Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/537,135, filed Mar. 29, 2000, now U.S. Pat. No. 6,530,814, which is a continuation of application Ser. No. 09/145,831, filed Sep. 2, 1998, now U.S. Pat. No. 6,116,974, issued Sep. 12, 2000. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with United States Government support under contract No. DABT63-93-C-0025 awarded by the Advanced Research Projects Agency (ARPA). The United States Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to display devices, such as field emission displays, plasma displays, and flat panel cathode ray tubes, and methods for making and using the same. Specifically, the invention relates to improved spacers for such display devices and methods for making and using the same. 
     Display devices visually present information generated by computers and other electronic devices. One category of display devices is electron emitter apparatus, such as a cold cathode field emission display (FED). A FED uses electrons originating from emitter tips on a base plate to illuminate a cathodoluminescent display screen and generate an image. A gate electrode, located near the emitters, and the base plate are in electrical communication with a voltage source. Electrons are emitted when a sufficient voltage differential is established between the emitters and the gate electrode. The electrons strike a phosphor coating on the display screen which emits photons to form the visual image. 
     In a FED, spacers separate the base plate and the display screen. Numerous processes have been developed for forming spacers for FED devices. U.S. Pat. Nos. 5,509,840, 5,232,549, 5,205,770, and 4,923,421, and Holloway et al.,  Production and Control of Vacuum in Field Emission Flat Panel Displays, Solid State Technology , August 1995, pp. 47-54, all incorporated herein by reference, disclose exemplary processes for forming such spacers. 
     Numerous materials have been used in the spacers, including ceramic, glass, glass-ceramic, ceramic-reinforced glass, devitrified glass, amorphous glass, metal with electrically insulating coating, bulk resistivity metals such as titanium, aluminum, or chromium oxide, silicon dioxide, silicon nitride, polyamide, or a variation of polyimide, such as Kapton. U.S. Pat. Nos. 5,530,582 and 5,658,832, both incorporated herein by reference, disclose that materials in the form of an aerogel or xerogel may be employed as spacers. 
     Manufacturers have recognized that a good emitter condition is important to effectively operate display devices. If the emitters become contaminated or oxidized by gases in the display device, the work function of the emitter increases and reduces the current emission, thus degrading the performance of the display device. Several articles discuss the impact of such harmful gases on emitter performance. See Schwoebel et al.,  Field - Emitter - Array Performance Enhancement Using Glow Discharge Processing,  6 th International Vacuum Micro Electronics Conference , Jul. 12-15, 1993; Itoh et al.,  Influence of Various Gases on the Emission of Field Emitter Arrays, Futaba Corporation , (date unknown); and Schwoebel et al.,  In Situ Enhancement of Field - Emitter Array Performance,  7 th International Vacuum Micro Electronics Conference , Jul. 4-7, 1994, p. 378; all incorporated herein by reference. 
     One method of reducing the amount of harmful gases in a display device has been to use gettering materials. See Giorgi et al.,  High - Porosity Thick - Film Getters, IEEE Transactions on Electron Devices , Vol. 36, No. 11, November 1989, pp. 2744-2747. Getters absorb gases, both those generated by components and those leaking in from the atmosphere, thereby minimizing harmful gas in the display device. The getter is often placed in peripheral regions of the display device, such as inactive regions outside the active display area between the base plate and display screen. When the getter is positioned outside the active display area, the size of the respective peripheral area must be increased, reducing the effective display area. Moreover, getters merely absorb gases, and are not used to desorb gases. 
     U.S. Pat. No. 5,684,356 discloses another method of reducing the damaging effects of harmful gases in a display device. This patent describes an FED device with an insulating layer comprising hydrogen silsesquioxane (HSQ). Under specific operating conditions in the display device, HSQ desorbs hydrogen, thus raising the partial pressure of hydrogen with respect to oxygen and keeping deleterious oxide from forming on the emitters. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of making a display device having at least one emitter by forming at least one spacer having a high concentration of an emitter-cleaning material and disposing the at least one spacer within the display device. The emitter-cleaning material preferably comprises hydrogen. The at least one spacer preferably exhibits a dual-sorption capability, allowing the at least one spacer to chemisorb gases contaminating or oxidizing an emitter and physisorb gases cleaning the emitter. 
     The present invention also provides a display device comprising a base plate containing at least one emitter, a display screen, and at least one spacer located between the base plate and display screen having a high concentration of an emitter-cleaning material. The at least one spacer preferably comprises a xerogel or aerogel material and the emitter-cleaning material preferably comprises hydrogen. The display device is preferably a field emission display device. 
     The present invention also provides a method for making at least one spacer for a display device containing at least one emitter by forming a sheet comprising a material having a high concentration of an emitter-cleaning material and then removing portions of the sheet to form the at least one spacer. The at least one spacer preferably comprises a xerogel or aerogel material, formed by making a substrate comprising a spacer material, removing a portion of the substrate to form the at least one spacer, and then increasing the concentration of an emitter-cleaning material. Preferably, the emitter-cleaning material comprises hydrogen. 
     The present invention further provides at least one spacer for a display device containing at least one emitter, the at least one spacer having a high concentration of an emitter-cleaning material. Preferably, the at least one spacer comprises a xerogel or aerogel material. The emitter-cleaning material preferably comprises hydrogen absorbed from a gas or mixture of gases containing hydrogen. The at least one spacer preferably exhibits the dual-sorption capability mentioned above. 
     The present invention moreover provides a method of operating a display device containing at least one emitter and at least one spacer having a high concentration of an emitter-cleaning material by causing the at least one spacer to desorb the material to clean the at least one emitter. The present invention also provides a method for cleaning an emitter in a display device by causing at least one spacer having a high concentration of an emitter-cleaning material to desorb the material to clean the at least one emitter. 
     The present invention provides several advantages over the prior art. The spacer absorbs gases during processing or packaging of the display device and then desorbs helpful gases, such as hydrogen, during operation thereof. The beneficial gases aid in cleaning emitter tips during processing and help keep the tips clean during operation. The beneficial gases, therefore, retain the work function at a low level and keep the emission current high, prolonging the usefulness of the display device. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Certain aspects of the present invention are illustrated in the accompanying drawings in which: 
     FIGS. 1-4 depict a method of forming spacers according to the present invention; 
     FIGS. 5A-5D depict another method of forming spacers according to the present invention; and 
     FIG. 6 depicts an assembled FED containing spacers formed according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a display device and methods for making and operating the same. In particular, the present invention provides spacers for display devices, such as field emission displays, plasma displays, and other electron-emitting apparatus, and a method for making and using the same. Specifically, the spacers of the present invention are formed of any material, such as an aerogel or xerogel, capable of absorbing gases during fabrication of the display device and releasing beneficial gases, such as hydrogen, during operation of the display device. 
     The following description provides specific details, such as material thicknesses and types, in order to provide a thorough understanding of the present invention. The skilled artisan, however, will understand that the present invention may be practiced without employing these specific details. Indeed, the present invention can be practiced with conventional fabrication techniques employed in the industry. 
     The process steps and structures described below neither form a complete process flow for manufacturing display devices nor a completed device. Only the process steps and structures necessary to understand the present invention are described. 
     The spacers of the present invention may be formed by any process resulting in the properties described below. For example, the spacers may be formed by a process similar to the process described in U.S. Pat. No. 5,658,832. Preferably, however, the spacers of the present invention are formed by the process illustrated in FIGS. 1-4 or FIGS. 5A-5D. 
     As shown in FIG. 1, substrate  10  is formed of a sheet of spacer material with thickness T, length L, and width W. A preferred spacer material of the present invention is gel materials, especially aerogel and xerogel materials. Aerogels and xerogels are generally considered as solid materials having a gas dispersed therein. In substrate  10 , for example, thickness T may be on the order of 10 μm to 1000 μm, while length L and width W will be on the order of inches to feet. Different methods of formulating aerogels and xerogels are known in the art and can be employed to form the spacers of the present invention. 
     In a preferred method of formulating the aerogel or xerogel spacer, a sol or solution of the gel material is formed. The solution can be one component or a multi-component. If the aerogel or xerogel is a silica-based material, there are many silicon alkoxides, such as tetraethylorthosilicate (TEOS), that undergo a sol-gel transition by hydrolysis and polymerization. The hydrolysis and polymerization reaction is initiated by mixing TEOS and water in a solvent, such as ethanol, at a constant temperature. 
     The polymerization continues until the mixture becomes an elastic gel. The gel contains a solvent phase which must be removed by drying to leave a low density structure. Whether an aerogel or xerogel is formed depends on how the solvent is removed. For aerogels, the drying is performed at a temperature and pressure above the solvent critical point, thus bypassing the liquid-vapor interface of the solvent. The vapor is then vented, leaving a network of material with about 95% porosity and a pore size on the order of about 10 to 50 nm (100 to 500 Å). During subsequent processing, this network is de-aired and the pores are closed by heat treatment. 
     Xerogels are similar to aerogels, but are dried by naturally evaporating solvent and water to the atmosphere. While the liquid is evaporating, the gel structure is collapsing on itself. Xerogels are generally denser than aerogels and have smaller pores, i.e., a pore size on the order of 2 to 5 nm (20 to 50 Å), yet are simpler to manufacture than aerogels. 
     Substrate  10  can be formed with the desired dimension and geometrical configuration using a suitable mold. Following formation of substrate  10 , a photopatterning and dry etch process can be used to pattern openings  16  and channels  18  in substrate  10  as depicted in FIGS. 2-4 and described below. 
     After forming substrate  10 , etch mask  12  is formed on substrate  10 . Etch mask  12  may be a photoresist layer patterned by passing ultraviolet light, or another form of radiant energy, through a reticle containing the desired pattern. The photoresist is then developed for removing either the exposed portions of resist for a positive resist or the unexposed portion for a negative resist to form a pattern of openings  14 . 
     As depicted in FIG. 3, etch mask  12  containing openings  14  is used to etch openings  16  through substrate  10 . Substrate  10  may be etched using a dry etch process, such as reactive ion etching (RIE) or plasma etching. Suitable gas etchants for etching aerogels and xerogels include fluorine species such as CF 4 , SF 4 , and SF 6 . Aerogels and xerogels are easily etched and, therefore, high aspect ratio features (i.e., high ratio of length to diameter) can be formed. Following this etch process, etch mask  12  is stripped by using suitable wet chemicals, such as a solution of sulfuric acid or hydrogen peroxide. 
     Openings  16  in substrate  10  have a generally conical shape with a diameter that decreases from a top surface  20  to a bottom surface  22  of the substrate  10 . In the assembled FED  34  shown in FIG. 6, openings  16  allow electrons emitted from emitters  40  of the FED  34  to pass through the substrate  10  to display screen  48 . In a plasma display device, the openings  16  would provide a space for generating a plasma. 
     Substrate  10  includes borders  32  along the periphery thereof Borders  32  are relatively thicker than the remainder of substrate  10  and can be formed by an etch process similar to the above-described process for forming openings  16 . Borders  32  provide a framework or support structure. 
     As illustrated in FIG. 4, channels  18  are formed in the top surface  20  of substrate  10 . Channels  18  interconnect openings  16  with one another and to borders  32  of substrate  10 . Channels  18  provide a conduit for gas removal during evacuation of the assembled FED, as explained below. Channels  18  may be formed by a photopatterning and etch process similar to the process described above for forming openings  16  in substrate  10 . 
     Instead of a dry etch process for forming openings  16  and channels  18  in substrate  10 , a laser ablation process may be used. Similar to the dry etch process, the laser ablation process uses an etch mask carrying the desired pattern on substrate  10 . A laser is then directed at substrate  10  to ablate excess substrate material and form openings  16  and channels  18 . The laser can also be preprogrammed to scribe excess material, thus eliminating the patterning step. 
     FIGS. 5A-5D illustrate another process for forming the spacers of the present invention. As shown in FIG. 5A, mask  12 ″ is placed on substrate  10 ″, which is formed in the same manner as substrate  10 , but containing a photosensitive material. Exposure to a source of radiation, such as collimated light  60 , forms latent image  16 ″ similar to opening  16  described above. 
     As depicted in FIG. 5B, mask  12 ″ is removed and latent image  16 ″ is developed by heating to a temperature ranging from about 500° to about 600° C. Next, as shown in FIG. 5C, substrate  10 ″ is flooded with uncollimated UV light  62 . No mask is required for this process, which exposes the clear areas of substrate  10 ″. 
     As illustrated in FIG. 5D, substrate  10 ″ is etched to form opening  16 ″. Depending on the substrate material, this etching may use a wet etchant, such as dilute HF acid. Following the etching, substrate  10 ″ can be further processed as required. For example, if photosensitive glass material is used as substrate  10 ″, it can be heated to a temperature of about 850° C. to convert the glass material to a ceramic. 
     Channels similar to channels  18  can then be formed in the same manner as openings  16 ′ by controlling the depth of the etch. Furthermore, borders similar to borders  32  can be formed by etching a rectangular area in substrate  10 ′ to a required depth. 
     The spacers of the present invention exhibit the following properties. The spacers are sufficiently non-conductive to prevent electrical breakdown between the cathode of the display screen and the anode of the display screen, in spite of the close spacing and voltage differential between the electrodes. The spacers also have sufficient mechanical strength so as to exhibit very little creep (slow deformation over time) and withstand atmospheric pressure sufficient to prevent the two electrodes from collapsing. Since electrons are generated at the emitters, the spacers also withstand electron bombardment without deleterious effects. Further, the spacers of the present invention withstand “bakeout” temperatures (required to create the vacuum between the base plate and display screen) of about 400° C. Moreover, the spacers are small enough so as not to visibly interfere with the display operation-if the spacer is not small enough, electrons from the emitters (being closely packed to obtain a high resolution) will be intercepted before striking the phosphor coated display face, thus degrading the brightness of the display. Additionally, the spacers of the present invention maintain the emitters and opposed display screen insulated from one another at a relatively small and uniform distance, thereby assuring the desired thinness and high resolution. 
     The spacer must also be able to exhibit dual-sorption. As used in the present invention, “dual-sorption” means that the spacer chemisorbs and physisorbs gases. Physisorption is a reversible process in which gases are absorbed on the surface of the substrate but may be released, or desorbed, at a later time. Chemisorption is a permanent process, similar to gettering, in which the gases are chemically absorbed. 
     To operate efficiently, a vacuum of about 10 −7  torr must be maintained within the FED cavity formed by the display screen and base plate. Over time, the pressure in the cavity increases from outgassing of components and from the atmosphere leaking into the cavity. The gases from the outgassing and atmospheric leakage can help or hinder the emission characteristics of the display device, depending on the gas. 
     Helpful gases include, but are not limited to, hydrogen, methane (CH 4 ), and carbon monoxide (CO). It is believed, especially in the case of hydrogen, that these gases reduce oxidation and contamination of the emitter tips, as well as maintain the phosphor luminescence. It is believed that these gases treat the emitter surface and condition the surface, improving and stabilizing the emitter performance. Oxidation and contamination of the metal of the emitter changes their work functions, often by as much as 25-50%. Harmful gases include, but are not limited to, sulfur dioxide, oxygen, water vapor, sulfur, and hydrogen sulfide (H 2 S). 
     Accordingly, the spacers of the present invention exhibit dual-sorption: they physisorb helpful gases and chemisorb harmful gases during fabrication of the FED. The spacers then desorb the helpful gases during operation of the display device. Preferably, in the case of aerogels and xerogels, the dual-sorption capability is helped by the porosity of the spacer. 
     Once substrate  10  is formed, the concentration of a material desorbing the helpful gases is increased in the substrate to obtain a high concentration of this material. A “high concentration” in the present invention means that the spacer contains a sufficient concentration of the desired material to desorb in the form of a helpful gas during operation of the FED. The concentration of the material, using hydrogen as the exemplary helpful gas, can be increased by numerous methods, including those described below. 
     In one method of increasing the concentration, hydrogen or methane is present in the ambient atmosphere during fabrication of the display device, especially the atmosphere present during the bake and seal-in period. If not already present, the gas can be injected into the processing chamber. As the hydrogen concentration increases in the ambient atmosphere, more hydrogen will be absorbed by the spacers. During operation of the display device, because the hydrogen has only been physisorbed, it will be released by the spacers. 
     As an alternative to the step of increasing the concentration of material desorbing helpful gases once the spacer is formed, a sufficiently high concentration of material desorbing helpful gases may be incorporated directly into the spacer itself during its formation. For example, when hydrogen is the helpful gas to be desorbed, the spacer could comprise a metal hydride. Other gas-releasing materials encompassed by the present invention include HSQ. 
     In another method of increasing the concentration, a layer of the desired gas-desorbing material can be coated on the spacer. For example, after the spacer is formed, the gas-desorbing material, such as HSQ, could be coated on the spacer. Alternatively, such material could be coated on the spacer material prior to patterning and etching the conventional spacer material. 
     After forming the spacer and increasing the concentration of the gas-desorbing material, the display device is assembled. As depicted in FIG. 6, the preferred FED  34  is assembled with substrate  10  (or  10 ″) functioning as a spacer. The assembled FED  34  includes base plate  36  formed with a conductive layer  38 . An array of electron-emitting emitter sites  40  is formed on the conductive layer  38 . A gate electrode structure, or grid  42 , is associated with the emitter sites  40 . The grid  42  and base plate  36  are connected to an electrical source  44  which establishes a voltage differential for initiating an electron emission from emitters  40 . Grid  42  is separated from base plate  36  by insulating layer  56 . Insulating layer  56  provides support for grid  42  and prevents the breakdown of the voltage differential applied by source  44 . Electrons  46  emitted by emitters  40  impinge on a cathodoluminescent display screen  48 . Display screen  48  includes external glass face  50 , transparent electrode  52 , and phosphors  54 . 
     In assembled FED  34 , substrate  10  is placed between base plate  36  and display screen  48 . Following assembly of FED  34 , substrate  10  functions as an interelectrode spacer. During the assembly process, openings  16  in the substrate  10  are precisely aligned with the emitters  40 . This allows a free flow of electrons from the emitter sites  40  to the display screen  48 . 
     Following assembly of FED  34 , the interior of FED  34  is evacuated to a pressure of 10 −6  Torr or less using an evacuation pump or similar apparatus. During the evacuation process, the FED  34  is typically heated to a temperature of around 400-550° C. to create a high vacuum between base plate  36  and display screen  48  of the FED  34 , thereby sealing base plate  36  and display screen  48  together. If necessary, a suitable sealing material can be provided at peripheral portions of base plate  36  and display screen  48  to aid in sealing them together. During the evacuation process, channels  18  formed in substrate  10  provide a conduit for gas removal from openings  16  and from interior of FED  34 . Channels  18  terminate at borders  32  of substrate  10  and can thus be placed in direct flow communication with the evacuation pump. 
     Emitters  40  in FED  34  are energized by applying a positive voltage potential to gate electrode  42  and conductive layer  38  operating as the cathode, thereby inducing an electric field which draws electrons from the tip of the emitter. The emitted electrons are accelerated toward transparent electrode  52 , which is positively biased by applying a larger positive voltage. Energy from the electrons attracted to the anode are transferred to particles of the phosphor coating  54 , resulting in luminescence. 
     While the preferred embodiments of the present invention have been described above, the invention defined by the appended claims is not to be limited by particular details set forth in the above description, as many apparent variations thereof are possible without departing from the spirit or scope thereof. For example, although the method of the invention has been described as forming interelectrode spacers for a FED, the skilled artisan will understand that the process and spacers described above can be used for other display devices, such as plasma displays and flat cathode ray tubes.

Technology Category: 5