Method and device for uniforming luminosity and reducing phosphor degradation of a field emission flat display

Abstract: Nonuniformities of luminance characteristics in a field emission display (FED) are compensated pixel by pixel by storing a matrix of correction values, determined by testing, and by applying a corrected drive signal through the relative column drive stages. The individual pixel's correction factor that is applied to the corresponding video signal may be stored in digital or analog form in a nonvolatile memory array. Various embodiments are described including the use of a second updatable RAM array wherein pixel's correction factors are calculated and stored at every power-on to provide an opportunity of trimming-up the luminance of the display for compensating long term decline of luminance due to the phosphors ageing process.

Description: This invention relates to a method and a circuital device for regulating the cathode current of flat panel displays (FPD) of the field emission type (FED) compensating for nonuniformities resulting from the process of fabrication as well as reducing the display ageing and degrading process. The continuous evolution towards portable electronic products such as laptop computers, personal organizers, pocket TVs and electronics games, has created an enormous market for monochromatic or color display screens of small dimensions and reduced thickness, having a light-weight and a low dissipation. Especially the first two requirements cannot be met by conventional cathode ray tubes (CRTs). For this reason, among the emerging technologies, let alone those related to liquid-crystal-displays (LCD), flat panel field emission display technology has been receiving increasing attention by the industry. A remarkable research and development work has been carried out in the past few decades on field emission displays (FED) employing a cathode in the form of a flat panel provided with a dense population of emitting microtips co-operating with a grid-like extractor essentially coplanar to the apexes of the microtips. The cathode-grid extractor structure is a source of electrons that are accelerable in a space, evacuated for ensuring an adequate mean free-path, towards a collector (anode) constituted by a thin and transparent conductor film upon which are placed luminescent phosphors excited by the impinging electrons. Emission of electrons is modulately excitable pixel by pixel through a matrix of columns and rows, constituted by parallel strips of said population of microtips and parallel strips of said grid-like extractor, respectively. The fundamental structure of these display systems and the main problems related to the fabrication technology, reliability and durability, as well as those concerning the peculiar way of exciting individual pixels of the display system and various proposed solutions to these problems, are discussed and described in a wealth of publications on these topics. Among the pertinent literature the following publications may be cited: US 5,391,259; Cathey, et al. US 5,387,844; Browning US 5,357,172; Lee, et al. US 5,210,472; Casper, et al. US 5,194,780; Meyer US 5,064,396; Spindt US 4,940,916; Borel, et al. US 4,857,161; Borel, et al. US 3,875,442; Wasa, et al. US 3,812,559; Spindt, et al. US 3,755,704; Spindt, et al. US 3,655,241; Spindt, et al. "Beyond AMLCDs: Field emission displays?", K. Derbyshire, Solid State Technology, Nov. 94; "The state of the Display", F. Dawson, Digital Media, Feb.-Mar. 94; "Competitive Display Technologies", 1993, Stanford Resources, Inc.; "Field-Emission Display Resolution", W. D. Kesling, et al., University of California, SID 93 DIGEST 599-602; "Phosphors For Full-Color Microtips Fluorescent Displays", F. Lévy, R. Meyer, LETI - DOFT - SCMM, IEEE 1991, pages 20-23; "Diamond-based field emission flat panel displays", N. Kumar, H. Schmidt, Solid State Technology, May 1995, pages 71-74; "Electron Field Emission from Amorphic Diamond Thin Films", Chenggang Xle, et al., Microelectronics and Computer Technology Corporation, Austin, TX; University of Texas and Dallas, Richardson, TX; SI Diamond Technology, Inc., Houston, TX; "Field Emission Displays Based on Diamond Thin Films", Natin Kumar, et al:, Microelectronics and Computer Technology Corporation, Austin, TX; Elliot Schlam Associates, Wayside, NJ; SI Diamond Technology, Inc., Houston, TX; "U.S. Display Industry on the Edge", Ken Wemer, Contributing Editor, IEEE Spectrum, May 1995; "FEDs: The sound of silence in Japan", OEM Magazine, Apr. 1995, pages 49, 51; "New Structure Si Field Emitter Arrays with low Operation Voltage", K. Koga, et al., 2.1.1, IEDM 94-23. The major advantages of FEDs compared to modem LCDs are: low dissipation; same color quality of traditional CRTs; visibility from any viewing angle. FED technology has developed itself on the basic teachings contained in US Patent No. 3,665,241; 3,755,704 and 3,812,559 of CA. Spindt and in US Patent No. 3,875,442 of K. Wasa, et al.. FED technology connects back to conventional CRT technology, in the sense that light emission occurs in consequence of the excitation of the phosphors deposited on a metallized glass screen bombarded by electrons accelerated in an evacuated space. The main difference consists in the manner in which electrons are emitted and the image is scanned. A concise but thorough account of the state of modem FED technology is included in a publication entitled "Competitive Display Technologies - Flat Information Displays" by Stanford Resources. Inc., Chapter B "Cold Cathode Field Emission Displays". A schematic illustration contained in said publication and giving a comparison between a conventional CRT display and a FED (or FED array) is herein reproduced in Fig. 1. In a traditional CRT there is a single cathode in the form of an electron gun (or a single cathode for each color) and magnetic or electrostatic yokes deflect the electron beam for repeatedly scanning the screen, whereas in a FED the emitting cathode is constituted by a dense population of emission sites distributed more or less uniformly over the display area. Each site is constituted by a microtip electrically excitable by means of a grid-like extractor. This flat cathode-grid assembly is set parallel to the screen, at a relatively short distance from it. The scanning by pixel of the display is performed by sequentially exciting individually addressable groups of microtips by biasing them with an adequate combination of grids and cathode voltages. As shown in Fig. 2, a certain area of the cathode-grid structure containing a plurality of microtips and corresponding to a pixel of the display is sequentially addressed through a driving matrix organized in rows and columns (in the form of sequentially biasable strips, into which the cathode is electrically divided and of sequentially biasable strips into which the grid extractor is electrically divided, respectively). A typical scheme of the driving by pixel of the cathodic structure of a FED is shown in Fig. 3. This figure illustrates the driving scheme of a fragment of nine adjacent pixels through a combination of the sequential row biasing pulses for the three rows R1, R2, R3, relative to a certain bias configuration of the three columns C1, C2 and C3. A typical cross-sectional view of a FED structure is shown in Fig. 4. The microtip cathode plate generally comprises a substrate of an isolating material such as glass, ceramic, silicon (GLASS BACKPLATE), onto which is deposited a low resistivity conductor layer as for example a film of aluminum, niobium, nickel or of a metal alloy (NICKEL ELECTRODE), eventually interposing an adhesion layer for example of silicon (SILICON FILM) between the substrate and the conductor layer. The conductor layer (NICKEL ELECTRODE) is photolithographically patterned into an array of parallel strips each constituting a column of a driving matrix of the display. A dielectric layer, for example of an oxide (SILICON DIOXIDE), is deposited over the patterned conductor layer. A conductor layer (NIOBIUM GATE METAL), from which the grid extractor will be patterned, is deposited over the dielectric layer. The grid structure is eventually defined in parallel strips, normal to the cathode parallel strips (NICKEL ELECTRODE). According to a known technique, microapertures or wells that reach down to the surface of the underlying patterned conductor layer (NICKEL ELECTRODE) are defined and cut through the grid conductor layer (NIOBIUM GATE METAL) and through the underlying dielectric layer (SILICON DIOXIDE). Onto the surface of the conductor layer exposed at the bottom of the "wells", are fabricated microtips (MOLIBDENUM MICROTIPS) that will constitute as many sites of emission of electrons. On the inner face of a glass faceplate of the display is deposited a transparent thin conducting film, for example of a mixed oxide of indium and tin (ITO CONDUCTOR) upon which is deposited a layer of phosphors (monochromatic phosphor or color phosphors) excitable by the electrons accelerated toward the conducting layer (ITO CONDUCTOR) acting as a collector of the electrons emitted by the microtips. Emission that is stimulated by the electric field produced by suitably biasing the grid conductor and the cathode tips. In order to improve color resolution, the realization of a "switched" anodic (collector) structure for separately biasing adjacent strips, each covered with a phosphor of a different basic color, has been suggested in a publication entitled: "Phosphors For Full-Color Microtips Fluorescent Displays" by F. Lévy and R. Mayer, LETI - DOFT - SCMM, Grenoble - Cedex - France. Fabricating processes of microtips cathode plates are described in US Patents No. 4,857,161; 4,940,916; 5,194,780 and 5,391,259. As an alternative to microtip cathode structures, the utilization of a film of amorphous diamond formed on a patterned conductor layer supported by a plate of dielectric material, by flash evaporation of carbon atoms from a target of graphite or other carbon material, has recently been suggested. The amorphous diamond film seems to offer extraordinary field emission characteristics (low extraction energy of electrons). Emission current densities of up to 100 mA/mm 2 have been recorded with a bias of less than 20V/µm that can be complied with by simply imposing a manageable voltage difference between the emitting cathodic surface constituted by the amorphous diamond film and a phosphor coated screen (anode), without requiring the formation of emitting microtips and of an extraction grid . In this case, the pixels are addressable through a driving matrix of orthogonal lines simply constituted by cathodic (columns) and anodic (rows) strips. The intrinsic emission properties of an amorphous diamond film give way to a substantial structural simplification of the panel. Nevertheless, this alternative technique of constituting a FED device still presents remarkable problems in terms of ensuring an acceptable uniformity of the characteristics of the cathodic structure. In general terms there exist a well recognized problem of ensuring an uniform luminance (brightness) of the display that involves a control of the emission current of each single microtip (commonly in the order of several hundreds) that belong to a singularly addressable pixel (area) of the display. While on one hand an intervened reduction of the emission efficiency of single tips is not in itself a cause of failure in view of the fact that each pixel is constituted by a multiplicity of microtips working in parallel, on the other hand, the presence of microtips having an emission efficiency much higher than average is particularly detrimental, because of the following effects: a) screen faults in the form of intensely bright spots; b) (in extreme cases) severe damage to the panel by evaporation of the tip and of the adjoining grid. Essentially the current emitted by each single tip depends on two factors: i) the radius of curvature of the microtip which depends on the fabrication technique; ii) the electric field at the vertex of the microtip that depends on the distance between the tip and the adjoining grid. Both these geometric parameters depend on the process of fabrication and therefore present a certain statistic dispersion (spread). Even in the case of cathode structures based on the use of a highly emissive film of amorphous diamond deposited over parallel conducting strips defined on the surface of a dielectric substrate, the field emission characteristics of a given portion of surface, defined by the addresses of the driving matrix (that is of the area sustaining the electric field of excitation of the corresponding pixel), are subject to a rather marked spread because of the difficulties of controlling the uniformity of deposition of the amorphous diamond film. Another problem is tied to the ageing of the display, that is, the reduction of the luminance resulting from a progressive reduction of the phosphors' efficiency. This ageing process is closely connected to the electronic bombardment and therefore tends to become faster in correspondence of bright spots and is generally faster the higher is the initial pixel current. The problem of nonuniformity is well recognized and various solutions have been proposed. In particular a solution that is widely used is that of interposing between the conductive layer of the strips into which the cathode is subdivided (which constitute the columns of the pixel driving matrix) and the microtips a resistive layer, generally constituted by amorphous and/or polycrystalline silicon, suitably doped, as described in US Patent No. 4,940,916 and in the publication entitled "Current limiting of field emitter array cathodes" by K. J. Lee, University of Georgia." Even this solution has its drawbacks because it is extremely difficult to control the resistivity of a necessarily thin layer of polycrystalline and/or amorphous silicon in a satisfactory manner for limiting the current emitted by the microtips of an addressed pixel of the display. Under certain aspects, a more efficient solution consists in using electrodes (e.g. cathode conductors) patterned in a grating-like, form where the conductive and the associated resistive portions are substantially coplanar as described in the US Patent No. 5,194,780. The increased uniformity is obtained at the expenses of a remarkable increase of the complexity of the fabrication process and of costs. The document FR-A-2 683 365 discloses a field emission display (FED) wherein the column scanning circuitry comprises a correction memory array programmable during a testing phase of the display for storing relative emission efficiency values of each individual pixel of the display for compensating disuniformities due to the fabrication spread. The biasing conditions of excitation of each pixel are modulated in function of the correction value for the pixel read from the correction memory array. Altogether these known solutions have little repercussions on the related problem of the ageing of the phosphors. In addition, as stated above, the scanning of a FED screen takes place by selecting one row (that is one strip of the grid extractor) at the time, in succession, while the luminance (brightness) of the single pixels is controlled by modulatedly biasing the columns (connected to the cathode electrodes) which are scanned in sequence. According to a first technique, the pixel current is controlled by the voltage difference existing between cathode tips of the selected pixel and the respective grid extractor strip (selected row). According to an alternative technique, the luminance (brightness) is controlled by regulating the excitation time of the pixel, that is in scanning the columns with signals having the same amplitude but different duration. The method and the device of the invention for compensating for the nonuniformities of intrinsic luminance characteristics of a FED consist in processing the signal that is used for driving the display pixels and will be described in depth by referring to the case of video signals. However, the concept can be easily expanded to alphanumerical applications or to computer monitors. According to a first aspect of the present invention an effective compensation of nonuniformities is attained by using a correction matrix of the pixel driving signals, constituted by a memory cell array of adequate dimensions, wherein data stored in the memory in the form of words (bytes) of a certain number of bits have a biunivocal correspondence with the pixels of the screen. Each of these data or words contains a calibration value that is used for correcting the level of the signal (for instance a video signal) that drives the corresponding pixel. This calibration value Is tied to a previously measured emission efficiency of the corresponding pixel. In this way, intrinsic nonuniformities of emission characteristics of the various pixels are compensated (corrected), thus ensuring a uniform luminance (brightness) of the FED. The stored datum for each pixel can be a word of relatively short length (small number of bits): for example a length of four bits will permit to obtain sixteen different correction values. This, number of different correction values is quite sufficient for compensating even relatively large differences of intrinsic brightness that are likely to occur among the pixels of the cathodic structure (that is the cathode and extractor grid assembly). Moreover, compensation may be implemented according to a linear law, or in different modes, as for example according to a logarithmic law, should the differences (spread) to be compensated for, which may be typical of a certain fabrication process, be particularly large. The invention is useful for compensating nonuniformities of a FED structure. The invention is equally useful for a FED device based on a microtip cathodic structure comprising an extractor grid as well as for a FED device based on the use of an emitting cathodic structure constituted by biasable strips of a high emissivity amorphous diamond film. In the course of the present description, eventual references or illustration of a microtip cathodic structure should be considered as purely indicative and in no way limitative, because the invention is equally useful even in the case of cathodic structures of different type but having similar problems of nonuniformity among individual pixel's emission characteristics throughout the whole surface of the display. Figure 1 is a general comparative scheme between a conventional CRT display device and a field emission flat array display device (FED); Figure 2 shows the general architecture of a FED device and of the relative driving circuitry; Figure 3 is an illustration of how every single pixel of a FED device is driven; Figure 4 is a schematic cross sectional illustration of the structure of a FED device; Figure 5 shows the architecture of a FED device of the invention, provided with individual pixel brightness compensation system; Figure 6 shows an alternative architecture of the correction system of the invention, particularly suited for video systems. According to a first embodiment, schematically shown in Fig. 5, the memory for storing the correction matrix is a nonvolatile, read only, memory of the EPROM, OTP or FLASH-EPROM type. As a matter of example, for a monochromatic display having 360 columns and 288 rows, it may be required to store a total of about 104,000 words. Assuming a word length of 4 bits, a memory of 0.5 Mbit will be appropriate. This size is quite compatible with nonvolatile memories available nowadays in either EPROM, FLASH-EPROM or EEPROM technology. Of course, for a color display the correction memory size requirement may grow by a factor of 3, yet remaining within a commonly available commercial range. First data acquisition for the correction matrix, can take place during a final quality control testing of the fabricated display. This can be undertaken according to an aspect of the method of this invention by monitoring the emission current under fixed bias values of cathode and grid voltages for each singularly addressed pixel, and by storing in the correction matrix a number proportional to the ratio between the measured emission current and a certain given reference value. This number can be used directly, or after processing it through an appropriate algorithm, as an "attenuation" factor to be applied through the relative drive circuits to the signal that actually drives the corresponding pixel, whether such a drive signal is modulated in terms of amplitude (voltage) or duration (time). In this manner, an almost perfect compensation of the emission nonuniformities of the cathode, resulting from the process spread, can be achieved. According to a different embodiment, the pixel correction factors stored in a nonvolatile memory array are proportional to the intensity of the light signal picked up by a photocell suitably placed in front of the FED undergoing testing, while the pixels are sequentially excited (scanned one by one) by a biasing signal of a fixed preset level. In this way, the correction factor that will be applied to each individual pixel is directly proportional to the measured pixel luminance. Therefore it is possible to correct the compounded effects of the disuniformities of the cathode structure and of the possible disuniformities of the light emitting phosphor layer of the display, that is of the anode structure which also comprises the layer or layers of phosphors. The method of this invention permits to reduce the problem connected with screen degradation in time. Accordingly, a matrix of correction values is stored in an electrically alterable memory array, for example an EEPROM or DRAM or SRAM memory array and a matrix of correction data is generated and stored in the memory each time the display is switched-on. According to this embodiment, during a self-testing phase that is automatically run at power-on, a control logic circuitry performs a sequential stimulation of all the rows with a constant amplitude signal, monitors the current flowing in the column drivers, and stores in the memory a digital value proportional to the ratio between the read current and a reference current level. By suitably fixing a reference value (current level) to be just lower than the maximum obtainable, and preferably as close as possible to an acceptable intermediate value, a new screen will be advantageously excited at the start of its operating life by a signal of an intermediate level. Of course, the correction attenuation values may be gradually increased to compensate for the ageing of the cathode due for instance to a degradation of the microtips as a result the presence of residual gases, that tend to reduce their emission efficiency, just by trimming up the reference value. In this way, besides correcting nonuniformities and retarding the ageing process by lowering the level of the "initial" current, a compensation, up to a certain limit of the ageing is made possible, thus extending the operational life of the screen overall. Obviously, this embodiment does not lend itself to provide a compensation of nonuniformities of the phosphors. Beside the type of memory, it is also possible to choose among different configurations of the memory array. For example, a standard video signal conceived for a conventional CRT, is essentially sequential and includes row and screen-refresh synchronization signals. The driving of a flat panel display (FPD) whether of the field emission type (FED) or of the liquid crystal type (LCD), requires a partial parallelization of a standard video signal by loading the frame relative to the scanning of a row on a buffer, which is then unloaded in parallel over the respective column drivers. According to an important aspect of the present Invention, in this type of (video) application, the correction values can be read sequentially from the correction memory by using, for example, the row and screen synchronization signals present in the incoming video signal for activating an address generator and introducing a correction in the serial signal before its parallelization, as illustrated in Fig. 5. According to this embodiment, the circuitry may be simplified and memories of a standard type can be used, through a rather high memory reading speed and a correction algorithm capable of being implemented at high speed are required. According to a further alternative embodiment of this invention, a conceptually simpler solution is that of using a memory array structured in rows and columns, as for example a CCD, whose internal organization in rows and columns "duplicates" that of the pixels of the FED screen itself. In this case, the correction values can be loaded in parallel on the column drivers of the screen, as depicted in Fig. 6. The speed requirements are in this case less stringent, even if at the expenses of a greater circuitry complexity, because the correction circuitry must be duplicated for each column driver (CORRECTION CIRCUIT). Further to what has been described above, the correction memory can be of the analog type, for instance based on a CCD or on an analog EEPROM, and correction implemented directly by an analog circuitry. Obviously, various combinations are possible. According to a preferred embodiment, use is made of a first nonvolatile memory for example of the EPROM type, in which a matrix of initial correction values, which may preferably compensate for disuniformities of the anodic structure (phosphors) too, is stored. This basic (or initial) correction matrix may be established by testing as described above. In a second updatable memory array, for example of the DRAM or CCD type, a "map" of the pixel currents is stored at every power-on. The correction signal (value) for each pixel may be obtained by combining the respective correction factors stored in the two distinct memory arrays, or in a simpler and less dissipative way, by generating an updated matrix of correction values in the second memory array by writing therein the resulting value of the combination of the pixel current level read at power-on with the corresponding basic correcting parameter stored in the nonvolatile memories. In this way, pixel signal processing would be performed once at power-on and the correction system will read only one memory at each screen refresh.

Claims: A method of controlling the cathode current in a field emission display (FED), characterized in that it comprises the steps of: determining a value representative of the relative emission current referred to a certain reference value of each individual pixel during a pre-operative phase at each turning-on of the display; modulating during the functioning of the display the bias of each scanned pixel in function of a video signal and of a correction signal function by said pre determined value. The method according to claim 1, characterized in that it comprises the steps of combining said relative emission current value of each pixel determined at each turning-on with a relative light emission value of the same pixel read from a non volatile memory written when testing the display and storing the resultant value in a second electrically alterable memory; modulating during the operation of the display the bias of each scanned pixel in function of a video signal and of a correction signal function of said stored resultant value. Field emission display (FED) comprising a cathodic structure in the form of conductive strips defined on a dielectric substrate and having field stimulable electron emission microtips distributed over their surface, each strip being individually biasable in sequence by a column scanning circuitry of a pixel driving matrix of the display, which driving matrix comprises conductive extractor strips orthogonal to said columns, selectable in sequence by a row selection circuitry, an anodic structure including a conductor film on which luminescent phosphors are excited by impinging electrons, said column scanning circuitry further comprising a non volatile correction memory programmable during a testing phase of the display for storing relative light emission values of each individual pixel of the display, biunivocally addressable with the corresponding excited pixel during the functioning of the display, and a correction circuit of a column driving signal capable of modulating the bias of the selected column as a function of a video signal and of the values stored in said correction memory for each selected pixel, characterized in that it comprises further a logic circuit for sequentially stimulating each row of pixels with a constant biasing signal, reading the value of the current through each column driving stage, generating a value of relative emission current referred to a certain reference current value for each pixel, and combining said value of relative emission current with the value stored in said first nonvolatile correction memory for the same pixel; at least a second electrically alterable memory array for storing said combined values for each pixel during a pre-operative phase at each turning-on the display; the combined values stored in said second memory constituting a matrix of correction values that are read at every screen refreshing. The field emission display (FED) according to claim 3, characterized in that at least said second electrically alterable memory has said combined correction values organized in a matrix of rows and columns similarly to the pixels of said driving matrix of the display and a correction circuit for each column. The field emission display (FED) according to claim 3, characterized in that said first non volatile memory is chosen from the group composed of EPROM, OTP and FLASH-EPROM and said second electrically alterable memory is chosen from the group composed of EEPROM, RAM and SRAM.

Inventors: BALDI LIVIO, BALDI, LIVIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: G09G3
IPC codes: H01J31, G09G3