FLAT PANEL DISPLAY UTILIZING LINEAR ARRAY OF FIELD EMISSION CATHODES

Abstract: A flat panel display (11), having at one edge a linear array of electron guns (16) incorporating field emission cathodes (17, 18, 19). Each of the field emission electron guns is individually addressable, and all are simultaneously addressable. A deflecting structure (28) is provided to deflect electron beams issuing simultaneously from the guns toward a phosphor-coated display screen (12) to simultaneously address an entire column of pixels on the screen. All columns of a raster scan are sequentially addressed to thereby form the full raster scan. A beam alignment/calibration structure is included to enable correction of any deviation of a beam from a desired intensity or alignment.

Description: FIAT PJUTE DISPLAY OTILIΣING LI EλB ARRAY OF FIELD EMISSION CATHODES The present invention relates to flat panel displays and, store particularly, to an improved display of this type utilizing field emission cathodes. Cathode ray tubes (CRTs) generally are used in monitors for computers, in television sets, etc., to visually display information, such as video images graphics, or alphanumeric characters. Such tubes include one or store thermionic cathodes or the like, to generate one or more beams of electrons. Beam forming, modulation, and initial acceleration elec¬ trodes typically are combined with the cathode(s) to assure that appropriate, discrete electron beams are formed. The combination of such electrodes and the cathodes art referred to in the art as electron guns. Deflection coils or electrodes sweep each electron beam across the surface of a phosphor-coated anode screen. (Magnetic coils rather than electro¬ static electrodes are often selected for deflection control, because an electromagnetic field provides better beam characteristics.) Phosphor on the screen reacts to beam impingement by emitting visible radia¬ tion. While monochromatic (black and white) CRTs often only have one electron gun, most color CRTs have three guns (or three beams issuing from a single gun) which respectively bombard a pattern of red, green and blue phosphor dots or strips on the anode screen with differing intensities, to provide desired color. Each small area of the screen displaying a discrete element 4 of the image typically is termed a pixel (picture element). The number of pixels per unit area of a screen, is a measure of the resolution provided by the tube. For example, a typical cathode ray tube now in a color television receiver provides approximately 150,000 pixels, which pixels are individually bombard¬ ed in a raster pattern by three scanning electron beams. The art of cathode ray tubes is highly developed, and they provide a satisfactory display for many purposes. For example, CRTs can display informa¬ tion in many different formats, from alphanumeric characters to high resolution pictorial displays. They do so with pictures that are reasonably bright. Cathode ray tubes do require, though, significant depth in order to have the distance required between the electron gun assemblies and the screen necessary to obtain adequately controlled scanning. lowever, there are a number of important uses which require coopactness, aυch as to provide a display for portable computers. Thus, there has been significant interest and much research and development expended in an effort to provide satisfactory so-called active "flat panel displays" or "quasi flat panel displays" not having the depth requirement of a typical cathode ray tube, but having comparable display characteristics, e.g., brightness, resolution, versatility in display, etc., and power requirements. Most success in achiev¬ ing such characteristics has been with arrangements in which the display screens are phosphor-coated (cathodo- luminescent) rather than plasma discharge or other types. Flat panel display arrangements requiring the formation of electron beams, generally have one of two main configurations. In one configuration, a plurality of electron beams are formed by a matrix of cathodes or other electron beam sources, opposed to the display screen. The individual electron beam in this matrix approach is dedicated to one or more selected pixels to energize the same. In some of such arrangements, the electrons are actually generated by parallel "strip" cathodes and then formed into a matrix of individual beams by appropriate electrodes or shielding structurss. A major difficulty with this approach is that typically a relatively large amount of power is required to energize the cathodes to produce the required matrix of beams. Some of such arrangements also do not provide uniform brightness and some have thermal problems, i.e., generate heat which must be dissipated for proper operation. More¬ over, it is difficult to provide a matrix of individ¬ ually addressable cathodes or other sources that provides acceptable resolution, and yet is sufficient¬ ly large for an acceptably large display. Most of such arrangements also utilize X-Y addressing tech¬ niques which prevent individual pixels from being energized for an extended duration of time. This limits screen brightness. (In some arrangements, the pixel addressing means include circuitry for long duty cycle energisation to overcome this problem. It will be appreciated that this is a quite complicated solution to.the problem and introduces its own set of problems, e.g., low resolution.) Examples of the matrix approach are provided by the display arrange¬ ments described in U.S. Patent Mo. 3,363,240; 3,447,043; 3,500,102; 3,624,273; 3,855,499, 3,935,500; 4,029,984; 4,075,535; 4,077/054; 4,158,210; 4,178,531; 4,341,980; 4,404,493; 4,408,143; 4,417,184; and the paper entitled Flat Cathode-Ray-Tube Display presented at the 1978 SID International Symposium, San Francisco (April 17-21, 1978). In the other electron-beam configuration common to flat panel display technology, the electron beam source, i.e., one or more electron gun assem¬ blies, typically is mounted at the side or back of a display screen. The beam(s) are then deflected by appropriate techniques, e.g., electromagnetic, electro¬ static, or both, to impinge on the surface of the screen at desired pixel locations. A design of this nature is referred to herein as an "edge mounted" design. A major difficulty with this approach is low brightness, since a minimal number of beams must address the entire screen area sequentially and repetitively to form a desired image. Another diffi¬ culty is that there are significant electron-optical problems. That is, it is difficult to adequately control the path of the electron beams to assure pixels are properly positioned for a high resolution display. Examples of this approach can be found in U.S. Patent Hos. 2,978,601; 4,205,252; 4,394,599; 4,031,421} 4,263,529; and 4,374,343. Suaπary Of The Invention The present invention provides a flat panel display of an edge mounted design having some of the desirable structural characteristics of a matrix approach. This is made possible because of the attributes of field emission cathodes of the type described in, for example, Spindt et al. U.S. Patent Mo. 3,789,471. Field emission cathodes of this type typically have a significant number of electron emission sites in a very small area. For example, field emission cathodes are now available which have about 15 emission sites in an area of only about .005 inches in diameter. The flat panel display of the invention includes a linear array of electron guns incorporating field emission cathodes, positioned at one edge of a display screen to emit when energized, an array of electron beams which travel along discrete paths generally parallel to one another, adjacent the surface of the screen. Means are provided to deflect the beams to Impact on areas of the screen defining individual pixels. Most desirably, the beams are deflected simultaneously to excite pixels falling in a linear array (preferably a column) which are to be excited to define a portion of the image to be displayed, and sequential arrays of pixels are excited to cover the entire area of the screen. Thus, beams need only be deflected in one. irection in order to address the screen in two directions and form a two-dimensional array of excited pixels. In other words, the field emission cathode guns eliminate the need for scanning in one direction, and a deflection control arrangement need only be provided to deflect the array in another, angularly related direction. Moreover, the deflection means can be a simple electrostatic arrangement, provided in the best mode by a plurality of generally parallel deflector electrodes which are in a plane that also is generally parallel to the display screen surface. Electrostatic deflection requires signifi¬ cantly less power than electromagetic deflection. It will be appreciated from the above, that the invention includes not only the apparatus which is described, but also a unique scanning method. Area scanning is achieved even though a deflection arrange¬ ment for only one direction is provided. A major advantage provided by this scanning technique is that it allows a full linear array of pixels to be ener¬ gized all at one time, i.e., a multiple number of pixels are energized simultaneously rather than individually. In this connection, the intensity of each of the individual beams is individually modulated to include the desired, individual pixel information. This considerably enhances the brightness achievable, since there is a relatively long duty cycle of energiz¬ ation for each pixel. The invention also includes a mechanism for enhanced beam alignment and/or calibration. That is, means are provided adjacent a second edge of the screen area for detecting electron beams emitted by the cathodes. The detection mechanism enables feed¬ back both of beam position and beam intensity informa¬ tion, for controlling the generation and direction of the individual electron beams. The invention includes other features and advantages which will be described or will become apparent from the following more detailed description. Brief Description Of The Drawings FIG. 1 is an isometric view of a preferred embodiment of the display panel of the invention; FIG. 2 is a diagrammatic illustration of an enlarged, broken away elevational view of the panel of the preferred embodiment; FIG. 3 is a diagrammatic end view of the panel of FIG. 1; FIG. 4 is an enlarged, sectional view of a portion of the construction of the electron gun assemblies incorporated into the preferred embodiment, illustrating the relationship of a linear array of field effect cathodes thereto; FIG. 5 is an enlarged and diagrammatic illustration of a portion of the panel of FIG. 1 schematically showing alignment/calibration elements and electronics; FIG. 5A is an enlarged diagrammatic illustra¬ tion of the detection anodes shown in FIG. 5; and FIG. 6 is a schematic representation of electron beam control electronics for the preferred e embodiment of the invention. Detailed Description Of A Preferred Embodiment A simplified representation of a flat panel incorporating the invention, is generally referred to in the drawings by the reference number 11. Flat panel 11 includes a display screen 12 which, as is conventional, is responsive to electron impact by emitting visible radiation. The screen 12 has an anode, cathodolu inescent interior surface, i.e., a surface which is attractive to electrons and responds to electron impact by fluorescing or phosphorescing. That is, the screen is a transparent, conductively coated glass having a thin-film coating of one or more phosphors on its interior surface. It will be recog¬ nized by those skilled in the art, though, that other screen arrangements could be used, such as one having a conductive, reflective coating through which elec¬ trons must pass to impact on a phosphor-coating providing the desired cathodoluminescence. * As illustrated in FIG. 2 panel 11 also includes a shielding grid 13 adjacent the interior surface of the display screen. Such grid is shown broken away for simplicity and, as is conventional, can be a conductive open mesh or made up of strip electrodes as shown. Its purpose is to shield the beam array during deflection, prior to accelerating the deflected electron beams toward the phosphor- coated surface of screen 12, to increase their energy and angle of impact. It will be appreciated by those skilled in the art that in some instances it may not be preferred to include such a shielding grid. Also, in some instances it will be desirable to provide such a grid with adjacent elements insulated from one another to provide micro positioning of deflected beams for, for example, color selection. In keeping with the invention, an array 16 of individually addressable electron guns incorporat¬ ing field emission cathodes is provided as a part of the flat panel display, positioned at one edge of the screen 12. The array 16 includes a multiple number of field emission cathodes, preferably as many of such cathodes as lines of display are desired in the direction perpendicular to the array. Array 16 most simply is a linear array. That is, the guns of such array are arranged in a single line. As used herein, though, a linear array of electron guns is meant to encompass an array of guns positioned to address a linear array (a single line) of pixels. In this connection, it will be recognized that a multiple number of pixels may share an electron gun at differ¬ ing times. For example, in an arrangement in which interlaced field lines are provided to construct an image frame as is typical of common full screen television scanning, one electron gun may be individu¬ ally modulated to impact adjacent pixels sequentially for different fields of the frame. Moreover, it is preferred that the line be a straight line and in the preferred embodiment be a column of pixels. In order to reduce the number of electron guns necessary for array 16, it is preferred that the edge of the display at which the array is positioned, be. one of the opposed edges at which the lesser number of lines of pixel elements to be excited originate. This minimizes the number of individual field emission cathode guns needed. It also simplifies the process- « ing of a standard display signal. FIG. 4 symbolically illustrates three adjacent field emission cathode guns 17, 18 and 19. The cathodes of such guns are of the type disclosed in U.S. Patent Hos. 3,665,241; 3,755,704; and 3,789,471, naming Charles A. Spindt as one of the inventors, the disclosures of which are incorporated by reference. Such cathodes include a common base 20 from which a plurality of electron emitters 21 project for each gun. Each gun cathode includes a counter-electrode or gate 22, 23 and 24, respectively, to cooperate with the tips 21 for the production of electrons. The counter-electrodes are supported spaced from the emitter tips by a layer 30 of insulating material. The counter-electrodes are spaced from one another aε shown at 25, to delineate the separate cathodes 17, 18 and 19. There are several advantages to use of gun assemblies having field emission cathodes generating the electron streams. As discussed previously, each field emission cathode includes a plurality of elec¬ tron emission sites enabling relatively high energy beams, and yet is quite small in size. Moreover, activation of such cathodes does not rely on heat, thus eliminating both heat shielding requirements and the necessity of providing power to generate thermal energy. Means are provided to assure that the electrons emitted by each of the cathodes in the array form discrete electron beams. Such means is represent¬ ed by inclusion of an aperturing electrode 26 and a plurality of accelerating and focusing electrodes 27 as part of the electron guns. The electrodes 26, 27 as well as the base 20, are common to all of the guns. The focusing electrodes provide dynamic focusing for all of the beams, for optimum focusing. Means are provided for deflecting the beams for bombardment on selected areas of the screen. In this preferred embodiment, such means takes the form of a plurality of elongated electrodes 28 (FIG. 2) positioned rearwardly of the space through which the beams are to travel. The electrodes 28 provide an -electrostatic deflecting arrangement, i.e., an arrange¬ ment for establishing deflecting electrostatic fields. As mentioned previously, electrostatic arrangements generally require less power than electromagnetic arrangements. As illustrated, electrodes 28 generally define a plane parallel to the display screen surface. Most simply.the electrodes 28 are provided by deposit¬ ing etalized strips, e.g., of aluminum, on a sub¬ strate sheet 29 of an insulating material, e.g., a sheet of glass. * It will be appreciated that although not included in this preferred embodiment, in some situa¬ tions it may be necessary to provide supporting - structure between, for example, the screen 12 and the back substrate sheet 29 to counteract pressure due to vacuum loading. Each of the cathodes is individually provided with an energization voltage and, hence, produces an electron beam having a desired intensity, depending upon the particular pixel it is intended to form. To this end, the counter-electrode of each field emission cathode has a separate lead 31 for connection to individual voltage sources 32 as illus¬ trated in FIG. 6 * . The beam intensity and the corres¬ ponding voltages will of course be dependent upon the desired image to be displayed, etc., as is conven- tional. Image processing,, timing and the like for the 12 cathode voltages is represented in FIG. 6 by a control¬ ler 33 which receives as its input, a display signal as represented at 34. Bach of the deflection electrodes 28 is also individually addressable. This is represented in FIG. 6 by the inclusion of separate Z deflection voltage sources 37, 38 and 39 for the three electrode strips 28 illustrated in FIG. 2. Timing of the application of voltages to the deflection electrodes and to the cathodes, is synchronized. This is represented in FIG. 6 by inclusion of a timing generator 40 receiving input from controller 33 as represented at 41, and having individual output leads 42 connected to the deflection voltage sources 37, 38 and 39. The timing is synchronized so that the deflection system deflects the beam emanating from each of the cathodes to strike the pixel for which the cathode beam intensity is determined at the moment. Most desirably, all of the cathodes are simultaneously energized to produce a plurality of beams which travel together on paths adjacent the screen. The beams are then most desir¬ ably all deflected together toward the screen to impact simultaneously a column of adjacent pixels. This operation is repeated sequentially. Deflection of the beams together in this manner results in a sequence of pixel columns being energized in one direction, i.e., "scanning" in such direction. As mentioned earlier, this results not only in a simplir fied scanning system, it enables brightness to be enhanced since the pixel elements can be "scanned" at a field or frame rate, rather than at a line rate. It should be noted that the fact that the individual electron beams are formed by individually addressable cathodes is an important aspect of this portion of the invention. It also should be noted that although previously cited U.S. Patent No. 4,031,427 mentions sequential line-by-line scanning, the structure described in the patent simply will not allow such scanning to be achieved with any meaningful resolu¬ tion. The invention includes an arrangement enabling precise beam alignment and calibration. That is, adjacent the edge of the screen opposite the edge having the cathode array 16, there is an array 44 of beam detectors, a corresponding detector for each of the beams to be aligned and/or calibrated. The detectors of array 44 are positioned to intercept the corresponding beams when such beams travel adjacent the display surface without being deflected. FIG. 5 illustrates two detector arrange¬ ments 46 and 47 positioned to intercept electron beams represented at 48 and 49 emanating from, for example, cathodes 17 and 18. Each of such detectors most simply is a plurality of anodes 51-54. As is best illustrated in the enlarged view of FIG. 5A, there are four anodes in each detector, arranged in a square. (It will be appreciated that in FIG. 5 anodes 51 and 52 and their leads are respectively shielded from view by anodes 53 and 54 and their leads.) The anodes 51-54 are positioned so that a beam to be detected, if properly directed, will strike equally on all four of such anodes. If a beam is slightly misdirected, there will be an imbalance in the current received by the anodes. Thus, a compari¬ son of the current received by the anodes will indi¬ cate a misdirection and also provide information 14 defining the orientation of the misdirection. A comparator 56 is connected to the outputs of the anodes of each detector and develops signal informa¬ tion indicative of any misdirection. An output lead 57 from each of the comparators is connected to appropriate voltage sources for controlling beam alignment electrodes for the individual beams. This is diagrammatically represented in FIGs. 4 and 6 by the input leads 57 to block 58 providing as repre¬ sented by output leads 59, the individual voltages required to control operation of appropriate beam alignment electrodes for the individual beams. The electrodes for each gun 17, 18 and 19 are respectively represented symbolically in FIG. 4 at 61, 62 and 63. It will also be appreciated that the sum of the currents of all of the anodes.of a given beam detector will be a measure of the intensity of the striking beam. An adder 64 is Illustrated connected to the outputs of all of the anodes of each of the detectors to develop a signal representative of such intensity. The output of each of the adders 64 is fed back as control information to each of the sources of cathode voltage. This is represented by output leads 66 emanating from each of the adders providing an input at €7 to an associated cathode voltage source 32. Although not illustrated, it will be recog¬ nized that other detection arrangements may be used. For example, detector plates are available which provide differential outputs on output leads position¬ ed strategically at the edges of an electrically resistive plate, depending upon the location on the plate at which a beam impinges. Such a detector arrangement could be used for a plurality of different beams if such beams are sequentially energized and the outputs of the detector arrangement are synchronized and programmed to Identify the particular beam responsible for particular outputs. It will be appreciated that although for simplicity of description the electronics has been illustrated and described as if it is exterior to the mechanical structure of the flat panel display, it is preferred that it be encapsulated therein to minimize the number of feed-throughs that are necessary. This is particularly true with respect to the electron gun control and.beam intensity/alignment arrangements discussed above. While the invention has been described in connection with a preferred implementation thereof, it will be appreciated by those skilled in the art that various changes and modifications can be made without departing from its spirit. It therefore is intended that the coverage afforded applicant be limited only by the claims and their equivalents.

Claims: What we claim is: 1. A flat panel display comprising: a display screen responsive to electron bombardment by emitting visible radiation, which screen has a surface for receiving electron bombardmen ; an array of electron guns incorporating field emission cathodes at one edge of said screen positioned to emit when energized an array of electron beams along paths which are adjacent to said surface of said screen; and means for deflecting electron beams emitted by said array to impact said beams on selected areas of said surface of said screen. 2. A flat panel display according to claim 1 wherein said paths of said beams are generally parallel to one another. 3. A flat panel display according to claim 1 wherein said paths are generally parallel to said surface of said screen. 4. A flat panel display according to claim 1 further including means adjacent said surface inter¬ posed between said paths and said screen surface for shielding said electron beams from an accelerating field adjacent said screen surface. 5. A flat panel display according to claim 1 wherein said surface of said display screen which is responsive to electron bombardment by emitting visible radiation from said screen Is a cathodolumineβcent surface. 6. A flat panel display according to claim 1 wherein said array of field emission cathode electron guns are positioned at said one edge of said screen to emit a plurality of discrete electron beams to travel respectively on each of said paths. 7. A flat panel display according to claim 6 further including means for commonly controlling the focus of said discrete beams. 8. A flat panel display according to claim 1 wherein said array of field emission cathode electron guns is a linear array extending along said one edge of said screen. 9. A flat panel display according to claim 1 wherein said means for deflecting electron beams emitted by said array comprises means for establishing electrostatic fields. 10. A flat panel display according to claim 9 wherein said means for establishing electrostatic fields comprises a plurality of generally parallel deflector electrodes which define a plane parallel to said display screen surface. 11. A flat panel display according to claim 1 further including means for individually controlling the operation of cathodes of said array. 12. A flat panel display according to claim 1 wherein each of said field emission cathodes includes a plurality of emission sites. 13. A flat panel display according to claim 1 further including means adjacent a second edge of said screen for detecting electron beams emitted by said cathodes. 14. A flat panel display according to claim 13 wherein said detection means includes anodes positioned to intercept each of said electron beams. 15. A flat panel display according to claim 13 wherein said detection means.detects any variation between the actual and a desired direction for an electron beam, and means are included ' responsive to a signal indicative of any misdirection by correcting for the same. 16. A flat panel display according to claim 13 wherein said detection means detects any variation between the actual and a desired intensity of an electron beam, and means are included responsive to a signal indicative of any difference by correcting for the same. 17. A flat panel display comprising: a display screen having a cathodolu¬ minescent surface responsive to electron bombardment by emitting visible radiation from said screen; a linear array of field emission cathodes at one edge of said screen positioned to emit when energized an array of electron beams which travel on paths which are generally parallel to one another, each of which cathodes has a plurality of emission sites; and a plurality of generally parallel deflector electrodes generally defining a plane parallel to said display screen surface for deflecting electron beams toward said surface. 18. A method of addressing an area of a display screen which reacts to electron beam bombard¬ ment by emitting visible radiation, comprising the steps of: providing means to form an array of individually addressable electron beams along one edge of said screen area; energizing said sources to simultaneous¬ ly produce a plurality of electron beams which travel together on paths adjacent said screen; deflecting said beams together toward said screen to simultaneously excite a first linear array of pixels on said screen to be excited to define a portion of a desired image; and thereafter deflecting said beams together toward said screen to simultaneously bombard a second linear array of adjacent pixels on said screen, whereby said area of said screen is addressed in two generally orthogonal directions. 19. A method according to claim 18 wherein said step of deflecting said beams together toward said screen to simultaneously excite a first linear array of pixels, comprises deflecting said beams together toward said screen to simultaneously excite a first straight linear array of pixels on said screen; and said step thereafter of deflecting said beams together toward said screen to simultaneously excite a second linear array of pixels on said screen, com¬ prises deflecting said beams together toward said screen to simultaneously excite a second straight linear array of said pixels. 20. A method according to claim 18 wherein said step of providing means to form an array of individually addressable electron beams along one edge of said screen area comprises providing an array of individually addressable field emission cathodes along said edge. 21. A method according to claim 18 further including the step of sequentially deflecting said beams together toward said screen to excite additional linear arrays of adjacent pixels on said screen simultaneously. 22. A flat panel display comprising: a display screen responsive to electron bombardment by emitting visible radiation, which screen has a surface for receiving electron bombardment; electron generating means at one edge of said screen positioned to emit when energized an array of electron beams on paths which are generally parallel to said surface of said screen; means for deflecting said electron beams toward said surface of said screen; and means adjacent a second edge of said screen for detecting electron beams emitted by said cathodes. 23. A flat panel display according to claim 22 wherein said detection means includes a plurality of anodes positioned to intercept predetemined ones of said electron beams.

Inventors: CONE ROY DONALD, SPINDT ALVIN CHARLES, CONE, ROY, DONALD, SPINDT, ALVIN, CHARLES
Applicants: STANFORD RES INST INT, SRI INTERNATIONAL
Application year: 1985
Main classification: H01J31
IPC codes: H01J29, H01J31