Abstract:
A method for forming a cathodoluminescent screen by forming cathodoluminescent films on the inner surface of screen panel for a field emission display by a screen printing, a spray, or an electrodeposition process. The field emission display cathodoluminescent particles for improving a luminescent emission efficiency, wherein the improved cathodoluminescent particles are formed by coating a uniform phosphor material on the surfaces of cathodoluminescent particles by an atomic layer epitaxy.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method for implementing a microencapsulation of phosphor particles or transparent conducting particles using a phosphor material to improve a luminous efficiency of a cathodoluminescent and a method for forming a cathodoluminescent screen using the same for a field emission display, and more particularly to a method for forming a cathodoluminescent screen provided with uniform cathodoluminescent phosphor-coated particles for a field emission display by a method such as an electrodeposition, a screen printing, or a spray by using the phosphor-coated particles.  
           [0003]    2. Description of the Related Art  
           [0004]    Phosphor particles are used in a variety of applications such as a flat panel display and decoration, cathode ray tube, and fluorescent lighting fixture. Luminescence or light emission by phosphor-coated particles may be stimulated by application of heat, light, high energy radiation, or electric fields.  
           [0005]    It has been recognized that various improvements in the performance of phosphors can be obtained if the phosphor material is coated with a protective film or pigment. Numerous attempts have been made to coat the outer surfaces of individual particles with a protective coating material.  
           [0006]    U.S. Pat. No. 4,508,760 discloses an encapsulation technique of phosphor particles by a vacuum deposition using a certain polymer.  
           [0007]    U.S. Pat. No. 4,515,827 is achieved by disclosing rotating phosphor particles coated in a vacuum chamber while vapor-depositing the color modifying material on the rotating particles.  
           [0008]    U.S. Pat. No. 4,585,673 discloses the formation of a continuous protective coating on phosphor particles by gas-phase chemical vapor deposition while the phosphor particles are suspended within a fluidized bed.  
           [0009]    U.S. Pat. No. 4,515,827 discloses encapsulated cathodoluminescent phosphor particles by a vapor phase hydrolysis reaction of oxide precursor material.  
           [0010]    U.S. Pat. No. 5,156,885 discloses encapsulated phosphor particles by a low temperature vapor phase hydrolysis reactions and deposition process.  
           [0011]    In the above coating techniques, the movement, rotation, or vibration of the particles was necessary for an uniform film growth because it was very hard to deposit uniform, continuous, and stoichiometric thin film or particles.  
           [0012]    Also, the cathodoluminescent films of the cathodoluminescent screen of a cathode-ray tube, such as a color cathode-ray tube or a monochromatic cathode-ray tube, are formed principally by a slurry process, and the cathodoluminescent films of the cathodoluminescent screen of some cathode-ray tubes are formed by a screens printing, a spray, or an electrodeposition process.  
           [0013]    Generally, ZnO, ZnGa 2 O 4 :Mn, ZnGa O   4 :Eu, YAG:Tb, Y 2 SiO 5 :Ce, Y 2 O 3 :Eu, Y 2 O 2 S:Tb, Gd 2 O 2 S:Tb, Srs:Ce, SrTe:Ce, SrS—Sc 2 S 3 , ZnS:Ag, ZnS:Pr, SrGa 2 S 4 , ZnCdS:Cu, Al are used for cathodoluminescent materials of the cathodoluminescent screen of a field emission display.  
           [0014]    However, in the conventional cathodoluminescent particles, the surfaces of the particles are continuously polished and are exposed to a dilution liquid, so that the surface of the same may be changed, thus forming a dead layer and decreasing the characteristic of the cathodoluminescent.  
           [0015]    Therefore, in the industry, the technique for reducing the dead layer formed on the surface of the cathodoluminescent particle becomes an important technique.  
           [0016]    The conventional invention of the thick film type will be explained with reference to FIGS. 1A and 1B.  
           [0017]    [0017]FIGS. 1A and 1B are cross-sectional views showing conventional phosphor particles and a cathodoluminescent screen using the same for a field emission display, respectively.  
           [0018]    As shown in FIG. 1B, a transparent conduction layer  2  is formed on the transparent substrate  1 . A cathodoluminescent films  3   a  composed of phosphor particles  3  of the FIG. 1 on the transparent conduction layer  2  is formed by phosphor-coated particles  3  of fine powder types having a diameter of less than 10 micron in average particle size by a screen printing, a spray, or an electrodeposition process.  
           [0019]    [0019]FIG. 2 is a cross-sectional schematic drawing of a field emission display showing an electron emission tip, or field emission cathode, surrounded by the self-aligned gate structures.  
           [0020]    Referring to FIG. 2, the electron emission tip  4  is integral with the single crystal semiconductor substrate  5 , and serves as a cathode conductor. Gate  8  serves as a low potential anode or grid structure for its respective cathode  4 . A dielectric insulating layer  7  is deposited on the conductive cathode layer  6 . The insulator  7  also has an opening at the field emission site location.  
           [0021]    A field emission display employing a cold cathode is depicted. The substrate  5  can be comprised of glass, for example, or any of a variety of other suitable materials, In the preferred embodiment, a single crystal silicon layer serves as a substrate  5  onto which a conductive material layer  6 , such as doped polycrystalline silicon has been deposited. At a field emission site location, a conical micro-cathode  4  has been constructed on the top of the substrate  5 . Surrounding the micro-cathode  4 , is a low potential anode gate structure  8 . When a voltage is applied between the cathode  4  and the gate  8 , a stream of electrons  9  is emitted toward a phosphor-coated screen  1 . Screen  1  is an anode and includes cathodoluminescent material  3  on its surface. The display faceplate cover with the included cathodoluminescent layer is distally disposed with respect to the electron emission structure, the purpose of the electron emitting structure being to emit electrons, at least some of which will impinge upon the cathodoluminescent material, such that at least some of the energy of the emitted electrons is converted to photon energy as visible light. The visible light is transmitted through the transparent conduction layer  2  and the transparent substrate  1  of the display to the viewer.  
           [0022]    The purity and intensity of light is determined by composition, uniformity and surface state of the phosphor particles. Luminous efficiency of the cathodoluminescent films formed by the thick film type is particularly determined by uniformity of the particle size and surface state of the particle. Also, the photoemission of the cathodoluminescent layer for a field emission display is emitted by electrons at the thinnest surface within several score nanometer.  
           [0023]    When a conventional phosphor particles of the FIG. 1 does powder type, the particle surface is formed a dead (non-luminescent) layer by continuously polishing or by opening in the delude with destructive state. Therefore, such conventional invention of the thick film type has been reduced luminous efficiency.  
           [0024]    Also, if acceleration voltage by the electron-ray is low, a photoemission region is thinly formed, the thinnest surface of the cathodoluminescent layer is emitted. Therefore, the surface state of the phosphor particles have influence on the luminous efficiency.  
           [0025]    Disadvantages associated with these known methods are eliminated by the method of the present invention by which a thin film coating having a desired substantially uniform thickness is formed by an atomic layer epitaxy on the outer surface of the phosphor particles or the transparent conducting particles.  
         SUMMARY OF THE INVENTION  
         [0026]    Accordingly, an object of the present invention is to provide a method of providing phosphor-coated particles of the transparent conducting particles by an atomic layer epitaxy to improve the luminous efficiency of the cathodoluminescent films for a field emission display and which does this while avoiding the disadvantages of the prior art.  
           [0027]    It is another object of the present invention to provide a method for forming a cathodoluminescent screen by forming cathodoluminescent films on the inner surface of screen panel for a field emission display by a method such as a screen printing, a spray and an electrodeposition process.  
           [0028]    To achieve the above objects, there is provided field emission display cathodoluminescent particles for improving a luminescent emission efficiency according to a first embodiment of the present invention, wherein the improved cathodoluminescent particles are formed by coating a uniform cathodoluminescent material on the surfaces of cathodoluminescent particles by an atomic layer epitaxy.  
           [0029]    To achieve the above objects, there is provided a field emission display cathodoluminescent layer forming method including a transparent substrate and a transparent electrode layer formed on the transparent substrate according to a second embodiment of the present invention, wherein the improved method is directed to forming cathodoluminescent particles, on which a uniform cathodoluminescent material is coated by an atomic layer epitaxy, on the surface of a field emission display unit by an electrophoretic deposition method.  
           [0030]    To achieve the above objects, there is provided a field emission display cathodoluminescent layer forming method including a transparent substrate and a transparent electrode layer formed on the transparent substrate according to a third embodiment of the present invention, wherein the improved method is directed to forming cathodoluminescent particles, on which a uniform phosphor material is coated by an atomic layer epitaxy, on the surface of a field emission display unit by a screen printing method.  
           [0031]    To achieve the above objects, there is provided a field emission display cathodoluminescent layer forming method including a transparent substrate and a transparent electrode layer formed on the transparent substrate according to a fourth embodiment of the present invention, wherein the improved method is directed to forming cathodoluminescent particles, on which a uniform phosphor material is coated by an atomic layer epitaxy, on the surface of a field emission display unit by a spray method.  
           [0032]    To achieve the above objects, there is provided field emission display cathodoluminescent particles for improving a cathodoluminescent efficient according to a fifth embodiment of the present invention, wherein the improved field emission display cathodoluminescent particles are formed by coating a uniform cathodoluminescent material on the surfaces of transparent conducting particles by an atomic layer epitaxy.  
           [0033]    To achieve the above objects, there is provided a field emission display cathodoluminescent layer forming method including a transparent substrate and a transparent electrode layer formed on the transparent substrate according to a sixth embodiment of the present invention, wherein the improved method is directed to forming cathodoluminescent particles, which are formed by coating an uniform cathodoluminescent material on the surface of transparent conducting particles by an atomic layer epitaxy, on a secreen of the field emission display unit by an electrophoretic deposition method.  
           [0034]    To achieve the above objects, there is provided a field emission display cathodoluminescent layer forming method including a transparent substrate and a transparent electrode layer formed on the transparent substrate according to a seventh embodiment of the present invention, wherein the improved method is directed to forming cathodoluminescent particles, which are formed by coating an uniform cathodoluminescent material on the surface of transparent conducting particles by an atomic layer epitaxy, on the surface of a field emission display unit by a screen printing method.  
           [0035]    To achieve the above objects, there is provided a field emission display cathodoluminescent layer forming method including a transparent substrate and a transparent electrode layer formed on the transparent substrate according to en eighth embodiment of the present invention, wherein the improved method is directed to forming cathodoluminescent particles, which are formed by coating an uniform cathodoluminescent material on the surface of transparent conducting particles by an atomic layer epitaxy, on the surface of a field emission display unit by a spray method.  
           [0036]    ALE is a chemical thin film deposition method based on saturated surface reactions. The unique feature of ALE is that reactant vapors—elements or compounds—are pulsed onto the substrate alternately, one at a time. Between the reactant pulses the reactor is either purged with an inert gas or evacuated. With a proper adjustment of the experiment conditions, i.e. substrate temperature, reactant doses and lengths of pulse and purge sequence, and an exact chemisorbed monolayer of the first reactant is retained on the substrate after the purge sequence. This chemisorbed monolayer reacts subsequently with the other precursor dosed onto the substrate resulting in a solid film and, if compounds are exploited as precursors, gaseous byproducts. By repeating this deposition cycle the film is grown layer-by-layer. However, due to steric hindrances of bulky precursor molecules and surface reconstructions, the surface density of chemisorbed species remains often too low for a formation of a complete crystal layer of the film during one cycle. Nevertheless, the film thickness is still only a function of the number of deposition cycles repeated. As a result, the growth is said to be self-controlled or self-limited.  
           [0037]    Also, in accordance with a preferred embodiment of the present invention, there is provided a method of forming a cathodoluminescent screen for a field emission display by depositing particles of a cathodoluminescent material or cathodoluminescent materials on the inner surface of a screen panel by an elctrodeposition process.  
           [0038]    A method of forming a cathodoluminescent film by an electrodeposition process in accordance with the present invention forms on the inner surface of a screen panel for a field emission display comprising: immersing the screen panel in an electrodeposition solution in which particles of a cathodoluminescent material is dispersed, applying a negative voltage on the transparent conduction layer and a positive voltage a counter electrode immersed opposite to each other in an electrodeposition solution prepared by dispersing particles of a cathodoluminescent material in an electrolyte for positively or negatively charging the particles of the cathodoluminescent material, a negative voltage and a positive voltage are applied respectively to the transparent electrode of the screen panel and the counter electrode when the cathodoluminescent material is positively charged to deposit the cathodoluminescent material over the surface of the electrode, Washing and drying the screen panel after the cathodoluminescent film has been formed.  
           [0039]    Also, a method of forming a cathodoluminescent film of the cathodoluminescent screen by a screen printing or a spray deposition process in accordance with the present invention for a field emission display comprises: confusing a paste or a solvent and a phosphor-coated particles, forming on the inner surface of a screen panel by the screen printing or the spray method. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]    The above and other objects, features and advantages of the present invention will become apparent from the following description of embodiments with reference to the accompanying drawings, in which:  
         [0041]    [0041]FIGS. 1A and 1B are cross-sectional view showing conventional phosphor particles and a cathodoluminescent screen using the same for a field emission display, respectively.  
         [0042]    [0042]FIG. 2 is a cross-sectional schematic drawing of a field emission display.  
         [0043]    [0043]FIGS. 3A and 3B are cross-sectional view showing phosphor-coated particles and transparent conducting particles by an atomic layer epitaxy and a cathodoluminescent screen using the same for a field emission display respectively, in a preferred embodiment according to the present invention.  
         [0044]    [0044]FIG. 4A is a cross-sectional view showing a method of forming a cathodoluminescent screen for a field emission display by an electrodeposition process in accordance with an embodiment of the present invention.  
         [0045]    [0045]FIG. 4B is a cross-sectional view showing a method of forming the slurry of cathodoluminescent phosphor particles for a screen for a field emission display by a screen printing or a spray process in accordance with an embodiment of the present invention.  
         [0046]    [0046]FIG. 5 is a schematic diagram of ALE process used in the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]    Embodiments of the present invention will be explained with reference to the drawings.  
         [0048]    Automic layer epitaxy (ALE) is a chemical thin film deposition technique based on saturated surface reactions. The unique feature of ALE is that reactant gas or reactant vapors carried with inert gas—elements or compounds—are pulsed onto the substrate alternately, one at a time. Between the reactant pulses the reactor is purged with an inert gas and evacuated. The schematic diagram of A-LE process of reactants A and B is shown in FIG. 5. The pulsing sequence is source A—purge gas—source B—purge gas. With a proper adjustment of the experimental conditions, i.e., reactor temperature, reactant doses and lenghts of pulses and purge sequence, and an exactly chemisorbed monolayer of the first reactant A is retained on the surface after the purge sequence. This chemisorbed monolayer reacts subsequently with the other precursor B dosed onto the surface resulting in a solid film and, if compounds are exploited as precursors, gaseous byproducts. By repeating this deposition cycle the film is grown layer-by-layer.  
         [0049]    For the embodiment of the present invention, as shown in FIG. 5, the reactor with filled with the phosphor particles or transparent conducting particles and supported by a supporting materials. The supporting material in the leading part of the reactor can distribute the gas flow evenly. The supporting material in the trailing part of the reactor supports the particles. The supporting materials should not disturb the flow of reactant gases and purge gases.  
         [0050]    Reffering to FIGS.  3 A, the cathodoluminescent (or phosphor-coated) particles  30  are formed by coating a uniform cathodoluminescent material  30   b  on the surface of phosphor particles  30   a  by using the above-mentioned ALE.  
         [0051]    In the present invention, the ALE for forming the cathodoluminescent films  30   b  on the particles  30   a  is carried out using at least one of halide precursors of Al, Ga, Sr, Ca, Si, tansition metal elements and rare earth elements including Zn, Y, Gd, Te, Sc, Cd, and Ta. Also, the ALE for forming the cathodoluminescent films  30   b  on the particles  30   a  is carried out using at least one of organometallic precursors of Al, Ga, Sr, Ca, Si, tansition metal elements and rare earth elements including Zn, Y, Gd, Te, Sc, Cd, and Ta.  
         [0052]    Preferably, the materials of the cathodoluminescent or phosphor particles  30   a  are ZnO, ZnGa 2 O 4 , Y 2 SiO 5 , Y 2 O 3 , Y 2 O 3 S, Y 3 Al 5 O 12 , Gd 2 O 2 S, Ga 2 O 3 , SrS, SrTe, SrS—SC 2 S 3 , ZnS, SrGa 2 S 4 , ZnCdS, Ta 2 Zn 3 O 8 , and mixtures thereof. The meterial is doped with transition metal or rare earth elements as the luminescent center.  
         [0053]    Also, the cathodoluminescent coating materials  30   b  are ZnO, ZnGa 2 O 4 , Y 2 SiO 5 , Y 2 O 3 , Y 2 O 3 , Y 2 O 3 S, Y 3 Al 5 O 12 , Gd 2 O 2 S, Ga 2 O 3 , SrS, SrTe, SrS—Sc 2 S 3 , ZnS, SrGa 2 S 4 , ZnCdS, Ta 2 Zn 3 O 8 , and mixtures thereof or multilayers thereof, doped with transition metal or rare earth elements as the luminescent center. More preferably, the cathodoluminescent coating materials  30   b  can used same as the materials of cathodoluminescent particles  30   a . The diameter of chathodoluminescent particles  30   a  is raging from 0.51 μm to 20 μm and the thickness of cathodoluminescent coating film  30   b  is in the range of 1-100 nm.  
         [0054]    Meanwhile, the cathodoluminescent particles can be formed by coating a uniform cathodoluminescent material on the surface of transparent conducting particles (not shown) by automic layer epitaxy. At this time, the material of the transparent conducting particles is used In-doped SnO 2 , Al-doped ZnO, Sb-doped SnO 2 , conducting polymer, or mixtures thereof. Also, the diameter of the transparent conducting particle is ranging from 0.5 μm to 20 μm.  
         [0055]    Next, as shown in FIG. 3B, the cathodoluminescent layer  30   a  composed by the phosphor-coated particles  30  is formed on the transparent conduction layer  20  of the transparent substrate  10   
         [0056]    [0056]FIG. 4A is a cross sectional view showing a method of forming a cathodoluminescent screen for a field emission display by electrodeposition process in accordance with an embodiment of the present invention.  
         [0057]    First, as shown in FIG. 4A, a transparent conduction layer  20  is formed on the transparent surface  10  for a field emission display. The negative electrode and the positive electrode  40  through the terminal  50  and  60  are connected to the transparent conduction layer  20  and the positive electrode  40  of carbon or the like, respectively. The negative and positive electrode is immersed in chamber  80  confused with an electrodeposition solution  70  and a phosphor-coated particles  30 . A negative voltage and a positive voltage are applied respectively to the transparent conduction layer  20  of the transparent substrate and the counter electrode  40  of carbon or the like in an electrolyte  70  for positively or negatively charging the phosphor particles  30  of the cathodoluminescent material. The cathodoluminescent material is positively charged to deposit the cathodoluminescent material over the surface of the electrode. A positive particles are deposited on the transparent conduction layer  20  of the transparent substrate  10 .  
         [0058]    Also, as shown in FIG. 4B, phosphor-coated particles  30  is confused in a paste or a solvent  90 , and cathodoluminescent film is formed by the screen printing or the spray method on the transparent conduction layer.  
         [0059]    As has been described above, the present invention can be used to widen a coating range of the particle which can be realized by an epitaxial method capable of precise control of the film thickness uniformity and of composition of the phosphor particle, each having a large effect on photoemission characteristics in units of atom layers, in an epitaxy technique for realizing a fine cathodoluminescent screen structure expected to perform a high luminous efficiency. Also, this enables growth of various types of compound semiconductors, therefore, makes it possible to grow a hetero structure, essential in realization of a device. As a result, it is expected that the atom layer epitaxy method is put into particle use, and the range of its applications is widened.  
         [0060]    As is apparent from the forgoing description, the method of forming a cathodoluminescent screen by using phosphor-coated particles for a field emission display by electrodeposition, screen printing, or spray process in accordance with the present invention has a uniform thickness. Also, a cathodoluminescent screen forming method for forming a color cathodoluminescent screen can be selectively formed with green, blue and red cathodoluminescent materials by repeating an electrodeposition process.  
         [0061]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.