Abstract:
Apparatus and method for mounting a field emission device having emitters and an extraction grid in an electron gun are provided. The apparatus may be adapted from parts of a conventional electron gun that uses a thermionic emitter. Electrical connection to the grid is provided by bumps that are spring-loaded against a conducting surface, such as the second grid of a conventional electron gun.

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, apparatus and method that may use modified parts of conventional electron guns are provided for mounting field emission devices. 
     2. Description of Related Art 
     A conventional electron gun uses a thermionic emitter as the source of electrons. FIG. 1 illustrates a common configuration of the lower parts of an electron gun using a thermionic emitter. Parts of an electron gun having a cathode and two grids are generally shown at  10 . Both grids, along with other grids, are normally “beaded” into a structure that makes up the entire electron gun. Filament  12  is used to heat emissive coating  14  to cause electron emission. Cathode cap  16 , attached to cathode shank  18 , supports emissive coating  14  (usually a combination of one or more of barium, strontium or calcium carbonates, which are converted to oxides in a CRT). Cathode shank  18  is normally welded to interface ring  20 . Interface ring  20  may be formed integral in ceramic cathode mount  22 . Ceramic cathode mount  22  is free to slide in grid  24  but is fastened in place by welding during final electron gun assembly. Grid  24  (G 1 ) and Grid  26  (G 2 ) are used to focus and accelerate the electrons produced by emissive coating  14 . The grids are biased electrically in a way to create a source of electrons that can be further focused and modulated by other grids to satisfy CRT requirements. 
     The technology to allow replacement of cathodes based on thermionic emission with cold cathodes based on field emission of electrons (Field Emission Devices or FEDs) has been developing in recent years. The emission of electrons from cold cathodes may occur from microtips that are fabricated from molybdenum, silicon or, in very recent years, carbon-based materials. It has been demonstrated that the carbon-based material or diamond-like material 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)). The use of field emission devices with the extraction gate built-in eliminates the need for two of the electrodes in an electron gun built on thermionic emission, G 1  and G 2  of FIG. 1 Elimination of these electrodes simplifies the gun and also reduces its length. The application of the integrated carbon-like cathode and electrodes into an electron gun has been described in a pending and commonly assigned patent application entitled “Compact Field Emission Electron Gun and Focus Lens,” filed Jul. 19, 1999, Ser. No. 09/356,851, with named inventors Rich Gorski and Keith D. Jamison, which is incorporated herein by reference. A segmented cathode has been disclosed in pending and commonly assigned patent application “Segmented Gate Drive for Dynamic Beam Correction in Field Emission Cathodes,” filed Dec. 31, 19999, Ser. No. 09/476,051, with named inventors Keith D. Jamison and Donald E. Patterson, which is incorporated herein by reference. A package structure for mounting a field emitting cathode into an electron gun is described in a pending and commonly assigned patent application entitled “Package Structure for Mounting a Field Emitting Device in an Electron Gun”, filed Jan. 28, 2000, Ser. No. 09/493,379, with named inventors Randolph D. Schueller, Kent R. Kalar and Anthony A. Kloba, which is incorporated herein by reference. 
     Structures for incorporating field emission cathodes into an electron gun are known. but these structures require either considerable re-engineering or replacement of the parts of electron guns using thermionic emitters or newly designed parts. The electron gun design changes and gun part tool modifications are both costly and time-consuming. The use of field emission devices (FEDs) as cathodes is new in the cathode ray tube (CRT) industry and demonstration of their benefits in a CRT using prior methods or apparatus would require costly and time-consuming electron gun design changes. Once the advantages of FEDs are demonstrated, these changes are expected to prove well worth the expense. What is needed is a structure and method that require minimal modification of the parts of already commercial electron guns that use thermionic emission, i.e., that do not require extensive gun design and part tooling changes, while serving all the requirements of FEDs. Use of the structure in a CRT should also allow minimum changes in electrical connections to the CRT. 
     BRIEF SUMMARY OF THE INVENTION 
     An electron gun structure for use of a field emission cathode having an integral extraction grid is provided. The structure includes a spring confined in a hollow member, which may be the grid (G 1 ) of a conventional electron gun, that acts on a shank, which may be the cathode shank of a conventional electron gun, to press an electrical contact area around an emitting array against a second surface, which may be the grid (G 2 ) of a conventional electron gun. The electrical contact area is electrically connected to the extraction grid of a field emission array. The grid may be integrally formed with the emitters of the array. The field emitting array is formed on a substrate and is preferably carbon-based. In another embodiment, a support plate is placed beneath the substrate to increase the mechanical contact area between the cathode cap and the FED substrate. In yet another embodiment, the cathode shank is cut off, which may reduce angular mounting errors in some applications. 
    
    
     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 following drawings in which like reference numbers indicate like features and wherein: 
     FIG. 1 shows a configuration of the lower parts of a prior art electron gun using a thermionic emitter. 
     FIG. 2 shows a cathode including a field emission array suitable for use in an electron gun. 
     FIG. 3 shows one embodiment of an electron gun using a field emitting cathode. 
     FIG. 4 shows a second embodiment of an electron gun using a field emitting cathode. 
     FIG. 5 shows a third embodiment of an electron gun using a field emitting cathode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 2, a view of a cathode is shown generally at  30 . Field emission array  32  has been formed on substrate  34 . The field emission array may be carbon-based as disclosed in commonly assigned pending patent applications Ser. Nos. 09/169,908 and 09/169,909, filed Oct. 12, 1998; Ser. No. 09/356,856, filed Jul. 19, 1999; Ser. No. 09/476,651, filed Dec. 31, 1999 and Ser. No. 09/493,379, filed Jan. 28, 2000, all of which are incorporated herein by reference. The array is preferably formed with carbon-based emitters  36  and integrally formed gate layer  38 . Bumps  40 , which may be gold stud bumps, may be formed on gate contact layer  38 , the bumps to serve as electrical contact sites as well as mechanical standoffs to prevent shorting of the edge of the substrate when mounting procedures explained below are used. Insulating layer  37  separates gate layer  38  from emitters  36 . 
     Referring to FIG. 3, parts of an electron gun using thermionic emission such as shown in FIG. 1 have been modified to provide an electron gun for use with FEDs, shown generally at  50 . Parts are “beaded” together using conventional techniques. The emissive coating  14  of FIG. 1 has been removed or omitted from cathode cap  16  and field emission array substrate  34  is bonded to cathode cap  16  so as to form electrical contact between cap  16  and field emission array  32 . A conductive graphite coating or “Aquadag” or a high-temperature conductive adhesive such as ABLEBOND 71-1 or ABLEBOND 2106 may provide the adhesive, electrical, high-temperature and high-vacuum properties required for CRT manufacturing. The aperture of grid  24  (G 1 ) of FIG. 1 has been enlarged to form cylinder  52 , which allows clear passage of joined substrate  34  through the aperture. Of course, any suitably sized hollow member may be used for cylinder  52 . Cathode shank  18  is welded to interface ring  20  before insertion of the assembly into cylinder  52 . Ceramic cathode mount  22  is placed in cylinder  52 , then coil spring  54  is inserted into cylinder  52  and retaining spring disk  56  is inserted and fastened (preferably by welding) in cylinder  52 . Coil spring  54 , held in place by retainer  56 , serves to press bumps  40  against cylinder  58  with a force to make electrical contact with gate layer contact  42 , shown in FIG.  2 . The modified electron gun is then formed from the parts shown in FIG.  3  and any other grids that may be needed. The modified gun can be sealed into a CRT using conventional techniques. 
     FIG. 4 shows a second embodiment of an electron gun adapted for use with FED cathodes. In this embodiment, shown generally at  60 , the top of cathode shank  18  and cathode cap  16  of FIG. 1 have been cut off level with the top surface of ceramic cathode mount  22 . Metal support plate  62  (preferably nickel or stainless steel) may be spot welded to the exposed end of metal-to-ceramic interface cylinder  20 . Support plate  62  can also be bonded to the top of ceramic cathode mount  22  using a conductive graphite coating or “Aquadag” or a high-temperature conductive adhesive such as ABLEBOND 171-1 or ABLEBOND 2106. This provides mechanical attachment of support plate  62  to ceramic cathode mount  22  and provides electrical connection of support plate  62  to metal-ceramic interface ring  20  and cathode shank  18 . Field emission substrate  34  may be bonded to metal support plate  62  using the adhesives previously mentioned. Electrical contact is thus established between support plate  62  and field emission array  32  and cathode shank  18 . The support plate is used in some applications to increase the electrical contact area. 
     FIG. 5 shows a third embodiment of an electron gun for FED cathodes. In this embodiment, shown generally at  70 , cold cathode emitting array  32  and substrate  34  are attached to ceramic cathode mount  22 . Substrate  34  is bonded directly to cathode ceramic cathode mount  22  using conductive adhesive such as described above. This provides an electrical connection of cold cathode array  32  to metal-ceramic interface ring  20  and subsequently to cathode shank  18 . In each of the embodiments (FIGS. 3,  4  and  5 ), coil spring  54  and retaining spring assembly  56  are inserted into the body of cylinder  52  or  72  behind ceramic cathode mount  22  and retainer spring assembly  56  is welded or otherwise fixed into place. This secures the cathode assembly and provides a consistent force between bumps  40  and the face of cylinder  58 . A metal ribbon or wire (not shown) may be welded between shank  18  and all grids and the appropriate stem pins before the electron gun is sealed into a CRT, as is practiced in current methods using electron guns in CRTs. The modified electron gun may then be sealed into a CRT using conventional techniques. 
     To operate an electron gun with an FED in a finished CRT, gate potential may be applied to the former stem pin for grid  26  (G 2 ) of FIG. 1, which is now electrically connected to cylinder  58 , while the former stem pins for grid  24  (G 1 ) and cathode shank  18  of FIG. 1 are connected in common and grounded. Alternatively, the former stem pins for grid  24  and shank  18  may be given a negative bias while the pin for former grid  26  may be grounded. All other electron gun connections may be unchanged. Cathode heaters or filaments are not required for the modified gun. Field emitting cathodes can be used interchangeably with thermionic cathodes without extensive changes to conventional electron gun designs. 
     A conventional electron gun was modified in accord with the procedures described above to produce the apparatus illustrated in FIG. 3 with a carbon-like field emission array. Referring to FIG. 1, the aperture of grid  24  was enlarged by enough to allow free passage of the modified assembly (or from 0.005 inch to 0.250 inch). Filament  12  was removed. Emissive coating  14  was removed. Ceramic cathode mount  22  was welded to cathode shank  18 . Now referring to FIG. 3, field emission array substrate  34  was attached to cathode cap  16  by ABLEBOND 71-1. The substrate and array were carbon-based and formed in accord with procedures described in commonly assigned and co-pending applications cited above. Size and configuration of the contacts were modified to produce the part illustrated in FIG.  2 . Ceramic cathode mount  22  was inserted in cylinder  52  and bumps  40  were moved to contact cylinder  58 . Then coil spring  54  was inserted in cylinder  52  and retaining spring assembly  56  was inserted and welded in place. Electrical connections were attached to the cathode shank and all electron gun grids and the electron gun was tested in a laboratory vacuum chamber and found to operate satisfactorily. 
     The foregoing disclosure and description of the invention are explanatory and illustrative thereof, and various changes in the construction and method of operation and assembly may be made without departing from the spirit of the invention.