Patent Publication Number: US-6218777-B1

Title: Field emission display spacer with guard electrode

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application relates to and claims priority on provisional application serial No. 60/052,345 filed Jul. 11, 1997 and entitled “Field Emission Display Spacer With Guard Electrode”. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to insulative spacers provided between parallel plates between which there is an electric potential. The insulative spacers of the invention may reduce the likelihood of surface electron flashover between the parallel plates. 
     BACKGROUND OF THE INVENTION 
     Parallel plate type electron beam arrays are known. Presently, such arrays are being provided in the form of microminiature field emitters, which are known in the microelectronics art. These microminiature field emitters are finding widespread use as electron sources in microelectronic devices. For example, field emitters may be used as electron sources in flat panel displays for use in aviation, automobiles, workstations, laptops, head wearable displays, heads up displays, outdoor signage, or practically any application for a screen which conveys information through light emission. Field emitters, as well as other types of electron beam arrays, may also be used in non-display applications such as power supplies, printers, and X-ray sensors. 
     Referring to FIG. 1, the cross-section of a parallel plate type electron beam emission device  10  is shown. The device includes a bottom plate  100 , a spacer structure  200 , and a top plate  300 . The bottom plate  100  may comprise a substrate  110  and a conductive element  120 . The bottom plate  100  may include additional elements in the interior of the device  10  including conductive gates, which are useful for emitting electrons in the direction of the top plate  300 . The top plate  300  may comprise a substrate  310  and a conductive element  320 . The top and bottom plates may be connected along their respective outer edge regions with the spacer structure  200 . The spacer structure  200  may itself comprise an insulator frame or ring  210  bonded to the top and bottom plates with an upper glass frit  220  and a lower glass frit  230 , respectively. 
     In order to achieve a beam of electrons, from the bottom plate  100  to the top plate  300 , of a predetermined velocity, the upper conductive element  320  may be maintained at a high positive voltage relative to the source of electrons located on the bottom plate  100 . Thus the upper conductive element  320  may also be referred to as an anode. If the device  10  is a display, the anode  320  may be implemented by a thin transparent conductive layer. 
     In order to operate the device  10 , the space between the bottom plate  100  and the top plate  300  should be evacuated. Typically, this space may be of the order of 0.5 to 5 millimeters. To maintain the vacuum between the top and bottom plates, they are sealed to one another along their respective edges by the spacer structure  200 . After being sealed, the space between the two plates,  100  and  300 , may be evacuated of air or gas and sealed off from the outside atmosphere. 
     It is imperative to the operation of the device  10  to have as near to a perfect vacuum in the device as possible. The reason being that gas molecules within the device may become ionized as a result of being bombarded by the electrons in the device. If the gas pressure is high enough, there will be a growth in the ionization leading to a gas-discharge (breakdown flashover) between the anode  320  and the elements of the bottom plate  100 . In devices in which the potential between the anode  320  and the bottom plate  100  is in the range of thousands of volts, such flashover may be catastrophic to the device  10 . The flashover problem is particularly noticeable during the burn-in of new displays. Burn-in is carried out by operating a display at anode voltages well above those that would be experienced by the display during normal operation. It is at this time that displays are particularly susceptible to flashover. 
     The susceptibility of a display to flashover may be related to the density of gas in the region of the display where the flashover occurs. The density of gas molecules close to the display wall tends to be high on a short time scale. If the product (p)(d) of the local gas pressure (p) in the vicinity of the walls and the distance (d) between the anode and the gate is sufficient for a Paschen breakdown, then a cumulative ionization leading to a gas discharge (flashover) will occur between the anode and the gate. The flashover between the anode and the gate can trigger a flashover between that gate and corresponding emitters. For this reason most flashovers take place close to the sidewalls in a field emission display. 
     Prior to the present invention, adequate flashover control at high voltages (e.g., ≧6KV) has been difficult. The primary method of combating flashover has been to reduce the operating potential between the anode  320  and the elements of the bottom plate  100 . By decreasing the potential to levels of only a few hundred volts, the occurrence of flashover may be reduced, although it is far from eliminated. 
     Ise, U.S. Pat. No. 5,448,133 (issued Sep. 5, 1995) for a Flat Panel Field Emission Display Device with a Reflective Layer, touts the advantages of reducing the potential between the anode and cathode in a Field Emitter Display (FED). Ise states that a reduction of the operating voltage of a FED will reduce power consumption, which reduces battery size, and enables portability. Ise states that presently the low end threshold for anode to cathode potential is about 400 volts. Ise reports operation of his FED at as low as 100 volts of cathode to anode potential. 
     Reduction of the bottom plate to anode potential, however, as suggested by Ise, may reduce FED lifespan. Lifespan may be reduced because the luminous efficiency of the FED phosphors depends on the coulomb charge per unit volume applied to the phosphors over a period of time. The application of charge to the phosphors seems to dislocate activators from their sites in the phosphor host lattice, and thus decreases the activator excitation efficiency (by increasing the vacancy density). A phosphor layer of certain thickness, if operated by high voltage and low current, tends to have low values of coulombs per unit volume due to the increased penetration depth of the charge delivering electrons. On the other hand, if the same layer is operated with low voltage and high current (maintaining the same power) the coulombs per unit area increases because of the increased current, and the coulombs per unit volume increases even more due to the decreased penetration of the electrons (charge concentration at the surface of the layer). Increased coulomb density resulting from low voltage operation is more detrimental to the activators than high voltage operation over a given time span. Consequently the luminous efficiency decreases more rapidly for low voltage FED&#39;s. A decrease in light output may also occur in low voltage FED&#39;s due to the intervening passive thickness of the phosphor layer between the observer and the active surface layer. 
     The problems associated with sidewall induced flashovers, discussed above, may also arise in the interior portions of large sized screen FED&#39;s when low internal device pressure is maintained. Internal spacers are commonly used in FEDs to prevent the FED screen from bowing inward as a result of the pressure difference between atmosphere outside and the vacuum conditions of the FED interior. While the spacers beneficially keep the screen from bowing or breaking, the spacers also provide a surface linking the gate and anode which can facilitate flashovers. Trace residual gas or gas buildup on these surfaces can support plasma arcs. 
     Accordingly, there is a need for new methods and apparatus for reducing the occurrence of flashover, without reducing the level of anode voltages. There is also a need for methods and apparatus for reducing the magnitude of damage suffered from the occurrence of flashovers during the initial burn-in and operation of the device. There is a particular need for a device which does not readily support surface flashovers along the interior surfaces and/or internal spacers of the device. The present invention meets this need, and provides other benefits as well. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide methods and apparatus for reducing the occurrence of flashovers in parallel plate electron beam arrays. 
     It is another object of the present invention to provide methods and apparatus for reducing the amount of damage suffered from the occurrence of flashovers in parallel plate electron beam arrays. 
     It is a further object of the present invention to provide methods and apparatus for reducing the occurrence of flashovers which are supported by spacers in parallel plate electron beam arrays. 
     It is still yet another object of the present invention to provide methods and apparatus for increasing anode voltages in a parallel plate electron beam array without increasing the occurrence of flashovers in the array. 
     It is still a further object of the present invention to provide a spacer structure in an FED that includes a conductive member for shunting away a flashover discharge. 
     Additional objects and advantages of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention. 
     SUMMARY OF THE INVENTION 
     In response to the foregoing challenge, Applicants have developed an innovative, economical field emitter display having top and bottom plates separated by a spacer, a spacer structure comprising an insulative member adapted to separate said top and bottom plates; and a conductive member spaced from said top and bottom plates, said conductive member extending through said spacer structure. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated herein by reference, and which constitute a part of this specification, illustrate certain embodiments of the invention, and together with the detailed description serve to explain the principles of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view in elevation of the edge region of an electron beam array device. 
     FIG. 2 is a cross-sectional view in elevation of the edge region of a first electron beam array embodiment of the invention. 
     FIG. 3 is an alternative embodiment of the spacer structure shown in FIG.  2 . 
     FIG. 4 is a second alternative embodiment of the spacer structure shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to a preferred embodiment of the present invention, an example of which is illustrated in the accompanying drawings. A preferred embodiment of the present invention is shown in FIG. 2 as the edge portion of device  20 . Device  20  may be any parallel plate electron beam array, including a field emitter display. 
     Device  20  comprises a bottom plate  100 , a top plate  300 , and a spacer structure  200 . The spacer structure  200  includes an insulator frame or ring  210 . Typically the insulator frame  210  may be made of glass, however, other insulative materials may be used. The insulator frame  210  may include plural insulative members  212  and  214  connected or fused together. For example, an upper insulative member  212  may be fused to a lower insulative member  214  using a glass frit therebetween to join the two insulative members. 
     A conductive member  250  may also be provided between the two insulative members  212  and  214 . The conductive member  250  may be used to shunt flashover arcs, which otherwise might carry current from the high voltage anode  320  all the way to the conductive element  120 . In this way, the current flows into conductive member  250  rather than into conductive element  120 . 
     The insulative members  212  and  214  may have different cross-sectional dimensions as illustrated in FIG.  2 . This permits contact to be made easily between the conductive member  250  and an electrical sink (not shown) outside of the device  20 . 
     The fusing together of the insulative members  212  and  214  with the conductive member  250  therebetween may be carried out at a temperature of 350-450° C. This temperature should be low enough to avoid significant distortion to the top and bottom plates,  300  and  100  respectively. The frit glass used to fuse the insulative members together should be chosen such that it will wet the top and bottom plates,  300  and  100 , the insulative members,  212  and  214 , and the conductive member  250 , without dissolving the conductive member. A lead oxide frit glass has been found to suffice when the top and bottom plates are glass. 
     The conductive member  250  may be made of any conductive material. In the embodiment illustrated by FIG. 2, the conductive member  250  may comprise a conductive frit made of a mixture of metallic particles and glass. Silver metallic particles have been used in particular. 
     With regard to FIG. 3, the spacer structure  200  may be provided in an alternative embodiment by an insulator frame  210  having a conductive metal foil  260  extending therethrough. The metal foil may have a tab  262  that extends beyond the insulator frame  210 . This tab may be especially useful for connecting the metal foil  260  to an electrical sink (not shown) when the insulative members  212  and  214  have the same cross-sectional widths. The metal foil may also be provided with an enlarged head portion  264 . The head portion  264  may increase the amount of surface area of the foil exposed within the display to a discharge. 
     With regard to FIG. 4, the spacer structure may be provided in another alternative embodiment by an insulator frame  210  with a metal coating  240  on an upper portion of the frame. The metal coating  240  is applied such that it covers portions of the sidewalls  216  of the insulator frame  210  without contacting the conductive element  120 . The metal coating  240  may also include a tab (not shown) similar to that shown in FIG. 3 for connecting the coating to an external electrical sink. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the construction, configuration, and/or operation of the present invention without departing from the scope or spirit of the invention. For example, in the embodiments mentioned above, various changes may be made to the sealing materials used to connect the insulator frame with the top and bottom plates of the device. Further, changes may be made to the order in which the top and bottom plates are sealed to the insulator frame, and to which of the elements (the frame or the plates) the sealing means is first applied. Changes may also be made to the shape, size, and wall width of the insulator frame without departing from the scope or spirit of the invention. Further, it may be appropriate to make additional modifications or changes to the location of the conductive member relative to the insulator frame. Thus, it is intended that the present invention cover the modifications and variations of the invention provided they come within the scope of the appended claims and their equivalents.