Abstract:
Apparatus and method are provided for a package structure that enables mounting of a field-emitting cathode into an electron gun. A non-conducting substrate has the cathode attached and the cathode is electrically connected to a pin through the substrate. Other pins are electrically connected to electrodes integral with the cathode. Three cathodes may be mounted on a die flag region to form an electron gun suitable for color CRTs. Accurate alignment of an emitter array to the apertures in the electron gun and other electrodes such as a focusing lens is achieved. The single package design may be used for many gun sizes. Assembly and attachment of the emitter array to the electron gun during construction of the gun can lower cost of construction.

Description:
RELATED REFERENCES 
   This application is a divisional of application Ser. No. 09/493,379, filed Jan. 28, 2000 now U.S. Pat. No. 6,469,433. 

   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, a package structure for mounting a field-emitting device into an electron gun is provided. 
   2. Description of Related Art 
   A cathode ray tube (CRT) and other devices requiring an electron beam normally include an electron gun having a thermionic emitter as the cathode. The electron gun is assembled to include the cathode and other electrodes that focus and accelerate the electron beam. Such electron guns are typically assembled manually and the cathode is inserted after the gun is assembled. Such assembly can be costly. It would be an advantage to include the cathode portion of the gun in the initial assembly. 
   The technology to allow replacement of cathodes based on thermionic emission with cold cathodes based on field emission of electrons has been developing in recent years. The field emission of electrons occurs 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. Elimination of these components 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 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. 
   Devices heretofore known for assembling cathode structures and electron guns using field emission cathodes are described in U.S. Pat. No. 5,898,262. This patent describes a way for packaging a field-emitting device to construct a color cathode ray tube. An insulating piece with an indentation for a single field emitting device that has three emission areas is provided. U.S. Pat. No. 5,869,924 provides an insulating material (plastic) that is created by filling in an external case, with pins extending through the insulating material. The cathode device is wire-bonded directly to the head of the pins. U.S. Pat. No. 5,905,332 discloses additional portions of an electron gun beyond the field-emitting cathode itself. The aperture spacing in the focusing portion of the gun is larger than the spacing between the field emitting devices. 
   The cathode, accelerating and focusing elements of electron guns may be assembled by alignment with a centering tool, spaced apart with shims and held in place by a nonconductive ceramic that is sintered onto the outer edge of the elements. This sintering of the elements to a ceramic structure is called a “glass beading operation.” The shim spacers are then removed to provide the electrically isolated elements of the electron gun. When thermionic emitters are used, a barium coated cathode is separately placed into the gun after this assembly operation. This is necessary because the fragile barium coating is not able to withstand the high temperatures at atmospheric pressure required in the glass beading operation. 
   One of the advantages of a field emission electron source is that the robustness of the cathode can allow the electron gun to be fully assembled before the glass beading operation. This eliminates the secondary step of inserting the thermionic component after the gun is assembled. A packaging technique is needed that takes advantage of the fact that the field-emission cathode can be placed at high temperature at atmospheric pressure without damage. The packaging should lower assembly costs of electron guns based on field emission cathodes. The package must also be constructed to allow precise alignment of the cathode in the electron gun. The structure resulting also should allow the use of the electron gun in a wide range of CRT neck-diameters. 
   BRIEF SUMMARY OF THE INVENTION 
   An electron gun cathode assembly having a field-emitting cathode and a method for assembling are provided. The field-emitting cathode may be carbon-based. A non-conductive substrate, normally a ceramic material in the form of a disk, has electrical connection such as provided by a die region to the back of a field-emitter die. An emitting array has been grown on the die by known methods. Three field-emitting dies may be spaced on the substrate to form an electron gun for a color CRT. Electrical connections for the die and for electrodes integral with the cathode are made to conductive traces that are connected to pins that pass through the substrate. A disk or can having one or more apertures is spaced apart from the emitting array by a separate spacer ring or the spacing may be created by forming the disk or can. Normally the substrate, spacer and disk or can are joined by welding or brazing, by adhesive or mechanically. The cathode assembly, including the disk or can and spacer, can be aligned and also aligned with a separate focusing electrode with an aligning tool that fits in apertures in the disk or can and lens. The entire assembly can then be glass beaded using known techniques. 

   
     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: 
       FIG. 1   a  shows a cross-section view of one embodiment of a cathode structure for an electron gun. 
       FIG. 1   b  shows a cross-section view of a second embodiment of a cathode structure for an electron gun. 
       FIG. 1   c  shows a cross-section view of a third embodiment of a cathode structure for an electron gun. 
       FIG. 2  shows a cross-section view of a fourth embodiment of a cathode structure for an electron gun. 
       FIG. 3  shows an aligning tool being used for alignment of a cathode structure and an electrode. 
       FIG. 4  shows glass beading of a cathode structure and an electrode. 
       FIG. 5  shows a top view of the single cathode and electrical connections on a substrate that is shown in cross-section in  FIG. 1   a , but with the top disk removed. 
       FIG. 6  shows a top view of three cathodes and electrical connections on a substrate. 
       FIG. 7  shows a top view of one embodiment of an alignment disk for a three-cathode structure. 
       FIG. 8  shows a cross-section view of one embodiment of the three-cathode structure shown in FIG.  7 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1   a , one embodiment of an electron gun cathode assembly is generally shown at  10 . Ceramic substrate  12  supports a field emitting device, which includes die  13  on which field emitting array  14  has been formed. Array  14  has been formed using methods for forming field emitting arrays such as disclosed in Ser. No. 09/169,909, filed Oct. 12, 1998 and commonly assigned, or Ser. No. 09/169,908, filed Oct. 12, 1998, and commonly assigned, or other field emitting devices which are known in the art and disclosed, for example in U.S. Pat. No. 5,869,924, all of which are incorporated by reference herein. Die  13  may be accurately placed on substrate  12  at a selected position using fiducials. Electrical connector  16 , which may be a conductive trace made by well known techniques, connects field emitting die  13  to pin  26 . High-temperature adhesive layer  17  enables a conductive path to the backside of die  13 . The adhesive used to form adhesive layer  17  may be, for example, Ablebond  71 - 1  or Ablebond  2106 . Conductive trace  18  connects to a bond pad to which wire  20  is attached. Wire  20  connects to an extraction electrode in emitting array  14 . Array  14  may also include focus electrodes integrally formed in the array, as disclosed in the applications and patents cited above in this paragraph. The electrodes may be electrically connected to conductive pads on ceramic substrate  12  using a wire bonding process well known in industry. Ring  22  is a spacer between alignment disk  24  and ceramic substrate  12  Ring  22  may be metal or ceramic. The thickness of ring  22  is selected to place aperture  25  in disk  24  at a distance from array  14  so as to focus an electron beam emanating from array  14  by applying a selected voltage to disk  24 . This distance is generally in the range from about 0.1 mm to about 10 mm. The aperture  25  in disk  24  is aligned with array  14  and disk  24  may be attached to the ring  22  through a weld or braze. Ring  22  may be attached to ceramic substrate  12  by brazing to metallized trace  32  on substrate  12 . Metallized trace  32  is electrically connected to pin  21  through via  28 . Alignment disk  24  is extremely important since it primarily is used for insuring that field emitting array  14  is properly centered with other elements of an electron gun. The outer edge of the alignment disk is used for mounting the cathode in an electron gun, as discussed below. Electrical connection to disk  24  through pin  21  allows disk  24  to be used as a focus lens in an electron gun. The diameter of alignment disk  24  may be sized for placement in a wide range of sizes of CRTs or other devices. Disk  24  is typically formed from stainless steel. Conductive trace  16  is connected to pin  28 . Pins such as  26  and  28  may be formed from iron or copper-based alloys, for example. Contact wires (not shown) may be spot welded to the pins during the final stemming operation. The wires provide electrical connection outside of the CRT. 
   A second embodiment of an electron gun cathode assembly is shown generally at  11  in  FIG. 1   b . This embodiment is similar to that in  FIG. 1   a , with the spacing ring  22  removed. In this embodiment, the alignment disk is formed with cylindrical side  9  to enable the required spacing between the alignment disk and field emitting die. The alignment disk may be mechanically and electrically attached to metallized trace  32  on the substrate with either a weld or an adhesive bond. Metallized trace  32  is electrically connected to pin  21 . This embodiment may be more cost-efficient since it eliminates the need to manufacture spacing ring  22 . 
   A third embodiment of an electron gun cathode assembly is shown generally at  45  in  FIG. 1   c . This embodiment is similar to  1   a , however it instead allows for a cathode connection to die  13  through wire  46  to metallized trace  16  that is connected to pin  26 . This is the preferred embodiment in those instances in which the backside of die  13  is not conductive. 
   A fourth embodiment of an electron gun cathode assembly is shown generally at  30  in FIG.  2 . In this embodiment, cylinder-shaped can  34  having wings  36  is used for insuring that field emitting array  14  is properly centered with other elements of an electron gun and for mounting the cathode in an electron gun. The diameter of can  34  or the dimensions of wings  36  may be selected to allow placement of assembly  30  in a wide range of sizes of CRTs or other devices. Ring  22  is inserted in can  34  and acts as a spacer to provide the optimal separation between emitting array  14  and aperture  25  in can  34 . Ring  35  is inserted after the cathode assembly and is used to lock the cathode in place. Braze  29  may be used to fasten pins into ceramic substrate  12 . Can  24  is typically formed of stainless steel. One advantage of the cylinder-shape can is that easier mechanical alignment is attained by tightly fitting substrate  12  into the opening in can  34 . Another advantage is that this package assembly allows the additional option of glass bead attaching the can  34  to the other elements of the electron gun prior to insertion of the cathode assembly. 
     FIGS. 3 and 4  show how electron gun cathode assembly  30  may be aligned with other electrodes in a CRT or other device and glass beaded into place (cathode assemblies  10 ,  11 , and  45  would be assembled in similar manner). Aligning tool  38 , sized to fit into aperture  25  of gun  30  and into the aperture of focusing lens  39  or any other grids to be placed in the device, is placed in the apertures and aligned along the axis of the device. Spacing between the gun elements may be achieved with the use of temporary shims  47  that are later removed. Gun  30  and other lenses or grids are then fixed in place using ceramic  40  (FIG.  4 ). Aligning tool  38  and shim  47  are then removed. Alternatively, gun  10  or  11 , for example, may be assembled in place of gun  30  using the same procedure. 
   Referring to  FIG. 5 , a top view is shown of the cathode assembly shown in cross-section in  FIG. 1   a  but with disk  24  removed. Ceramic substrate  12 , field emitting die  13  and field emitting array  14  are shown from the top. Field emitting die  13  is bonded to die flag region  42  with conductive cement, as disclosed above. The electrically conductive adhesive enables a conductive path to the back side of field emitting array  14 . Die flag region  42  may be formed on ceramic substrate  12  by a metal, typically tungsten, although copper or other metals would be satisfactory. The metal is typically screened on and fired with the ceramic. The ceramic is typically aluminum oxide, although other ceramics would be satisfactory. Such ceramic substrates are available from Kyocera, Coors or other suppliers. Wire bond pads  44  provide a terminal for fixing wire bond connections between elements of the electron gun and pins such as  26  and  27 . Wire leads such as  20  of  FIG. 1  are typically joined to wire bond pads by thermosonic bonding, using techniques well known in industry. If in integral focus electrode is present in array  14  a separate wire lead is connected to the focus electrode and to a pad such as  44 . Conductive traces such as  16  and  18  electrically connect pins to various components of the electron gun or to the die flag region. Spacer ring  22  was discussed above. It may be formed from KOVAR, a steel alloy or may be eliminated and the alignment plate formed to create the required spacing as in  FIG. 1   b . A single field emission array is illustrated in  FIG. 5 , which would be used in a CRT for a monochrome gun. 
     FIG. 6  illustrates one embodiment of an assembly of three emission arrays to be used in an electron gun to be placed in a color CRT. In this case, one die is needed for red, one for green, and one for blue. All three dies  14  are mounted on die flag region  42 . Additional pins will be connected through conductive traces such as  18  to wire bond pads such as  44 . Die flag region  42  is used for connecting through field emission dies as described above. With three emission areas, a disk such as disk  24  of  FIG. 1   a  or a can such as can  34  of  FIG. 2  would have three openings, each to be placed opposite arrays  14 . The disk or can may then be fixed to ring  22 . 
   Referring to  FIG. 7 , alignment disk  50  to be used in another embodiment of a packaging device including three emission arrays is illustrated. Disk  50  serves the same purpose as alignment disk  24  of  FIG. 1   a  or  FIG. 1   b . The disc includes three apertures  52 . Also shown are three formed impression areas  54  that may be used to provide the correct spacing of the aperture to the field emitting array  14  of FIG.  6 . Each cathode assembly, including field emitting array  14  would be centered under aperture  52 . Alternatively, the impression areas may not be present and ring  22  of  FIG. 5  may be joined by welding or cementing to alignment disk  50 . A side view of the same assembly is shown in FIG.  8 . Disk  50  may be glass beaded into an electron gun package using techniques described above whereby at least one aperture in the disk may be used for alignment. 
   The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the details of the illustrated apparatus and construction and method of operation and assembly may be made without departing from the spirit of the invention.