Patent Publication Number: US-7223148-B2

Title: Method of fabricating tetra-polar field-emission display

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates in general to a field-emission display, and more particularly, to a method for configuring an opening of a shadow masking converging electrode of a tetra-polar field-emission display, so as to optimize the converging effect. 
     FIG. 1  shows a converging electrode of a conventional tetra-polar field-emission display. As shown, a substrate  11   a  having an anode electrode structure formed thereon is provided. The anode electrode structure has a first conductive layer  12   a  and a second conductive layer  13   a  enclosing the first conductive layer  12   a  therein. The first and second conductive layers  12   a  and  13   a  serves as the electrode layer  14   a  on which electron beams will impinge, so as to generate light. The field-emission display further includes a cathode electrode structure, which comprises a substrate  21   a , a first insulating layer  22   a  on the substrate  21   a , a gate conductive layer  23   a  on the first insulating layer  22   a , a second insulating layer  24   a  on the gate conductive layer  23   a , and a converging layer on the second insulating layer  24   a . The converging layer  25   a , the second insulating layer  24   a , the gate conductive layer  23   a , and the first insulating layer  22   a  are patterned to form a window  26   a  from which the substrate  21   a  is exposed. The area of substrate exposed by the window  26   a  is denoted by the numeral reference  27   a  as shown in  FIG. 1 . A first conductive layer  28   a  is then formed on the exposed substrate  26   a , and a second conductive layer  29   a  is formed on the first conductive layer  28   a , such that a gate electrode layer  30   a  is formed as shown. 
   The above structure is formed using a metal shadow mask, and the gate conductive layer  23   a  is formed with a specific thickness between about 50 microns to about 200 microns, which is relative thick compared to the converging electrode fabricated by photolithography or screen printing process. The metal shadow mask is advantageous on mass production, however, it has the following drawbacks in addition to the relative thick feature. 
   Firstly, the electron beam starts diffusing after being drained by the gate layer. Therefore, the thicker the converging layer is, the longer path the electron beam is to propagate through the converging electrode. As a result, a portion of the electrons is absorbed by the converging electrode, such that the current density is reduced. 
   Secondly, to avoid the loss of the electron beam, the opening of the converging electrode is designed larger than the opening of the gate layer. Thereby, the space between the apertures of the gate shadow mask is reduced, and it is difficult to implement high-resolution array. 
   Thirdly, when the opening of the converging electrode is larger than that of the gate layer, higher voltage is required for the converging electrode for converging the electron beam. 
   It is therefore a substantially need to provide a method for fabricate a field-emission display of which the absorption of electron beam by the converging electrode is reduced, which the voltage provided to the converging electrode is not increased. 
   BRIEF SUMMARY OF THE INVENTION 
   To resolve the above drawbacks, a tetra-polar field-emission display is provided. The field-emission display has a redesigned opening of a converging electrode, such that the converging effect is optimized without causing loss of the electron beam. 
   A method of fabricating a tetra-polar field-emission display is provided. A shadow mask is used to form an opening of a converging electrode, so as to improve converging effect of an electron beam propagating through the opening. An anode electrode structure and a cathode electrode structure are formed. The cathode electrode structure includes a first dielectric layer, a gate layer, a second dielectric layer and a converging layer on a substrate. The converging layer, the second dielectric layer, the gate layer and the first dielectric layer are patterned to form a window exposing the substrate. A cathode electrode layer is formed on the substrate exposed by the window. The converging layer is patterned into n converging electrode having a top surface, a bottom surface, and a pair of side surfaces. The side surfaces are so configured that the window is gradually reduced from the top surface towards a turning point between the top surface and the bottom surface, and then gradually enlarged from the turning point towards the bottom surface. 

   
     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  shows a conventional tetra-polar field-emission display; 
       FIG. 2  shows an embodiment of a tetra-polar field-emission display; 
       FIG. 3  shows a first embodiment of the emission of the electron beam through the shadow mask converging electrode; 
       FIG. 4  shows a second embodiment of the emission of the electron beam through the shadow mask converging electrode; 
       FIG. 5  shows a third embodiment of the emission of the electron beam through the shadow mask converging electrode; 
       FIG. 6  shows a fourth embodiment of the emission of the electron beam through the shadow mask converging electrode; 
       FIG. 7  shows a fifth embodiment of the emission of the electron beam through the shadow mask converging electrode; and 
       FIG. 8  shows another embodiment of a tetra-polar field-emission display. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 2 , a tetra-polar field-emission display as provided comprises an anode electrode structure  1  and a cathode electrode structure  2  aligned with the anode electrode structure  1 . In the tetra-polar field-emission display, the converging electrode is fabricated from shadow mask process, such that the converging effect can be optimized. 
   The anode electrode structure  1  includes a substrate  11 , a first conductive layer  12  formed on the substrate  11 , and a second conductive layer  13  formed on the first conductive layer  12 . The first conductive layer  12  includes an indium tin oxide (ITO) layer, and the second conductive layer  13  includes a phosphor layer, for example. Therefore, an electron beam impinges on the anode electrode layer  14  comprising the first and second conductive layers  12  and  13  can generate light therefrom. 
   The cathode electrode structure  2  includes a substrate  21 , a first insulating or dielectric layer  22  formed on the substrate  21 , a gate conductive layer  23  formed on the first dielectric layer  22 , a second dielectric layer  24  formed on the gate conductive layer  23 , and a converging (focusing) layer  25  formed on the second dielectric layer  24 . The substrate  21  is fabricated from glass material, for example. The converging layer  25 , the second dielectric layer  24 , the gate conductive layer  23 , and the first dielectric layer  22  are patterned to form a window  26  which exposes a portion of the substrate  21 . The area of the window  26  is denoted as the reference number  27  as shown in  FIG. 2 . A first conductive layer  28  such as a silver paste is formed on the exposed substrate e 21  in the area  27 . A second conductive layer  29  is then formed on the first conductive layer  28 . The second conductive layer  29  is preferably formed of carbon nanotube by spray coating or photolithography. The first and second conductive layers  28  and  29  form a cathode electrode layer  30  of the cathode electrode structure  2 . 
   The converging layer  25  is patterned to form a pair of lateral protrusions  31  and  31 ′ extending towards the window  26 . The protrusion  31  has two slanted side surfaces  32  and  33  extending from the top and bottom corners  31   a  and  31   c  to merge at the tip  31   b , and the protrusion  31 ′ has two slanted side surfaces  32 ′ and  33 ′ extending from the top and bottom corners  31   a ′ and  31   c ′ to merge at the tip  31   b ′. In this embodiment, the protrusions  31  an  31 ′ are in the form of triangles having bottom sides extending between the top and bottom corners  31   a  and  31   c  and  31   a ′ and  31   c ′, respectively, and top angles  31   b  and  31   b ′ pointing at each other above the area  27  within the window  26 . That is, the converging layer  25  is patterned into a plurality of converging electrodes in the form of a hexagon, which comprising two side triangle portions extending towards the windows  26 . As shown in  FIG. 2 , the thickness of the converging layer  25  is a+b, and the width of the window  26  is narrowed by  2   c  at the tips  31   b  and  31   b ′, which is the total height of the protrusions  31  and  31 ′. 
   Therefore, the dimension of the window  26  is gradually reduced from the top corners  31   a  and  31   a ′ towards the tips,  31   b  and  31   b ′. The window  26  is then gradually enlarged from the turning points, that is, the tips  31   b  and  31   b ′ of the protrusions  31  and  31 ′ towards the bottom corners  31   c  and  31   c ′. The width of the window  26  at the top corners  31   a  and  31   a ′ is d, the width of the window  26  at the tips  31   b  and  31   b ′ is e, and the width of the window  26  at the bottom corners  31   c  and  31   c ′ is f. The distance between the tip  31   b  and the edge of the second dielectric layer  24  is h. The width of the top surface  251  of each converging electrode is i, and the width of the bottom surface  252  of each converging electrode is j. The width of the gate conductive layer  23  is k. 
   The following conditions provide the optimum converging effect: 
   1. e=g, such that the opening of the converging electrodes equals to the diameter of the electron emission source, that is, the second conductive layer  29  of the cathode electrode layer; 
   2. d&gt;e, f&gt;e, d≧f; and 
   3. a:b=0.8 to 1.2. 
   The condition of e and f larger than e can be achieved by etching, such that the opening area of the converging electrode is minimized, and the electron loss is minimized. By having f&gt;e, the equi-potential lines provide properly converging force to the electron beam. By having d&gt;e, the electron beam is diverged. However, the reduction of the local dimension avoids loss of electrons of the electron beam. 
   The ratio of a and b is to adjust the focus. 
   Empirical data shows that, as shown in  FIG. 3 , when b=0 and a specific voltage is provided to the converging electrode, diverging effect occurs and the converging effect for the electron beam is poor. Thereby, a larger converging voltage is required. However, when an excessive converging voltage is applied, the performance of the gate is replaced by the converging electrode. 
   Referring to  FIG. 4 , the electron beam of the second type of opening of shadow mask opening converging electrode is illustrated. A specific voltage is applied to the converging electrode. When a:b=0.2, converging effect can be obtained. However, the converging effect is obtained with relatively larger converging voltage, such that color effusion occurs. 
   Referring to  FIG. 5 , the electron beam propagating through another type of opening is illustrated. In  FIG. 5 , a:b=0.2 to 5. When a specific voltage is applied, the electron beam has a uniform distribution, such that the converging voltage can be controlled for converging the electron beam. 
   In  FIG. 6 , a:b=8. when a specific voltage is applied, the electron beam is excessively converged, such that the impinging area of the phosphor is too small. Although a small voltage is required for achieving converging effect, the voltage may be too small to be adjusted. 
   In  FIG. 7 , a=0, such that pre-focus effect occurs to the electron beam, and the donut distribution of impinging area occurs. 
   In  FIG. 8 , the bottom surface of the converging electrode is larger than the gate conductive layer  23 , such that the gate conductive layer  23  can be formed on the converging layer  25 . 
   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.