Abstract:
A panel structure is provided in which a dielectric layer having no voids thereinside can be formed by a vapor deposition method. A layered film of plural metal layers that constitute an electrode covered with a dielectric layer is formed to have a stepped shape in which a width is smaller from a bottom layer to an uppermost layer for each layer in order. The stepped shape is formed by projecting an edge portion of a lower layer by design compared to an upper layer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to plasma display panels and is characterized by structures of an electrode and a dielectric layer for covering the same.  
         [0003]     2. Description of the Related Art  
         [0004]     AC type plasma display panels have a dielectric layer covering display electrodes. Dielectric layers are generally made of low-melting glass and are formed by a thick film process in which low-melting glass paste is applied and burned.  
         [0005]     Vapor deposition methods (also called vapor growth methods) have recently received attention as methods for forming dielectric layers. Japanese Unexamined Patent Publication No. 2000-21304 describes forming dielectric layers made of silicon dioxide or organic silicon oxide by the plasma CVD (Chemical Vapor Deposition) method that is one type of chemical vapor deposition methods. When the vapor deposition methods are used, it is possible to obtain thin dielectric layers having a uniform thickness and to form dielectric layers made of low dielectric constant materials that are advantageous to reduction in interelectrode capacitance at lower temperatures compared to a burning process.  
         [0006]     Metal films of a three-layer structure of Cr—Cu—Cr are well known as a structure of display electrodes. Copper as an intermediate layer is a main conductor and chromium as a lower layer serves to enhance adhesion to a glass substrate or a transparent conductive film. Chromium as an upper layer serves to prevent a chemical reaction between low-melting glass that is a material of a dielectric and copper that is a material of an electrode.  
         [0007]     Metal films having a three-layer structure are formed by laminating three layers on the entire screen using a film deposition method such as sputtering, and then by patterning the three layers all together. In the patterning, an etching mask having a predetermined pattern is formed by the photolithograph process and one etching mask thus formed is shared for etching of the three layers. Accordingly, the three layers are basically equal to one another in plane pattern and size. In other words, the three layers fully overlap with one another in usual cases.  
         [0008]     Conventional plasma display panels have a drawback that a void is apt to be found in the vicinity of a plural-layered metal film that constitutes a display electrode when a vapor deposition method is used to form a dielectric layer. A void is generated, in patterning of a metal film, when an upper layer has a pattern width larger than a lower layer has. This is because, in an overhanging structure in which an edge portion of an upper layer projects over a lower layer, a void generated below the projecting edge portion does not deposit materials of a dielectric.  
         [0009]     A void inside a dielectric layer causes dielectric breakdown or improper control of discharge. Influence due to a void increases with decreasing the thickness of the dielectric layer. This is because as the layer is thinner, the void is larger relative to the layer thickness. In addition, as a screen size increases, the difficulty of equalizing an etching amount in patterning of electrodes increases, causing excessive progress of side etching locally in many cases. As side etching progresses, a projection amount of an upper layer increases, so that a void gets larger.  
         [0010]     The problem that a void is formed inside a dielectric layer arises also when a dielectric layer is formed by a thick film process. In particular, when a lamination method, which is one for attachment of a sheet-like material, is used to apply low-melting glass, air remains in an overhanging portion of a metal film in the attachment. Accordingly, a void is apt to be generated.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention is directed to solve the problem pointed out above, and therefore, an object of the present invention is to provide an electrode coating structure in which a void is less likely to be generated inside a dielectric layer. Another object of the present invention is to enhance practicability in formation of a dielectric layer using a vapor deposition method.  
         [0012]     According to one aspect of the present invention, a layered film of plural metal layers that constitute an electrode covered with a dielectric layer is formed to have a stepped shape in which a width is smaller from a bottom layer to an uppermost layer for each layer in order. More specifically, compared to an upper layer, an edge portion of a lower layer is formed to project outward by design. This eliminates an eaves-shaped structure hindering deposition, so that no voids are generated inside a dielectric layer even when a chemical vapor deposition method or a physical vapor deposition method is used to form a dielectric layer.  
         [0013]     According to the present invention, a void is less likely to be formed inside a dielectric layer, leading to increase in reliability of plasma display panels.  
         [0014]     These and other characteristics and objects of the present invention will become more apparent by the following descriptions of preferred embodiments with reference to drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a diagram showing an example of a cell structure of a three-electrode surface discharge type plasma display panel.  
         [0016]      FIG. 2  is a diagram showing a planar shape of a display electrode.  
         [0017]      FIG. 3  is a diagram showing a cross-sectional structure in the arrow direction taken along the line a-a of  FIG. 2 .  
         [0018]      FIG. 4  is a diagram showing a layered structure of the display electrode.  
         [0019]      FIGS. 5A-5E  schematically show a process of forming a metal film of the display electrode. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     The present invention can be suitably applied to three-electrode surface discharge type plasma display panels that are used as color display devices.  
         [0021]      FIG. 1  is a diagram showing an example of a cell structure of a three-electrode surface discharge type plasma display panel. For easy understanding of an internal structure,  FIG. 1  illustrates a part corresponding to 3×2 cells in a plasma display panel  1  with a front panel  10  being detached from a rear panel  20 .  
         [0022]     The plasma display panel  1  includes the front panel  10  and the rear panel  20 . The front panel  10  and the rear panel  20  are structural elements of the plasma display panel  1 . A base of each of the front panel  10  and the rear panel  20  is a glass sheet that is larger than the screen and has a thickness of approximately 3 mm. The front panel  10  corresponds to a substrate in the present invention. The front panel  10  includes a glass sheet  11 , display electrodes X and Y as row electrodes, a dielectric layer  17  and a protection film  18 . The display electrodes X and Y are covered with the dielectric layer  17  and the protection film  18 . The rear panel  20  includes a glass sheet  21 , address electrodes A as column electrodes, an insulation layer  24 , a partition  29  as a mesh-patterned discharge barrier and fluorescent material layers  28 R,  28 G and  28 B for color display. The partition  29  is a structure in which plural vertical walls  291  for defining columns in the screen are integral with plural horizontal walls  292  for defining rows in the screen. The fluorescent material layers  28 R,  28 G and  28 B are excited by ultraviolet rays emitted from a discharge gas so as to emit light. Alphabet letters R, G and B in parentheses in  FIG. 1  denote light emission colors of the fluorescent materials.  
         [0023]     Each of the display electrodes X and Y includes a transparent conductive film  41  that is patterned to have a wide ribbon-like shape and a metal film  42  that is patterned to have a narrow ribbon-like shape. The metal film  42  is a bus conductor for reducing electrical resistance of an electrode. A set of a display electrode X and a display electrode Y that are adjacent to each other makes an electrode pair (an anode and a cathode) for a surface discharge. The display electrode X and the display electrode Y are equal to each other in structure.  
         [0024]      FIG. 2  shows a planar shape of a display electrode and  FIG. 3  shows a cross-sectional structure in the arrow direction taken along the line a-a of  FIG. 2 . The display electrode X is illustrated as a typical example in  FIGS. 2 and 3 .  
         [0025]     The transparent conductive film  41  of each of the display electrodes X and Y has a ribbon-like shape in which plural rectangular holes  45  are provided on both sides of respective central parts of portions overlapping with the horizontal wall  292  and the rectangular holes  45  are spaced out along the horizontal wall  292 . The metal film  42  has a straight ribbon-like shape with a constant width and overlaps with a middle portion of the transparent conductive film  41 . Two of ladder portions x 1  and x 2 , which are obtained by dividing the display electrode X into two parts in the column direction, are respectively engaged in display of one row.  
         [0026]      FIG. 4  shows a layered structure of a display electrode. The reference character X (or Y) in the drawing indicates that a structure including relevant elements is a common structure to the display electrode X and the display electrode Y.  
         [0027]     As described above, each of the display electrode X and the display electrode Y includes the ribbon-like transparent conductive film  41  and the metal film  42  having a width smaller than the transparent conductive film  41  has. The transparent conductive film  41  includes tin oxide as a main constituent and is a single-layer film having a thickness of approximately 5000 Å. The metal film  42  is a layered film having a two-layer structure in which a main conductor layer  422  overlaps with a base layer  421 . The base layer  421  is made of chromium (Cr) and has a thickness of approximately 500 Å. The main conductor layer  422  is made of copper (Cu) and has a thickness of approximately 3 μm.  
         [0028]     Note that materials of the electrodes are not limited to the exemplified materials. Materials that are suitable for the main conductor layer  422  and are superior in electrical conductivity include, for example, silver (Ag) and aluminum (Al). Materials of the base layer  421  that enhance adhesion to the main conductor layer  422  include molybdenum (Mo), tungsten (W), nickel (Ni) and titanium (Ti).  
         [0029]     The layered structure in each of the display electrodes X and Y is characterized in that the metal film  42  is formed to have a stepped shape in which a width is smaller from a lower layer to an upper layer for each layer in order. More specifically, in the metal film  42 , the main conductor layer  422  as an upper layer of a layered film has a width W 2  smaller than a width W 1  of the base layer  421  as a lower layer, and both ends of the base layer  421  project outward beyond the main conductor layer  422  respectively. The projection length of the base layer  421  is preferably a value ranging from approximately 1 to 10 μm and such a value is sufficiently smaller than a typical value of each of the widths W 1  and W 2 , i.e., a value ranging from 50 to 80 μm.  
         [0030]     A low dielectric constant material is desirable for a material of the dielectric layer  17  covering the display electrode X. In particular, silicon dioxide (SiO 2 ) is preferable for a material of the dielectric layer  17 . Even if silicon dioxide contacts copper, no significant chemical reactions take place. Accordingly, it is unnecessary to form the metal film  42  in the form of three-layer structure by forming an anti-reaction layer on the main conductor layer  422  made of copper. The small number of layers contributes to reduction in the cost of production.  
         [0031]     The dielectric layer  17  made of silicon dioxide is formed by the plasma CVD method. Since the plasma CVD method is one for depositing materials on a formation surface in the same direction, a surface layer of the dielectric layer  17  has steps reflecting irregularities on the formation surface. In forming the dielectric layer  17 , a technique for producing compressive stress, which is disclosed in Japanese Unexamined Patent Publication No. 2000-21304, is adopted to prevent cracking.  
         [0032]      FIGS. 5A-5E  schematically show a process of forming a metal film of a display electrode.  
         [0033]     As shown in  FIG. 5A , chromium that is a material of a base layer and copper that is a material of a main conductor layer are formed on the patterned transparent conductive film  41  in order of mention by sputtering, so that two layers  421   a  and  422   a  are formed. Then, a resist film  50   a  for patterning is overlaid on the film  422   a . The resist film  50   a  is patterned by photolithography. Then, as shown in  FIG. 5B , a resist mask  50  is formed which has a pattern of covering portions corresponding to the metal film  42  in the layers  421   a  and  422   a . On this occasion, allowing for a side etching amount that is described later, it is necessary to optimize a pattern width of the resist mask  50 .  
         [0034]     A first etchant that dissolves copper selectively is used to remove portions that are not masked in the layer  422   a  (see  FIG. 5C ). For example, ferric chloride is used as the first etchant. When the layer  422   a  is etched, etching time is controlled to progress side etching intentionally, so that a main conductor layer  422  is formed which has a pattern width substantially smaller than that of the resist mask  50 . After that, the process goes to a patterning step of the layer  421   a  with the resist mask  50  remaining.  
         [0035]     A second etchant that dissolves chromium selectively is used to pattern the layer  421   a . For example, hydrochloric acid is suitable for the second etchant. At the starting point of patterning, since the pattern width of the main conductor layer  422  that is already formed is smaller than that of the resist mask  50  as described above, there is a void between the layer  421   a  to be etched and the resist mask  50 . An etching rate in this void is, however, substantially lower compared to an etching rate in a non-masked area that is not covered with the resist mask  50 , because the main conductor layer  422  has a thickness of a few microns in practice. Accordingly, non-masked portions of the layer  421   a  are removed practically, so that a base layer  421  is formed which has a pattern width larger than that of the main conductor layer  422  as shown in  FIG. 5D . The layer  421   a  is patterned and after that the resist mask  50  is removed. Thus, formation of a display electrode is completed (see  FIG. 5E ).  
         [0036]     In the process discussed above, one resist mask  50  is used to form the base layer  421  and the main conductor layer  422  both of which have different pattern widths. Accordingly, the above-described process needs less man-hour in comparison with a case where resist masks are used individually for the base layer  421  and the main conductor layer  422 . It is noted that if anisotropic dry etching is adopted for patterning of the layer  421   a , it is possible to form a base layer  421  reliably whose edge projects outward beyond the main conductor layer  422 .  
         [0037]     The present invention is useful for improvement in reliability of AC type plasma display panels that include an electrode having at least two metal layers and a dielectric layer covering the electrode.  
         [0038]     While example embodiments of the present invention have been shown and described, it will be understood that the present invention is not limited thereto, and that various changes and modifications may be made by those skilled in the art without departing from the scope of the invention as set forth in the appended claims and their equivalents.