Redundant conductor electron source
BACKGROUND OF THE INVENTION 
     The present invention relates, in general, to electron emission display devices, and more particularly, to a novel extraction grid for an electron emission source. 
     Field emission devices (FEDs) are well known in the art and are commonly employed for a broad range of applications including image display devices. An example of a FED is given in U.S. Pat. No. 5,142,184 issued to Robert C. Kane on Aug. 25, 1992. FEDs typically employ at least two electrodes, a cathode conductor and a gate or extraction grid. Generally, the extraction grid and the cathode conductor are formed at right angles to facilitate utilizing row and column addressing to stimulate electron emission from emission tips or emitters. The cathode conductor and the extraction grid typically are electrically isolated by a dielectric layer. During the FED formation, pinholes can form in the dielectric layer and result in electrical shorts between the extraction grid and the cathode conductor. Because of the electrical short, the cathode conductor and the extraction grid are forced to the same potential thereby preventing a column of emitters and the row from being energized. The shorted column of emitters can not generate an image, thus, a display device formed with such electrical shorts usually appear as a dark or continually bright line where the shorted emitters are positioned. 
     Accordingly, it is desirable to have an electron source that remains functional if the extraction grid is shorted to the cathode conductor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates an enlarged cross-sectional portion of a display device in accordance with the present invention; 
     FIG. 2 schematically illustrates a plan view of a portion of an extraction grid in accordance with the present invention; and 
     FIG. 3 illustrates a plan view of a portion of a cathode conductor in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates an enlarged cross-sectional portion of a field emission display device 10 that has a novel electron source with redundant conductors. The electron source includes a novel redundant conductor scheme for an extraction grid 17 and for column conductors of the electron source. As will be more apparent in the subsequent discussion of FIG. 2, grid 17 has a plurality of extraction elements such as an extraction element 27 shown in FIG. 1. Device 10 has a substrate 11 on which other portions of device 10 are formed. Substrate 11 typically is an insulating or a semi-insulating material, for example, silicon having a dielectric layer or glass. In the preferred embodiment, substrate 11 is glass. The electron source of device 10 includes a resistive layer that generally is formed into a plurality of resistive sections on substrate 11 such as a resistive section 12 as will be seen hereinafter. The plurality of resistive sections typically are utilized as ballast resistors. The electron source also has a column conductor that includes a first column conductor strip 14 which is utilized to provide electrical contact between an emitter 13 that is formed on section 12 and an external voltage source (not shown). As will be seen hereinafter in FIG. 3, the electron source includes a second column conductor strip 25 that is not shown in FIG. 1. Although only one emitter 13 is shown in the portion illustrated in FIG. 1, the electron source has a plurality of emitters 13 as will be seen hereinafter. Grid 17 is disposed on a dielectric layer 16 to electrically isolate grid 17 from substrate 11, strip 14, and section 12. Grid 17 has an emission opening 15 that is substantially centered to emitter 13 to permit electrons to travel from emitter 13 to a distally disposed anode 18 and form an image thereon. The surface of anode 18 facing emitter 13 typically is coated with a phosphor in order to provide a display as electrons strike anode 18. 
     FIG. 2 schematically illustrates an enlarged plan view of a portion of extraction grid 17 shown in FIG. 1. Elements of FIG. 2 having the same reference numbers as FIG. 1 are the same. Device 10 (FIG. 1) has a plurality of emitters 13 as indicated in the discussion of FIG. 1. Emitters 13 are arranged in groups wherein each group is within a pixel area such as a first pixel area 28 and a second pixel area 36. The emitters within one pixel area are utilized to form a single pixel image on anode 18 (FIG. 1). Pixel areas 28 and 36 usually occur where grid 17 overlies emitters 13 and the associated column conductors such as the column conductor that includes strips 14 and 25 shown hereinafter in FIG. 3. 
     Grid 17 is formed as a plurality of conductors that are electrically isolated so that a short between one conductor of grid 17 and either of strips 14 or 25 (FIG. 3) still allows the other conductor of grid 17 to function. To accomplish this, grid 17 has a plurality of extraction elements within each pixel area wherein at least one extraction element generally is electrically connected to one of the plurality of conductors of grid 17. Each of the plurality of conductors of grid 17 may have a plurality of such extraction elements within each pixel area. 
     In the preferred embodiment, the plurality of conductors of grid 17 includes a first conductor strip 21 that is positioned near an edge of pixel areas 28 and 36, and a substantially parallel second conductor strip 22 that is spaced a distance 29, illustrated by an arrow, from conductor strip 21. Distance 29 is approximately twelve to twenty-five microns in order to obtain the desired pixel density. Strips 21 and 22 are approximately two to one hundred microns wide in order to have a low resistance to minimize switching time, and to match the pixel size. Strip 22 is positioned near an edge of pixel areas 28 and 36 that is opposite of strip 21. Within pixel area 28, grid 17 has a first extraction element 23, illustrated by a dashed box, and a second extraction element 26, also illustrated by a dashed box. Element 23 is formed in the portion of conductor strip 21 overlying emitters 13, and element 26 is adjacent to and substantially parallel to strip 22. Element 26 is electrically connected to strip 21 by an &#34;L&#34; shaped conductor extension of strip 21. A third extraction element 27, illustrated by a dashed box, is formed in the portion of conductor strip 22 overlying emitters 13, and a fourth extraction element 24, illustrated by a dashed box, is adjacent to and substantially parallel to strip 21 and is between strip 21 and element 26. Element 24 is electrically connected to strip 22 by an &#34;L&#34; shaped conductor extension of strip 22. Consequently, element 26 is a second distance 37 from element 23, and element 24 is a third distance 38 from element 23 such that distance 37 is less than distance 29, and distance 38 is less than distance 37. Elements 23, 24, 26, and 27 can have other shapes, for example, each conductor strip 21 and 22 may have only one large square projecting from each of conductor strips 21 and 22. Each element 23, 24, 26, and 27 has a plurality of emission openings 15 wherein each opening corresponds to an emitter of plurality of emitters 13 as indicated in the discussion of FIG. 1. 
     Grid 17 also has, within pixel area 36, extraction elements 31, 32, 33, and 34 that are similar to elements 23, 24, 26, and 27, respectively. It should be noted that the portion of device 10 shown in FIG. 1 is a cross-section that cuts through element 27 so that only the portion that includes an emitter 13 that is near strip 14 is shown in the FIG. 1 cross-section. 
     If either strip 21 or 22 is shorted to an underlying cathode conductor, then the external grid voltage (not shown) can be applied to the remaining non-shorted strip of strips 21 and 22 in order to provide an image on anode 18 (FIG. 1). The shorted strip of strips 21 and 22 is not utilized. The short can be determined when device 10 (FIG. 1) is tested prior to connecting all external electronics (not shown) to display 10. 
     FIG. 3 schematically illustrates novel redundant cathode conductors 39 and 40 that also facilitate using device 10 when an electrical short occurs. Elements of FIG. 3 that are the same as FIG. 1 and FIG. 2 have the same reference numbers. Conductor 40 includes strip 14 and strip 25 that a substantially parallel and along opposite sides of area 28. A plurality of resistive sections 12, 19, 20, and 30 are formed on substrate 11 between strips 14 and 25 in area 28. Sections 12, 19, 20, and 30 are formed in a pattern to underlie extraction elements 27, 26, 24, and 23 (FIG. 2), respectively. Sections 12, 19, 20, and 30 can be formed by applying a continuous resistive layer and etching the layer as is well known to those skilled in the art. Strip 14 connects sections 12 and 20 into a pattern that corresponds to elements 27 and 24, respectively, while strip 25 connects sections 19 and 30 into a pattern that corresponds to elements 26 and 23, respectively. Emitters 13 are then formed on sections 12, 19, 20, and 30. For simplicity of the drawing, only six emitters are illustrated on each section 12, 19, 20, and 30 in FIG. 3. 
     Similarly, conductor 39 is within area 36 and includes a conductor strip 41 and a conductor strip 42 that corresponds to strips 14 and 25, respectively. Area 36 also has sections 43, 44, 46, and 47 that are similar to sections 12, 19, 20, and 30, and that correspond to the pattern of elements 34, 33, 32, and 31 (FIG. 2), respectively. 
     Utilizing grid 17 (FIG. 2) together with the redundant conductor cathode conductor of FIG. 3 provides several possible usable connections if a short occurs. If strip 14 shorts to strip 21 (FIG. 2), then strip 25 and strip 22 (FIG. 2) are still usable to form an image on anode 18 (FIG. 1). Also, using the redundant cathode conductor of FIG. 3 provides an advantage over prior art cathode conductors even when the redundant cathode conductor is used with a prior art single conductor extraction grid. In such a case, the prior art extraction grid can short to one of strips 14 or 25 yet the non-shorted one of strips 14 and 25 remains available to be used for emitting electrons. For example, if a prior art extraction grid shorts to strip 14, strip 25 may not be shorted. When an external voltage is applied to the prior art extraction grid, strip 14 and emitters 13 on resistive sections 12 and 20 are at the same potential as the prior art extraction grid. But, strip 25 and emitters 13 on resistive sections 19 and 30 are at a different potential, thus, emitters 13 on sections 19 and 30 can emit electrons. 
     By now it should be appreciated that there has been provided a novel redundant conductor electron source that facilitates using the electron source even if there is an electrical short within the electron source. Forming the extraction grid of the electron source into a plurality of electrically isolated conductor strips permits using non-shorted grid conductors to create an image. Similarly forming a redundant cathode conductor facilitates using non-shorted portions of the cathode conductor to create an image. Consequently, display devices that have shorted conductors can be used instead of discarded thereby increasing the yield and lowering the display device costs.