Patent Publication Number: US-2002000771-A1

Title: Flat panel display with improved micro-electron lens structure

Description:
CROSS REFERENCE TO RELATED APPLICATION  
     [0001] This application is related to an application filed on the same day and by the same Applicants as this application, the related application entitled “DISPLAY DEVICE WITH IMPROVED GRID STRUCTURE,” which is referred to herein as the companion application. The companion application is incorporated herein by reference in its entirety. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] This invention relates in general to flat panel displays and in particular, to a flat panel display device employing an improved micro-electron lens structure.  
       [0003] Many flat panel display devices have been proposed. In U.S. Pat. No. 5,083,058 toNonomura et al., for example, a flat panel display device is proposed where one or more layers of struts formed by a screen printing method are used as spacers between cathodes on or near a back plate and an anode on or near a front plate.  
       [0004] The display device proposed by Nonomura et al. is disadvantageous for several reasons. The display device employs a complicated and complex control grid structure which is difficult to manufacture, especially for high resolution displays. For example, it is tedious and impractical to screen print high aspect ratio struts with fine pitch. Nonomura et al.&#39;s device employs intermediate electrode structures comprising multiple beam control grids with top and bottom electrodes on insulating plates with holes. It may be difficult to accurately align the multiple beam control layers with the struts on the front and back plates, especially for high resolution displays.  
       [0005] None of the flat panel displays currently on the market or proposed are entirely satisfactory. It is, therefore, desirable to propose a flat panel display device where the above-described difficulties are alleviated.  
       SUMMARY OF THE INVENTION  
       [0006] This invention is based on the recognition that by simply employing a layer of an electrically conductive material with a two-dimensional array of holes therein and by applying an electrical potential to the layer, the layer forms a micro-electron lens for focusing and/or imaging that greatly improves the performance and increases manufacturing tolerance of the display. Preferably the front and back plates of the display are separated by spacers that permit the spacing between the anode and cathodes to be of a desired value. This permits the paths of electrons to be focused and/or imaged by the micro-electron lens structure.  
       [0007] Since the electrical potential applied to this micro-electron lens structure can be altered to achieve the desired focusing and/or imaging effects, the alignment between the cathode elements and pixel dots can be relaxed so that the flat panel display device made employing such structure has a higher tolerance for misalignment during manufacture. Furthermore, the holes in the structure can be made to be of considerable size to permit a high percentage of electrons generated by the cathode elements to pass.  
       [0008] The electrons generated by a set of cathode elements are focused by a corresponding micro-electron lens to form an image of the set at the luminescent layer. In this context, each set of cathode elements has an image at the luminescent layer. In one embodiment, the lateral dimensions of at least one of the holes are preferably at least one-tenth of the lateral extent of a corresponding set of cathode elements. More preferably the lateral dimensions of at least one of the holes are at least one-third of the lateral extent of a corresponding set of cathode elements.  
       [0009] One embodiment of the invention is directed towards a flat panel display device for displaying images when viewed in a viewing direction, comprising a front face plate and an anode on or near the front face plate. A first layer of luminescent material on or near the anode is employed. The layer comprises an array of rows and columns of sets of pixel dots of luminescent material. Each set of pixel dots contains at least one pixel dot. Each of the pixel dots emits red, green or blue light in response to electrons. An array of field emitter cathode elements on a cathode substrate is employed, where the array has rows and columns of sets of cathode elements. Each set of cathode elements contains at least one cathode element. A micro-electron lens structure is used including at least one layer of electrically conductive material between the anode and cathodes, where such layer defines a two-dimensional array of holes therein. Each set of pixel dots in the luminescent layer substantially overlaps an image of a corresponding set of cathode elements at the luminescent layer through a corresponding hole in the layer of the micro-electron lens structure. A controller applies an electrical potential to the anode, a scanning electrical potential sequentially to rows or columns of the cathode elements, data electrical potentials to columns or rows of the elements and a focusing and/or imaging electrical potential to the micro-electron lens structure. This causes electrons from each set of the cathode elements to reach its corresponding image of the corresponding set of pixel dots of the luminescent layer for displaying desired images. In the preferred embodiment, the micro-electron lens structure is an integral, unitary, one-piece structure, so that substantially the same electrical potential is applied at or near each of the holes therein.  
       [0010] Another embodiment of the invention covers a flat panel display device for displaying images when viewed in a viewing direction, comprising a front face plate, and an anode on or near the front face plate. A layer of luminescent material is disposed on or near the anode, where the layer includes an array of rows and columns of sets of pixel dots of luminescent material. Each set contains at least one pixel dot emitting red, green or blue light in response to electrons. An array of field emitter cathode elements on a cathode substrate is employed, where the array has rows and columns of sets of cathode elements, each set containing at least one cathode element. A micro-electron lens structure including a layer of electrically conductive material is employed between the anode and cathodes, where such layer defines a two-dimensional array of holes therein. Each set of pixel dots in the luminescent layer substantially overlaps a corresponding image of a set of the cathode elements through a corresponding hole in a layer of the micro-electrons structure. An array of grid electrodes is also employed between the anode and the cathode elements. A controller applies a focusing and/or imaging electrical potential to the layer of the micro-electron lens structure, an electrical potential to the anode, and addressing and data electrical potentials to the sets of cathode elements and the array of grid electrodes. This causes electrons emitted by each set of cathode elements to reach its corresponding image at the set of pixel dots of the luminescent layer for displaying desired images. In the preferred embodiment, the micro-electron lens structure is an integral, unitary, one-piece structure, so that substantially the same electrical potential is applied at or near each of the holes therein. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1 is a cross-sectional view of a portion of a flat panel display device employing a micro-electron lens to illustrate a first embodiment of the invention where each hole of the lens corresponds to a corresponding pixel dot.  
     [0012]FIG. 2 is a cross-sectional view of a portion of a flat panel display device where a micro-electron lens structure is used for focusing and imaging to illustrate a second embodiment of the invention where each hole of the lens corresponds to a plurality of corresponding pixel dots.  
     [0013]FIG. 3A is a perspective view of a flat panel display device with a portion cut away to illustrate a third embodiment of the invention.  
     [0014]FIG. 3B is a portion of the display device of FIG. 3A in more detail.  
     [0015]FIG. 4 is a cross-sectional view of a flat panel display device employing a micro-electron lens structure with two or more layers of conductive material to illustrate a fourth embodiment of the invention.  
     [0016]FIG. 5 is a cross-sectional view of a portion of a flat panel display device similar to that of FIG. 1 except that it has a set of grid electrodes to illustrate a fifth embodiment of the invention.  
     [0017] For simplicity in description, identical components are identified by the same numerals in this application.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0018]FIG. 1 illustrates a cross-sectional view of a portion of a flat panel display device  10  which includes an anode plate or front face plate  12  and a cathode substrate  16 . On or in the top surface of the cathode substrate or plate and facing the front face plate is a two-dimensional array of sets of field emitter cathode elements  14 . On the inside or bottom surface of the anode plate facing the cathode substrate is a phosphor layer  33  comprising pixel dots, where each pixel dot emits red, green or blue light when impinged upon by electrons. On the phosphor layer  33  is a conductive layer  32 , such as an aluminum coating, which forms the anode  32 . Layer  32  is typically thin enough so that electrons from the cathodes would penetrate the layer to reach the pixels dots for generating light. An electrically conductive layer  50  is held in place between the anode and the cathodes by cathode spacers  56  and anode spacers  60 , where anode spacers  60  are attached to layer  50  and the anode by means of glass frit  58 . Cathode spacers  56  may be formed by a process such as stencil printing onto layer  50  and attached to substrate  16 .  
     [0019] The thickness of the cathode spacers is such that the spacing between layer  50  and the cathode substrate is of the order of about 10 to 500 microns, and more preferably about 30 to 250 microns, and is of the order of about 100 microns in the embodiment of FIG. 1 and of the order of about 200 microns in the embodiment of FIG. 2. The anode spacers  60  preferably have heights such that layer  50  and anode  32  are spaced apart by not less than 0.5 millimeters, and more preferably by not less than about 1 millimeters, so that a relatively high potential difference of the order of 200 to 10,000 volts (more preferably 1,000 to 10,000 volts) may be applied between the anode  32  and cathodes  14 . Since the cathodes  14  are typically operated at a low voltage, a relatively high voltage in a range of about 200 to 10,000 volts is then applied to the anode layer  32 . This permits the phosphor layer  33  to be operated at or close to its optimum efficiency and lifetime.  
     [0020] For some applications, it may be possible to omit the anode spacers  60 . If anode spacers are not used, it may be necessary to increase the thickness of the anode or front face plate  12  so that the housing of the display  10  comprising the anode plate  12  and any back plate are strong enough to withstand atmospheric pressure. If the potential difference between the cathode and the anode is not too high (e.g. not more than 300 volts), it may be possible to exchange the position between the aluminum coating  32  for the anode and the phosphor layer  33 , so that the electrons from the cathodes  14  directly impinge upon the phosphor layer  33  without having to pass through the aluminum coating  32 .  
     [0021] Pixel dots  33  are arranged in a two-dimensional array, where each pixel dot overlaps a corresponding set of field emitter cathode elements  14  and a corresponding hole in a two-dimensional array of holes in layer  50  when viewed in the viewing direction  52 . As shown in FIG. 1, the pixel dot  33 R overlaps the set  14   r  of cathode elements and a corresponding hole in layer  50 , the pixel dot  33 G overlaps the set  14   g  of cathode elements and a corresponding hole in layer  50 , and the pixel dot  33 B overlaps the set  14   b  of cathode elements and a corresponding hole in layer  50 . The array of sets of field emitter cathode elements  14  form a two-dimensional array of rows and columns. Power supply in controller  44  applies (not shown) a scanning electrical potential sequentially to the rows of the sets of cathode elements and data electrical potentials to columns of the sets of elements to accomplish XY (two-dimensional) addressing and brightness control of the two-dimensional array of pixel dots  33  in order to display video images. Alternatively, the scanning electrical potentials may be applied sequentially to columns of sets of such cathode elements and data electrical potentials may be applied to the rows of such sets of elements for the same purpose. In addition, a focusing electrical potential may be applied to layer  50  to focus the electrons generated by each set of field emitter cathode elements to its corresponding and overlapping pixel dot in layer  33 . Preferably the layer  50  is an integral, unitary one-piece structure, so that any electrical potential applied thereto will cause such potential to be applied at or near each hole in the layer.  
     [0022] As shown in FIG. 1, the pixel dots are grouped into clusters, each cluster containing a pixel dot emitting red light, a pixel dot emitting green light and a pixel dot emitting blue light in response to electrons, and the holes in layer  50  and the set of field emitter cathodes also form corresponding clusters. It will be understood, however, that the pixel dots may be grouped to form other types of clusters, such as where each cluster includes a pixel dot emitting red light, two pixel dots emitting green light and one pixel dot emitting blue light; all such variations in this and other embodiments of this invention are within the scope of the invention. Layer  50  forms a micro-electron lens that focuses the electrons emitted by each set of field emitter cathode elements to the corresponding pixel dot, to reduce the effect of any misalignments between the set of field emitter cathode elements, and its corresponding hole and pixel dot.  
     [0023] To further reduce cross-talk, cathode spacers  56  have thicknesses in the range of about 10 to 500 microns, and more preferably in the range of about 30 to 250 microns, and the lens layer  50  is at least about 10 microns from the cathode elements. Preferably, the thicknesses of the cathode spacers  56  are such that the spacing between layer  50  and the cathodes  14  is of the order of about 100 microns in the embodiment of FIG. 1. By maintaining each set of cathode elements at a close distance to its corresponding hole in layer  50 , cross-talk is much reduced.  
     [0024]FIG. 2 is a cross-sectional view of a portion of a flat panel display device  100  substantially similar to display  10  of FIG. 1, except that the micro-electron lens  50 ′ is different from layer  50  of FIG. 1. In display  10  of FIG. 1, the electrons passing through each hole in layer  50  originate from essentially a single set of field emitter cathode elements and the electrons generated from each set of field emitter cathode elements are directed towards substantially a single pixel dot. In contrast, each hole of layer  50 ′ of FIG. 2 passes electrons originating from three or more sets of field emitter (FE) cathode elements where such electrons are directed towards three or more pixel dots. Electrons originating from the set  14   r  of field emitter cathode elements are focused and imaged by lens  50 ′ onto pixel dot  33 R, those from set  14   g  focused and imaged onto pixel dot  33 G and those from set  14   b  focused and imaged onto pixel dot  33 B. When external voltage sources are provided to the anode, cathode elements and the micro-electron lens (conceptual structure  66  enclosed with dotted lines), equipotential lines (not shown) representative of electric fields originated from the anode, cathodes and micro-electron lens exist in the region of the micro-lens structure. Electrons are emitted from the FE cathodes  14  by applying an externally suitable voltage. These emitted electron beams are accelerated through the electric field in the region of the micro-electron lens and preferably collected at the anode.  
     [0025] The emitted electrons&#39; transit trajectories (electron beams) originating from the FE cathodes  14  are controlled by the electric field in the spatial region of the micro-electron lens. The electric field in the region of the micro-electron lens is a function of the applied voltages to the anode, FE cathode elements and layer  50 ′ as well as the distance between the anode, FE cathode elements and layer  50 ′ and the aperture shape of the holes in layer  50  forming the micro-electron lens. In this respect, the emitted electron beams are focused and imaged onto the anode by structural characteristics and the parametric conditions of the micro-electron lens.  
     [0026]FIG. 3A is a perspective view of a portion of a flat panel display device  150  with a portion cut away to illustrate the third embodiment of the invention. FIG. 3B is an exploded view of a portion of the device in FIG. 3A. The embodiment of FIGS. 3A and 3B is taken from the companion application. Rim or sealing frame  26  may be attached to the micro-electron lens layer  50 ″ to form electrode structure  20 , where the structure  20  is attached to the anode plate  12  by means of glass sealing frit  58 . Anode spacer  60  may be attached to the layer  50 ″ and the anode  32  by means of glass sealing frit. For ease of assembly, these anode spacers may first be attached to layer  50 ″, so that the rim  26  and anode spacers  60  may be attached to the anode or anode plate in a single process. Cathode spacers  62  may also be formed on layer  50 ″. The perimeter portion  50   b  of layer  50 ″ and cathode spacers  62  may then be attached to the cathode plate  16  in a single process. When structure  20  is attached to the anode and cathode plates, the layer  50 ″ is properly aligned with the rows and/or columns of field emitter cathodes on the cathode plate  16  and with pixel dots on the anode. Once so aligned and the electrode structure  20  is attached to the cathode and anode plates, accurate alignment has been achieved.  
     [0027] As noted above, the focusing and imaging characteristics of the micro-electron lens  50 ′ of FIG. 2 depend on the structural shape of the holes and the voltages applied to the anode, cathodes and micro-electron lens as well as distances between the anode, cathode elements and micro-electron lens, and the aperture shape of the micro-electron lens. For improved focusing, a composite micro-electron lens structure may be used. In the preferred embodiment, it may be desirable to employ multiple layers of conductive materials instead of a single layer of conductive material  50 ′ as in FIG. 2 to form a composite micro-electron lens. Such new configuration is shown in FIG. 4. Thus, as shown in FIG. 4, display device  200  is substantially similar to display  100  of FIG. 2, except that a multi-layer electrode structure  202  is employed instead of a single layer  50 ′ as in FIG. 2. As shown in FIG. 4, lens  202  includes two layers  202   a,    202   b,  each made of an electrically conductive material, where the two layers are separated by an insulating layer  204 . By employing two or more electrically conductive layers, it is possible to more accurately fabricate the apertures or holes therein so that their sizes and shapes are of the desired accuracy. Furthermore, different electrical potentials may be applied to layers  202   a,    202   b,  further increasing the versatility and the control of the focusing and imaging functions of the micro-electron lens structure  202 . By such focusing and/or imaging functions, the electrons emitted by the field emitter cathode elements in a small area on the cathode substrate or plate may be focused and imaged onto a larger area of the phosphor layer. Thus the addressing capability of the micro-electron lens can be realized by the composite structure of FIG. 4 in which additional control electrodes are formed onto the basic single layer micro-electron lens structure. The control electrodes combined with the specific basic functions of the single layer micro-electron lens structure are used for focusing and imaging, focusing and addressing as well as focusing, imaging and addressing.  
     [0028] The spacing between layers  202   a,    202   b  is at least about 1 micron, while in the preferred embodiment, the spacing is at about 20 microns. For some applications, the holes or apertures in layer  202   a  are preferably smaller than those in layer  202   b;  for other applications, they may be of substantially the same size and shape. The lateral dimensions (i.e. the dimensions in a plane substantially parallel to the anode and cathode elements) of at least one of the holes are preferably at least one-tenth of the lateral extent (i.e. the dimensions of the two-dimensional area that the set of cathode elements occupies in a plane substantially parallel to the anode and cathode elements) of a corresponding set of cathode elements. In other words, the dimensions of holes  98 ,  99  in FIGS. 2, 4 are preferably at least one-tenth of the lateral extent of a corresponding set of cathode elements (e.g. set  14   b ) on the substrate  16 . More preferably the dimensions of at least one of the holes are at least one-third of the lateral extent of a corresponding set of cathode elements. In other words, if the lateral extent of the corresponding set of cathode elements along a first direction (e.g. X) parallel to the surface of the cathode substrate  16  is x, then the dimension of the hole along such direction is preferably at least one-tenth of x and more preferably one-third of x. If the lateral extent of the corresponding set of cathode elements along a second direction orthogonal to the first direction (e.g. Y) parallel to the surface of the cathode substrate  16  is y, then the dimension of the hole along such direction is preferably at least one-tenth of y and more preferably one-third of y.  
     [0029] As in the embodiments described above, the height of the anode spacers  60  may be such that the spacing between layer  202   a  and the anode layer is spaced apart by at least about 0.5 millimeters, and preferably by at least about one millimeter. Preferably, the spacing between layer  202   b  and the cathode elements is at least about 20 microns. In the preferred embodiments, the spacing between layer  202   b  and the cathodes is within the range of 30 to 250 microns.  
     [0030]FIG. 5 is a cross-sectional view of display  250  which is substantially similar to display  10  of FIG. 1, except that device  250  includes an additional layer of grid electrodes which may be useful for controlling the addressing or brightness data control of device  250 . Thus, power supply and controller  44  may apply scanning or data electrical potentials to the grid electrodes  252 . This, together with the data or scanning electrical potentials applied to the rows or columns of sets of field emitter cathode elements, are adequate to control the addressing and brightness data control for displaying video images. Layer  252  is separated from layer  50  by means of an insulating layer  254 . A similar layer of grid electrodes may also be formed on layer  50 ′ of FIG. 2 and on layer  202   b  of FIG. 4. Such and other variations are within the scope of the invention.  
     [0031] The above-described displays are particularly easy to manufacture compared to prior art displays such as that proposed by Nonomura et al. described above. The micro-electron lens may be formed simply by a layer of metal, such as layers  50  and  50 ′ of FIGS. 1 and 2. Even if the micro-electron lens structure includes multiple layers, such multi-layer structure is also simple to manufacture. Fabrication of the anode and cathode spacers and the alignment of the micro-electron lens structure with the pixel dots and sets of cathode elements can also be accomplished in a single process and in a simple manner.  
     [0032] While the invention has been described by reference to various embodiments, it will be understood that different changes and modifications may be made without departing from the scope of the invention which is to be defined only by the appended claims.