Patent Publication Number: US-2006008578-A1

Title: Method of fabricating electrode structure of field-emission display

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates in general to a method of fabricating an electrode structure of a field-emission display, and more particularly, to a method of polishing an electrode layer made of low-cost silver ink, by which the planarity of the electrode layer is improved.  
      The conventional method for forming the electrode layer and the electron emission layer of a cathode of a field-emission display typically uses thin-film or thick-film technique. The thin-film technique normally provides higher planarity and precision. However, it is more costly compared to the thick-film technique which uses low-cost process such as screen printing or thick-film photolithography for fabricating partial structure of the field-emission display as disclosed in the Taiwanese Patent No. 502395 and 511108, for example.  
      In the example of forming the electrode layers of the cathode of a field-emission display by screen printing or thin-film photolithography, silver ink is printed and patterned, followed by printing ink containing carbon nanotube for forming an electron emission source. Alternatively, the electron emission source can be formed by spray, photolithography, electrophoresis or other electrochemical process.  
      The above thin-film photolithography including sputtering or evaporation for forming an electrode layer is very costly. Although a great planarity is obtained, the thickness of the film is typically limited to tens or hundreds of nanometers. Under a high voltage, the electrode often flares or broke down. The thick-film technique provides a thickness of the electrode layer up to about 1 micron. The hardness and breakdown voltage also meet with the circuit requirements. Therefore, thick-film technique has been commonly used for forming the electrode layers of the field-emission display.  
      Although screen printing can greatly reduce fabrication cost, such process is restricted to reticulation, knots and emulsion to result in non-uniform planarity. The accumulated planarity error of the stack of the electrode layers can be more than 5 microns. Microscopically, the photolithography process is also limited to the specification and cost of the material. For example, the silver ink typically used for forming the electrode layer has a grain size more than one micron, which consequently cause a planarity error over one micron.  
      The insufficient planarity uniformity of the electrode layer does not only result in an uneven electron-emission source, but also case light scattering in exposure step of the subsequent photolithography process. The precision of the photoresist layer formed by the photolithography process is thus greatly degraded.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention uses surface polishing technique following formation of an electrode layer of a field-emission display fabricated from low-cost ink material by thick-film technique, such that the planarity of the electrode layer is enhanced, and the subsequent process can be performed with higher precision.  
      As provided, screen-printing technique or a thick-film technique is used to apply low-cost silver ink on a substrate to form an electrode layer. The silver ink includes particles having diameters ranged between 0.1 microns and 10 microns. By a sintering process, the particles of the silver ink are converted into crystals with diameters between one micron and 10 micron. The electrode layer made of the silver ink is the polished to control the surface planarity error under 0.1 microns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above objects and advantages of the present invention will be become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIG. 1  is process flow showing the method of fabricating an electrode layer of a field-emission display;  
       FIG. 2  shows the electrode layer;  
       FIG. 3  shows the polishing process performed on the electrode layer; and  
       FIG. 4  illustrates the polished electrode layer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to  FIGS. 1 and 2 , as provided, the method for fabricating an electrode structure of a field-emission display includes polishing the electrode layer to enhance the planarity thereof.  
      To fabricate the electrode structure, a substrate  1  such as a glass is provided.  
      An electrode layer  11  is formed on the substrate  1  by screen-printing or photolithography  2 . A low-cost silver ink  3  is selected as the material for forming the electrode layer  11 . The silver ink includes particles of which the diameters range from about 0.1 microns to about 10 microns, for example. It will be appreciated that particles in other sizes may also exist in the silver ink. Preferably, the ink  3  includes silver particles and glass particles having diameters ranging between 2 microns and 6 microns.  
      When the silver ink  3  is patterned to form the electrode layer  11 , a sintering process  4  is performed at a predetermined temperature. In this embodiment, the predetermined temperature is about 400° C. After the sintering process, the particles of the electrode layer  11  are converted to crystal grains having diameters between about one micron and about 10 microns. Preferably, the diameters range from 2 microns to 3 microns.  
      A polishing step  5  is then performed on the electrode layer  11 , such that the surface planarity error can be controlled under 0.1 microns.  
      After the polishing step, the electrode is rinsed by water  6 , baked  7 , and subjected to a high-pressure air  8  to remove any unwanted residual medium or particles thereon. The cathode structure of the field-emission display is thus formed.  
      Referring to  FIGS. 3 and 4 , when the electrode structure  10  is formed, a polishing member  201  of a polisher  20  is used to perform polishing with a high rotation speed of about 1000 rpm. Meanwhile, polishing medium  301  is sprayed on the first and second electrode layers  11  and  12  via a spray tool  30 . The polishing medium  301  includes slurry made of hard metal oxide particles suspension. The metal oxide includes aluminum oxide, zirconium oxide, manganese oxide, or selenium oxide. In this embodiment, solution of selenium oxide suspension is used as the polishing medium. The diameter of the selenium oxide particles is under 1 micron. Therefore, the planarity error can be controlled under 0.1 microns.  
      The polishing step provides a planarized surface of the electrode layer  11 , such that when a photoresist layer is formed by photolithography subsequently, scatter of exposure light is not caused by uneven surface of the electrode layer  11 . Therefore, a high precision can be obtained.  
      While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those of ordinary skill in the art the various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.