Abstract:
A flat panel display is provided including a baseplate for carrying a first potential, the baseplate having emitters for emitting electrons positioned thereon and a faceplate for carrying a second potential, the faceplate having phosphors thereon. The baseplate and the faceplate are hermetically sealed around the periphery to define an evacuated volume. A gate electrode for carrying a third potential causes the emitter to selectively emit electrons, which cause the phosphors to emit light and which ionize contaminant gases in the evacuated volume. A gettering material is disposed in housing connected to the evacuated volume and has a getter connection connecting the gettering material to the baseplate for applying the first potential to the gettering material, which causes the ionized contaminant gases to be attracted to and absorbed by the gettering material. The getter connection extends outside the vacuum to allow for testing of the ionized contaminant gases.

Description:
TECHNICAL FIELD 
     The present invention relates generally to flat panel displays and more particularly to flat panel displays with gettering systems which assist in evacuating and maintaining the evacuation of flat panel displays. 
     BACKGROUND ART 
     Cathode-ray tube (CRT) displays have been the predominant display technology for purposes such as home television and computer systems. For many applications, CRTs have advantages in terms of superior color resolution, high contrast and brightness, wide viewing angles, fast response times, and low manufacturing costs. However, CRTs also have major drawbacks such as excessive bulk and weight, fragility, high power and voltage requirements, strong electromagnetic emissions, the need for implosion and x-ray protection, undesirable analog device characteristics, and a requirement for an unsupported vacuum envelope that limits screen size. 
     To address the inherent drawbacks of CRTs, alternative display technologies have been developed. These technologies generally provide flat panel displays, and include liquid crystal displays (LCDs), both passive and active matrix, electroluminescent displays (ELDs), plasma display panels (PDPs), vacuum fluorescent displays (VFDs) and field emission displays (FEDs). 
     The FED offers great promise as an alternative flat panel display technology. Its advantages include low cost of manufacturing as well as the superior optical characteristics generally associated with the CRT display technology. Like CRTs, FEDs are phosphor based and rely on cathodoluminescence as a principle of operation. FEDs rely on electric field or voltage induced emissions to excite the phosphors by electron bombardment rather than the temperature induced emissions used in CRTs. To produce these emissions, FEDs have generally used row-and-column addressable cold cathode emitters of which there are a variety of designs, such as point emitters (also called cone, microtip, or “Spindt” emitters), wedge emitters, thin film amorphic diamond emitters, and thin film edge emitters. 
     Each of the FED emitters is typically a miniature electron gun of micron dimensions. When a sufficient voltage is applied between the emitter and an adjacent gate, electrons are emitted from the emitter into a vacuum which is located between a baseplate, upon which the emitters are mounted, and a faceplate having a transparent anode surface to which the phosphors are applied. The emitters are biased as cathodes and the emitted electrons are attracted and accelerated to strike the phosphors on the anode surface. The phosphors then emit visible light which form picture elements, or pixels, which make up the images on the face of the FED. 
     Electron emissions in FEDs require a hard vacuum to avoid serious problems, such as vacuum degradation, emission current degradation, and/or plasma generation or ionization which can lead to non-uniform brightness of the display or shortening of the working life of the display. 
     The FED is conventionally hermetically sealed in air and then evacuated through a tube which is pinched or melted shut after evacuation in a process called “tubulation”. To assist in the evacuation process and to maintain the hard vacuum, a “gettering material” is used which absorbs contaminant gases by various chemical reactions. There are basically two different types of gettering materials. One type is an evaporable gettering material, which is capable of being deposited by an evaporative deposition process. The other type is a non-evaporable gettering material, which is formed into the configuration in which it will be used. Non-evaporable getters are manufactured in various geometries, such as metal wires or strips covered by a porous coating of gettering material. 
     One approach of using an evaporable gettering material is to deposit it in the portion of the tube between the flat panel display and the pinch or melt point of the tubulation process. This has the disadvantage of the tube being accidentally broken off during the handling which accompanies manufacturing. 
     Another approach is simply forming an evaporable getter at a location along the interior surface of baseplate or/and faceplate. This is disadvantageous because a getter typically needs a substantial amount of surface area to perform the gas collection function. However, it is normally important that the ratio of active display area to the overall interior surface area be quite high in an FED. Because an evaporable getter is formed by evaporative deposition, a substantial amount of inactive area along the interior surface of the baseplate or/and the faceplate structure would normally have to be allocated for a getter, thereby significantly reducing the active-to-overall area ratio. In addition, the active components of the FED easily become contaminated during the gettering material deposition process and some of the active FED components could become short-circuited. 
     A non-evaporable getter is an alternative to an evaporable getter. A non-evaporable getter typically consists of a pre-fabricated unit. As a result, the likelihood of damaging the components of an FED during the installation of a non-evaporable getter into the FED is considerably lower than with an evaporable getter. While a non-evaporable getter does require substantial surface area, the pre-fabricated nature of a non-evaporable getter generally allows it to be placed closer to the actual display elements than an evaporable getter. 
     For flat panel displays with both these gettering systems, it has been determined that certain gases remain and are difficult to remove by the gettering system even after long periods of time. Knowing that the contaminant gases cause severe problems, those skilled in the art have long sought a system by which the gettering effect could be improved, but they have been unsuccessful. 
     DISCLOSURE OF THE INVENTION 
     The present invention provides a flat panel display having a cathode carrying baseplate hermetically sealed to an anode-coated, phosphor-bearing, faceplate with a vacuum between the baseplate and the faceplate. Electron emitters are mounted on the baseplate in contact with the cathode and a gettering material is disposed in a housing open to the vacuum and adjacent to the baseplate. The gettering material is conductively connected to the cathode on the baseplate to charge the gettering material to attract contaminant gas ions so that they can be absorbed by the gettering materials to maintain the vacuum. 
     The present invention provides a flat panel display having a cathode carrying baseplate hermetically sealed to an anode-coated, phosphor-bearing, faceplate. A vacuum is located between the baseplate and the faceplate. Electron emitters connected to the cathode are mounted on the baseplate and a gettering material is disposed in a housing open to the vacuum and adjacent to the faceplate. The gettering material is conductively connected to the cathode on the baseplate by a conductive connection which extends outside the vacuum to allow checking the quantity of residual gas ions present in the vacuum. 
    
    
     The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 (PRIOR ART) is a close-up cross section of a field emission display for a single picture element; 
     FIG. 2 (PRIOR ART) is a schematic cross section of a field emission display having a housing containing gettering material; 
     FIG. 3 is a schematic cross section of a field emission display having a gettering material charged in accordance with the present invention; 
     FIG. 4 is a schematic cross section of a field emission display having a gettering material connected to an electrode in accordance with the present invention; 
     FIG. 5 is a schematic cross section of a field emission display having a gettering material connected to a gate electrode in accordance with the present invention; and 
     FIG. 6 is a schematic cross section of a field emission display having a gettering material connected to a focus plate in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 (PRIOR ART), therein is shown a close-up cross section of a portion of a flat panel display, such as a field emission display (FED)  100  for a single picture element, or pixel  101 . The FED  100  includes a baseplate  102  and a faceplate  104  separated by a focus plate  106  and a wall spacer  108  and surrounded by a hermetic seal  148 . The space between the baseplate  102  and the faceplate  104  is a hard vacuum  110  of about 10 −7  torr containing traces of contaminant gases (not shown). 
     The baseplate  102  includes an insulating plate  114  upon which a base electrode, or conductive “row” electrode  116 , has been deposited. A resistive layer  118  is deposited on the conductive row electrode  116  and is covered by an insulating layer  120  which has a cavity  122  formed therein. Inside the cavity  122  is an electron emissive element such as an emitter  124 . The emitter  124  is deposited on the resistive layer  118  in the cavity  122  and is concentric with holes  126  patterned into an upper base electrode or conductive column electrode of which a portion is designated as a gate electrode  128 . The gate electrode  128  is deposited over the insulating layer  120  and is connected to a column electrode (not shown). 
     The faceplate  104  includes a transparent plate  130  of a material, such as glass or plastic, coated with phosphors  132  having a thin electrode  134  of a material such as aluminum deposited on the phosphors  132 . 
     A gettering system  140  is positioned adjacent the baseplate  102 . Those skilled in the art would understand that the gettering system could be in any position, and could be of any configuration. The gettering system  140  includes a housing  142  having an opening  144  connected to the vacuum  110 . Gettering material  146  is disposed in the housing  142 . Examples of gettering materials are aluminum (Al), barium (Ba), cobalt (Co), chromium (Cr), iron (Fe), manganese (Mn), nickel (Ni), tantalum (Ta), titanium (Ti), vanadium (V), tungsten (W), combinations thereof, and compounds thereof. 
     In operation, the baseplate  102  is charged to become the cathode and the faceplate  104  is charged to become the anode. More specifically, a negative voltage is imposed on the conductive row electrode  116 . The negative voltage is imposed through the resistive layer  118  to the emitter  124 . A positive voltage is imposed on the thin electrode  134 . When a suitable voltage, generally around 10 volts less negative than the negative voltage on the emitter  124 , is applied to the gate electrode  128 , the emitter  124  emits electrons into the vacuum  110  at various angles. The emitted electrons, under the influence of electric fields from the focus plate  106 , follow parabolic trajectories indicated by the lines  150  to impact on the thin electrode  134 , which has the anode voltage impressed upon it. The phosphors  132  behind the thin electrode  134  struck by the emitted electrons will produce light of a color consistent with a particular phosphor selected. The light will be for one picture element, or pixel  101 . 
     Referring now to FIG. 2 (PRIOR ART), therein is shown a schematic of a FED  100  with the baseplate  102 , the faceplate  104 , the emitters  124 , the gettering system  140 , and the gettering material  146 . Between the baseplate  102  and the faceplate  104  are shown various contaminant gases which remain after the hard vacuum of the vacuum  110  is formed. Representative gases are oxygen (O 2 )  214 , carbon monoxide (CO)  216 , nitrogen (N 2 )  218 , hydrogen (H 2 )  220 , vaporous water (H 2 O)  222 , carbon dioxide (CO 2 )  224 , and methane (CH 4 )  226 . 
     Also shown are electrons  230 ,  232 , and  234  being emitted from the emitters  124 . The electron  230  is shown striking the thin electrode  134  on the faceplate  104 . The electron  232  is shown striking the CH 4  molecule  226 . The electron  234  is shown striking and breaking a CH 4  molecule  236  into hydrogen ions (H + )  240 - 243  and a carbon (C +)  ion  244 . The H + ions  240 - 243  and the C +   244  have positive charges and are attracted towards the negatively charged, cathode, or the baseplate  102  as indicated by the wide arrows. After accumulating near the baseplate  102 , the ions will recombine to form a CH 4  molecule  246 . A CH 4  molecule  248  indicates that recombined molecules having a neutral charge will again enter the vacuum  110  to cause various previously enumerated problems. Due to its neutral charge, the CH 4  molecule  248  may or may not enter the gettering system  140  since it will move randomly. 
     In the past, a common gettering material  146  was barium (Ba), which absorbs various contaminant gases to maintain the vacuum  110  during the life of the FED  100  through the following series of reactions: 
     Phase 1: 
     
       
         2Ba+O 2 ⇄2BaO 
       
     
     
       
         3Ba+2CO⇄2BaO+BaC 2   
       
     
     
       
         3Ba+N 2 ⇄Ba3N 2   
       
     
     
       
         Ba+H 2 ⇄BaH 2   
       
     
     
       
         Ba+H 2 O⇄BaO+H 2 ↑  Equation 1 
       
     
     
       
         5Ba+2CO 2 ⇄4BaO+BaC 2   
       
     
     . . . etc. 
     Phase 2: 
     
       
         BaO+CO 2 ⇄BaCO 3   
       
     
     
       
         BaO+H 2 O⇄Ba(OH) 2   
       
     
     
       
         BaC 2 +H 2 O⇄BaO+C 2 H 2 ↑  Equation 2 
       
     
     
       
         2BaH 2 +O 2 ⇄2BaO+2H 2 ↑ 
       
     
     
       
         Ba 3 N 2 +3H 2 O⇄3BaO+2NH 3 ↑ 
       
     
     . . . etc. 
     Phase 3: 
     
       
         Ba(OH) 2 +CO 2 ⇄BaCO 3 +H 2 ↑ 
       
     
     . . . etc. 
     During life testing of the FED  100 , it was found that the life expectancy was disproportionately shorter for the flat panel displays which ran 6 kV than the flat panel displays that run at 4 kV. An explanation of this shortening is that life expectancy is proportional to emission current from the emitter, which depends on work functions. The work functions are based on the intensity of the electric field on top of the emitters and the pressure in the flat panel displays. It is believed that the emission currents, and thus life expectancy, are decreased by ion sputters of contaminant gases (the force of each ion impact is based on f=EQ where f is force, E is the electric field, and Q is the electric charge of the ion), which soften the vacuum in the FED  100 . It also appears that detrimental arcing increases where there are contaminant ions in the FED  100 . 
     In investigating further into the types of contaminant gases which might be present, it was discovered that CH 4  appeared as a contaminant gas over the life of the flat panel display. The source of this contaminant gas was unclear, but it appeared that the gettering material  146  was not absorbing CH 4  in sufficient quantities to remove it from the vacuum  110  during the life of the FED  100 . 
     Referring now to FIG. 3, therein is shown the same structure as shown in FIG. 2 (PRIOR ART) with the same numbers being used to designate the same elements. Of particular interest is the CH 4  As previously mentioned, the source of this contaminant gas was unclear. 
     In examining the various chemical reactions in the three Phases above, and in particular the reactions indicated by Equations 1 and 2, it appeared that Ba functions as a catalyst to make CH 4  from CO 2 , CO, and H 2 O by the reactions: 
     
       
         Ba+H 2 O⇄BaO+H 2 ↑  Equation 1 
       
     
     
       
         BaC 2 +H 2 O⇄BaO+C 2 H 2 ↑  Equation 2 
       
     
     
       
         C 2 H 2 +3H 2 →2CH 4 ↑  Equation 3 
       
     
     Basically, two of the reactions produce H 2  gas and C 2 H 2  gas, which combine to produce CH 4  as shown in Equation 3. Further, in none of the reactions of Phases 1-3 does CH 4  gas combine with the Ba in the gettering material  146  so as to be absorbed. Thus, even if the CH 4  gas migrated into the gettering system  140  of FIG. 2, it would not be removed from the vacuum  110 . 
     After much analysis it was realized that, if the CH 4  molecule  226  could be ionized into C +  ion  244  and H +  ions  240 - 243  by electron impact, the C +  ion  244  and H +  ions  240 - 243  might be absorbed by the gettering material  146 . However, the difficulty is that the C +  and H +  ions tend to recombine into CH 4  before reaching the gettering material  146  in the gettering system  140  of FIG. 2 (PRIOR ART). 
     The above analysis led to the further realization that the gettering system  140  was electrically neutral and, by charging the gettering system  140  to form a charged gettering system, it would be possible to attract ions, such as C +  ion  244  and H +  ions  240 - 243 , as indicated by the broad arrows, to the vicinity of the gettering material  146  where it could be absorbed. It was further deemed that adding the charge directly to the gettering material  146  would further assure absorption by attracting the positively charged ions into direct contact with the negatively charged gettering material  146 . 
     The charge could be applied as a voltage from the FED power supply (not shown) through a conductive connection  250  to the gettering material  146  in the charged gettering system  249 . 
     As shown in FIG. 3, when the conductive connection  250  is in operation, the gettering material  146  will have a negative charge, which causes positive ions, such as H +  ions  252 - 255  and C +  ion  256 , to be attracted into the gettering system  249  to be absorbed by the gettering material  146  before it can recombine into CH 4 . 
     The above arrangement has been determined to be extremely efficacious in removing the CH 4  gases from the vacuum  110  in the FED  100 . 
     As would be evident to those skilled in the art, the above arrangement will work for any positively charged ion resulting from the ionization of any of the other gases. This renders the vacuum  110  of the present invention even harder than that of the conventional flat panel display with regard to other gases than the CH 4  gas, which is used as an example above. 
     Referring now to FIG. 4, therein is shown schematic cross section of a preferred embodiment of the FED  100  having the gettering material  146  connected to the lower base electrode  116  by a conductive connection  260 . 
     An additional advantage of the present invention may be obtained by extending a conductive connection  261  (shown as an alternative connection by the dotted line) outside of the FED  100  where it may be accessed for testing purposes to determine the real time hardness of the vacuum  110  for quality control and life test purposes. This feature was previously not obtainable. 
     Referring now to FIG. 5, therein is shown a schematic cross section of the FED  100  having the gettering material  146  connected in an alternate embodiment to the gate electrode  126  by a conductive connection  262 . The gate electrode  126  is not as highly charged as the conductive row electrode  116 , but may be easier to access in some designs. 
     Referring now to FIG. 6, therein is shown a schematic cross section of the FED  100  having the gettering material  146  connected in an alternate embodiment to the focus plate  106  by a conductive connection  264 . The focus plate  106  may be the easiest to access for making the conductive connection  264 . 
     It will be understood that the terms “row” and “column” may be interchanged and the terms “upper” and “lower” are used just as a matter of convenience and may be different based on the orientation of the FED  100 . 
     While the invention has been described in conjunction with a specific best mode, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations which fall within the spirit and scope of the included claims. All matters set forth herein or shown in the accompanying drawings are to be interpreted in an illustrative and non-limiting sense.