Abstract:
A method of fabricating an LCD is provided. According to the method, an alignment film is printed and hardened on a first substrate and a second substrate. Subsequently, the alignment film is subject to a primary alignment process and a subsequent secondary alignment process. The primary alignment process includes a rubbing process and the secondary alignment process includes irradiating light on the entire surface of the rubbed alignment film. Finally, retarder LC materials are coated and hardened on the light-irradiated alignment film to form a first compensation film and a second compensation film.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a liquid crystal display device (LCD), and more particularly, to a method of fabricating an LCD having an improved alignment characteristic with a coating-type compensation film.  
         [0003]     2. Description of the Related Art  
         [0004]     Liquid crystal (LC) molecule has anisotropy. The anisotropy of a LC cell having the LC molecules or a film may be changed according to distribution of the LC molecules and/or tilt angles relative to a substrate. The change of the anisotropy also changes the polarization of light relative to a viewing angle from which the LC cell is viewed. Due to unique characteristics of the LC cell, brightness and contrast ratio of a display may be different depending on a viewing angle from a top/bottom position and a left/right position.  
         [0005]     A compensation film may be attached to compensate the anisotropy distribution due to the viewing angle of the LC cell. The compensation film is manufactured to have anisotropy distribution opposite to the anisotropy distribution of the LC cells so that a retardation difference due to the viewing angle may be removed when the compensation film is attached on the cells.  
         [0006]     The compensation film using a high molecular film generates a phase difference of the transmitted light. The compensation film, which is extension-processed to a predetermined direction, has a birefringence due to the anisotropy of the molecules.  
         [0007]     When an external magnetic field is applied to a twisted nematic (TN) LCD of a normally black mode, the LC molecules arrange in response to the electric field, so that light transmission occurs according to the following equation: 
 
 I=I   O  sin 2 [θ(1 +u   2 )½ ], u=πR/θλ, R=Δn·d    (1)
 
 where I is intensity of transmitted light, I O  is luminosity of incident light, Δn is an birefringence index, d is the thickness of an LC cell, λ is the wavelength of the transmitted light, θ is the twisted angel of twisted nematic LC, and R is a phase difference. As shown in the above equation, the phase difference has the close relationship with the viewing angle. Therefore, the phase difference may be compensated so as to improve the viewing angle. 
 
         [0008]     The compensation film that compensates for the phase difference and disposed between a substrate and a polarization plate may be either uniaxial material or biaxial material. Both uniaxial material and biaxial material have an anisotropic refractive index.  
         [0009]      FIGS. 1A  to  1 C illustrate an ellipse having an anisotropic refractive index of a phase-difference compensation film. Assuming that the refractive indices of x, y, and z directions in an orthogonal coordinate system are nx, ny, and nz, respectively, the uniaxial property and the biaxial property are determined depending on whether nx and ny are identical or not.  
         [0010]     Referring to  FIG. 1A , in case where the refractive indices of two directions are the same and different from the refractive index of the remaining direction in the orthogonal coordinate system, it is defined “uniaxial.” For example, in the uniaxial material, the relationship of three refractive indices are nx=ny&gt;nz. On the contrary, referring to  FIGS. 1B and 1C , in case where the refractive indices are different from one another such as nx&gt;nz&gt;ny and nx&gt;ny&gt;nz, it is defined “biaxial.” 
         [0011]     A compensation film is generally formed of the uniaxial material having an anisotropic refractive index. Such compensation film has an arrangement that a longer axis of the ellipse is parallel or perpendicular to the surface of the film.  
         [0012]     The compensation film is fabricated using a method of stretching a high molecular film along one axis or two-axes. The birefringence can be obtained by having the optical axis of the compensation film form an arbitrary angle with respect to the progression direction of the film. Another method of forming the compensation film is directly coating the compensation film on a substrate. This method is often referred to as a coating-type compensation film.  
         [0013]     The coating-type compensation film is manufactured as follows. First, after an alignment film is formed on a substrate, an alignment process is performed. The alignment process allows the optical axis of the compensation film to have an arbitrary angle afterwards.  
         [0014]     Subsequently, an LC of optical hardness is coated with a coatable retarder material on the alignment-processed alignment layer. The LC-coated substrate is irradiated with light, so that a nematic LC of optical hardness is hardened and fixed as a film. Another alignment film for aligning the LC interposed between the upper and lower substrates of the LCD is additionally formed on the coating-type compensation film.  
         [0015]     In the related art method of fabricating the coating-type compensation film, the alignment film initially formed on the substrate is formed by printing an organic high molecular material such as polyimide and polyamide on the substrate as an alignment material and hardening the same.  
         [0016]     The alignment process performed on the hardened alignment film applies a rubbing method of rubbing an alignment film in a predetermined direction using a rubbing cloth of a predetermined shape. As a result, grooves may be formed in a predetermined direction on the surface of the alignment film.  
         [0017]     The above rubbing method has advantages of easy alignment and stable alignment. Further, the rubbing method is also suitable for mass production. However, the rubbing method has a problem that a rubbing defect occurs when a roller having a defective rubbing cloth attached thereon is used during the rubbing process. Because the rubbing method using the rubbing cloth requires a direct contact between the alignment film and the rubbing cloth, it may contaminate LC due to particle attachments and a non-uniformity of the alignment upon application to a large-sized LCD. Accordingly, there is a need of a fabricating method of an LCD that overcomes drawbacks of the related art fabrication method.  
       SUMMARY OF THE INVENTION  
       [0018]     By way of introduction only, a method of fabricating an LCD includes printing an alignment film on a first substrate and a second substrate and hardening the same; performing a primary alignment-process of the alignment film by rubbing the alignment film; performing a secondary alignment-process of the rubbed alignment film by irradiating light on an entire surface of the rubbed alignment film; and forming a first compensation film and a second compensation film by coating a retarder liquid crystal (LC) material on the light-irradiated alignment film and hardening the same.  
         [0019]     A liquid crystal display includes a first substrate having a color filter layer; a second substrate having array devices including thin film transistors (“TFTs”); an LC layer filling a space formed by the first and second substrates spaced a predetermined interval; a first polarization plate and a second polarization plate attached on outer surfaces of the first and second substrates, respectively; and a first compensation film and a second compensation film coated on inner surfaces of the first and second substrates, respectively. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:  
         [0021]      FIGS. 1A  to  1 C illustrate an ellipsis having an anisotropic refractive index of a phase-difference compensation film;  
         [0022]      FIG. 2  is a schematic view of an LCD using a coating-type compensation film; and  
         [0023]      FIGS. 3A  to  3 D illustrate a fabricating process of a coating-type compensation film. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0025]      FIG. 2  is a schematic view of an LCD  100  using a coating-type compensation film. Referring to  FIG. 2 , the LCD  100  includes an upper substrate  120  having a color filter layer  122  and a lower substrate  110  having array devices  112  including TFTs. An LC layer  130  fills a space formed by the upper and lower substrates  120  and  110 . A first polarization plate  121  and a second polarization plate  111  are attached on outer surfaces of the upper and lower substrates  120  and  110 . The first and second polarization plates  121  and  111  are parallel to each other. A first compensation film  123  is coated on the inner surface of the upper substrate  120  and a second compensation film  113  is coated on the inner surface of the lower substrate  110 . These coating-type compensation films  113  and  123  are formed by directly coating on the upper and lower substrates  120  and  110 .  
         [0026]     Although not shown, the color filter layer  122  formed on the upper substrate  120  and the array devices  112  formed on the lower substrate  110  are formed on the first and second compensation films  123  and  113 . The array devices  112  include TFTs operating as switching devices and pixel electrodes are formed in a matrix type at intersections between the gate lines and the data lines.  
         [0027]     A black matrix (BM), a color filter, and a common electrode are sequentially formed on the color filter layer  122 . The first polarization plate  121  and the second polarization plate  111  transmit light having a direction parallel to an optical transmittance axis to convert natural light into linearly polarized light. The first and second polarization plates  121  and  111  are disposed on the outer surfaces of the upper plate  120  and the lower plate  110 , i.e., the upper surface of the upper substrate  120  and the lower surface of the lower substrate  110 , respectively.  
         [0028]     The first compensation film  123  and the second compensation film  113  are formed by coating a retarder material on the inner surface of the substrate. The first compensation film  123  of the upper substrate  120  and/or the second compensation film  113  of the lower substrate  110  are formed by forming an alignment film made of high molecular material such as polyimide and then performing a primary alignment process and a secondary alignment process on the formed alignment film.  
         [0029]     The primary alignment process is a rubbing process and a light-irradiation process or an ion-beam irradiation process is additionally applied as the secondary alignment process. An LC of optical hardness such as nematic LC is coated with the retarder material on the alignment-processed film. Subsequently, the coated substrate is irradiated with light, so that the nematic LC is hardened to be used as a film.  
         [0030]      FIGS. 3A  to  3 D illustrate a fabrication process of a coating-type compensation film. First, referring to  FIG. 3A , to align LC molecules to a predetermined direction, organic high molecular materials such as polyimide are spread on the substrate  200  to form an alignment layer  210 . Solvent is evaporated at the temperature range of 60-80° C. and the LC molecules are aligned and then hardened at the temperature range of 80-200° C.  
         [0031]     Referring to  FIG. 3B , a primary alignment-process applies to the alignment layer  210 . The primary alignment process includes a rubbing process of forming the alignment direction by rubbing the alignment layer  210  made from polyimide with a rubbing cloth  233 . For example, the rubbing cloth  233  may include velvet, rayon, and/or nylon. Various other kinds of a rubbing cloth are possible.  
         [0032]     In  FIG. 3C , the secondary alignment process is performed by irradiating light or an ion-beam on the alignment-processed alignment film  210 . For example, a plasma-beam or an electron-beam may be used. Various other beams are possible. The direction of the light irradiation may be perpendicular to that of the rubbing and the direction of the ion-beam irradiation may be the same as that of the rubbing. The light may be linearly polarized light or partially polarized light. The method of irradiation light may be an inclined irradiation method or a vertical irradiation method. Various other polarization and/or irradiation methods are possible.  
         [0033]     The ion used in the ion-beam is an argon ion. Various other ions are possible. An apparatus for irradiating the ion-beam or light is an entire-surface irradiation apparatus for irradiating an ion or light on the entire surface of the substrate  200 . In other embodiment, a scan type apparatus that irradiates light by scanning the substrate  200  may be used.  
         [0034]     When the primary alignment process, which is the rubbing process, is performed on the alignment film  210  and the secondary alignment process is performed, alignment uniformity may be secured, so that a coatable retarder with an improved contrast ratio may be formed. The optical axis of the fabricated compensation film may have an arbitrary angle with respect to the progression direction of the film. The control of the alignment direction of the alignment film also may be arbitrary.  
         [0035]     Referring to  FIG. 3D , a coatable retarder material  220  is coated on the alignment film  210  processed with the primary and secondary alignment process. The substrate  200  coated with the coatable retarder material is hardened using non-polarized ultraviolet light to be fixed as a film. In other embodiments, the fabrication process described above may be for use with an LCD having a coatable retarder of a rear structure and optical films such as various compensation films, as well as an LCD forming an in-Cell type coatable retarder.  
         [0036]     The rubbing process is performed on the alignment film, and the process of irradiating light or an ion-beam is further performed. As a result, the coatable retarder LC is aligned to improve alignment uniformity and prevent deterioration in an image quality. The alignment of the coatable retarder LC used for the compensation film may properly operate in the LCD having the coating-type compensation film.  
         [0037]     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.