Abstract:
A field emission cathode providing for dynamic adjustment of beam shape is disclosed. Beam shape adjustment is accomplished by segmenting the gate electrode of a gated field emission cathode and independently driving the various gate segments to form the desired beam shape. Segments can be turned on and off as the beam is deflected allowing dynamic correction of aberrations in the beam. A focus lens can be placed on the gated cathode to produce a parallel electron beam. In addition, a hollow cathode can be produced to minimize space charge repulsion in a beam.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to electron guns for devices such as cathode ray tubes (CRTs). More particularly, it relates to improved field emission arrays having integral electrodes. 
     2. Description of Related Art 
     A cathode ray tube (CRT) and any other device requiring an electron beam normally contains a hot filament to cause thermionic emission from a cathode. There has long been an interest in developing cold cathodes, depending on field emission of electrons, to replace hot cathodes. For low current devices, such as scanning electron microscopes, there are a large number of patents describing field emission electron guns. There are also a large number of patents for field emission flat panel displays where the field emitter has a low duty cycle. For higher current applications, such as TV displays, prior art field emission cathodes, generally based on molybdenum and silicon, have not proven sufficiently robust for commercial applications. Tip damage occurs from ion back scattering caused by the presence of background gases and the tips fail when driven at high current densities. 
     It has been demonstrated that carbon-based microtip cathodes can be fabricated and used as a replacement for molybdenum- or silicon-based microtip field emission cathodes. It has also been demonstrated that the diamond can be monolithically integrated with gated electrodes in a self-aligned structure, using integrated circuit fabrication techniques (“Advanced CVD Diamond Microtip Devices for Extreme Applications,” Mat. Res. Soc. Symp. Proc., Vol. 509 (1998)). 
     Much of the work in field emission cathode development was directed to electron sources for use in flat panel displays. U.S. Pat. No. 3,753,022 discloses a miniature directed electron beam source with several deposited layers of insulator and conductor for focusing and deflecting the electron beam. The deposited layers have a column etched through to the point field emission source. The device is fabricated by material deposition techniques. U.S. Pat. No. 4,178,531 discloses a cathode ray tube having a field emission cathode. The cathode comprises a plurality of spaced, pointed protuberances, each protuberance having its own field emission-producing electrode. Focusing electrodes are used to produce a beam. The structure produces a plurality of modulated beams that are projected as a bundle in substantially parallel paths to be focused on and scanned over the screen of a CRT. Manufacture using a photoresist or thermal resist layer is disclosed. U.S. Pat. No. 5,430,347 discloses a cold cathode field emission device having an electrostatic lens as an integral part of the device. The electrostatic lens has an aperture differing in size from the first aperture of the gate electrode. The electrostatic lens system is said to provide an electron beam cross section such that a pixel size of from approximately 2 to 25 microns may be employed. Computer model representations of the side elevation view of prior art electron emitters are shown. 
     U.S. Pat. No. 5,786,657 proposes a method to minimize the nonuniform influence of surrounding electric potential on an electron beam from field emitters. A hole in the emitting surface and electrodes with suitable potentials are used to minimize beam distortion. A recent paper discusses the use of field emitter electron guns in a CRT. (“Field-Emitter Array Cathode-Ray Tube,” SID 99 Digest, pp. 1150-1153, 1999) The paper discusses means for decreasing beam diameters by making smaller diameter gates and other adjustments. Also, the problem of limited pixel definition at the periphery of an ellipse-shaped beam is discussed and fabrication and use of segmented or divided focus electrodes to improve beam focus is described. 
     Space charge, beam deflection, beam size and position, and other factors influence the shape of the beam when a beam passes through electron optics and is focused onto an object. The shape of the beam may also vary with the angle of deflection when the beam is magnetically or electrostatically deflected. Improvement in dynamic beam-shaping methods and apparatus will provide added value for field emitter arrays for use in CRTs or other devices. The dynamic beam shaping method should be widely adaptable to a variety of conditions where the final beam-shape needs improvement, such as when an electron beam is deflected by a magnetic field. The dynamic beam shaping method should allow for the continued adjustment at different deflection angles of the beam. 
     BRIEF SUMMARY OF THE INVENTION 
     Apparatus and method are provided for dynamically adjusting the emitted beam shape from a field emission cathode having a gate electrode. The cathode emitter may be carbon-based, but other emitter materials may be used. The gate electrode in an array of field emission sources is independently controlled for each emitter or group of emitters in different areas of the array. Control of voltage on the gate electrode allows emission to be turned off and on or to be adjusted in intensity from different areas. This control allows for dynamic correction of aberrations in the beam by adjusting the emission area and shape in the emitted beam from the cathode array. Control voltages may be supplied from drive circuitry that may be controlled by a microcontroller. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein: 
     FIGS. 1A,  1 B and  1 C are illustrative views of an area of a field emission array having a monolithically integrated segmented gate electrode with individual control of each emitter in an array. 
     FIGS. 2A and 2B are illustrative views of an area of a field emission array having a monolithically integrated segmented gate electrode for separate control of areas of an array. 
     FIGS. 3A and 3B are illustrative views of a field emission array having monolithically integrated segmented gate electrode and an integrated focus electrode. 
     FIG. 4 shows the fabrication procedures used to form an emitter array with integrated extraction and focus electrodes with control of areas of the extraction electrode. 
     FIG. 5 illustrates the application in a CRT of an emission array with control of areas of the array by circuitry. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1A, an illustration of an area of a field emitter cathode is generally shown at  10 . Emitter material  12  has been used to form an array of tips  14  on the emitter material, using procedures described hereafter. In one embodiment, emitter material  12  is carbon-based material as disclosed in commonly assigned pending applications Ser. No. 09/169,908 and Ser. No. 09/169,909, filed Oct. 12, 1998 which are incorporated by reference herein. In other embodiments, emitter material  12  is tungsten, molybdenum, silicon or other materials that are commonly used for field emission sources or a wide bandgap emitter such as gallium nitride or aluminum gallium nitride. Insulating layer  16  is grown on the emitter material and then gate electrode  17  is deposited on the insulating layer. Gate holes are then defined around each emitter using etch techniques as described in the co-pending patent applications Ser. No. 09/169,908 and Ser. No. 09/169,909. Gate electrode  17  is shown in FIGS. 1A and 1B as segmented or isolated for each emission point. Via  18  connects a segmented extraction electrode to a wire pad  19 . A wire (not shown) attached to a pad may supply voltage to control emission from each point. With the large number of emission points normally present in an array, this embodiment requires a large number of vias, pads, wires and control voltage sources. Any method for connecting the controlled voltage to each extraction gate may be used. Vias may extend to the edge of the array. Direct wire bonding to the gate surfaces may be used. Dynamic beam adjustment can be carried out as explained below with the greatest control over beam shape. 
     FIG. 1B shows a cut-away section of cathode  10 . Gates  17  are thin layers of metal on top of dielectric layer  16 . FIG. 1C illustrates a cross-section of the device showing electron beams  15  emitted from tips  14 . Voltage on gate electrode  17  is selected to obtain the desired beam current. Although cathode  10  is shown as a circular design, it should be understood that the cathode may generally be square, rectangular, or any other desired shape. 
     In FIG. 2A an illustration of an area of a field emitter array is generally shown at  20 . Materials may be the same in the illustration of FIG. 2 as illustrated in FIG. 1, but in FIG. 2 extraction gates are ganged together in selected segments over the area of the emitter array to form voltage control areas, as shown by area  22 . Voltage control areas  22  are selected to achieve the desired ability to dynamically control beam shape, as explained further below. Areas such as area  22  may be shaped to provide optimum results. The number of areas is greater than one and less than the total number of microtips. Areas may be in stripes across the array, in concentric patterns, or in any other shape. Pads may be present on such an array, as illustrated in FIG. 1, but alternatively wire bonding may be applied to areas such as area  22 . FIG. 2B shows a cut-away view of an area of array  20 . 
     In either case (ganged or unganged gate electrodes), an additional integrated focusing lens layer may be added over the segmented gate layer. Extraction gates determine the areas of the structure that are actually on and emitting electrons; focusing lenses tend to produce a parallel beam of electrons from each emission tip. FIG. 3 illustrates an area of a segmented field emitter array generally at  30 , which includes integral focus lens  32 . Extraction electrode  17  is present but dielectric layer  16  now extends above electrode  17 . Pads  34  have been exposed on the perimeter of an area such as to allow wire bonding to selected segments of extraction electrode  17  of FIG. 1 or areas  22  as shown in FIG.  2 . Pads may be electrically connected to integral focus lens  32  and wire bonding may be applied directly to the lens segments. FIG. 3B shows a cross-section of an area of the array. The quantity of current in electron beam  36  is controlled by extraction gate  17  and each beamlet is focused by focusing electrode  32  around each point  14 . Gate electrode  17  determines which tips are turned on. 
     The fabrication processes used for producing the segmented or individual extraction gates disclosed herein include a particular combination of standard field emission array fabrication steps along with steps described in co-pending and commonly assigned applications Ser. No. 09/169,908, Ser. No. 09/169,909 and in the application titled “Compact Electron Gun and Focus Lens,” Filed Jul. 19, 1999, all of which are incorporated by reference herein. FIG. 4 shows steps of the fabrication processes that may be used. An emitter array is fabricated from a suitable material such as a carbon-based material or other material disclosed herein. Normally such an array will be grown in selected parts of the surface of a wafer that is later cut into dies, each having an array of emitting tips, as is well known in the art. After tips are grown, a dielectric or insulating layer, often composed of silicon oxide, is grown or deposited over the tips. A conducting metal layer is then deposited, using known techniques. Then a photoresist layer is deposited as part of a standard photolithographical process to form a desired pattern for the extraction gate structure, vias and connecting wire pads. These steps result in structures such as shown in FIG.  1  and FIG. 2 before the hole is etched around the tip. To form the structure shown in FIG. 3, a second insulating layer is deposited over the extraction electrode, then a second metal layer that will form the focusing lenses is grown. Then a second photoresist layer is deposited, but it is not to be patterned as was the first such layer. Rather, this layer is used to form a self-aligned focus lens structure. The resist layer is spun to a thin layer and the resin of the photoresist material cured. The photoresist layer is thinner over the microtips of the array, which cause protrusions over each microtip. This feature allows a controlled dry etch to expose the second metal layer only on the tips of the protrusions. Then a series of wet and/or dry etches allows etching through successive conducting and insulating layers until emitter tips are exposed. The overall structure resembles a tip at the bottom of a well. 
     After the emitter tips are exposed, the focus layer is photolithographically patterned to form the final device structure. Each device is composed of one segmented array. Excess metal on the wafer between what will be different cathode devices may then be etched away. Vias to gate structure contact pads are subsequently etched to expose gate electrode contact pads such as pads  34  of FIG.  3 A. Preferably, tiers are formed as shown in FIG. 3A such that dielectric layer  16  extends to the edge of emitting material  12 . Emitting material is preferably in the form of a die that is cut from a wafer after arrays of field emitting points are grown on the wafer at selected locations. Similarly, to minimize short circuits, focusing electrode  32  preferably does not extend to the edge of dielectric layer  16  of FIG.  3 . Although circular areas of an emitting array are shown in FIGS. 1,  2  and  3 , dies are often cut into rectangular or other shapes. The field-emitting array on each die may likewise be rectangular, circular or any other desired shape. 
     FIG. 5 illustrates the application of a segmented field-emitting array in a cathode ray tube (CRT). CRT  50  is of conventional design except for the cathode. The usual thermionic emission cathode has been replaced with a field emitting cathode structure shown generally at  52 . Referring to FIG. 5B, ceramic substrate  53  supports and is electrically connected to die  54  that has segmented emitting array  56 , which has been described above. Wires  58  electrically connect the cathode or the electrodes to pins  62 . Wires  58  may be joined by wire bonding their ends to pads or pins  62 . Pins  62  pass outside CRT  50  through glass seal  64 . Pins  62  may then be wire bonded by wire  66  to pads  68  on an electronic card or circuit  70 . Drive circuitry  72  (FIG. 5A) delivers selected voltages to each pad  68  as preselected synchronous signals. The voltages control emission from each point or each selected ganged area of electron emission from array  56 . By turning on or off or altering beam current from each selected segment of the array, the shape of a the total electron beam from cathode structure  52  is modified. This can be used to dynamically change the beam at different angles during magnetic deflection, for example. The voltage changes may be synchronized such that beam shape is selected for each deflection angle. This provides a beam-shaping capability not heretofore available; one that can be achieved by field emission cathodes and not by thermionic cathodes. 
     In one embodiment, the beam adjustments necessary to avoid distortion of the beam when the electron beam from the field emission cathode structure  52  is deflected to a selected portion of a display are determined experimentally by measuring the beam shape of a spot on the screen of the CRT at a fixed selected location. The beam is deflected to the selected portion of display screen  75  of CRT  50  and beam shape is measured on the screen. Voltage is decreased or turned off to the gate electrode for selected tips and increased at other tips while beam dimensions are measured. Optimum beam dimensions are obtained by selectively turning off or on of gate electrode voltages to selected tips or segments of tips. Preferably, when voltage is decreased at tips to decrease electron beam current from those tips, voltage is increased at other tips to maintain total beam current at approximately a constant value. Adjustments of gate electrode voltages may be controlled by a microprocessor that is programmed in accord with the measurements of beam dimensions for different areas of the display. The microprocessor turns on various segments or areas of the array depending on where the spot caused by the beam is located in the display. The microprocessor may be programmed initially to apply various patterns of voltages to different areas of an emitting array and measurements of beam area, taken either manually or by well known photosensitive instruments, may be used to select a final sequence of voltage changes during a sweep cycle of the beam. 
     In another embodiment, beam dimensions are calculated using known mathematical methods for electron beam simulation. Such Electron Beam Simulation (EBS) methods are discussed, for example, in the co-pending and commonly assigned application titled “Compact Field Emission Electron Gun and Focus Lens,” filed Jul. 19, 1999, and incorporated by reference herein. Such calculation may be performed with selected areas of an array emitting no beam current or a selected beam current. The size and shape of the beam on a display at a selected distance may then be calculated. Deflection of the beam may also be simulated and included in the calculation of beam dimensions. In addition, a hollowbeam pattern can be produced by control of extraction electrode voltages in the center of an array to eliminate or minimize electron current from that area of an array. This beam pattern would minimize space charge repulsion in a beam. 
     While the foregoing disclosure and description for fabricating the segmented gate drive has concentrated mainly on a “self-aligned” fabrication process, the fabrication of segmented gate drives can easily be added as a modification to processes for fabricating other types of field emission cathode structures. U.S. Pat. Nos. 3,755,704, 3,789,471, 3,812,559, and 3,970,887, all of which are incorporated by reference herein, are representative of other prior art techniques used to fabricate field emission cathodes. Having fabricated a prior art field emission cathode, our segmented gate structure would be added by photolithographically defining the segmented structure into the existing extraction gate structure through a series of photolithography and metal etch steps. The focus electrode could then also be added to prior art cathodes in the manner disclosed herein. 
     The foregoing disclosure and description are illustrative and explanatory thereof, and various changes in the details of the illustrated apparatus and construction and method of operation may be made without departing from the spirit of the invention.