Abstract:
An apparatus is provided for reducing color bleed in a flat panel display. The apparatus comprises an anode ( 30 ) with a plurality of phosphors ( 28 ) of at least two colors sequentially disposed thereon. A cathode ( 14 ) is arranged in parallel opposed position to and separated from the anode ( 30 ) and contains a plurality of pads ( 40 ) of emitters. Each pad ( 40 ) is disposed on the cathode ( 14 ) in spaced relationship to and aligned with one of the at least two colors, respectively, wherein electrons from each of the plurality of pads of emitters that drift from its intended phosphor ( 28 ) are encouraged to drift toward an adjacent phosphor ( 28 ) of the same color.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a flat panel display and more particularly to a cold cathode display. 
     BACKGROUND OF THE INVENTION 
     Field emission displays include an anode and a cathode structure. The cathode is configured into a matrix of rows and columns, such that a given pixel can be individually addressed. Addressing is accomplished by placing a positive voltage on one row at a time. During the row activation time, data is sent in parallel to each pixel in the selected row by way of a negative voltage applied to the column connections, while the anode is held at a high positive voltage. The voltage differential between the addressed cathode pixels and the anode accelerates the emitted electrons toward the anode. 
     Color field emission display devices typically include a cathodoluminescent material underlying an electrically conductive anode. The anode resides on an optically transparent frontplate and is positioned in parallel relationship to an electrically conductive cathode. The cathode is typically attached to a glass backplate and a two dimensional array of field emission sites is disposed on the cathode. The anode is divided into a plurality of pixels and each pixel is divided into three subpixels. Each subpixel is formed by a phosphor corresponding to a different one of the three primary colors, for example, red, green, and blue. Correspondingly, the electron emission sites on the cathode are grouped into pixels and subpixels, where each emitter subpixel is aligned with a red, green, or blue subpixel on the anode. By individually activating each subpixel, the resulting color can be varied anywhere within the color gamut triangle. The color gamut triangle is a standardized triangular-shaped chart used in the color display industry. The color gamut triangle is defined by each individual phosphor&#39;s color coordinates, and shows the color obtained by activating each primary color to a given output intensity. 
     So long as the pixels are sufficiently large, relative to a given electron beam size, the color gamut available at the frontplate of the display is only limited by color output of a given phosphor. Under ideal operating conditions, electrons emitted by the addressed emitter subpixels on the cathode only strike the intended subpixel on the anode. However, in many practical systems of interest, such as high-voltage displays, the beam width of the emitted electons is not confined to a particular subpixel on the anode. At the relatively large cathode to anode separation distances used in high voltage displays, the electron beam spreads and stray electrons can strike adjacent subpixels on the anode. This phenomenon is known as “color bleed”. As the color bleed increases, the available color gamut of the display is decreased. The color purity is reduced and the image resolution and sharpness is reduced. 
     To overcome the loss of color gamut, switched anode techniques in combination with frame sequential addressing have been developed. A switched anode provides separate circuits for subpixels of the same color, but located in adjacent pixels. The groups of subpixels on the anode are electrically connected to form two separate networks. An electronic control system is provided for sequentially addressing alternating rows and columns of pixels on the anode and on the cathode. Adjacent pixels are assigned an odd or even designation in order to separate the activation of the same color subpixels located in adjacent pixels on the anode. 
     Another method used to overcome color bleed is to add additional electrodes in the cathode to focus the emitted electron beam. The electron beam spreading is controlled by electrostatically confining the electron beam, such that the beam strikes the intended subpixel on the anode. 
     While the switched anode techniques and additional focusing structures improve color performance, these can be difficult to implement in a high voltage display and they require more complicated electronics, which add to the expense of the display. Furthermore, additional processing steps are often necessary, which increase the manufacturing cost of the display. Accordingly, a need exists for a low-cost, color field emission display having improved color performance. 
     Accordingly, it is desirable to provide a cathode design that substantially reduces color bleed. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
     BRIEF SUMMARY OF THE INVENTION 
     An apparatus is provided for reducing color bleed in a flat panel display. The apparatus comprises an anode with a plurality of phosphors of at least two colors sequentially disposed thereon. A cathode is arranged in parallel opposed position to and separated from the anode and contains a plurality of pads of emitters. Each pad is disposed on the cathode in spaced relationship to and aligned with one of the at least two colors, respectively, wherein electrons from each of the plurality of pads of emitters that drift from its intended phosphor are encouraged to drift toward an adjacent phosphor of the same color. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a partial isometric schematic view of a known carbon nanotube display device; 
         FIG. 2  is a partial schematic bottom view of an anode and cathode of the device of  FIG. 1 ; 
         FIG. 3  is a partial schematic view of a subpixel of the device of  FIG. 1 ; 
         FIG. 4  is a partial schematic view of a subpixel of an array of adjacent emitters arranged in accordance with an embodiment of the present invention; 
         FIG. 5  is a partial schematic view of an array of red, green, and blue subpixels in accordance with an embodiment of the present invention; 
         FIG. 6  is a comparison of beam profiles of the devices of  FIGS. 4 and 5 ; 
         FIG. 7  is a beam profile of the device of  FIG. 4  versus red, green, and blue frequencies; 
         FIG. 8  is a graph of distance versus normalized intensity for the embodiment of  FIG. 4  and the known device of  FIG. 3 ; 
         FIG. 9  is a graph comparing electron drift versus normalized intensity for the embodiment of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
     Using nanotubes as field emission sources in field emission displays is expected to substantially reduce the manufacturing costs of high voltage displays. A primary cost-saving component is the use of less precise, lower cost lithography than previous field emission display technology. However, the trade-off for this cost savings is that more device real estate is required to define the same number of ballasted emitter pads. Since, the area containing nanotube emitters is larger, there is a comparatively smaller margin between the edge of the nanotube emitter structures and the edges of the phosphor to which their electron beams must be restricted. Consequently, it is more important than ever to substantially reduce the color bleed of the electron beam in order to obtain a good image. The eye is sensitive to cross-talk between colors of less than 3% in static images. 
     Referring to  FIG. 1 , a known carbon nanotube field emission device  10  includes a cathode electrode  14  positioned on a substrate  12 . A ballast resistive layer  16  is positioned between a dielectric layer  18  and the cathode electrode  14 . A catalyst material  20  is positioned on the ballast resistive layer  16  for allowing higher quality growth of carbon nanotubes  22  thereon. A gate electrode  24  is positioned on the dielectric layer  18  for drawing electrons from the carbon nanotubes  22  in a manner known to those skilled in the art. 
     The catalyst material  20  comprises pads  26  (or pads) of carbon nanotubes  22 . In  FIG. 1 , while three pads  26  are shown, it should be understood that many pads  26  are typically used. Each group of pads  26  is aligned with an area of phosphor  28  of one of three colors, e.g., red, on the anode  30  ( FIG. 2 ). A plurality of pads designated as directing electrons at a given phosphor of one color are referred to as subpixels. As electrons are emitted from the carbon nanotubes  22 , the electrical attraction of the gate electrode  24  “pulls” the electrons in the ‘x’ direction. The closer the gate electrode  24  is to the carbon nanotubes  22 , the stronger it pulls the electron beam and, therefore, the more it pulls the electron beam toward neighboring subpixels in the ‘x’ direction. In addition to the electrons being pulled toward the gate electrode  24 , the carbon nanotubes  22  themselves will be pulled, or slant, in the direction of the gate electrode  24 . As the carbon nanotubes  22  slant, the electrons are “aimed” in that direction away from the desired phosphor  28 , i.e., the ‘x’ direction. Note also that since there is a smaller gap between phosphors  28  in the ‘x’ direction than in the ‘y’ direction, color bleed in the ‘x’ direction has even more of an impact. 
     Referring to  FIG. 2 , the device  10  is shown overlying areas of phosphor  28  on the anode  30 . As electrons are pulled by the gate electrode  24  in the ‘x’ direction, some of the electrons may stray into the adjacent phosphor  28  of a different color. For example, electrons intended for the red phosphor  32  may stray into a green  34  and/or blue  36  phosphor. This color bleed significantly degrades the color image of the field effect device. 
     The subpixel array of  FIG. 3  is one known embodiment that includes three columns of pads  26  positioned on the ballast resistor  16  and surrounded by the gate electrode  24 . The three columns of pads  26  paint electrons on a single color providing redundancy in case one pad  26  does not function properly. It is noted that the area of the gate electrode  24  is significantly larger and closer in the ‘x’ direction from each pad, thereby creating the “pull” in the ‘x’ direction. 
     Referring to  FIG. 4 , and in accordance with the present invention, pads  40  of carbon nanotubes  22  are positioned in a 4 by 8 configuration on the ballast resistive layer  42  to form the subpixel  46 . While a 4 by 8 configuration is illustrated, any sized matrix may be used within the scope of this invention. While the preferred embodiment comprises carbon nanotubes, any cold cathode device that emits electrons, such as metal tips, an emitting film, or any carbon like nanostructure, could be used with the present invention. In this invention, the electric field required to extract electrons from the emitter pads by the gate electrode  44  is applied predominantly from the ‘y’ direction (there is more of the gate electrode  44  material in the ‘y’ direction). In this way, the pull from the electrode on the electron beam occurs predominantly in the ‘y’ direction and any electron drift is thus “encouraged”, as defined herein, to drift in the ‘y’ direction and not the ‘x’ direction. Additionally, the re-orientiation of emitters (tilting of emitters due to the pull of the field) like carbon nanotubes also occurs predominantly in the y-direction. As a result, the electron beam deflection that results from the extraction electrodes occurs substantially in the ‘y’ direction toward subpixels  46  of the same color and does not contribute to color mixing by pulling the electrons in the ‘x’ direction towards subpixels  46  of another color. 
     In the embodiment in  FIG. 4 , it is necessary to connect the gate electrode  44  to a common voltage source. This is accomplished by busing the gate electrode  44  together with a gate bus line  47  on the far +x and −x sides of the emitter pads  40 . Structurally, the gate bus line  47  is just a part of the gate electrode  44 , but functionally it is not spaced to the emitter pads  40  close enough to extract electrons. The gate bus line  47  produces a small deflection field in the ‘x’ direction, which is not desired. In order to minimize the role of the gate bus lines  47 , they must be placed as far as possible from the edges of the emitter pads  40 , and they must be as narrow as the design allows. The gate bus line  47  is placed at least twice the distance to the pad in the ‘x’ direction as the gate electrode  44  is in the ‘y’ direction. Preferably this distance would be a multiple of four. At twice the distance, it is assured that the electric field due to the gate bus line  47  is at least half the value in the ‘x’ direction as in the ‘y’ direction. In terms of the physics of the device, this means in general that the field in the ‘x’ direction from the gate bus line  47  is insufficient to induce field emission at the pads  40  at the operating voltage of the gate electrode  44 , if the gate electrode  44  in the ‘y’ direction were absent. The gate bus line  47  is not acting as an extraction electrode. The pull of the electron beam by the gate bus line  47  is further minimized by making the bar as thin as design rules for conductor lines allow so that the electron beam encounters its potential for only a short period of time. 
     Optionally, column electrode lines  45 , which is coupled to the pads  40 , may be positioned at the sides of the subpixel  46 . Since the potential of the pads  40  is from 0 to approximately 15 volts above the cathode electrode line  45 , column electrode lines  45  provides some co-planar focusing in the x-direction (towards the pads  40  and away from the column electrode lines  45  and the neighboring phosphor of another color). 
     Referring to  FIG. 5 , the column electrode line  52  can be used to shield the field from the gate bus line  47 . By running an exposed section of this electrode between the pads  40  and the gate bus line  47 , a stronger co-planar focusing effect can be realized from the column electrode line  52 . Also, the ballast resistor in the region between the end pad and the gate bus line  47  is at a potential lower than the gate electrode  44 , and thereby partially shields the field from the gate bus bar. 
     Referring to  FIG. 6 , another embodiment has the gate bus line  47  running through the middle of the pad area and no gate electrode  44  in the ‘x’ direction from the pads  40 , thereby providing absolutely no pull of the electron beam (or emitters in the case of carbon nanotube emitters) in the x-direction. In this case, the end pads  40  are closer to the neighboring pixel  46  in the x-direction, but there is no gate bus line  47  in the region at the far sides of the row of pad  40 . Consequently, there is no field contribution from the gate electrode  44  near the edges of the subpixel  48 . Preferably, the gate bus line  47  down the middle would also be twice the distance from the nearest pads than the distance from the gatel electrode  44  along the rows. However, if the gate electrode  44  is closer and provides a significant pulling field, or even a field large enough to induce electron emission, the affect on color purity is minimal because the affected beams are in the middle of the subpixel  48 . 
     In the embodiments where a pixel is square, each color subpixel will be rectangular and the long direction will be in the ‘y’ direction. In this configuration it is highly desirable to apply the present invention. With the gate electrodes pulling in the ‘y’ direction in preference to the ‘x’ direction, the electron beam from each pad is pulled more along ‘y’. Because ‘y’ is a much longer direction than x, the percentage of the beams that impinge on the proper phosphor area is larger than it would be if the pixel were comparatively shorter in the ‘y’ direction. In summary, this embodiment allows the composite electron beam for each subpixel to better match the corresponding phosphor area, thereby reduced bleed over and electrons which strike the black surround areas of the anode. This improves the device efficiency and brightness. 
     In addition, anode designs which leave room for a spacer between pixels in the y-direction have a larger gap between pixels in the y-direction than in the x-direction. This larger gap in the ‘y’ direction makes the phosphor in the ‘y’ direction less sensitive to electron bleedover from the adjacent subpixel (in y). If there are any electrons reaching the pixel in the ‘y’ direction, there will be no color error. In fact, the uniformity of the image may be enhanced. 
     Referring to  FIG. 7 , subpixels  46  are positioned in alignment with phosphor region  28  on anode  30 . Since any “color bleed”, or pull of electrons, is in the ‘y’ direction, any straying electrons will move into the adjacent phosphor in the ‘y’ direction of the same color instead of moving in the ‘x’ direction into a phosphor of a different color. This encouragement of any drifting electrons towards adjacent phosphors of the same color instead of adjacent phosphors of a different color significantly reduces color bleed and improves the color gamut. It should be understood that the phosphor regions  28  in the preferred embodiment are red  32 , green  34 , and blue  36 , they may comprise other colors as well. 
     Referring to  FIG. 8 , the electron drift  62  of the known device of  FIG. 3  and the electron drift  64  of the device of the present invention of  FIG. 4  are plotted as distance versus normalized intensity. It may be seen that the present invention provides a substantially more focused beam in the x-direction for a given anode distance. The present invention reduces the beam width by nearly a factor of two without reducing the area in which the pads reside. Since the intrinisic beam size from the pads can be substantially reduced, the present invention allows for higher resolution geometries. Additionally, more pads can be disposed in the subpixel area without causing bleed over, thereby improving the brightness and short range subpixel to subpixel uniformity of the display. The short range uniformity is improved because the increase in the number of pads provides additional statistical averaging. When more pads are accommodated in the emitting area, the device designer can also choose to maintain the same brightness level. In this case the extraction voltage to achieve a given brightness is reduced. This, in turn, reduces the beam size in the ‘y’-direction. 
     Referring to  FIG. 9 , electron drift  64  of the device of the present invention is plotted as distance versus normalized intensity against a background with areas  32 ,  34 , and  36  representing red, green, and blue, respectively. This electron beam profile measured from one of the devices, built with the design depicted in  FIG. 4 , uses a 726 micrometer square subpixel, the size used for a 42″ diagonal 1280x 720 HDTV display. It can be seen that there is minimal electron drift from green to the neighboring colors of red and blue in the x-direction, so the application of this invention is sufficient to provide the required color purity. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.