Abstract:
A method of forming an alignment layer for a liquid crystal display device including: attaching a transfer film on a printing roll, the transfer film including a convex portion having first-sized halftone dots and second-sized halftone dots; positioning a substrate on a printing table; providing aligning solution on an anilox roll using a doctor roll; and transferring the aligning solution on the anilox roll to the transfer film while moving the printing table and rotating the printing roll, and then coating the aligning solution on the substrate to thereby form an alignment layer on the substrate.

Description:
The present invention claims the benefit of Korean Patent Application No. 2002-88327 filed in Korea on Dec. 31, 2002, which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display (LCD) device and more particularly, to a method of forming an alignment layer for the liquid crystal display device. 
     2. Discussion of the Related Art 
     In general, a liquid crystal display (LCD) device includes two substrates that are spaced apart and face each other with a liquid crystal material layer interposed between the two substrates. Each of the substrates includes electrodes that face each other. Voltages applied to each of theses electrode induces an electric field between the electrodes and within the liquid crystal material layer. 
     The liquid crystal material layer includes a dielectric anisotropic material having spontaneous polarization properties. Accordingly, when an electric field is induced in the liquid crystal material layer, the liquid crystal molecules form a dipole due to the spontaneous polarization characteristic of the liquid crystal material. Thus, the liquid crystal molecules of the liquid crystal material layer are arranged to correspond with the direction of the applied electric field. Optical modulation of the liquid crystal material layer occurs in accordance with to the arrangement of the liquid crystal molecules. Therefore, images are produced on the LCD device by controlling light transmittance of the liquid crystal material layer using optical modulation. 
       FIG. 1  is an exploded perspective view of a liquid crystal display (LCD) device according to the related art. In  FIG. 1 , an LCD device  11  has upper and lower substrates  5  and  10 , which are spaced apart from and facing each other. A liquid crystal material layer  9  is interposed between the upper and lower substrates  5  and  10 . 
     The upper substrate  5  includes a black matrix  6 , a color filter layer  7 , and a transparent common electrode  18  disposed on a surface of the upper substrate  5  that faces the lower substrate  10 . The black matrix  6  has openings such that the color filter layer  7  is in each of the openings of the black matrix  6 . Each opening of the black matrix  6  has a color filter layer  7  including one of the three sub-color filters of red (R), green (G), and blue (B). Accordingly, the upper substrate  5  can be referred to as the color filter substrate. 
     A gate line  14  and a data line  22  are formed on a surface of the lower substrate  10  facing the upper substrate  5 . The gate line  14  and the date line  22  cross each other to define a pixel area P. A thin film transistor T is formed adjacent to the crossing of the gate line  14  and the data line  22 . The thin film transistor T has a gate electrode, a source electrode, and a drain electrode. A pixel electrode  36 , which is connected to the thin film transistor T, is formed within the pixel area P and corresponds to one of the sub-color filters. The pixel electrode  36  is made of a transparent conductive material, such as indium-tin-oxide (ITO). The lower substrate  22  can be referred to as an array substrate. 
     In the LCD device of the related art, the array substrate is fabricated by forming a plurality of switching elements, such as thin film transistors, a plurality of pixel electrodes corresponding to the plurality of switching elements, the gate and data lines crossing each other and connected to the switching elements, and pads disposed at ends of the gate and data lines through deposition, photolithography, and etching processes. The color filter substrate is manufactured by sequentially forming the black matrix, the color filter layer and the common electrode on a surface facing the array substrate. The array substrate and the color filter substrate are then attached to each other. The LCD device is completed by injecting liquid crystal material into a space between the array substrate and the color filter substrate. 
     The LCD device utilizes electro-optical effect of the liquid crystal material, which are determined by anisotropies of the liquid crystal material and the arrangement of liquid crystal molecules in the liquid crystal material. Therefore, controlling the arrangement of the liquid crystal molecules has a large effect on imaging properties of an LCD device. To make an initial arrangement of the liquid crystal molecules uniform, an aligning process is performed. The aligning process may be accomplished by a rubbing method, for example. An alignment layer of a predetermined thickness may be formed on the array substrate and then hardened. In addition or in the alternative, the alignment layer can be formed on the color substrate. To arrange the surface of the alignment layer in a specified direction, the hardened alignment layer is rubbed with a particular cloth, which is referred to as a rubbing fabric. Thus, polymer chains in the alignment layer are arranged in the specified direction, so that the initial arrangement of the liquid crystal molecules may be made uniform. 
     The alignment layer makes the liquid crystal molecules possess the order and uniformity of single crystal. An alignment layer can be categorized as an inorganic alignment layer and an organic alignment layer. Polyimide (PI) is an organic alignment layer that is widely used. 
     The alignment layer may be generally formed by a roll coating method, which uses a printing apparatus including a plurality of rolls and a printing table and a transfer film attached to the printing apparatus. Formation of the alignment layer will be explained in detail with reference to  FIG. 2 .  FIG. 2  shows a process of printing an alignment layer using a roll coating method according to the related art. 
     In  FIG. 2 , a printing apparatus  1  includes a printing table  2  that reciprocates in a plane, a printing roll  5  that rotates in correspondence with a reciprocation of the printing table  2 , a transfer film  15  attached on the printing roll  5 , an anilox roll  8  that transfers aligning solution to the transfer film  15 , and a doctor roll  10  for uniformly coating the anilox roll  8  with the aligning solution. A substrate  13  is positioned on the printing table  2 , and then the printing table  2  is moved toward the printing roll at a constant speed. When the printing roll  5  engages with the substrate  13 , the printing roll rotates and the transfer film  15  attached on the printing roll  5  contacts the substrate  13  on the printing table  2  so that the aligning solution on the transfer film  15  is transferred to the substrate  13 . Thus, an alignment layer (not shown) is formed on the substrate  13 . The alignment layer is dried to remove moisture therein and then cured to be hardened. Next, the hardened alignment layer is rubbed by a rubbing fabric under constant pressure so that polymer chains in the alignment layer are arranged in a specified direction. 
       FIG. 3A  is a plan view of a related art transfer film and  FIG. 3B  is an enlarged view of the region M of  FIG. 3A .  FIG. 4A  is a cross-sectional view of the related art transfer film and  FIG. 4B  is an enlarged view of the region N of  FIG. 4A . As shown in  FIGS. 3A ,  3 B,  4 A and  4 B, the transfer film  15  includes convex portions  17  and a ground portion  19 . Each convex portion  17  possesses the aligning solution and transfers the aligning solution to the substrate  13  of  FIG. 2 . The ground portion  19  surrounding the convex portions  17  does not contact the substrate  13  of  FIG. 2 . In general, the convex portion  17  has an average thickness T 1  within a range of about 2.24 mm to about 2.84 mm, and the ground portion  19  has an average thickness T 2  within a range of about 1.4 mm to about 2.0 mm. 
     The convex portion  17  includes a plurality of halftone dots  21 , each of which has a shape of a truncated cone for containing the aligning solution of uniform quantity, and grooves  23 , in which the aligning solution resides therein, are formed between the halftone dots  21 . The plurality of halftone dots  21  has substantially the same height and the same size and spaces between the halftone dots  21  are also equal. That is, the grooves  23  have substantially the same depth and the same width. In addition, the halftone dots  21  of the convex portion  17  have sides inclined with respect to the lower surface of the transfer film  15 . The convex portion  17  may includes the halftone dots of about 400 meshes. 
       FIG. 5A  is a cross-sectional view of showing an alignment layer formed on a substrate according to the related art, and  FIG. 5B  is an enlarged view of the region E 1   FIG. 5A . As shown in  FIGS. 5A and 5B , an alignment layer  32  is coated on a substrate  30 , which may be either an upper substrate or a lower substrate of a liquid crystal display device, and is then cured. The alignment layer  32  has a thickness within a range of about 700 Å to about 1,100 Å. However, the alignment layer  32 , which is formed by the transfer film  15  illustrated in  FIGS. 3A to 4B , may have a non-uniform thickness. That is, although the alignment layer  32  should have the thickness within a range of about 700 Å to about 1,100 Å, the alignment layer  32  may have a thickness within a range of about 1,400 Å to about 3,300 Å at edges thereof. Therefore, the alignment layer  32  at the edges has a thickness that is about two to about three times greater as compared with the central portion thereof. This problem causes a non-uniform gap between the color substrate and the array substrate in the liquid crystal display device. Thus, spots having different degrees of brightness occur in the displayed images when signal voltages are applied. 
     This problem of the alignment layer having a thicker thickness at the edges than at the central portion may be caused by a marginal phenomenon. The marginal phenomenon is as follows. In the printing apparatus of the alignment layer, the transfer film having halftone dots of the same size is attached on the printing roll, and the printing roll rotates with the attached transfer film while the table having the substrate thereon moves at a constant speed along with the surface of the transfer roll. As the convex portion of the transfer film including the halftone dots contacts the substrate, the aligning solution filled in the grooves of the convex portion of the transfer film is transferred to the substrate by the anilox roll. The aligning solution is gathered at the edges of the convex portion by the anilox roll as the printing roll rotates. Thus, the aligning solution is thicker at the grooves about leading and wayward edges of the convex portion. This phenomenon can be referred to as the marginal phenomenon. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a method of forming an alignment layer for a liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide a method of forming an alignment layer that has a uniform thickness for a liquid crystal display device. 
     Another object is to provide a transfer film for forming an alignment layer of a liquid crystal display device that has a uniform thickness. 
     Additional features and advantages 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 the invention. The objectives and other advantages 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. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of forming an alignment layer for a liquid crystal display device includes: attaching a transfer film on a printing roll, the transfer film including a convex portion having first-sized halftone dots and second-sized halftone dots; positioning a substrate on a printing table; providing aligning solution on an anilox roll using a doctor roll; and transferring the aligning solution on the anilox roll to the transfer film while moving the printing table and rotating the printing roll, and then coating the aligning solution on the substrate to thereby form an alignment layer on the substrate. 
     In another aspect of the present invention, a method of forming an alignment layer for a liquid crystal display device using a printing apparatus having a printing table, a printing roll, an anilox roll and a doctor roll, includes: attaching a transfer film on the printing roll, the transfer film including a convex portion, wherein the convex portion has first-sized halftone dots and second-sized halftone dots; positioning a substrate on the printing table; providing aligning solution on the anilox roll using the doctor roll; and coating the aligning solution on the substrate from the anilox roll via the transfer film on the printing roll. 
     In yet another aspect, a transfer film to be dispersed on a printing roll for transferring an alignment solution to a substrate for forming an alignment film of an LCD device, the transfer film including a convex portion having halftone dots and a ground portion surrounding the convex portion, wherein a first part of the convex portion having first-sized halftone dots and a second part of the convex portion having second-sized dots, the first-sized halftone dots being larger than the second-sized halftone dots. 
     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 the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding 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 the invention. 
         FIG. 1  is an exploded perspective view of a liquid crystal display (LCD) device according to the related art. 
         FIG. 2  is a schematic view showing a process of printing an alignment layer using a roll coating method according to the related art. 
         FIG. 3A  is a plan view of a related art transfer film and  FIG. 3B  is an enlarged view of the region M of  FIG. 3A . 
         FIG. 4A  is a cross-sectional view of the related art transfer film and  FIG. 4B  is an enlarged view of the region N of  FIG. 4A . 
         FIG. 5A  is a cross-sectional view of showing an alignment layer formed on a substrate according to the related art and  FIG. 5B  is an enlarged view of the region E 1  of  FIG. 5A . 
         FIG. 6A  is a plan view of a transfer film according to an exemplary embodiment of the present invention, and  FIG. 6B  is a cross-sectional view along the line VI—VI of  FIG. 6A . 
         FIG. 7A  is a cross-sectional view of an alignment layer formed by using a transfer film according to the present invention and  FIG. 7B  is a schematic enlarged view of the region E 2  of  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the illustrated embodiment of the present invention, which is illustrated in the accompanying drawings. 
       FIG. 6A  is a plan view of a transfer film according to an exemplary embodiment of the present invention, and  FIG. 6B  is a cross-sectional view along the line VI—VI of  FIG. 6A . As shown in  FIG. 6A , the transfer film  100  includes convex portions  105  and a ground portion  110 . Each convex portion  105  of the transfer film  100  contains aligning solution and transfers the aligning solution to a substrate when the transfer film contacts the substrate. The ground portion  110  surrounds the convex portions  105  and has a thinner thickness than the convex portions  105 . In embodiments of the present invention, each convex portion  105  is composed of a central part  103  and an edge part  102 . The central part  103  has different shapes than the edge part  102 . That is, sizes and/or heights of halftone dots or meshes are different in the central part  103  than in the edge part  102  of the convex portion  105 . 
     “Meshes” is the number of halftone dots formed in a square having a width and a length of 1 inch, i.e., 2.54 cm. For example, a transfer film of 400 meshes includes 400 halftone dots in a square of 2.54 cm long and 2.54 cm wide, which may be defined in the convex portion  105 . 
     As stated above, in embodiments of the present invention, the central part  103  and the edge part  102  have different meshes. For example, the central part  103  can have 400 meshes, while the edge part  102  has more than 400 meshes. That is, the edge part  102  may have 400 meshes to 600 meshes. If the number of meshes varies, the spaces between the halftone dots and the sizes of the halftone dots also change. In addition, a height of the halftone dot in the central part  103  can be higher than a height of the halftone dot in the edge part  102 . The edge part  102  may have widths W 1  and W 2  within a range of about 0 mm to 100 mm. 
     As shown in the embodiment of  FIG. 6B , halftone dots  120   a  and  120   b  having different sizes are formed in the central part  103  and in the edge part  102 , respectively, centering on a border portion BA between the central part  103  and the edge part  102 . The letter A indicates a first space between halftone dots  120   a  of the central part  103 . The letter B designates a second space between halftone dots  120   b  of the edge part  102 . The alphanumeric T 3  designates a height of the halftone dots  120   a  in the central part  103 . The alphanumeric T 4  designates a height of the halftone dots  120   b  in the edge part  102 . 
     As shown in  FIG. 6B , the convex portion  105  of the transfer film  100  is formed so as to satisfy the following conditions: A&gt;B, T 3 &gt;T 4 . Since sizes of grooves  125   a  and  125   b  are proportional to the sizes of the halftone dots  120   a  and  120   b , the groove  125   a  of the central part  103  is larger and deeper than the groove  125   b  of the edge part  102 . Therefore, the central part  103  can contain more aligning solution than the edge part  102 . 
     The convex portion  105  of the transfer film  100  may have an average thickness H within a range of about 2.24 mm to about 2.84 mm. Although the halftone dots in the central part  103  and the edge part  102  may have different heights, top surfaces of the halftone dots in the central part  103  and the edge part  102  are on the same plane. 
     A fabricating method of the transfer film will be explained hereinafter. The transfer film may be formed by attaching photo-polymer, which may be referred to as APR (Asahi kasei Photosensitive Resin), with a polyethylene film, which may be referred to as a base film. An exposer is used to form the transfer film, wherein the exposer comprises an ultraviolet (UV) lamp and a substrate, which can be made of quartz or crystal. The substrate of the exposer is situated over the lamp. A negative film, which is patterned with circle patterns having as many as halftone dots to be later formed in the convex portion, is disposed on the substrate of the exposer. The circle patterns have different sizes depending on corresponding parts, i.e., either central part or edge part. The circle patterns corresponding to the central part have lager diameters than the circle patterns corresponding to the edge part. 
     Next, APR of a certain thickness is coated on the negative film. A transparent base film made of polyethylene is arranged over and is attached to the coated APR. The APR attached to the base film is exposed to the UV lamp under the substrate of the exposer. Portions of APR corresponding to circle patterns are exposed to UV and hardened. Thus, the circle patterns of the negative film are transferred to the APR through UV exposure. The APR attached to the base and having the halftone dots is cleaned so that the hardened portions may remain and the non-exposed portions of APR may be removed. The hardened APR and attached base film becomes a transfer film. 
     The APR transfer film is dried through a dry process and the surface of the APR transfer film is further hardened via an exposure from the backside. The resulting APR transfer film has convex portions comprising a central part and an edge part, where sizes, heights and the number of halftone dots are different. 
     The sizes and heights of the halftone dots are controlled by exposing time and the sizes of the circle patterns on the negative film during the UV exposure. If the circle pattern on the negative film is small, the size of the halftone dot is also small and the height of the halftone dot or a depth of a groove is small. 
     Next, an alignment layer is coated on a substrate through a printing process using the above-mentioned transfer film. The process will be described hereinafter. The transfer film, which includes an edge part having smaller-sized halftone dots, is attached to a printing roll of a printing apparatus and a substrate is located on a printing table. Aligning solution is provided using a doctor roll such that aligning solution is uniformly transferred and coated on the surface of the anilox roll by the doctor roll, which is made of rubber and shakes while it rotates. The aligning solution coated on the surface of the anilox roll is coated on the substrate by the printing roll when the table is moved such that the transfer film having the edge part with smaller-sized halftone dots is engaged. The substrate including the aligning solution thereon is dried and cured to form an alignment layer. The alignment layer formed through the above-mentioned processes has uniform thicknesses in the edge regions and in the central region. 
       FIGS. 7A and 7B  show a cross-section of an alignment layer formed by using a transfer film according to embodiments of the present invention.  FIG. 7B  is a schematic enlarged view of the edge region E 2  of  FIG. 7A . As shown in  FIGS. 7A and 7B , an alignment layer  155  is formed on a substrate  150  by using a transfer film having edge part with smaller-sized halftone dots and a central part with regular-sized halftone dots. That is, the convex portion includes halftone dots of different sizes in the central part than in the edge part. The alignment layer  155  is formed using a transfer film with a convex portion film having an edge part with smaller-sized halftone dots and an central part with regular-sized dots to have a uniform thickness within a range of about 700 Å to about 1,100 Å. Accordingly, the alignment layer  155  has uniform thickness in the central region and in the edge region. 
     In embodiments of the present invention, the alignment layer is formed by a roll coating method using a transfer film with a convex portion film having an edge part with smaller-sized halftone dots and a central part with regular-sized dots. Thus, the marginal phenomenon can be prevented and the alignment layer can have a preferable thickness within a range of about 700 Å to about 1,100 Å. Accordingly, the alignment layer has a uniform thickness across the substrate, and images of the device are improved. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the fabrication and application of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.