Abstract:
A plasma display panel having a plasma discharge structure in a gap between a first substrate and a second substrate may include an align mark formed on a surface of the first substrate opposing the second substrate. The align mark may include a plurality of cavities. Protrusions may be located within the cavities.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of Korean Patent Application No. 10-2003-0093812, filed on Dec. 19, 2003, which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     (a) Field of the Invention  
         [0003]     The present invention relates to a plasma display panel (PDP), and more particularly, to a PDP having align marks structured so that they are not formed with defects. The present invention relates also to a method and apparatus for forming the align marks using an offset process.  
         [0004]     (b) Description of the Related Art  
         [0005]     A PDP is a display device that displays images by exciting phosphors using plasma discharge. Vacuum ultraviolet (VUV) rays emitted from plasma obtained by gas discharge excite phosphor layers. The phosphor layers then emit visible light forming images. With its potential for high resolution and large screen sizes, PDP technology may become the leading next-generation flat screen technology.  
         [0006]     In the basic structure of the conventional PDP, address electrodes, barrier ribs, and phosphor layers are formed on a rear substrate, and display electrodes comprised of scan electrodes and sustain electrodes are formed on a front substrate. Each of the scan electrodes and sustain electrodes includes a transparent electrode made of a material having a degree of transmissivity (e.g., indium tin oxide), and a metal bus electrode.  
         [0007]     The address electrodes and the display electrodes are covered by a first dielectric layer and a second dielectric layer, respectively. An MgO protective layer is formed on the second dielectric layer. A discharge cell is formed in a discharge space where the address electrodes intersect the display electrodes, and a discharge gas (typically a Ne-Xe compound gas) fills the discharge cells.  
         [0008]     The scan electrodes are mounted opposite the sustain electrodes with predetermined discharge gaps between them. The discharge gaps correspond to centers of the discharge cells. The barrier ribs are formed in stripes in the same direction the address electrodes are formed such that the discharge cells are connected in this same direction.  
         [0009]     Precise arrangement of the electrodes on the substrates is necessary to accurately align the substrates with each other. With the increased complexity of the transparent electrodes recently, it is increasingly important that the unit cells be properly aligned. Increasing panel size exacerbates the problem of deformation in the glass used in PDPs and in the transparent electrodes. This further complicates the alignment processes during PDP manufacture.  
         [0010]     To perform alignment, align marks are formed on the substrates. The align marks may be formed simultaneously during the formation of the electrodes, dielectric layers, and other elements.  
         [0011]     Screen printing and photolithography methods are used to form the bus electrodes. Lift-off and thin film methods can also be used to form the bus electrode. There is a recent preference to use offset printing.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention can provide an align mark formation method and apparatus that may minimize the amount of align mark paste that is transferred, and may prevent deformation of the align marks during transfer onto a substrate.  
         [0013]     It is another object of the present invention to provide a PDP having align marks in which defects in the formation and positioning of align marks are prevented to thereby ensure accurate alignment of the elements of the PDP.  
         [0014]     A plasma display panel having a plasma discharge structure in a gap between a first substrate and a second substrate may include at least an align mark formed on a surface of the first substrate opposing the second substrate. The align mark may include a plurality of cavities.  
         [0015]     The cavities may be arranged in a substantially uniform pattern with predetermined spaces between adjacent cavities. The spaces may be interconnected to form a lattice pattern.  
         [0016]     In another aspect, an outer boundary of the align mark may be defined by an edge, and the cavities at the edge may be closed off by the edge. In yet another aspect, the cavities of the align mark are formed in a lattice pattern.  
         [0017]     Each of the cavities may have a cross-sectional shape that is circular or polygonal.  
         [0018]     The align marks may be formed using an offset printing process.  
         [0019]     An align mark formation method using an offset printing process may include forming in a gravure a concavity in the shape of an align mark to be printed, and simultaneously forming a plurality of protrusions in the concavity. It may further include filling the concavity with a paste for aligning marks, transferring the paste to a printing blanket from the concavity, and transferring the paste to a substrate of a plasma display panel from the printing blanket.  
         [0020]     The protrusions may be formed using an etching process. The gravure may be in the form of a plate or in the form of a cylinder.  
         [0021]     An align mark formation apparatus may include a gravure having a concavity filled with a paste used to form align marks, a blanket for transferring the paste to a substrate, and a plurality of protrusions formed in the concavity of the gravure plate. The protrusions may have a cross-sectional shape that is circular or polygonal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a fragmentary exploded perspective view of a conventional PDP.  
         [0023]      FIG. 2  is an exploded perspective view of front and rear substrates of a PDP having align marks according to an embodiment of the present invention.  
         [0024]      FIG. 3  is a plan view of an align mark of  FIG. 2 .  
         [0025]      FIG. 4  is a plan view of an align mark according to another embodiment of the present invention.  
         [0026]      FIG. 5  is a plan view of an align mark according to yet another embodiment of the present invention.  
         [0027]      FIG. 6  is a schematic view of an align mark formation apparatus that utilizes an offset process according to an embodiment of the present invention, illustrating the align mark formation apparatus in a state of use.  
         [0028]      FIG. 7  shows sectional views of sequential processes involved in forming an align mark on a substrate according to an embodiment of the present invention.  
         [0029]      FIG. 8  is a schematic view of an align mark formation apparatus that utilizes an offset process according to another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0030]     As shown in  FIG. 1 , address electrodes  3 , barrier ribs  5 , and phosphor layers  7  may be formed on a first substrate (rear substrate)  1 . Display electrodes  15  including scan electrodes  11  and sustain electrodes  13  may be formed on a second substrate (front substrate)  9 . Each of the scan electrodes  11  may include a transparent electrode  11  a made of a material such as ITO with a high transmissivity and a bus electrode  11   b  made of metal. Thus the scan electrode  11  may be conductive.  
         [0031]     Similarly, each of the sustain electrodes  13  may include a transparent electrode  13   a  made of a material such as ITO with a high transmissivity, and a bus electrode  13   b  made of metal. Thus the sustain electrode  13  may be conductive. The address electrodes  3  and the display electrodes  15  may be covered by a first dielectric layer  17  and a second dielectric layer  19 , respectively. An MgO protective layer  21  may be formed on the second dielectric layer  19 . Discharge cells may be formed in a discharge region and where the address electrodes  3  intersect the display electrodes  15 . A discharge gas (typically an Ne-Xe compound gas) may fill the discharge cells.  
         [0032]     As shown in  FIG. 2 , discharge structures of the PDP may be formed in a display region  26  positioned within an area where a front substrate  21  and a rear substrate  22  overlap. Align marks  24  may be formed to the exterior of the display region  26 . Align marks  24  may be used to align the front substrate  21  and the rear substrate  22  when they are sealed.  
         [0033]     The align marks  24  may be formed using an electrode paste during the formation of bus electrodes or address electrodes. The align marks  24  may also be used as points of reference during exposure processes.  
         [0034]     As shown in  FIG. 2 , each align mark  24  according to the present invention may include a plurality of cavities  24   a  formed within a predetermined area.  
         [0035]      FIG. 3  is a plan view of an align mark  24 , shown substantially along a direction normal to the front substrate  21  of  FIG. 2 . As shown in  FIG. 3 , the cavities  24   a  may be arranged in a uniform pattern with predetermined spaces provided between adjacent cavities  24   a . The spaces between the cavities  24   a  may be filled with a paste to realize the predetermined pattern.  
         [0036]     As an example, the spaces may be interconnected and form a lattice pattern as shown in  FIG. 3 .  
         [0037]     Grooves may be formed in a gravure plate used in an offset process. After protrusions are formed in the grooves, the grooves may be filled with a paste and printing may be performed. This process may result in the cavities  24   a . The cavities  24   a  may have a cross-sectional shape that is circular, square, rectangular, or the like.  
         [0038]     In the first exemplary embodiment, the cavities  24   a  have a cross-sectional configuration that is square.  
         [0039]     The align mark  24  may be interconnected along an edge. On a surface opposing the rear substrate  22 , it may be possible for the align mark  24  to be completely interconnected without forming cavities.  
         [0040]     As shown in  FIG. 4 , a plurality of cavities  25   a  may be formed in an align mark  25 , and predetermined spaces may be formed between the cavities  25  as in the previous embodiment. However, the cavities  25   a  in this embodiment may not all be identical in shape.  
         [0041]     The cavities  25   a  may be formed having a cross-sectional shape that is, for example, square, triangular, and trapezoidal. The outer boundary of the align mark  25  may be defined by an edge, and the cavities  25   a  at these areas may be closed off by this edge.  
         [0042]     As shown in  FIG. 5 , a plurality of cavities  26   a  may be formed in an align mark  26  as in the previous embodiments. However, in this embodiment, the cavities  26   a  may be formed in an interconnected lattice pattern.  
         [0043]     As shown in  FIG. 6 , a concavity  33  may be formed in a gravure plate  31 , and the concavity  33  may be filled with a paste. The paste may then be transferred to a blanket  35 . Next the paste may be transferred from the blanket  35  to a glass substrate  37 . In actual production, a plurality of the concavities  33  may be formed in the gravure plate  31 .  
         [0044]     Protrusions  40  may be formed in the concavity  33 . The protrusions  40  may have a cross-sectional shape that is circular, square, rectangular, polygonal, or the like.  
         [0045]     The concavity  33  and the protrusions  40  formed in the gravure plate  31  may be formed by an etching process. In such a case the steps involved may include deposition of a photoresist, exposure using a photomask, and developing. As shown in  FIG. 7 , a concavity  33  in the shape of an align mark to be printed may first be formed in a gravure plate  31 . A plurality of protrusions  40  may simultaneously be formed in the concavity  33 . The concavity  33  may then be filled with a paste  34 , after which a blade  32  may be used to remove excess portions of the is paste  34  (e.g., overflow paste).  
         [0046]     As a result of this formation of the align mark concavity  33  including the protrusions  40  formed at predetermined intervals as described above, the paste  34  may fill between the protrusions  40  rather than within the entire area encompassed by the concavity  33 . Hence, the amount of paste  34  required may be reduced by an amount equal to the volume occupied by the protrusions  40 .  
         [0047]     Next, the paste  34  filled in the concavity  33  may be transferred onto a printing blanket  35 . When transferred onto the printing blanket  35 , the resulting configuration of the paste  34  may be opposite to the shape of the concavity  33 . Locations corresponding to where the protrusions  40  are formed in the concavity  33  may be indented.  
         [0048]     Subsequently, the paste  34  transferred onto the printing blanket  35  from the concavity  33  may then be transferred onto a glass substrate  37 . During this process, the paste  34  may be squeezed between the printing blanket  35  and the glass substrate  37 . Nevertheless, the paste  34  may not undergo any significant outward deformation. This results from the relatively minimal use of the paste  34  as described above. The protrusions  40  thus may ensure that there are sufficient gaps in the paste  34  prior to transfer onto the printing blanket  35 .  
         [0049]     Thus, when the paste  34  is then transferred onto the glass substrate  37 , the squeezing pressure applied to the paste  34  as a result of being pressed between the printing blanket  35  and the glass substrate  37  may be applied in an inward direction toward the gaps formed in the paste  34 . This prevents an outwardly distorted formation of the align marks.  
         [0050]     In the offset printing process, the align marks are typically formed during electrode formation. The paste may be transferred onto a cylindrically shaped blanket made of silicone rubber, and the blanket may contact the substrate and roll on it. Thus the paste may be transferred onto the substrate.  
         [0051]     In the conventional process, with the pressure applied to the align marks in the direction of movement of the blanket, the align marks are not positioned correctly, and are frequently deformed.  
         [0052]     However, with the use of the align mark formation method of the exemplary embodiment of the present invention described above, problems in position or formation of the align marks may not occur.  
         [0053]     Following the transfer of the paste  34  onto the glass substrate  37 , drying and firing of the paste  34  may be performed to thereby complete the formation of the align marks. The align marks may be formed at the same time electrode formation takes place as described above.  
         [0054]     As shown in  FIG. 8 , concavities  38  may be formed in a gravure roll  39 , and the concavities  38  may be filled with a paste. Following filling of the concavities  38 , the paste may be transferred to a glass substrate  37 .  
         [0055]     As with the previous embodiment, an etching process may be performed on the surface of the gravure roll  39  to thereby form the concavities  38 . A plurality of protrusions  41  may be formed in each of the concavities  38 . The protrusions  41  may have a cross-section that is circular, square, rectangular, polygonal, or the like. The protrusions  41  may be formed at the same time as the concavities  38 .  
         [0056]     The concavities  38  may be filled with a paste  34 . Next, a blade  32  may be used to remove excess portions of the paste  34  (e.g., overflow paste). Subsequently, the paste  34  filled in the concavities  39  may be transferred onto a printing blanket  35 . The paste  34  may finally be transferred onto a glass substrate  37 . Drying and firing may then be performed to complete the align marks.  
         [0057]     Although embodiments of the present invention have been described in detail hereinabove, many changes may be made to the embodiments without departing from the scope of the invention.