG. 7. To maintain the drive circuitry at reasonable operating temperatures, it may be shielded from the heat of the resistor by means of a second bridge 44 as illu- 18 strated in FIG. 8. This second bridge is spaced from both the resistor bridge 30 and the drive circuit 32, and is formed from a thermally reflective material such as alumi¬ num. In addition to protecting the circuit elements from overheating, the reflective bridge 44 increases the pixel output power, since most of the IR radiation which it re¬ flects is transmitted through the thin resistor bridge to enhance the output IR radiation. FIG. 9 illustrates an alternate to the discrete re- flective bridge 44. In this embodiment an electrically insulative layer 46 such as Si0 2 is laid down over the drive circuit 32, followed by a thermally reflective layer 48 such as aluminum over the insulative layer. Additional area on the array may be conserved and de- voted to the radiating resistor elements by directing at least some of the lead lines under the bridge resistors. Such an arrangement is illustrated in FIG. 10. In this single layer metallization scheme, the image data, power and ground lines for each pixel 50 are run under the resis- tor bridge 30, while the strobe line extends along the side of the resistor. With a double layer metallization, all four lines could be separated by insulating layers and run under the bridge. Single and double layer metallization techniques are well known in the art. For further savings in real estate, adjacent pixels could share the same power and ground lines. The preferred technique for fabricating the array is to first fabricate the drive circuits on an SOS wafer. The whole substrate is then covered with a spacer layer, such as potassium chloride or aluminum, which can be selectively etched. A resist layer is then laid on top of the spacer layer to define the pixels. The resist at the intended resistor bridge locations is next exposed, the unexposed resist is washed off, and the spacer layer below the re- moved resist is etched away. After removing the remaining resist the bridge material is overcoated on the remaining spacer layer by a technique such as sputtering or electron beam evaporation, depending upon the bridge material and properties desired. A shadow mask technique might also be employed to form the bridges, but it is difficult to obtain desirably high pixel density and small bridge dimension with this technique. Finally, the remaining spacer materi¬ al is removed by standard techniques, such as dissolving away potassium chloride or etching away aluminum. With the described IR simulation system, 60-65% of the pixel area can be devoted to the resistor. As a result, the resistors do not need to be heated to as high a temper¬ ature to attain a given effective pixel temperature as with the prior system that employed much smaller resistors. The resistors can be tailored to impedance match the drive cir¬ cuits, thereby significantly improving the efficiency of the device. The problems of capacitive and electrical shorting associated with the lead lines is also substan¬ tially alleviated. The device is calculated to have an effective thermal dynamic range of l,000 β C in the 3-5 mic¬ ron region, and 600°-700 β C in the 8-12 micron region (with the substrate at room temperature) . Frame rates of 200 Hz are achievable without bothersome flicker. The bridge structure of the present invention can also be used as a sensitive and space-efficient emr detector for IR, microwave and other wavelengths. A particular range of wavelengths can be selected for sensing by an appropriate selection of the bridge dimensions, generally following the rule that the length of the bridge should be at least half the desired wavelength. IR wavelengths are quite small, so the bridges in this range will generally be considerably larger than a half wavelength. Microwave wavelengths are much longer, on the order of cms., so microwave detector bridges would not need to be much bigger than a half wave- length. With the use of a refractive lens or, in the case of microwaves, a focusing reflector, the emr beam can be at least partially focused to permit the use of smaller detec¬ tor bridges. FIG. 11 illustrates a resistance bridge detector that can be used for this purpose. The bridge 52 consists of a center span 54 with support legs 56 at either end extending up from a substrate 57, and a readout circuit 58 on the substrate under the chip and at least partially shaded thereby from incident emr. A preferred material for the bridge is amorphous sili¬ con, which has a relatively high temperature coefficient of resistance, is compatible with integrated circuit process¬ ing, and can be formed with controlled mechanical stresses. Other amorphous semiconductors are also suitable, such as Ge, SiGe, and SiC. Materials which are generally unsuit¬ able for the resistor bridge include metals because of high thermal conductivity and a low electrical resistance and low temperature coefficient of resistance, insulators be¬ cause of low conductivity, and cermets because of low tem- perature coefficients of resistance. The substrate 57 is preferably single crystal silicon for relatively small dim¬ ensions in the IR regime, or a silicon-on-insulator con¬ struction for larger microwave dimensions. A layer 60 of an emr absorbent material, such as metallic black or black paint for IR, is provided on the bridge center span, which is the emr target area, to in¬ crease the emr-induced temperature variation. Various geo¬ metric configurations for the bridge can also be designed to assist in thermally isolating the center span from the substrate, and thereby increase the potential degree of heating for the center span and thus the device's sensitiv¬ ity. This goal may be accomplished by making the cross- sectional areas of the legs which support the center span much less than the area of the center span emr target area. In FIGs. 12(a) and 12(b) the center span 54a is rectangu- lar, while a pair of support legs 56a extend from opposite sides of the center span and are quite narrow where they meet the center span. The support legs broaden out to the same width as the center span along the substrate to pro- vide a more secure bridge retention. By contrast, in FIGs. 13(a) and 13(b) the center span 54b has the same rectangu¬ lar configuration, but the support legs 56b consist of nar¬ row tabs 56b at each corner of the center span. In FIGs. 14(a) and 14(b) the center span 56 has a generally serpen- tine configuration to increase its effective length, and thereby enhance its thermal isolation, by increasing the length of the average thermal path between the center span and the substrate. Support legs 56c consist of narrow tabs at opposite ends of the center span. A thin metal layer, typically on the order of 100 Ang¬ stroms thick, is established over each of the support legs to provide an electrical shunt between the center span and the underlying readout circuit. This significantly reduces I 2 R heating in the legs. Because they are so thin, the metal layers themselves do not conduct heat significantly. FIG. 15 is a simplified schematic diagram of an output circuit that can be used to monitor the bridge resistance of each detector cell, and thereby the level of emr re¬ ceived by that cell. Similar readout circuits would be replicated for each of the individual cells. The circuit consists of a basic voltage divider formed by the variable bridge resistance B in series with a load resistor R L . Positive and negative bias voltages V b are applied across the series network, while the output at the midpoint of the two resistors is amplified by an amplifier Al and delivered as an output voltage V 0 . Any emr-induced change in R B will alter the bridge circuit, and adjust V 0 accordingly. A computer model has been devised to estimate the per¬ formance of a thin amorphous silicon bridge detector of this type. The resulting estimation for the bridge respom- 22 sivity to applied emr in a vacuum is 4xl0 3 volts/watt, for an amorphous silicon bridge having a thermal conductivity of about 0.0256 J/c /'K, a temperature coefficient of 2%, a resistivity of 100 ohm-cm, dimensions of 100x40x1 microns, and a bias voltage V b of 5 volts. Assuming Johnson (or resistive) noise is a limiting source of noise figure, the estimated noise equivalent power is near 3.4xl0 "10 W, and the corresponding detectivity is 1.8xl0 8 cm - Hz°- 5 /W for an effective bandwidth of 100 Hz. The minimum resolvable tem- perature difference at 300°K background is then close to 0.17"C, and the total power consumption for a 320x160 de¬ tector array is on the order of 2 Watts. This estimated performance is quite suitable for applications of an un- cooled IR imaging system. A more detailed schematic of an output circuit for each resistor bridge detector is provided in FIG. 16. Re¬ sistors Rl and R2 are chosen to maximize the product of the transistor transconductance and output resistances. The upper transistor T2 provides a large dynamic load, but rel- atively low static load, for the bottom amplifying transis¬ tor Tl, resulting in a large gain. Since the detector array will have a large number of output circuits of this type, one for each bridge detector, the output circuits are arranged in a matrix configuration with a strobe signal S applied to an output transistor T3 to obtain an output from each individual readout circuit at a desired strobe time. A capacitor C across the drain and gate of amplifier transistor Tl provides a large equivalent capacitance, and a corresponding low level of Johnson noise; the strobe readout process via T3 also discharges (resets) capacitor C. The equivalent capacitance of the circuit is given by the expression C(l+A) , where A is the amplifier gain. A physical construction by which Tl and C can be mono- lithically integrated into the same structure is shown in 23 FIG. 17. A gate metallization 58 is formed over an oxide layer 60 on the substrate, with a heavily doped semiconduc¬ tor layer 62 underlying a portion of metallization 58 and oxide 60, and extending laterally to one side. A gate con- nection is made to metallization layer 58, a drain connec¬ tion is made to doped semiconductor layer 62, and a source connection is made to a heavily doped region 64 on the opposite side of oxide layer 60 from drain layer 62. The transistor channel comprises the portion of the substrate between drain 62 and source 64, while capacitor C is estab¬ lished by the aligned portions of gate metallization 58 and drain layer 62. While the provision of a large equivalent capacitance in the circuit of FIG. 16 produces a significant reduction in Johnson noise, it is still a limiting factor in the operation of the circuit. An alternate readout circuit is shown in FIG. 18 which further reduces the Johnson noise level. This circuit employs a two-stage amplifier, with the capacitor C connected across the second amplifier A2 and an input resistor R2 for A2. Johnson noise is reduced by a factor of 7R2/RL. In addition, the capacitance of C can be reduced, with a corresponding reduction in the chip area required by the capacitor, to the extent the total amplification of the two-stage amplifier of FIG. 18 exceeds the amplification of the single-stage amplifier discussed above. FIG. 19 shows a different form of bridge emr detector which, as with the resistance bridge detector discussed above, can be dimensioned to detect various emr wave- lengths. The sensor is basically a theriαocouple bridge 66 which generates a thermally induced voltage in response to received emr. It consists of a bridge structure with alternating layers of dissimilar materials. While in theory any two different materials could be used to produce a thermal junction voltage, metal-semiconductor junctions have generally been found to produce the highest voltage levels. The bridge is preferably fabricated as a stacked layer, with thin metal layers 68 such as platinum alternat¬ ing with semiconductor layers 70 such as amorphous SiGe. The metal layers are preferably about 0.1-0.2 microns thick, while the semiconductor layers are about 0.5-2 microns thick to give sufficient mechanical strength. The thermocouple stack is surmounted by a heavily doped semi¬ conductor back contact layer 72, which in turn is capped by a layer 74 of radiation absorbing material. The back con¬ tact layer 72 can be formed by a low energy ion implant to a depth on the order of 0.1 microns. An opening or "via" 76 is formed in the radiation absorbent coating 74 to pro¬ vide a passage for an electrical lead line 78 to establish contact with back contact layer 72. A readout circuit 80 is provided on the substrate, below and at least partially shaded by the bridge 66, to monitor the emr-induced thermo¬ couple voltage. A simplified schematic of a readout circuit for this embodiment is given in FIG. 20. A load resistor R L is con¬ nected in series with the thermocouple junction, represent¬ ed as a variable voltage source 82. A bias voltage source 84 is connected across this voltage divider circuit, with the voltage V 0 across R L monitored as an indication of the emr-induced voltage across the junction. The detector array of the present invention can also be implemented as a Schottky junction bridge 86, illustrat¬ ed in FIG. 21. In this embodiment, a semiconductor layer 88 is formed over a bridge-shaped metal layer 90 to estab- lish a Schottky junction. The semiconductor layer 88 should have a small bandgap to obtain a high leakage cur¬ rent across the junction, with a corresponding high sensi¬ tivity. Semiconductors such as amorphous germanium or amorphous tin are suitable for this purpose. A heavily doped semiconductor that is substantially conductive can also be used instead of metal layer 90. As with the therm¬ ocouple embodiment, the bridge is topped by a doped back contact layer 92 and a radiation absorbent layer 94, with a lead 96 electrically contacting the back contact through a via 98 in the radiation absorbent coating. The readout circuit 100 for each bridge is again pref¬ erably located under the bridge, and at least partially shaded thereby. A simplified form of a suitable readout circuit is shown in FIG. 22. The Schottky junction bridge, represented by Schottky diode 102, is connected in series with a load resistor R L . A voltage source 104 is connected across the series circuit to reverse bias the Schottky junction. Changes in the Schottky leakage current appear as an output voltage V 0 across R L , which is monitored as an indication of the emr incident on the bridge. The detector bridges for each of the three embodi¬ ments, resistance, thermocouple and Schottky junction, are fabricated in a manner similar to the IR simulator bridge described previously. Spacer layers are established at the intended bridge locations, the bridges are formed over the spacer layers, and the spacer material is then dissolved or etched away. An emr source 106 can be imaged onto an array 108 of the described emr detectors by placing a collimating lens 110 in the path of the emr travelling towards the detector array. While only a small number of individual detector bridges are shown on detector array 108 for simplicity, in practice the arrays would have a large number of individual bridge detectors. The actual number for any particular application would depend upon the required bridge dimen¬ sions for the radiation to be detected, the available dim¬ ensions for the overall array, and the desired resolution. An array of 320 X 160 individual detector elements might be typical. In practice, the sensitivity of the resistance of the bridge resistor and that of the load resistor to variations in their dimensions, as well as variations in the output voltage of the pixel circuits due to nonuniformities in bias voltage and amplifier gains, makes it important to have an on-bσard compensation technique to correct for the resulting image nonuniformities. Because the dimensional changes are small and could in principle also be caused by mechanical and/or ambient thermal stresses, it is necessary to correct on a periodic basis. To correct for such non¬ uniformities, a shutter can be placed in front of the de¬ tector array and periodically closed to obtain a black readout. This readout is stored in a ROM or other conveni¬ ent storage device, and used to compensate readouts with the shutter open. The system operation is preferably per¬ formed in an analog format, with the calibration informa¬ tion stored digitally. With a video sample rate of 30 frames per second, recalibration might be performed perhaps once an hour. Electrical input and output lines can be conveniently run along the substrate, passing under the individual de¬ tector bridges to conserve substrate area, in a manner sim¬ ilar to that shown in FIGs. 2 and 10 for the IR simulator array. Power lines, ground lines, output lines and strobe lines would typically be employed. Since numerous variations and alternate embodiments will occur to those skilled in the art, it is intended that the invention be limited only in terms of the appended claims.

Claims: WE CLAIM: 1. A two-dimensional sensing array for electromag¬ netic radiation (emr) , comprising: a substrate, an array of emr detector cells in said substrate, each of said cells including an emr sensitive bridge struc¬ ture spanning a portion of the cell, said bridge structure being shaped to form a generally thermally insulative gap between the bridge structure and substrate, and having a defined characteristic which varies in accordance with the amount of emr received by the bridge, and means for monitoring said defined characteristic for each of said cells. 2. The emr sensing array of claim 1, said bridge structures comprising respective resistor bridges. 3. The emr sensing array of claim 2, said resistor bridges comprising a center span having a target area for receiving emr and a plurality of support legs elevating the center span above the substrate, the geometries of said center span and support legs being selected to enhance emr absorption by the center span relative to thermal losses from the center span to the substrate through the support legs. 4. The emr sensing array of claim 3, wherein the cross-sectional areas of said support legs are substantial¬ ly less than the areas of their respective target areas to thermally insulate the center span from the substrate. 5. The emr sensing array of claim 4, wherein a pair of support legs are provided for each center span, general¬ ly centered on opposite sides of the support span. 6. The emr sensing array of claim 4, said center spans generally comprising rectangles, wherein a support leg is provided adjacent each corner of said rectangles. 7. The emr sensing array of claim 3, said center spans having a generally serpentine configuration to in¬ crease their effective lengths and reduce thermal conduc¬ tion losses therefrom. 8. The emr sensing array of claim 2, said monitoring means, comprising respective resistors connected in series with the resistor bridges to form respective voltage divid¬ er circuits, means for applying a voltage to said voltage divider circuits, and means for monitoring the voltages across said resistors as a function of the emr received by said resistor bridges. 9. The emr sensing array of claim 8, said monitoring means comprising respective monitoring circuits at each cell, said monitoring circuits including multiple-stage amplifier readout circuits to reduce Johnson noise associ- ated with said voltage divider circuits. 10. The emr sensing array of claim 2, said monitoring means including respective monitoring circuits at each cell, said resistor bridges comprising a center span having a target area for receiving emr, a plurality of support legs elevating the center span above the substrate, and a thin layer of conductive material on said support legs electrically connecting said center span to said monitoring circuits, said layers of conductive material being thin enough to substantially maintain the thermal isolation of said center spans from the substrate. 11. The emr sensing array of claim 2, said resistor bridges being formed from an amorphous semiconductor mater¬ ial. 12. The emr sensing array of claim 1, said bridge structures comprising respective junction devices having an electrical characteristic which varies in accordance with the amount of emr received by the junction device, and said monitoring means comprises means for monitoring said elec¬ trical characteristic for each of said cells. 13. The emr sensing array of claim 12, said junction devices comprising junctions of unlike materials selected to generate a thermally induced voltage, said junction de¬ vices being configured to heat in response to applied emr within a desired range of wavelengths, and said monitoring means comprises means for sensing thermally induced volt¬ ages in each of said junction devices. 14. The emr sensing array of claim 13, said junction devices comprising semiconductor-metal junctions. 15. The emr sensing array of claim 14, said junction devices comprising a stacked plurality of alternating semi¬ conductor and metal layers. 16. The emr sensing array of claim 13, said monitor¬ ing means comprising respective resistors connected in series with said junction devices to form respective voltage divider circuits, means for applying a voltage to said voltage divider circuits, and means for monitoring the voltages across said resistors as a function of the emr received by said junction devices. 17. The emr sensing array of claim 12, said junction devices comprising Schottky diode bridge structures, and said monitoring means comprises means for reverse biasing said Schottky diode bridge structures and monitoring their leakage currents as a function of emr-induced heating. 18. The emr sensing array of claim 17, said Schottky contact bridge structures comprising adjacent layers of a semiconductor with a metal or doped semiconductor conduc¬ tor, said layers meeting along a Schottky contact junction. 19. The emr sensing array of claim 18, wherein said semiconductor is amorphous germanium or amorphous tin. 20. The emr sensing array of claim 1, said emr sen¬ sitive bridge structures including a layer of emr absorbing material for receiving emr and activating the remainder of the bridge structures to vary said defined characteristic in response to emr received by said absorbing material. 21. The emr sensing array of claim 1, said monitoring means including a readout circuit for each cell situated on the substrate at least partially under the bridge structure for that cell and at least partially shaded thereby from applied emr. 22. The emr sensing array of claim 21, said monitor¬ ing means including actuating and monitoring lead lines extending along the substrate for respectively actuating the bridge structures response to received emr and monitor¬ ing said responses, at least some of said lines extending under at least some of said bridge structures. 23. The emr sensing array of claim 1, further in¬ cluding a lens for imaging an emr source onto said array. 24. An emr detector, comprising: a substrate, an emr bridge structure spanning a portion of said substrate and separated therefrom by a generally ther- mally insulative gap, said bridge structure having a de¬ fined characteristic which varies in accordance with the amount of emr which it receives, means for elevating the bridge structures above the substrate, and means for monitoring said defined characteristic. 25. The emr detector of claim 24, said bridge struc¬ ture comprising a resistor bridge. 26. The emr detector of claim 25, said bridge struc¬ ture and elevating means comprising a center span having a target area for receiving emr and a plurality of support legs elevating the center span above the substrate, the geometries of said center span and support legs being sel¬ ected to enhance emr absorption by the center span relative to thermal losses from the center span to the substrate through the support legs. 27. The emr detector of claim 26, wherein the cross- sectional areas of said support legs are substantially less than the target area of said target area to thermally insu¬ late the center span from the substrate. 28. The emr detector of claim 27, wherein a pair of support legs are provided for the center span, generally centered on opposite sides of the support span. 29. The emr detector of claim 27, said center span generally comprising a rectangle, wherein a support leg is provided adjacent each corner of said rectangle. 30. The emr detector of claim 26, said center span having a generally serpentine configuration to increase its effective length and reduce thermal conduction losses therefrom. 31. The emr detector of claim 26, said monitoring means including a monitoring circuit on the substrate, and further comprising a thin layer of conductive material on said support legs electrically connecting said center span to said monitoring circuit, said layers of conductive material being thin enough to substantially maintain the thermal isolation of said center span from the substrate. 32. The emr detector of claim 25, said monitoring means comprising a resistor connected in series with the resistor bridge to form a voltage divider circuit, means for applying a voltage to said voltage divider circuit, and means for monitoring the voltage across said resistor as a function of the emr received by the resistor bridge. 33. The emr detector of claim 32, said monitoring means including a multiple-stage amplifier readout circuit to reduce Johnson noise associated with said voltage divid¬ er circuit. 34. The emr detector of claim 25, said resistor bridge being formed from an amorphous semiconductor materi¬ al. 35. The emr detector of claim 24, said bridge struc¬ ture comprising a junction device having an electrical characteristic which varies in accordance with the amount of emr received by the junction device, and said monitoring means comprises means for monitoring said electrical char¬ acteristic. 36. The emr detector of claim 35, said junction de¬ vice comprising a junction of unlike materials selected to generate a thermally induced voltage and configured to heat in response to applied emr within a desired range of wave¬ lengths, and said monitoring means comprises means for a thermally induced voltage in said junction device. 37. The emr detector of claim 36, said junction de¬ vice comprising a semiconductor-metal junction. 38. The emr detector of claim 37, said junction dev¬ ice comprising a stacked plurality of alternating semicon¬ ductor and metal layers. 39. The emr detector of claim 36, said monitoring means comprising a resistor connected in series with said junction device to form a voltage divider circuit, means for applying a voltage to said voltage divider circuit, and means for monitoring the voltage across said resistor as a function of the emr received by the junction device. 40. The emr detector of claim 35, said junction dev¬ ice comprising a Schottky diode bridge structure, and said monitoring means comprises means for reverse biasing said Schottky diode bridge structure and monitoring its leakage current as a function of emr-induced heating. 41. The emr detector of claim 40, said Schottky con¬ tact bridge structure comprising adjacent layers of a semi¬ conductor with a metal or doped semiconductor conductor, said layers meeting along a Schottky contact junction. 42. The emr detector of claim 41, wherein said semi¬ conductor is amorphous germanium or amorphous tin. 34 43. The emr detector of claim 24, said emr sensitive bridge structure including a layer of emr absorbing materi¬ al for receiving emr and activating the remainder of the bridge structures to vary said defined characteristic in response to emr received by said absorbing material. 44. The emr detector of claim 24, said monitoring means including a readout circuit situated on the substrate at least partially under said bridge structure and at least partially shaded thereby from applied emr.

Inventors: BRAATZ PAUL A, GRINBERG JAN, WELKOWSKY MURRAY S, WU CHIUNG-SHENG, BRAATZ, PAUL, A., GRINBERG, JAN, WELKOWSKY, MURRAY, S., WU, CHIUNG-SHENG
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1990
Main classification: H01L27
IPC codes: G01J1, G01J5, H01L27, H01L31, H01L33, H01L35