Abstract:
A faceplate in a flat panel display has attachment sites made with a method that includes steps of mixing frit and photoresist to form a mixture, applying the mixture to the substrate, softbaking the substrate and mixture, and exposing and developing the resist to define adhesion sites. Spacers are then attached to the faceplate at the adhesion sites.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of Ser. No. 08/764,485, filed now Dec. 12, 1996, U.S. Pat. No. 5,984,746. 
    
    
     STATEMENT OF GOVERNMENT RIGHTS 
     This invention was made with government support under contract No. DABT63-93-C0025 awarded by Advanced Research Projects Agency (ARPA). The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to displays, and more particularly to processes for creating spacer attachment sites for a field emission display (FED). 
     Referring to FIG. 1, in a typical FED (a type of flat panel display), a backplate (cathode)  21  has a substrate  10 , such as glass, on which conductive layers  12 , such as doped polycrystalline silicon or aluminum, are formed. Conical emitters  13  are formed on conductive layers  12 . A dielectric layer  14  surrounds emitters  13 , and a conductive extraction grid  15  is formed over dielectric layer  14 . When a voltage differential from a power source  20  is applied between conductive layers  12  and grid  15 , electrons  17  bombard pixels  22  of a phosphor coated faceplate (anode)  24 . Faceplate  24  has a transparent dielectric layer  16 , preferably glass, a transparent conductive layer  26 , preferably indium tin oxide (ITO), a black matrix grille (not shown) formed over conductive layer  26  to define regions, and phosphor coating over the regions defined by the grille. 
     Backplate  21  and faceplate  24  are spaced very close together in a vacuum sealed package. In operation, there is a potential difference on the order of 1000 volts between conductive layers  12  and  26 . Electrical breakdown must be prevented in the packaged FED, while the spacing between the plates must be maintained at a desired thinness for high image resolution. 
     A small area display, such as one inch (2.5 cm) diagonal, may not require additional supports or spacers between faceplate  24  and backplate  21  because glass substrate  16  in faceplate  24  can support the atmospheric load. For a larger display area, several tons of atmospheric force are exerted on the faceplate, thus making spacers important if the faceplate is to be thin and lightweight. 
     SUMMARY OF THE INVENTION 
     The present invention includes methods of making spacers in displays and particularly in field emission displays (FEDs). One method includes steps of mixing frit and photoresist together to form a mixture, applying the mixture to a surface of a portion of a faceplate or backplate, removing portions of the mixture to form adhesion sites at desired locations, and attaching spacers at the adhesion sites. In preferred embodiments, the mixture has about 2% frit and 98% photoresist and is provided on a grille and a transparent conductive layer of a faceplate, and is then removed except over portions of the grille. 
     With the method of the present invention, precise adhesion sites can be conveniently formed. Other features and advantages will become apparent from the following detailed description, drawings, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a known FED. 
     FIG. 2 is a cross-sectional view of a faceplate covered with a layer of frit and photoresist. 
     FIG. 3 is a cross-sectional view of the faceplate of FIG. 2 after the layer has been selectively etched and phosphor has been deposited. 
     FIG. 4 is a plan view of the faceplate of FIG.  3 . 
     FIG. 5 is a cross-sectional view of the faceplate of FIG. 3 with spacers attached. 
     FIG. 6 is a plan view illustrating a bundle of spacers over an adhesion site on a faceplate. 
    
    
     DETAILED DESCRIPTION 
     According to the present invention, frit (a glass powder) and a compatible photoresist are mixed together to form a mixture. Conventional frits, such as Corning 7572 or 7575, and known positive and negative photoresists, such as OCG SC negative photoresists, can be used. For Corning 7572 or Corning 7575, a resist such as OCG SC100 or a polyvinyl alcohol (PVA) based resist could be used. In an exemplary mixture of Corning 7572 and OCG SC100, the mixture is preferably about 1-5% by weight of frit and about 95-99% by weight of resist, and more preferably about 2% by weight of frit and about 98% by weight of resist. The resist and frit are mixed with a low shear technique until a substantially homogeneous mixture without bubbles or froth is obtained. For Corning 7572 and an OCG SC negative resist, the combination can be mixed for about 30-60 minutes. 
     Referring to FIG. 2, mixture  30  of frit and photoresist is applied with an even thickness to a faceplate  32  by using known techniques, such as spin coating or spraying. Faceplate  32  has a transparent dielectric layer  34 , preferably glass, and a transparent conductive layer  36 , such as tin oxide or indium tin oxide (ITO), coating dielectric layer  34 . Over conductive layer  36  is a patterned grille  38  made of an opaque, non-reflective material, such as cobalt oxide, manganese oxide, or diaqueous graphite (DAG). Grille  38  defines regions  40  where phosphor particles will later be coated. Mixture  30  thus covers grille  38  and regions  40  (which are not covered by grille  38 ). After applying the mixture to faceplate  32 , the assembly of faceplate  32  and mixture  30  is heated (softbaked) to cure the resist. If the mixture uses OCG SC negative resist, the substrate is heated to about 80-100° C. for a period of about 5-20 minutes. 
     Referring to FIG. 3, the resist is then exposed and developed to create desired regions of the mixture of frit and cured photoresist that serve as adhesion sites  42 . Exposure is performed according to known techniques, such as using an aligner to align a mask with the assembly and then exposing the masked assembly with known methods, such as projection lithography or contact printing. E-beam lithography could also be used. After exposure, the mixture is developed using an appropriate developing solvent, such as WNRD. The mixture can be developed with a dip-develop technique or a spray-develop technique. For the dip-develop technique, faceplate  32  with mixture  30  is immersed in developer for about two minutes with gentle agitation, and is then removed and put into a second tank with a rinse for about 30 seconds. It is then removed from the second tank and allowed to air dry, or it can be dried with forced gases and/or gentle heat. The developing and rinse times an vary depending on the thickness of the mixture, the softbake process, and ther parameters. The developing typically takes about 1.5 to 3 minutes, and he rinse lasts for about 30 seconds. 
     These steps produce a well defined, precise pattern of sites  42  with frit ixed with cured photoresist. The photoresist thus serves to bind the frit to the underlying faceplate. As shown in exemplary FIGS. 3-4, sites  42  are formed at desired alternating intersections of rows and columns of grille  38 . The sites could be formed at all intersections or at fewer intersections, or on portions of grille  38  between intersections. The number of adhesion sites with spacers will depend on the strength of the spacers and the size of the display. 
     After the frit mixed with cured photoresist is formed on the substrate, a glazing step may be performed to help the frit stick together, and to burn off organics in the mixture. This step is typically performed at about 400-450° C., but the temperature could be different depending on the frit used. 
     Referring to FIGS. 5 and 6, spacers  46  are then attached to faceplate  32  with the frit serving as the adhesive. One way to attach spacers is to provide glass spacers in bundles with binder fibers as described in detail in U.S. Pat. No. 5,486,126, and in application Ser. No. 08/528,761, both of which are expressly incorporated herein by reference for all purposes. Large numbers of spacers  46  are formed in bundles  50  and clamped with uniform pressure to the faceplate at adhesion sites  42  at the intersection of rows and columns of grille  38 . Bundles  50  and faceplate  32  are then heated sufficiently to soften the frit. When cooled, some spacers  46  in bundle  50  are firmly attached to grille  38  at sites  42 , and thus extend perpendicularly away from the faceplate. The spacers can then be further processed, e.g., with a planarization technique, such as chemical-mechanical planarization (CMP). 
     The faceplate with spacers is then assembled with the backplate/cathode in a vacuum-sealed package in a generally known manner to produce a display, such as a display similar in principle to that in FIG.  1 . The spacers extend to and rest on the extraction grid of the cathode, but preferably are not held there with adhesive; rather, the pressure differential holds the spacers in place. 
     Having described certain processes according to the present invention, it should be apparent that changes can be made without departing from the scope of the invention as defined by the appended claims. The mixture can also be provided to a backplate, preferably after conductive layers, a silicon layer, an oxide, and a conductive grid layer are formed, and prior to etching to form the emitter cones. The resulting adhesion sites are preferably on the conductive extraction grid. The faceplate need not have a matrix grille, and if it does, spacers can be provided before or after the grille is formed. While a devitreous frit is preferred for the mixture, a vitreous frit can be used.