Abstract:
An apparatus for coating polyimide layer includes a print table for affixing a substrate thereon, an inkjet head installed over the print table and having a jetting surface with a plurality of nozzles for jetting polyimide liquid onto the substrate, a polyimide liquid supply tank for receiving a polyimide liquid therein, a cleaning liquid supply tank for receiving cleaning liquid, a wiper cleaning bar adjacent to the cleaning liquid supply tank, and a wiper movable from one side of the jetting surface to an other side of the jetting surface while the wiper contacts the jetting surface of the inkjet head, and rotatable to dip the wiper into the cleaning liquid and then contact the wiper cleaning bar so as to remove cleaning liquid from the wiper.

Description:
This application claims the benefit of Korean patent application 10-2006-0028389 filed in Korea on Mar. 29, 2006, which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention relates to coating, and more particularly, to an apparatus and method for coating a polyimide layer. 
     2. Background of the Related Art 
     A liquid crystal display (LCD) device is an apparatus for displaying a desired image by adjusting quantity of light reaching a color filter substrate. The adjustment of the quantity of light is accomplished by changing intermolecular orientation of liquid crystal molecules interposed between a transparent insulating substrate serving as the color filter substrate and an array substrate. One type of LCD device is a thin film transistor liquid crystal display (TFT LCD) device, which uses thin film transistors (TFTs) as switching elements. 
     In general, an LCD device includes an LCD panel for displaying an image and a driver for driving the LCD panel by applying driving signals to the LCD panel. The LCD panel includes a color filter substrate and an array substrate bonded to each other with a predetermined gap therebetween. A layer of liquid crystal molecules is in the gap between the color filter substrate and the array substrate. The color filter substrate and the array substrate of the LCD panel are manufactured through a plurality of masking processes. Polyimide layers are formed on respective substrates after finishing the masking processes and before the substrates are bonded to each other. The polyimide layers are used as alignment films to arrange the liquid crystal molecules in a predetermined direction. 
     The polyimide layers can be coated on the substrates through a variety of methods, such as a spin-coating method, a spray-coating method, and an inkjet-coating method. Of the coating methods, the inkjet-coating method is the quickest and easiest to apply because of the use of an inkjet coating apparatus. A plurality of inkjet heads are used in an inkjet coating apparatus to jet polyimide liquid onto the substrates. 
       FIG. 1A  and  FIG. 1B  are schematic views illustrating an accumulation problem in an apparatus for coating a polyimide layer according to the related art. More specifically,  FIG. 1A  and  FIG. 1B  are schematic views illustrating the surface flatness of the inkjet head  10  before and after jetting polyimide liquid onto the substrates in the inkjet-coating method according to the related art. The inkjet head  10  moves in a predetermined direction above a print table on which a target substrate is placed while jetting polyimide liquid onto the target substrate, thereby coating a polyimide layer onto the substrate. As shown in  FIG. 1A , when the jetting surface of the inkjet head  10  is clean and does not have any residue thereon, polyimide liquid can be jetted uniformly so that a polyimide layer having uniform thickness can be formed on the substrate. However, as shown in  FIG. 1B , polyimide liquid residue can accumulate on the jetting surface A 1  of the inkjet head  10  even after only one inkjet coating process. 
     The accumulated polyimide liquid residue on the jetting surface of the inkjet head  10  interferes with jetting of the polyimide liquid during a subsequent inkjet coating process. Thus, a non-uniform polyimide layer may be coated on a substrate or, in extreme cases, no polyimide layer is coated on a substrate. The non-uniform polyimide layer can include pinhole faults and/or line blemishes. Even if the jetting surface of the inkjet head is cleaned using a cleaning bar, polyimide residue may still remain on the inkjet head. 
     SUMMARY OF THE INVENTION 
     Accordingly, embodiments of the invention is directed to an apparatus and method for coating a polyimide layer that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of embodiments of the invention is to provide an apparatus and method for coating a polyimide layer that includes the capability of effectively removing polyimide residue from the jetting surface of an inkjet head. 
     Another object of embodiments of the invention is to provide an apparatus and method for coating a polyimide layer that prevents pinhole faults and line blemishes from occurring in the polyimide layer. 
     Additional features and advantages of embodiments of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of embodiments of the invention. The objectives and other advantages of the embodiments of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     According to one aspect of embodiments of the invention, there is provided an apparatus for coating polyimide layer includes a print table for affixing a substrate thereon, an inkjet head installed over the print table and having a jetting surface with a plurality of nozzles for jetting polyimide liquid onto the substrate, a polyimide liquid supply tank for receiving a polyimide liquid therein, a cleaning liquid supply tank for receiving cleaning liquid, a wiper cleaning bar adjacent to the cleaning liquid supply tank, and a wiper movable from one side of the jetting surface to an other side of the jetting surface while the wiper contacts the jetting surface of the inkjet head, and rotatable to dip the wiper into the cleaning liquid and then contact the wiper cleaning bar so as to remove cleaning liquid from the wiper. 
     In another aspect of embodiments of the invention, there is provided method for coating polyimide layer that includes placing a substrate on a print table, loading polyimide liquid into a polyimide liquid supply tank, loading a cleaning liquid vessel filled with cleaning liquid into a cleaning liquid supply tank, jetting the polyimide liquid onto the substrate through a plurality of nozzles provided in the jetting surface of the inkjet head after the polyimide liquid is supplied to the inkjet head installed over the print table, wiping off the jetting surface with a wiper, dipping the wiper into the cleaning liquid, and removing the cleaning liquid remaining on the wiper with a wiper cleaning bar. 
     In yet another aspect of embodiments of the invention, a method for coating polyimide layer includes wiping off a jetting surface of an inkjet head after polyimide liquid is jetted from the inkjet head with a wiper, dipping the wiper into the cleaning liquid, rotating the wiper into a first direction so as to dip the wiper into the cleaning liquid, and rotating the wiper further in the first direction such that the wiper contacts the wiper cleaning bar. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of embodiments of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of embodiments of the invention. In the drawings: 
         FIG. 1A  and  FIG. 1B  are schematic views illustrating an accumulation problem in an apparatus for coating a polyimide layer according to the related art; 
         FIG. 2  is a schematic diagram illustrating an apparatus for coating a polyimide layer according to an embodiment of the invention; 
         FIG. 3A  through  FIG. 3C  are schematic views for showing operational movement of a wiper with respect to a wiper cleaning bar; 
         FIG. 4  is a flow chart showing a method of coating a polyimide layer according to another embodiment of the invention; 
         FIG. 5  shows a process for wiping an inkjet head in accordance with the process step of  FIG. 4 ; and 
         FIGS. 6A-6C  shows a process for cleaning a wiper in accordance with the process steps of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Like reference numerals in the drawings denote like elements. 
       FIG. 2  illustrates constitution of an apparatus for coating a polyimide layer according to an embodiment of the invention. As shown in  FIG. 2 , the apparatus for coating a polyimide layer according to an embodiment of the invention includes a print table  100 , an inkjet head  110 , a polyimide liquid supply tank  102 , and a wiper  130 . More specifically, the polyimide layer coating apparatus has the print table  100  on which the substrate  120  to be coated with a polyimide layer is placed and fixed. The substrate  120  can either be a color filter substrate or a thin film transistor array substrate of an LCD device. 
     A plurality of inkjet heads  110  are arranged in parallel with each other over the substrate  120 , and each inkjet head  110  is connected to the polyimide liquid supply tank  102 . The polyimide liquid supply tank  102  receives polyimide liquid from a pressure tank  101 , and supplies the polyimide liquid to the inkjet head  110  at a predetermined constant pressure and flow rate. Polyimide liquid has the advantage of being heat resistant, chemically stable and high reliability. 
     A gas connection pipe provides nitrogen (N 2 ) to the pressure tank  101 . A polyimide liquid recovery connection pipe provides polyimide liquid recovered from the inkjet head to the pressure tank  101 . A polyimide supply connection pipe through which polyimide liquid is provided to the polyimide supply tank  102  through a filter is connected to the pressure tank  101 . A back pressure supply connection pipe for providing back pressure to a cleaning liquid supply tank  103  is also connected to the pressure tank  101 . Further, each connection pipe is provided with a valve for controlling amount of the content passing through the connection pipes and the pressure in the connection pipes. 
     A jetting surface of each inkjet head  110 , which faces the substrate  120 , is provided with a plurality of nozzles  111  for jetting the polyimide liquid therethrough. The inkjet head  110  scans over the substrate  120  while jetting the polyimide liquid onto the substrate  120 , thereby forming a polyimide layer on the substrate  120 . After jetting the polyimide layer onto the substrate, the jetting surface of the inkjet head  110  is wet with the polyimide liquid. The polyimide liquid remaining on the jetting surface of the inkjet head  110  is then removed through a wiping method. Accordingly, the jetting surface of the inkjet head is maintained in a state such that polyimide liquid can be jetted uniformly. 
     The wiper  130  is installed in a manner such that it can move along a moving shaft  131  while the wiper  130  maintains contact with the jetting surfaces of the inkjet  110 , so that the wiper  130  can perform a wiping operation. After finishing the wiping operation, the wiper  130  rotates 180 degrees from a wiping position so as to be dipped into a cleaning liquid in the cleaning liquid supply tank  103 , and then further rotates another 180 degrees to go back to the wiping position. While returning to the wiping position, the wiper  130  comes into contact with the wiper cleaning bar  140 . Thus, polyimide liquid residue on the wiper  130  is removed as the wiper  130  is dipped into the cleaning liquid, and then any remaining polyimide and/or cleaning liquid on the wiper  130  is removed as the wiper  130  moves past the wiper cleaning bar  140 . Further, the wiper  130  can be dipped and wiped again before another wiping operation. 
     If polyimide liquid and cleaning liquid still remain on the surface of wiper  130  after the cleaning of the wiper, the wiping operation of the inkjet head  110  cannot be properly performed. For example, if the wiping operation is not properly performed, polyimide liquid may be partially jetted, non-uniformly jetted or not jetted at all. Such problems are prevented by dipping the wiper  130  into the cleaning liquid and then having the wiper  130  contact the wiper cleaning bar  140 . The cleaning liquid supply tank  103  receives a cleaning liquid vessel filled with cleaning liquid and supplies the cleaning liquid when cleaning the wiper  130 . The cleaning liquid can be an imide-based polar solvent N-Methyl pyrrolidone (NMP). The wiper cleaning bar  140  is installed in such a manner that the wiper cleaning bar  140  can come into contact with the surface of the wiper  130  after the wiper  130  is dipped into the cleaning liquid so that the wiper cleaning bar  140  removes cleaning liquid and any remaining polyimide from the surface of the wiper  130 . 
     The polyimide liquid supply tank  102  and the cleaning liquid supply tank  103  can be structured such that they respectively receive the polyimide liquid in a polyimide liquid filled vessel and a cleaning liquid filled vessel, which expel their contents in response to an external gas pressure. Accordingly, the polyimide liquid supply tank  102  and the cleaning liquid supply tank  103  can be a single tank that receives both the polyimide liquid vessel and the cleaning liquid vessel. In other words, the polyimide liquid vessel and the cleaning liquid vessel share a single tank. 
       FIG. 3A  through  FIG. 3C  are schematic views for showing operational movement of a wiper with respect to a wiper cleaning bar.  FIG. 3A  through  FIG. 3C  illustrate the process sequence in which the wiper  130  is cleaned using the cleaning liquid, and then residue on the wiper  130  is removed as the wiper  130  rotates on a moving shaft  131  so that the wiper  130  contacts the wiper cleaning bar  140 . Referring to  FIG. 3A , the cleaning bar  140  includes a vertical support  141  affixed to the support  104 , a first cleaning structure  143  attached to the vertical support  141  and having an inclined surface facing the wiper  130 , and a second cleaning structure  143  attached to the first cleaning structure  143  and having another inclined surface facing the wiper  130 . When the wiper  130  is rotated in a first direction from an initial position, the wiper  130  is dipped into the cleaning liquid. When the wiper  130  is then further rotated in the first direction, the wiper  130  contacts the wiper cleaning bar  140  as the wiper  130  is returned to its initial position. 
     Referring to  FIG. 3A  through  FIG. 3C , the first rotation in which the wiper  130  rotates to a cleaning position so that the wiper  130  dipped into the cleaning liquid is done in a clockwise direction when the upper surface of the wiper  130  goes from contacting the jetting surface of the inkjet head  110  to being dipped into cleaning liquid. When the wiper  130  is further rotated in the clockwise direction, the wiper  130  contacts the wiper cleaning bar  140  and then the upper surface of the wiper  130  contacts the jetting surface of the inkjet head  110 . The wiper cleaning bar  140  is configured so as to minimize a frictional force between the wiper  130  and the wiper cleaning bar  140  when the wiper  130  comes into contact with the wiper cleaning bar  140  by rotation of the wiper  130 . To minimize the frictional force between the wiper  130  and the wiper cleaning bar  140 , the first cleaning structure  142  has an angle θ in a range of 90 to 140 degrees with respect to the second cleaning structure  143 . The wiper cleaning bar  140  may be made of stainless steel, aluminum, carbon graphite, carbon steel, cast iron, Ethylene Propylene Dien Rubber (EPDM), Polytrafluore Ethylene (PTEE) or Fluorocarbon (FKM). 
       FIG. 4  is a flow chart illustrating the method for coating a polyimide layer according to another embodiment of the invention, and  FIG. 5  shows a process for wiping an inkjet head in accordance with the process step of  FIG. 4 ,  FIGS. 6A-6C  shows a process for cleaning a wiper in accordance with the process steps of  FIG. 4 . 
     As described in step  100  of  FIG. 4 , a substrate  120  is positioned and affixed to the print table  100 . The substrate  120  may be a thin film transistor array substrate or a color filter substrate of an LCD device. 
     Next, a polyimide liquid vessel filled with polyimide liquid is loaded into the polyimide liquid supply tank  102 , as described in steps S 110  and S 120  of  FIG. 4 . Subsequently, the polyimide liquid is supplied to the inkjet head  110  installed over the print table  100 , and then jetted on the substrate  120  through the nozzles  111  provided on the jetting surface of the inkjet  110 . 
     Next, referring to  FIG. 5 , the wiper  130  moves to contact the jetting surface of the inkjet  110  and wipes off the nozzles  111  of the inkjet head  110  in a wiping operation, as described in step S 130  of  FIG. 4 . After finishing the wiping operation, polyimide liquid residue A 1  may remain on the wiper  130 . 
     Next, as described in steps S 140  and S 150  of  FIG. 4 , a cleaning liquid vessel filled with cleaning liquid is loaded into the cleaning liquid supply tank, and then the wiper  130  rotates from an initial position, as shown in  FIG. 6A , to dip into the cleaning liquid so that the wiper  130  is cleaned. The cleaning liquid can be N-Methyl pyrrolidone imide-based polar solvent to remove both cured and un-cured polyimide liquid. 
     As described in step S 160  of  FIG. 4 , the wiper  130  rotates so as to come into contact with the wiper cleaning bar  140  so that polyimide liquid residue and/or any cleaning liquid remaining on the wiper  130  can be removed and then the wiper  130  further rotates to return to the initial position. 
       FIG. 6B  and  FIG. 6C  illustrate respective conditions of the wiper  130  before and after performing the step S 160 , as described in  FIG. 4 . As shown in  FIG. 6A  and  FIG. 6B , the cleaning liquid and the polyimide liquid residue A 2  remaining on the wiper  130  are removed when the wiper  130  comes into contact with the wiper cleaning bar  140 , so that a clean wiper  130  can be subsequently used to wipe off the inkjet head  110 . 
     As described above, the wiper  130  rotates in a first direction from an initial position to dip into then cleaning liquid, and then rotates further in the first direction back to the initial position after coming into contact with the wiper cleaning bar  140  to remove cleaning liquid from the wiper  30 . Thus, the inkjet head  110  of the polyimide layer coating apparatus can then be wiped off with a clean wiper  130  because the wiper cleaning bar  140  keeps the wiper  130  clean such that polyimide liquid residue remaining on the inkjet head  110  can be effectively removed. The polyimide layer coating apparatus and method according to embodiments of the invention can reduce pinhole faults and line blemishes by effectively removing polyimide liquid residue on the surface of inkjet when forming a polyimide layer through an inkjet coating method.