Patent Publication Number: US-6215243-B1

Title: Radioactive cathode emitter for use in field emission display devices

Description:
This application is a continuation-in-part of application U.S. Ser. No. 08/955,880, filed Oct. 22, 1997 now abandoned, which is a continuation-in-part of application U.S. Ser. No. 08/852,228, filed May 6, 1997 now U.S. Pat. No. 5,982,082. 
    
    
     This invention relates to electronic field emission display devices, such as matrix-addressed monochrome and full color flat panel displays in which light is produced by using cold-cathode electron field emissions to excite cathodoluminescent material. Such devices use electric fields to induce electron emissions, as opposed to elevated temperatures or thermionic cathodes as used in cathode ray tubes. 
     BACKGROUND OF THE INVENTION 
     Cathode ray tube (CRT) designs have been the predominant display technology, to date, for purposes such as home television and desktop computing applications. CRTs have drawbacks such as excessive bulk and weight, fragility, power and voltage requirements, electromagnetic emissions, the need for implosion and X-ray protection, analog device characteristics, and an unsupported vacuum envelope that limits screen size. However, for many applications, including the two just mentioned, CRTs have present advantages in terms of superior color resolution, contrast and brightness, wide viewing angles, fast response times, and low cost of manufacturing. 
     To address the inherent drawbacks of CRTs, such as lack of portability, alternative flat panel display design technologies have been developed. These include liquid crystal displays (LCDs), both passive and active matrix, electroluminescent displays (ELDs), plasma display panels (PDPs), and vacuum fluorescent displays (VFDs). While such flat panel displays have inherently superior packaging, the CRT still has optical characteristics that are superior to most observers. Each of these flat panel display technologies has its unique set of advantages and disadvantages, as will be briefly described. 
     The passive matrix liquid crystal display (PM-LCD) was one of the first commercially viable flat panel technologies, and is characterized by a low manufacturing cost and good x-y addressability. Essentially, the PM-LCD is a spatially addressable light filter that selectively polarizes light to provide a viewable image. The light source may be reflected ambient light, which results in low brightness and poor color control, or back lighting can be used, resulting in higher manufacturing costs, added bulk, and higher power consumption. PM-LCDs generally have comparatively slow response times, narrow viewing angles, a restricted dynamic range for color and gray scales, and sensitivity to pressure and ambient temperatures. Another issue is operating efficiency, given that at least half of the source light is generally lost in the basic polarization process, even before any filtering takes place. When back lighting is provided, the display continuously uses power at the maximum rate while the display is on. 
     Active matrix liquid crystal displays (AM-LCDs) are currently the technology of choice for portable computing applications. AM-LCDs are characterized by having one or more transistors at each of the display&#39;s pixel locations to increase the dynamic range of color and gray scales at each addressable point, and to provide for faster response times and refresh rates. Otherwise, AM-LCDs generally have the same disadvantages as PM-LCDs. In addition, if any AM-LCD transistors fail, the associated display pixels become inoperative. Particularly in the case of larger high resolution AM-LCDs, yield problems contribute to a very high manufacturing cost. 
     AM-LCDs are currently in widespread use in laptop computers and camcorder and camera displays, not because of superior technology, but because alternative low cost, efficient and bright flat panel displays are not yet available. The back lighted color AM-LCD is only about 3 to 5% efficient. The real niche for LCDs lies in watches, calculators and reflective displays. It is by no means a low cost and efficient display when it comes to high brightness full color applications. 
     Electroluminescent displays (ELDs) differ from LCDs in that they are not light filters. Instead, they create light from the excitation of phosphor dots using an electric field typically provided in the form of an applied AC voltage. An ELD generally consists of a thin-film electroluminescent phosphor layer sandwiched between transparent dielectric layers and a matrix of row and column electrodes on a glass substrate. The voltage is applied across an addressed phosphor dot until the phosphor “breaks down” electrically and becomes conductive. The resulting “hot” electrons resulting from this breakdown current excite the phosphor into emitting light. 
     ELDs are well suited for military applications since they generally provide good brightness and contrast, a very wide viewing angle, and a low sensitivity to shock and ambient temperature variations. Drawbacks are that ELDs are highly capacitive, which limits response times and refresh rates, and that obtaining a high dynamic range in brightness and gray scales is fundamentally difficult. ELDs are also not very efficient, particularly in the blue light region, which requires rather high energy “hot” electrons for light emissions. In an ELD, electron energies can be controlled only by controlling the current that flows after the phosphor is excited. A full color ELD having adequate brightness would require a tailoring of electron energy distributions to match the different phosphor excitation states that exist, which is a concept that remains to be demonstrated. 
     Plasma display panels (PDPs) create light through the excitation of a gaseous medium such as neon sandwiched between two plates patterned with conductors for x-y addressability. As with ELDs, the only way to control excitation energies is by controlling the current that flows after the excitation medium breakdown. DC as well as AC voltages can be used to drive the displays, although AC driven PDPs exhibit better properties. The emitted light can be viewed directly, as is the case with the red-orange PDP family. If significant UV is emitted, it can be used to excite phosphors for a full color display in which a phosphor pattern is applied to the surface of one of the encapsulating plates. Because there is nothing to upwardly limit the size of a PDP, the technology is seen as promising for large screen television or HDTV applications. Drawbacks are that the minimum pixel size is limited in a PDP, given the minimum volume requirement of gas needed for sufficient brightness, and that the spatial resolution is limited based on the pixels being three-dimensional and their light output being omnidirectional. A limited dynamic range and “cross talk” between neighboring pixels are associated issues. 
     Vacuum fluorescent displays (VFDs), like CRTs, use cathodoluminescence, vacuum phosphors, and thermionic cathodes. Unlike CRTs, to emit electrons a VFD cathode comprises a series of hot wires, in effect a virtual large area cathode, as opposed to the single electron gun used in a CRT. Emitted electrons can be accelerated through, or repelled from, a series of x and y addressable grids stacked one on top of the other to create a three dimensional addressing scheme. Character-based VFDs are very inexpensive and widely used in radios, microwave ovens, and automotive dashboard instrumentation. These displays typically use low voltage ZnO phosphors that have significant output and acceptable efficiency using 10 volt excitation. 
     A drawback to such VFDs is that low voltage phosphors are under development but do not currently exist to provide the spectrum required for a full color display. The color vacuum phosphors developed for the high-voltage CRT market are sulfur based. When electrons strike these sulfur based phosphors, a small quantity of the phosphor decomposes, shortening the phosphor lifetimes and creating sulfur bearing gases that can poison the thermionic cathodes used in a VFD. Further, the VFD thermionic cathodes generally have emission current densities that are not sufficient for use in high brightness flat panel displays with high voltage phosphors. Another and more general drawback is that the entire electron source must be left on all the time while the display is activated, resulting in low power efficiencies particularly in large area VFDs. 
     Against this background, field emission displays (FEDs) potentially offer great promise as an alternative flat panel technology, with advantages which would include low cost of manufacturing as well as the superior optical characteristics generally associated with the traditional CRT technology. Like CRTs, FEDs are phosphor based and rely on cathodoluminescence as a principle of operation. 
     Unlike CRTs, FEDs rely on electric field or voltage induced, rather than temperature induced, emissions to excite the phosphors by electron bombardment. To produce these emissions, FEDs have generally used a multiplicity of x-y addressable cold cathode emitters. There are a variety of designs such as point emitters (also called cone, microtip or “Spindt” emitters), wedge emitters, thin film amorphic diamond emitters or thin film edge emitters, in which requisite electric fields can be achieved at lower voltage levels. 
     Each FED emitter is typically a miniature electron gun of micron dimensions. When a sufficient voltage is applied between the emitter tip or edge and an adjacent gate, electrons are emitted from the emitter. The emitters are biased as cathodes within the device and emitted electrons are then accelerated to bombard a phosphor generally applied to an anode surface. Generally, the anode is a transparent electrically conductive layer such as indium tin oxide (ITO) applied to the inside surface of a faceplate, as in a CRT, although other designs have been reported. For example, phosphors have been applied to an insulative substrate adjacent the gate electrodes which form apertures encircling microtip emitter points. Emitted electrons move upwardly through the apertures and strike phosphor areas. 
     FEDs are generally energy efficient since they are electrostatic devices that require no heat or energy when they are off. When they operate, nearly all of the emitted electron energy is dissipated on phosphor bombardment and the creation of emitted unfiltered visible light. Both the number of exciting electrons (the current) and the exciting electron energy (the voltage) can be independently adjusted for maximum power and light output efficiency. FEDs have the further advantage of a highly nonlinear current-voltage field emission characteristic, which permits direct xy addressability without the need of a transistor at each pixel. Also, each pixel can be operated by its own array of FED emitters activated in parallel to minimize electronic noise and provide redundancy, so that if one emitter fails the pixel still operates satisfactorily. Another advantage of FED structures is their inherently low emitter capacitance, allowing for fast response times and refresh rates. Field emitter arrays are in effect, instantaneous response, high spatial resolution, x-y addressable, area-distributed electron sources unlike those in other flat panel display designs. 
     As described in pending parent application U.S. Ser. No. 08/852,228, filed May 6, 1997, the FED device has been improved to provide for a high-brightness field emission display with improved operating characteristics and durability. Phosphor biasing, electron emission amplification, and nitride barrier layers will contribute to the reduction of emitter to phosphor gap and vacuum requirements, while permitting a wider range of operating voltages as may be more efficient or otherwise desirable for improved brightness levels. Contamination control is also provided to extend emitter life and ion blocking is further used to extend the phosphor life. 
     While the FED technology holds out many promises, existing designs are not without drawbacks. Extensive research and development has been devoted to FEDs in recent years, and yet problems remain unsolved. It was against this background that the present invention has been conceived. 
     OBJECTS OF THE INVENTION 
     It is accordingly an object of this invention to provide a low cost, high efficiency field emission display having the superior optical characteristics generally associated with the traditional CRT technology, in the form of a digital device with flat panel packaging. 
     Another object of the invention is to provide a field emission display device, for either monochrome or full color applications, with improved light conversion efficiencies, and with greater cathode to anode voltage level flexibility. 
     Another object of the invention is a field emission display device using an improved cathode emitter made from a radioactive material resulting in an easier and cheaper manufacturing process for the device. 
     Another object of the invention is a field emission display device with improved light conversion efficiencies and in which requirements for an emitter to phosphor gap or an internal vacuum is reduced, in the case of an all-film emitter device structure. 
     Another object of the invention is a field emission display device in which a plurality of electron multiplier structures provide for a single pixel. 
     Another object of the invention is a field emission display in which a plurality of electron multiplier structures allow for an increased emitting area to minimize the effects of phosphor lumination caused by edges of conventional cathode emitters. 
     Another object of the invention is a field emission display in which an improved electron multiplier structure decreases cathode emitter tip erosion. 
     Another object of the invention is a field emission display in which an improved electron multiplier structure generates additional secondary electron emission from exposed surface edges of insulating layers. 
     SUMMARY OF THE INVENTION 
     The invention applies generally to field emission display devices which use cathodoluminescence of a light emitting layer as a principle of operation. In such devices, a field emitter cathode matrix may be opposed by a phosphor-coated, transparent faceplate that serves as an anode and has a positive voltage relative to the emitter array matrix. The devices will typically incorporate a transparent conductive layer such as indium tin oxide (ITO) applied to the inside surface of the faceplate, or between the faceplate and the phosphor coating, to provide the anode electrode for applicable biasing with respect to the cathode-emitters. The ITO and the phosphor coating may be masked or patterned on the faceplate to provide a matrix of x-y addressable pixels, with addressing provided via a selective cathode-emitter activation. The devices may use high voltage sulfur-based phosphors, or low voltage phosphors may also be used. Smooth deposited phosphor films on the order of about 1200 Angstroms in thickness are presently preferred for use with this invention, for improved light transmission. 
     In accordance with one aspect of the invention, an electron multiplier structure is used to generate secondary electron emissions. A plurality of electron multiplier structures may correspond to a single pixel in a FED device, as well as allow for an increased emitting area to minimize the effects of phosphor lumination caused by edges of conventional cathode emitters. Moreover, the use of the narrow channel structures reduces the possibility that a returning ion will strike an emitter, and thus decrease cathode emitter tip erosion. 
     The above-mentioned and other objects, features and advantages of the invention will become apparent from the further descriptions and the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional schematic view of an exemplary field emission display device within the prior art. 
     FIG. 2 is a cross sectional schematic view of an exemplary “all-film” field emission display device implementing a radioactive emitter. 
     FIG. 3 is a cross sectional schematic view of an exemplary field emission display device implementing a radioactive emitter with conventional emitter to phosphor spacing. 
     FIG. 4 is a cross sectional schematic view of an exemplary field emission display device implementing an electron multiplier structure in conjunction with a radioactive emitter. 
     FIG. 5 is a top view of an exemplary field emission display device implementing a plurality of electron multiplier structures for use as a single pixel. 
     FIG. 6 is a side view of an exemplary field emission display device implementing a plurality of electron multiplier structures for use as a single pixel. 
     FIG. 7 is a cross sectional schematic view of an exemplary field emission display device implementing an electron multiplier structure in conjunction with an edge emitter. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     FIG. 1 schematically depicts an exemplary field emission display (FED) device  10  found within the prior art. This flat panel display comprises an x-y electrically addressable matrix of cold-cathode microtip or “Spindt” type field emitters  12  opposing a faceplate  14  coated with a transparent conductor layer  16  and a phosphor light emissive layer  18 . A distance or gap  19 , generally on the order of 100 to 200 μm, is maintained between the emitters  12  and the phosphors  18  by spacers  20 . The volume of space between the emitters  12  and the phosphors  18  is evacuated to provide a vacuum environment with a pressure generally in the range of 10 −5  to 10 −7  Torr. This environment is generally gettered (by means not illustrated) to mitigate against contamination of the internal parts, and to maintain the vacuum. 
     As illustrated, each emitter  12  has the shape of a cone and is coupled at its base to an addressable emitter electrode conductor strip or layer  22 , through which the emitter  12  is biased as a cathode having a negative voltage, via power supply  9 , with respect to the conductor  16  which serves as the anode. Adjacent conductor strips  22  are electrically separated by extensions of a dielectric insulator structure  24  that also separates adjacent emitters  12 . A conductive electron extraction grid  26  is positively biased as a gate electrode with respect to the emitters  12 , and has apertures  28  through which emitted electrons  29  have a path from the emitters  12  to the phosphors  18 . The extraction grid  26  can be an addressable strip, orthogonal to the conductors  22 , for servicing a row or column of matrix groups of emitters  12 . In that case there would typically be a multiplicity of orthogonal extraction grids  26  and conductor strips used within the FED  10 . As shown, the extraction grid  26  is spaced and electrically isolated from the conductors  22  by the insulator structure  24 . The emitters  12  and the conductors  22  are formed on a substrate or base plate  30 . 
     When the FED  10  is operational, a group of emitters  12  is addressed and activated by application of a gate potential, usually on the order of about 15 to 50 volts, between the associated cathode electrode strip  22  and extraction grid  26 . With the resulting primary field emission of electrons from the emitters  12 , the emitted electrons are accelerated toward the anode conductor layer  16  to bombard the intervening phosphors  18 . The phosphors  18  are induced into cathodoluminescence by the bombarding electrons, emitting light through the faceplate  14  for observation by a viewer. The operational potential between the cathode electrode strip  22  and the anode conductor layer  16  at the faceplate  14  is generally on the order of 500 to 1000 volts for FEDs using high-voltage, sulfur-based phosphors. 
     As illustrated in FIG. 1, the phosphors  18  may be optionally patterned on the faceplate  14  with conventional black matrix separations  32  to better define dots or discrete pixel areas which may be digitally addressed and illuminated on the FED  10 . As shown, each pixel may be serviced by its own matrix or multiplicity of emitters  12  to provide redundancy in the event one or more of the emitters  12  prove inoperative. 
     By miniaturizing the size of the emitters  12 , modest voltages can cause electrons to emit out of the cone tips very efficiently, without heat. For this reason, these and operationally similar field emitters are often called “cold cathode” emitters. “Spindt” type emitters  12  are typically sized with cone heights on the order of about 1 μm, and pitched at about 10 microns or less, allowing packing densities on the order of about 10 6  emitters per cm 2 . Apertures  28  are typically sized with diameters on the order of 1 μm. 
     The illustrated field emitter structure, comprising the emitters  12 , the conductor strips  22 , the insulator structure  24 , and the extraction grid  26 , can generally be made at low cost for small size displays using semiconductor micro-fabrication technology. For example, the emitters  12  can be formed on the conductor strips  22  on a silicon substrate  30  and overlaid by sequential depositions of a layer of silicon dioxide and a conductive metal gate film for the insulator structure  24  and the extraction grid  26 . Resulting raised areas over the emitters  12  can be removed by polishing, and the silicon dioxide dielectric immediately surrounding the emitters  12  can be removed by wet chemical etching to define self-aligned apertures  28 , as is well known. This process becomes very difficult to manufacture as display size increases. 
     FIG. 1 is not drawn to scale, as a typical FED of the type illustrated would generally have  100  or more of the emitters  12  for servicing of each pixel area on the display. 
     FIG. 2 schematically illustrate the presently preferred embodiments of the invention with features which can be readily adapted to the type of FED device  10  shown in FIG. 1, as well as to other types of field emission display devices with other types of field emitters not illustrated. Referring to FIG. 2, this figure depicts exemplary film layering for an “all-film” FED device  40  having a cathode emitter stage  42 , a light emitting stage  44 , two amplification stages  46  and  48 , and a gate controlled amplification stage  49 . The emitter stage  42  comprises a radioactive emitter  50  constructed using radioactive material (e.g. 5,000 Angstroms thick layer) applied to a molybdenum substrate  52  and electronically coupled as a cathode. The radioactive material may include tritium (H 3 ), e.g., in the form of tritided scandium or titanium. It is also envisioned that the radioactive material may include amorphous silicon reacted with a silane doped with tritium. A radioactive emitter  50  will have a long half-life in the range of 12½ years and decays with a relatively constant, low-level release of negative beta particles (electrons) over the projected product life of the FED device  40 . While a single emitter is schematically illustrated for servicing of a single display pixel location, it will be understood that a matrix or multiplicity of cathode emitters may be used, such as was previously described with reference to FIG.  1 . 
     Since the radioactive emitter  50  is continuously “on”, the associated primary electron emissions must be at a sufficiently low level that they do not significantly penetrate to the phosphors when the device is “off”. For example, material with radioactivity levels at least 100 millicuries per square centimeter, comparable to those low levels of radioactivity found in some compasses, watches and other similar devices, can be used to produce primary electron emissions with energy levels on the order of 5.7 kev. To activate the phosphors, the FED device  40  uses one or more stages of amplification and applied electronic fields to produce secondary electron emissions having higher requisite energy levels. 
     A barrier layer  54  comprised of a thin film of insulator material, preferably silicon nitride, may be disposed between the radioactive emitter  50  and the first amplification stage  46 . The barrier layer  54  will generally be disposed directly on the anode side of the cathode emitter  50  as shown. Preferably, this is a thin silicon nitride layer applied directly on the radioactive emitter  50 , thin enough to permit the tunneling of electrons (on the order of 30-40 Angstroms thick). It is important to appreciate that silicon nitride is an effective blocker of ions, and that electron tunneling is exhibited in sufficiently thin films of silicon nitride. 
     Amplification stages,  46  and  48 , may include multiple film layers interposed between the emitter stage  42  and the light emitting stage  44 . Generally, amplification stages comprise an amplification material having a high amplification factor, for producing secondary emissions of electrons when bombarded by primary emissions of electrons from the cathode emitter. By way of example, the amplification factor for copper-beryllium is estimated to be approximately 4 to 6. This means that when bombarded with electrons of sufficient energy, for each electron reaching the copper-beryllium target, there will be 4 to 6 electrons emitted. (On this scale, the secondary emission amplification factors for most metals are less than two.) Silver-magnesium (e.g., Ag—Mg) films are similar to those of copper-beryllium. In the FED device  40  as shown in FIG. 2, primary electrons will bombard and enter the amplification stages from the side of the cathode emitter  50 , generating secondary electron emissions internally or on the side of the light emitting stage  44 . Presently preferred amplification materials include slightly oxidized films of copper-barium, copper-beryllium, gold-barium, silver-magnesium or tungsten-barium-gold. Also, gold-calcium would be a particularly effective amplification material to use, although its amplification properties may not have been heretofore well appreciated. It is also envisioned that rubidium antimony or other alkali compounds may be used as material for the amplification layers. 
     To achieve enhanced secondary electron emissions within the FED device  40 , an amplification enhancement layer or film  62  and  72  can be disposed on the light emitting stage side of the amplification layer  60  and  70 . Preferably, the amplification enhancement layer  62  and  72  will be a near mono-molecular layer, and will generally be applied directly over top of the amplification layer  60  and  70 , respectively. The material for amplification enhancement layer  62  and  72  is preferred to consist essentially of an oxide of barium, beryllium, calcium, magnesium or strontium. Preferred combinations of amplification layers and amplification enhancement layers include magnesium oxide  62  or  72  in association with an Ag—Mg layer  60  or  70 , a beryllium oxide  62  or  72  in association with a Cu—Be layer  60  or  70 , or a calcium oxide layer  62  or  72  in association with a gold-calcium layer  60  or  70 . Lastly, a copper-beryllium layer  60  or  70  overlaid on the light emitting stage side by a near mono-molecular layer  62  or  72  of magnesium oxide or beryllium oxide would also help increase secondary emissions as described herein. Amplification enhancement layers are generally less than 10 Angstroms in thickness. 
     The amplification layer  60  and  70 , and amplification enhancement layer  62  and  72  may be deposited by conventional sputtering from a conditioned alloy target or, for example, by a co-sputtering process. To illustrate, a lightly oxidized beryllium target may be prepared by moving a target from room-temperature, ambient conditions to an oven at about 250° C. for about 30 minutes, converting the exposed beryllium surface to Be—O. The resulting lightly oxidized target can then be introduced along with a second, copper target for use within a sputtering chamber which is evacuated and back-filled with argon to a pressure of approximately one to ten microns. By sputtering initially from the beryllium target only, a near mono-molecular beryllium oxide layer may be deposited. By then co-sputtering from the beryllium and copper targets simultaneously, a copper-beryllium layer can then be deposited to a thickness of approximately 120 Angstroms, which will contain a small amount of Beryllium oxide. 
     Additional dielectric barrier films may also be applied to a light emitting side of either an amplification layer  60  or an enhanced amplification layer  62 . These thin barrier films will increase the electron excitation voltages and inhibit current flow until a sufficiently high electric field is present. A first barrier film  64  and  74  is preferably comprised of silicon nitride or other similar dielectric material. A second (optional) additional intrinsic amorphous silicon (α-silicon) film  66  and  70  may also be interposed on the light emitting side of either amplification layer. 
     Adjacent the emitter stage  42  is a first amplification stage  46  comprising a amplification layer  60  of silver magnesium overlaid by an amplification enhancement layer  62  of magnesium oxide. A silicon nitride barrier layer  64  and second (optional) α-silicon barrier layer  66 , each on the order of 30-40 Angstroms thick, are shown deposited over the amplification enhancement layer  62 . A second amplification stage  48  having essentially the same film structure as the first amplification stage  46  is interposed between the first amplification stage  46  and the phosphor layer  80 . In addition, a gate controlled amplification stage  49  also having essentially the same film structure as the first amplification stage  46  is then interposed between the second amplification layer and the phospher layer  80 . This stage controls the number of electrons exciting the phosphor layer  80 . The gate input signal is applied to the amplification layer  76  of the gate controlled amplification stage  49 . When this signal is negative, electrons are repelled and unable to reach the phosphor film layer  80 . The gating action is similar to gating in a CRT. As will be apparent to one skilled in the art, two or more amplification stages can be incorporated into the FED device. 
     The light emitting stage  44  includes a light emitting layer  80 , an anode electrode layer  86 , a faceplate  88 , and additional barrier layers,  82  and  84 . The light emitting layer  80  preferably has a thickness on the order of about 1200 Angstroms, and preferably comprises smooth deposited phosphors that can be applied by atomic layer epitaxy (ALE) or by the vapor reaction technique taught by Cusano and Studer in U.S. Pat. No. 2,685,530. Phosphors such as Y 2 O 3 :Eu 3+  can be used, as can other cathodoluminescent phosphors such as oxide type (e.g., ZnO:Zn) or sulfur-based cathodoluminescent phosphors. The best thickness for a phosphor layer depends upon the conductivity of the phosphors. Generally, phosphor field strengths are preferred to be in excess of 5×10 4  volts/centimeter. Because of the high field strengths involved with electron tunneling, use of phosphor powders is not presently preferred. One of the reasons for this is related to the packing density of phosphors. Spherical phosphor particles pack more densely than polyhedral particles and would be the phosphor particle of choice. However, conventional commercially available phosphor powders generally have a polyhedral makeup. Preferably, the light emitting layer  80  will be masked or patterned as dots or otherwise on the faceplate  88  to provide a matrix of discrete pixel areas, with addressing provided via a selective cathode-emitter area activation. 
     As further shown in FIG. 2, the light emitting stage  44  may also incorporate a barrier layer  82  of a thin film of insulator material, preferably silicon nitride, disposed between the faceplate  88  and the light emitting layer  80 . The barrier layer  80  will generally be disposed directly on the anode side of the light emitting layer  80 . The barrier layer  82  functions to inhibit ion flow, migrations or depositions of anode material on or into light emitting layer  80 . Preferably, this is a thin silicon nitride layer that is thin enough to permit the tunneling of electrons but thick enough to inhibit the flow of ions by way of anode plating action into the phosphor when the device is activated. In addition, a second (optional) barrier layer  84  of a similar insulating material may also overlay the first barrier layer. 
     What may not be appreciated is the effect that such plating action may have on phosphor poisoning and lifetime degradations in a field emission display. Thus, the need for nitride barrier layer  82 . 
     A further advantage of the barrier layers  82  and  84  results from the tunneling characteristics of the nitride material, to enhance the non-linearity and luminous efficiency of the FED device  40 . Cathodoluminescent phosphors are generally very efficient under high accelerating voltages as compared to phosphors excited at low accelerating voltages. In fact, luminescence can for the most part disappear when the excitation voltage drops below a “dead voltage”, which can be as high as about 1500 volts for high voltage phosphors in conventional devices. This occurs because of a dead surface layer on the phosphors and charge build-up. What is important to realize is that there must be good electron penetration into the phosphor material to achieve good luminous efficiency. When phosphors are excited at low voltages, the current may be high but penetration is low, resulting in poor luminous efficiency. 
     With one or more barrier layers, the phosphor excitation voltage is effectively increased until tunneling occurs and the barrier layers  82  and  84  become conductive via tunneling. This increase in excitation voltage—prohibiting current flow until a high field is present—results in higher electron penetration into the light emitting layer  80  and increased phosphor efficiencies. Thus, barrier layers  82  and  84  each contribute to high brightness cathodoluminescence with improved light conversion efficiencies and phosphor lifetimes within a FED device  40 . 
     Chemical vapor deposition (CVD) and sputtering are two well known and acceptable techniques for the deposition of the barrier layers  82  and  84 , which are each to be deposited to a thickness on the order of about 30 to 40 Angstroms. For efficient electron tunneling through the barrier layers  82  and  84 , and for voltage drops of less than 10 volts across each layer, their combined thickness should be less than about 100 Angstroms. If phosphor biasing is implemented, the bias voltage can be on the order of about 20 to 35 volts with the barrier layers  82  and  84  being within this thickness range. Field strengths across the nitride barrier layers  82  and  84  are preferably on the order of 10 6  volts/centimeter for effective tunneling of electrons through the films. 
     While silicon nitride is the presently preferred material for barrier layer  82 , other dielectric materials such as silicon dioxide, magnesium fluoride or polyamide materials (e.g., Kapton™ polyamide films) may also be useable. Also, a semiconductor material, such as amorphous or polysilicon, can be used for the barrier layer. Whatever dielectric or insulator material is used it is preferred that the layers  82  and  84  be dense as opposed to porous. Standard thermal evaporated material films usually tend to be porous, while sputtered and CVD films are more dense and therefore preferred. 
     The faceplate  88  is generally transparent to allow transmission of emitted light from an inside surface of faceplate  88  to an outside surface of faceplate  88  for viewing. Electrical biasing of the faceplate  88  is accomplished by using an anode electrode  86  comprising a transparent layer of electrically conductive material, such as indium tin oxide (ITO), disposed between the inside surface of faceplate  88  and either (or both) first barrier layer  82  and second barrier layer  84 . The thickness of each barrier layer  82  and  84  is between 30-40 angstroms. Preferably, the conductive layer will be deposited ITO on the inside surface of the faceplate  88 , with a resistance of about 200 to 300 ohms per square, and a refractive index of less than 1.75, to permit at least 80% of directed emitted light to be transmitted through the anode electrode  86  and the faceplate  38 . The anode electrode  86  may be continuous or it may be patterned, for example, such as by having addressable strips to implement a full color display as taught in U.S. Pat. No. 5,225,820. 
     In operation, low-level primary electron emissions continuously occur through the radioactive cathode emitter. These in turn produce successively multiplied secondary electron emissions at higher energy levels by passing through the successive amplification stages, ultimately bombarding the phosphor layer with penetrating secondary electrons to produce emitted light for viewing through the face plate. A gating voltage  92  between the second amplification stage  48  and the light emitting stage  44  can be switched from negative to positive with respect to the emitter  50  or with respect to the second amplification stage  48  as a way to provide gating or selective activation and deactivation of phosphor pixel areas within the FED device. In this way, the amplification stage  49  serves as the gating layer of the device. 
     Although an “all-film” FED device using a radioactive emitter may reduce emitter to phosphor spacing and eliminate associated vacuums, FIG. 3 shows an embodiment employing a more conventional device structure. As shown in FIG. 3, a radioactive emitter (as discussed above) serves as the primary electron source. A distance or gap  98  of less than 100 μm is maintained between the emitter and the phosphors by spacers  99 . An internal working vacuum better than 10 −5  Torr, and an emitter-cathode to anode working potential less than 500 volts (for high voltage phosphors) may also be desired. One skilled in the art will recognize that this embodiment will otherwise have the same construction using the same process as a conventional FED device. 
     An alternative embodiment of the present invention is seen in FIG.  4 . An electron multiplier structure  102  is shown in conjunction with a field emission display device  100 . Although the electron multiplier  102  has been depicted in the context of a FED device  100 , the following description is intended to adequately teach one skilled in the art to make a similar electron multiplier for use in a variety of situations. Referring to FIG. 4, the cathode emitter  104  for providing primary electron emissions is constructed using a radioactive material (as discussed above). This radioactive emitter  104  is constructed by depositing a thin film of scandium or titanium onto a substrate  106 . Next, an amplification stage can be deposited onto the cathode emitter  104 . An amplification stage includes thin dynode layers  110  sandwiched between thin insulating layers  112 . Multiple layers, alternating between dynode layers  110  and insulating layers  112 , can be added to the amplification stage until achieving the desired amplification ratio for producing amplified secondary emission of electrons. Similar to the amplification layers previously discussed, dynode layers  110  comprise amplification materials having a high amplification factor with a thickness on the order of 3,000 Angstroms. Insulating layers  112  with an approximate thickness of 5,000 Angstroms are comprised of silicon nitride or other dielectric material. 
     Numerous materials exhibit high secondary electron emission required for the dynode layer  110 . Some of these materials have higher band gaps then what is commonly used in photomultiplier tubes. In this invention, materials with high band gaps are preferred because they are not as sensitive to fabrication problems as are low band gap materials, such as cesium or the other alkali&#39;s that are necessary for the visible light generation of electrons. One skilled in the art will recognize that due to migration problems, first column halides for use as dynode material should be chosen carefully. Although copper-beryllium, magnesium, aluminum or other first column halides can be used, copper-beryllium will be discussed in this application. It is also envisioned that rubidium antimony or other alkali compounds may be used as material for the dynode layer. 
     After the amplification stage has been deposited onto the cathode emitter layer  104 , it is coated with photoresist and patterned by techniques known to those skilled in the art. An electron multiplier structure  102  is formed by etching a channel or cavity  114  into the multiple amplification stage layers. These layers are subjected to RIE (reactive ion etch) to delineate the pattern as shown in the cross sectional view of FIG.  4 . The amplification protrusions  116  shown on the ends of the dynode layers  110  extending into the cavity  114  are formed by first etching through all of the layers via an anisotropic etch down to the cathode emitter layer  104 . The etch must not erode this bottom (i.e., cathode emitter) layer. Standard procedures known by those skilled in the art provide a means for forming this type of etch step. Next, an isotropic etch selective to silicon nitride or other applicable insulating material is performed to etch laterally into each insulating layer  112 , such that amplification protrusions  116  are formed extending into the cavity  114  of the electron multiplier structure  102 . Amplification protrusion  116  extend a distance  117  on the order of 1,000 Angstroms into the cavity  114 . While not essential to operation, these protrusions are desirable to raise efficiency. 
     A top view of the electron multiplier structure  102 , as seen in FIG. 5, shows each cavity  114  as a long narrow channels. A multiplicity of these cavities  114  are used for each pixel to reduce/eliminate strong edge phosphor illumination effects. In prior art emitters, a number of Spindt points or edge emitters are used for each pixel in generating a required number of electrons. As can be seen in FIG. 5, only one such electron multiplier structure  102  is required for each pixel. 
     At this point, the scandium or titanium film of a radioactive emitter  104  can be tritiated, if not already done so. After the cathode emitter  104  is tritiated, the exposed edge surface of amplification protrusions  116  are oxided for enhanced secondary electron emissions. The exposed edge surfaces are exposed to oxygen at approximately 150° C. for 10 minutes; this procedure will convert a small part of the copper-beryllium surface to oxide which is necessary for good secondary electron emission. Other materials can also be used such as magnesium which is oxidized to magnesium oxide or aluminum which is oxidized to aluminum oxide. Although these materials are not exciting for photo emission as required in a photomultiplier tube, they do serve as good secondary electron emitters when bombarded with primary electrons. 
     In operation, the electron multiplier  102  of the present invention is similar to a photomultiplier tube. Unlike the photomultiplier, the primary electrons in this embodiment are given off by a radioactive cathode emitter. Primary electrons from the cathode emitter are driven to the first dynode layer by an applied voltage source  120  coupled to electrically bias the cathode emitter with respect to the first dynode layer. Electrons striking the first dynode layer release more electrons as a result of secondary emission. With a secondary electron emission ratio of five, there will be five electrons liberated for each electron striking the first dynode layer. Because the voltage applied to the second dynode is positive with respect to the first dynode layer, the electrons will in turn be directed toward the second dynode layer. The number of amplified electrons generated is determined by the number of dynode layers multiplied by the secondary electron emission ratio. 
     To facilitate the operation of the electron multiplier structure  102 , a gate electrode  118  is shown in FIG.  4 . Preferably, the gate electrode  118  will be comprised from Molybdenum or Tantalum. A gating voltage source  122  is interconnected between the gate electrode  118  and ground. One could also connect the input voltage  122  between the gate electrode  118  and second dynode electrode  116  with or without ground being connected to second dynode  116  or to the radioactive emitter  104 . The gating voltage is modulated to control the number of electrons delivered to the light emitting layer (not shown). While this gating voltage is on the order of ten volts, it is dependent on the depth and spacing of the electron multiplier structure  102  along with deposited layer thicknesses. As will be apparent to one skilled in the art, the various configurations may be used to construct the light emitting layer (not shown) which overlays the gate electrode  118  to complete the field emission device  100 . As best seen in FIG. 6, the gate electrode  118  is delineated in a pattern which is intended to reduce input capacitance. 
     In addition, the electron multiplier structure described in this application also acts partially like a channel electron multiplier (CEM) or a microchannel plate (MCP) as taught in U.S. Pat. No. 5,378,960. The dynode/insulator layering structure of this invention is comparable to the conductive (resistive) “tunnel” in a CEM device. However, the insulating layer  112 , can also provide additional secondary emission effects. Although there is no conductive coating on the exposed edge surfaces of the silicon nitride, the mere presence of it being sandwiched between two conductors, creates a capacitive distribution of the applied voltage along its length. Therefore, the electron multiplier structure  102  of this invention benefits from secondary emission characteristics from the exposed edge surfaces of insulating layers  112 . 
     Alternatively, secondary emissive material could be deposited via chemical vapor deposition (CVD) onto the entire inner surface of the sidewalls in the electron multiplier structure  102 . In this manner, emissive material covering the exposed edge surfaces on the insulating layers  112  would be the same as material covering the dynode layers  110 . There would be no electrical continuity between the dynode layers  110 , rather a capacitive distribution would serve to boost electron amplification. The amount of electron amplification depends upon the ratio between the height of the sidewalls and the cavity opening width of the electron multiplier structure, such that the higher this ratio, the greater the amount of electron amplification. 
     Another embodiment of the electron multiplier structure  102  is in conjunction with electron edge type emitters. Rather than having electrons being released from a radioactive source, primary electrons are emitted from a point-like source, such as the edge of a thin film, as taught in U.S. Pat. No. 5,618,216. In FIG. 7, the edge emitter  134  is disposed between a substrate  136  and an insulating layer  142  of the electron multiplier structure  132 . To facilitate the etching process, an (optional) silicon nitride layer  135  may be disposed between edge emitter  134  and substrate  136 . The edge emitter  134  consists of a very thin conductive film protruding into the channel of the electron multiplier  132 , preferably having a high melting point and a low work function. Although low work function materials are usually low melting point materials, hafnium and tantalum are a few high temperature materials that have also been found with low work functions. The emitting protrusions  148  formed on the end of the edge emitter layer  134  extend a distance  149  of approximately 1,000 Angstroms into the cavity  144 . Similarly, the amplification protrusions  146  formed on the end of the dynode layers  140  extend into the cavity  144  a distance  147  on the order of 1,000 Angstroms. One skilled in the art will recognize that the edge emitter embodiment is constructed having the same characteristics, including a gate electrode layer  150 , and using the same process as discussed for the embodiment shown in FIG.  4 . Although not necessary, the protrusion increases efficiency. 
     As previously seen, when a high enough voltage is applied between the edge emitter and a first dynode layer and the edge emitter is negative with respect to the first dynode layer, electrons are driven off the point-like edge the same as in the well known Spindt cathodes. These field emission electrons are driven to a first dynode layer where they bombard it and produce more electrons by secondary emission. Again, this increased number of electrons are next directed toward a second dynode where they are collected and again amplified for eventual delivery to the light emitting layer of the display device (not pictured here). 
     Due to the lateral layering of the electron multiplier structure, there will be minimal damage to edge emitter(s) caused by sputtering ions returning to the emitter. Ions are generated by electrons moving toward the anode under high voltage. Colliding with gas molecules enroute to the anode, the gas molecule becomes a positive ion and an electron is emitted. A positive ion—being large in mass—races towards the cathode to find and recapture its lost electron. Upon impact with the cathode, cathode material is released by sputtering. In Spindt “cone shaped” microtips, erosion of the tip occurs. Thus, conventional FED devices employ a very high vacuum to reduce the number of gas molecules available to collide with. However, in addition to using a vacuum, the narrow electron multiplier structure of this invention significantly decreases the possibility that a returning ion will strike an edge emitter. 
     As described, the features of the FED device will provide for a high-brightness field emission display with improved operating characteristics and durability. The feature of a radioactive emitter in the context of an “all-film” device will contribute to the reduction of the emitter to phosphor gap, as well as eliminate or reduce vacuum requirements. An improved electron multiplier structure may decrease tip erosion of emitters, and improve secondary electron emissions. Finally, an overall simplified structure will decrease manufacturing costs for FED devices. 
     While the presently preferred embodiments of the invention have been illustrated and described, it will be understood that those and yet other embodiments may be within the scope of the following claims.