PATENT ABSTRACT
Faceplates for field mission displays having novel cathodoluminescent layers are disclosed. In one embodiment a faceplate includes a transparent conductive layer, and a cathodoluminescent layer formed on the transparent conductive layer, the cathodoluminescent layer having been scrubbed by electron irradiation from an electron source with an electron current having a duty cycle in excess of ten percent, the electron current having a current density of greater than one-tenth milliampere per square centimeter while a voltage less than a thousand volts is maintained between the cathodoluminescent layer and the electron source. In one aspect, the transparent conductive layer may be formed on a transparent insulating viewing screen. In alternate aspects, the voltage maintained between the cathodoluminescent layer and the electron source may be dithered to treat the cathodoluminescent layer to varying depths. Significantly, the scrubbed faceplate has significantly enhanced performance and increased useful life compared to faceplates that have not been scrubbed.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 09/079,138, filed May 14, 1998, now U.S. Pat. No. 5,338,663. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with government support under Contract No. DABT63-93-C-0025 awarded by Advanced Research Projects Agency (ARPA). The government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     This invention relates in general to field emission displays for electronic devices and, in particular, to improved cathodoluminescent layers for field emission displays. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 is a simplified side cross-sectional view of a portion of a display  10  including a faceplate  20  and a baseplate  21  in accordance with the prior art. FIG. 1 is not drawn to scale. The faceplate  20  includes a transparent viewing screen  22 , a transparent conductive layer  24  and a cathodoluminescent layer  26 . The transparent viewing screen  22  supports the layers  24  and  26 , acts as a viewing surface and forms a hermetically sealed package between the viewing screen  22  and the baseplate  21 . The viewing screen  22  may be formed from glass. The transparent conductive layer  24  may be formed from indium tin oxide. The cathodoluminescent layer  26  may be segmented into pixels yielding different colors to provide a color display  10 . Materials useful as cathodoluminescent materials in the cathodoluminescent layer  26  include Y 2 O 3 :Eu (red, phosphor P-56), Y 3 (Al, Ga) 5 O 12 :Tb (green, phosphor P-53) and Y 2 (SiO 5 ):Ce (blue, phosphor P-47) available from Osram Sylvania of Towanda PA or from Nichia of Japan. 
     The baseplate  21  includes emitters  30  formed on a surface of a substrate  32 , which may be a semiconductor such as silicon. Although the substrate  32  may be a semiconductor material other than silicon, or even an insulative material such as glass, it will hereinafter be assumed that the substrate  32  is silicon. The substrate  32  is coated with a dielectric layer  34  that is formed, in one embodiment, by deposition of silicon dioxide via a conventional TEOS process. The dielectric layer  34  is formed to have a thickness that is approximately equal to or just less than a height of the emitters  30 . This thickness may be on the order of 0.4 microns, although greater or lesser thicknesses may be employed. A conductive extraction grid  38  is formed on the dielectric layer  34 . The extraction grid  38  may be, for example, a thin layer of polysilicon. An opening  40  is created in the extraction grid  38  having a radius that is also approximately the separation of the extraction grid  38  from the tip of the emitter  30 . The radius of the opening  40  may be about 0.4 microns, although larger or smaller openings  40  may also be employed. 
     In operation, the extraction grid  38  is biased to a voltage on the order of 100 volts, although higher or lower voltages may be used, while the substrate  32  is maintained at a voltage of about zero volts. Signals coupled to the emitter  30  allow electrons to flow to the emitter  30 . Intense electrical fields between the emitter  30  and the extraction grid  38  then cause emission of electrons from the emitter  30 . A larger positive voltage, ranging up to as much as 5,000 volts or more but generally 2,500 volts or less, is applied to the faceplate  20  via the transparent conductive layer  24 . The electrons emitted from the emitter  30  are accelerated to the faceplate  20  by this voltage and strike the cathodoluminescent layer  26 . This causes light emission in selected areas, i.e., those areas adjacent to the emitters  30 , and forms luminous images such as text, pictures and the like. 
     When the emitted electrons strike the cathodoluminescent layer  26 , compounds in the cathodoluminescent layer  26  may be dissociated, causing outgassing of materials from the cathodoluminescent layer  26 . When the outgassed materials react with the emitters  30 , their work function may increase, reducing the emitted current density and in turn reducing display luminance. This can cause display performance to degrade below acceptable levels and also results in reduced useful life for displays  10 . 
     Residual gas analysis indicates that the dominant materials outgassed from some types of cathodoluminescent layers  26  include hydroxyl radicals. The hydroxyl radicals reacting with the emitters  30  leads to oxidation of the emitters  30 , and especially to oxidation of emitters  30  formed from silicon. Silicon emitters  30  are useful because they are readily formed and integrated with other electronic devices on the substrates  32  when the substrate is silicon. Electron emission is reduced when silicon emitters  30  oxidize. This leads to time-dependent and/or degraded performance of displays  10 . 
     In conventional cathode ray tubes (“CRTs”), some scrubbing of the cathodoluminescent screen is typically carried out after the tube is sealed using an electron gun of the type contained in a CRT. “Scrubbing,” as used here, means to expose the cathodoluminescent layers (e.g., cathodoluminescent layer  26 ) to an electron beam until a predetermined charge per unit area has been delivered to the cathodoluminescent layer  26 . This scrubbing is carried out at a very low duty cycle and at a very low current density because the electron beam is rastered over the area of the cathodoluminescent screen. It is also carried out at the same current levels that the CRT is expected to support in normal operation, typically 100 microamperes/cm 2  or less. However, this approach will not work for scrubbing cathodoluminescent layers  26  for the displays  10 , in part because the emitters  30  in the displays  10  are poisoned by the chemical species evolving from the cathodoluminescent layer  26  in response to the scrubbing operation. Moreover, the cathodoluminescent layer  26  is typically much less than a millimeter away from the emitters  30 , i.e., the mean free path for any gaseous chemical species evolving from the cathodoluminescent layer  26  is much larger than the distance separating the cathodoluminescent layers  26  from the emitters  30 . In contrast, the electron gun used to scrub cathodoluminescent layers in a CRT are not adversely affected by this chemical species and electron guns are, as a rule of thumb, displaced from the cathodoluminescent screen by a distance approximately equal to the diagonal dimension of the CRT screen. 
     There is therefore a need for a technique to prevent evolution of oxygen-bearing compounds from cathodoluminescent screens in field emission display faceplates. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a low voltage, high current, large area cathode for electron scrubbing of cathodoluminescent layers is described. The electron scrubbing is particularly advantageous for use with cathodoluminescent screens of field emission displays having silicon emitters. The present invention includes an apparatus to irradiate a cathodoluminescent layer in a vacuum with an electron beam and a device to move the cathodoluminescent layer relative to the irradiating apparatus. The irradiation is stopped when a predetermined total Coulombic dose has been delivered to the cathodoluminescent layer. Significantly, the scrubbing results in a cathodoluminescent layer that does not outgas materials that are deleterious to performance of silicon emitters. This results in a more robust display and extended display life. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified side cross-sectional view of a portion of a display. 
     FIG. 2 is a simplified plan view of a portion of a low voltage, high current scrubbing device according to an embodiment of the present invention. 
     FIG. 3 is a simplified side cross-sectional view, taken along section lines III—III of FIG. 2, of one portion of the cathode of FIG.  2 . 
     FIG. 4 is a simplified side cross-sectional view, taken along section lines IV—IV of FIG. 2, of another portion of the cathode of FIG.  2 . 
     FIG. 5 is a simplified side cross-sectional view of the scrubbing device of FIGS. 2-4 together with the faceplate of FIG. 1 according to an embodiment of the invention. 
     FIG. 6 is a flow chart describing steps in a scrubbing operation using the low voltage, high current cathode according to an embodiment of the present invention. 
     FIG. 7 is a simplified block diagram of a computer using the display having the scrubbed cathodoluminescent layer according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring again to FIG. 1, when the cathodoluminescent layers  26  for displays  10  are scrubbed with high current density electron beams (i.e., greater than 0.1 milliampere/cm 2 , typically between one and ten milliamperes/cm 2 , and about two milliamperes/cm 2  in one embodiment) in a high vacuum, the cathodoluminescent layers  26  darken in a reversible manner. When the darkened cathodoluminescent layers  26  are baked in atmosphere at 700° C., the darkening disappears. Repeating the scrubbing process causes the cathodoluminescent layers  26  to darken again. When faceplates  20  having the darkened cathodoluminescent layers  26  are sealed into displays  10  using silicon emitters  30 , the emitters  30  do not degrade as is observed when untreated cathodoluminescent layers  26  are used. The darkening of the cathodoluminescent layer  26  suggests that a change in chemical composition of the cathodoluminescent layer  26  has taken place. Because these cathodoluminescent layers  26  do not cause degradation of the emitters  30 , the changes in the cathodoluminescent layers  26  due to electron bombardment appear to be beneficial. Because these changes can be reversed by baking the bombarded cathodoluminescent layers  26  in atmosphere, it is likely that the substance or substances causing degradation of the emitters  30  are also present in the atmosphere. Additionally, when faceplates  20  having the transparent conductive layer  24  but not the cathodoluminescent layer  26  are bombarded by electrons in displays  10 , there is no degradation of the efficiency of silicon emitters  30  in those displays  10 . 
     These experiments show that the materials causing the efficiency degradation of silicon emitters  30  can be removed by prescrubbing the cathodoluminescent layers  26  with high current, low voltage electron beams prior to sealing the faceplates  20  with the cathodoluminescent layers  26  into the displays  10 . This process results in robust displays  10 . 
     One way of efficiently prescrubbing the cathodoluminescent layers  26  uses a low voltage, high current scrubbing device  70  described below in conjunction with FIGS. 2 through 4. FIG. 2 is a simplified plan view of a portion of the scrubbing device  70  according to an embodiment of the present invention. The scrubbing device  70  includes posts  72 , each having one end of a wire cathode  74  coupled to it. The scrubbing device  70  also includes spring loaded contacts  76  coupled to posts  78 . Flexure of the bend in the contact  76  provides the spring loading. Each spring loaded contact  76  is coupled to a second end of one of the wire cathodes  74 . The couplings between the ends of the wire cathodes  74  and the posts  72  and  78  may be formed through conventional spot welding or any other suitable coupling providing electrical contact and mechanical support. The posts  72  are electrically and mechanically coupled to a first conductive base  80 . The posts  78  are electrically and mechanically coupled to a second conductive base  82 . The conductive bases  80  and  82  are mounted on to an insulating base  84  and are fastened to the base  84  by conventional means such as a conventional glass or ceramic frit that is fired in an oven. 
     The wire cathodes  74  typically are tungsten wires having a diameter of 10-20 microns. The wire cathodes  74  are usefully coated with conventional “triple carbonate” to reduce the work function of the wire cathode  74  and thereby increase electron emissions by the wire cathodes  74  when the wire cathodes  74  are heated. 
     The wire cathodes  74  are heated by a current that is passed between the conductive bases  80  and  82  via interconnections  86  and  88 , respectively. Although the wire cathodes  74  are heated to a temperature lower than that required in order to make them red hot, the wire cathodes  74  begin to emit significant numbers of thermionic electrons at this temperature. The heating also causes expansion of the wire cathodes  74 . The sagging of the wire cathodes  74  that would otherwise occur is avoided by the tension provided by the spring loading of the contacts  76  coupled to the posts  78 . 
     A voltage is applied between the wire cathodes  74  and the transparent conductive layer  24  on the faceplate  20 . This voltage accelerates the thermionically-emitted electrons from the wire cathodes  74  towards the faceplate  20 . When these electrons arrive at the faceplate  20 , they have a kinetic energy equal to the voltage, but expressed in electron-volts. Optionally, a conductive plate  90  is formed on a surface of the insulating base  84 . A negative voltage applied to the conductive plate  90  may increase the efficiency of the scrubbing device  70  by repelling electrons that otherwise would travel from the wire cathodes  74  towards the insulating base  84 . 
     In normal use, the scrubbing device  70  is placed within a vacuum system  92 , represented in FIG. 2 by a rectangle surrounding the scrubbing device  70 . In one embodiment, the vacuum system  92  is a load-locked system having a conveyor system for transporting the faceplates  20 , including the cathodoluminescent layers  26 , past the scrubbing device  70 . In one embodiment, the faceplates  20  are placed on the conveyor system such that the cathodoluminescent layer  26  faces upward, and the scrubbing devices  70  are mounted just above a plane of cathodoluminescent layers  26  such that the wire cathodes  74  are the part of the scrubbing device  70  that is closest to the cathodoluminescent layer  26 . 
     Cathodes similar to scrubbing device  70 , but manufactured for use in vacuum fluorescent displays, and wire cathodes  74 , are commercially available from several sources. These cathodes may be ordered built to the buyer&#39;s specifications. 
     The bonding layer  96  of FIGS. 3 and 4 is realized, in one embodiment, by screening a frit on to the conductive bases  80  and  82  and/or the insulating base  84 . The conductive bases  80  and  82  are placed in the desired position on the insulating base  84 . Firing the composite assembly in an oven then provides a robust mechanical bond between the conductive bases  80  and  82  and the insulating base  84 . 
     FIG. 3 is a simplified side cross-sectional view, taken along section lines III—III of FIG. 2, of one portion of the scrubbing device  70  of FIG.  2 . This portion includes the post  72  with the wire cathode  74  electrically and mechanically coupled to a top end of the post  72 . A bottom end of the post  72  is electrically and mechanically coupled to the conductive base  80 . The conductive base  80  is mechanically coupled to the insulating base  84  via a bonding layer  96 . 
     FIG. 4 is a simplified side cross-sectional view, taken along section lines IV—IV of FIG. 2, of another portion of the scrubbing device  70  of FIG.  2 . This portion includes the post  78  with the wire cathode  74  electrically and mechanically coupled to the spring-loaded contact  76  formed at a top end of the post  78 . A bottom end of the post  78  is electrically and mechanically coupled to the conductive base  82 . The conductive base  82  is mechanically coupled to the insulating base  84  via the bonding layer  96 . 
     FIG. 5 is a simplified side cross-sectional view of the scrubbing device of FIGS. 2-4 together with the faceplate of FIG. 1 according to an embodiment of the invention. In the embodiment shown in FIG. 5, the vacuum system  92  encloses both the faceplate  20  and the scrubbing device  70  including the insulating base  84  and the wire cathode  74 . A voltage source  97  is electrically coupled between the wire cathode  74  of the scrubbing device  70  and the transparent conductive layer  24  of the faceplate  20 . The voltage source  97  supplies the bias that accelerates electrons from the wire cathode  74  to the cathodoluminescent layer  26 . In a first embodiment, the wire cathode  74  together with the other elements making up the scrubbing device  70  are moved above the faceplate  20 . In another embodiment, the scrubbing device  70  is maintained in a stationary position and the faceplate  20  is moved relative to the wire cathode  74 . In yet a third embodiment, both the scrubbing device  70  and the faceplate  20  may be in motion. In all of these embodiments, the objective is to deliver the predetermined electron dose to the cathodoluminescent layer  26 , and to do so in a way that is uniform across the area of the cathodoluminescent layer  26 . 
     FIG. 6 is a flow chart describing steps in a scrubbing process  100  using the low voltage, high current scrubbing device  70  of FIGS. 2 through 5. In step  102 , the cathodoluminescent-coated faceplates  20  are placed flat, with the cathodoluminescent layer  26  up, on a conveyor system. In step  104 , the faceplates  20  are moved through a load lock and into the vacuum system  92  of FIG.  2 . This arrangement is used in one embodiment because a peripheral portion of the surface bearing the cathodoluminescent layer  26  on the faceplate  20  includes a layer of glass frit (not illustrated) that will be used to seal the faceplate  20  to the remainder of the display  10 . Therefore, it may not be feasible to handle the faceplates  20  by other than their front surface (i.e., the transparent insulating layer  22 ) at this stage in manufacturing. 
     In step  104 , the faceplates  20  are swept along in the vicinity of (e.g., beneath) the scrubbing device or scrubbing devices  70 . Movement of the faceplates  20  relative to the scrubbing devices  70  tends to result in uniform electron doses and uniform scrubbing, despite local variations in electron flux. 
     In step  106 , the faceplates  20  are bombarded with electrons at a current density of one to ten and preferably about two milliamperes/cm 2 . A return path for this current is provided via an electrical contact (not illustrated) to the transparent conductive layer  24 . The accelerating voltage may be chosen to be between 200 and 1,000 volts, although higher or lower voltages may be employed. In contrast to the methods employed in scrubbing of CRT screens, the accelerating voltage for the scrubbing operation for cathodoluminescent layers  26  for displays  10  may be chosen to be higher or lower than the operating accelerating voltage of the completed display  10 . 
     In one embodiment, the scrubbing energy is varied in optional step  110  by dithering the acceleration voltage over a range that is preferably less than thirty percent, e.g., ten or twenty percent. In some applications, it may be desirable in step  110  to ramp the accelerating voltage, i.e., slowly vary the voltage from, e.g., 200 volts to 500 volts, and then reduce the voltage back to 200 volts. This causes the depth to which the particles forming the cathodoluminescent layer  26  are scrubbed to vary and allows removal of impurities from more than just the surface of the particles forming the cathodoluminescent layer  26 . 
     Step  108  (and optionally step  110 ) is preferably carried out for five to twenty hours until it is determined in a query task  112  that a dose in the range of from five to twenty five Coulombs/cm 2  has been delivered to the cathodoluminescent layer  26 , although higher or lower doses may be employed. In one embodiment, a dose of seven to twenty Coulombs/cm 2  is used. When the query task  112  determines that the desired dose has been achieved, the scrubbing operation  40  ends and the scrubbed faceplate  20  may be incorporated into a display  10  via conventional fabrication procedures, provided that the scrubbed faceplate  20  is not allowed to re-absorb the species that were removed via the process  100 . When the query task  112  determines that the desired dose has not yet been achieved, steps  106 - 112  are repeated. 
     The scrubbing process  100  may be accompanied by other processes for treating the cathodoluminescent layer  26 . The cathodoluminescent layers  26  may be vacuum baked at a temperature of 400 to 700° C. prior to the scrubbing process  100  to remove water and other contaminants. Atmospheric baking may be employed after a first scrubbing process  100  to remove contaminants and a second scrubbing process  100  may be carried out after the atmospheric baking. A hydrogen plasma may be used to clean and chemically reduce the cathodoluminescent layer  26  prior to or following the scrubbing process  100 . Chemical reduction reactions may also be employed, such as baking in a carbon monoxide atmosphere. 
     Cooling may be required for some types of faceplates  20  during the scrubbing process  100  if the energy delivered to the faceplates  20  during scrubbing heats the faceplates  20  to excessive temperatures, e.g., over 500° C. Cooling may be effectuated by use of a duty cycle of less than 100% (i.e., the scrubbing device  70  supplying current less than 100% of the time) or via thermal conduction from the faceplate  20  through the conveyor system or both. For example, a duty cycle of one percent, 10%, 50% or up to 100% could be employed in view of scrubbing current requirements, heating concerns and any other issues. 
     A number of scrubbing devices  70  may be “tiled” together to provide an arbitrarily large area for electron irradiation of the cathodoluminescent layers  26 . This allows cathodoluminescent layers  26  of any size to be scrubbed. For example, a rectangular or square faceplate  20  having a seventeen inch diagonal measurement may be scrubbed using an array of scrubbing devices  70  each individually having a smaller diagonal measurement but collectively providing a larger diagonal measurement. In such an arrangement, the scrubbing devices  70  are typically placed adjacent one another to provide a relatively: uniform current density over the total area of the faceplate  20 . 
     The wire cathode  74  may be oriented so that it extends along the direction of travel of the cathodoluminescent layer  26 . This orientation may result in uneven treatment of the area of the cathodoluminescent layer  26  because of variations in incident electron flux, leading to areal variations in total Coulombic dose delivered to the cathodoluminescent layers  26 . In another embodiment, the wire cathode  74  may be oriented perpendicular to the direction of travel of the cathodoluminescent layers  26 . In one embodiment, the wire cathodes  74  are oriented at an oblique angle between 5° and 85°, e.g., 45°, to the direction of travel of the cathodoluminescent layers  26 . This may be effected by moving the cathodoluminescent layer  26  at an angle that is oblique to wire cathodes  74  oriented as illustrated in FIG. 2, or by orienting the wire cathodes  74  at an oblique angle on the insulating base  84 . It will also be appreciated that the insulating base  84  need not be rectangular but could be any shape. 
     FIG. 7 is a simplified block diagram of a portion of a computer  120  using the display  10  fabricated as described with reference to FIGS. 2 through 6 and associated text. The computer  120  includes a central processing unit  122  coupled via a bus  124  to a memory  126 , function circuitry  128 , a user input interface  130  and the display  10  including the scrubbed cathodoluminescent layer  26 . The memory  126  may or may not include a memory management module (not illustrated). The memory  126  does include ROM for storing instructions providing an operating system and a read-write memory for temporary storage of data. The processor  122  operates on data from the memory  86  in response to input data from the user input interface  130  and displays results on the display  10 . The processor  122  also stores data in the read-write portion of the memory  126 . Examples of systems where the computer  120  finds application include personal/portable computers, camcorders, televisions, automobile electronic systems, microwave ovens and other home and industrial appliances. 
     Field emission displays  10  for such applications provide significant advantages over other types of displays, including reduced power consumption, improved range of viewing angles, better performance over a wider range of ambient lighting conditions and temperatures and higher speed with which the display  10  can respond. Field emission displays  10  find application in most devices where, for example, liquid crystal displays find application. 
     Although the present invention has been described with reference to a specific embodiments, the invention is not limited to these embodiments. Rather, the invention is limited only by the appended claims, which include within their scope all equivalent devices or methods which operate according to the principles of the invention as described.