Abstract:
Disclosed is an LCD preventing color shift and simultaneously improving the characteristics of viewing angle. The LCD comprises a substrate and a common electrode and a pixel electrode having same spiral shape formed on the substrate and being opposed each other, wherein an electric field formed between the common electrode and the pixel electrode is radial shape.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display device (hereinafter LCD), more particularly to an LCD having a wide viewing angle without color shift. 
     2. Background of the Invention 
     An LCD has the characteristics of light weight, thin thickness and low power consumption. It has been used in various information display terminals or visual equipment. The major operating mode for the LCD can be the twisted nematic(“TN”) and the super twisted nematic(“STN”). Though they are presently commercially used in various LCD means, the characteristic of narrow viewing angle has still remained unsolved. 
     An In-Plane Switching(IPS) mode LCD has been suggested to solve the foregoing problems. 
     FIG. 1 is a plane view showing a general IPS mode LCD. A common electrode  11  is formed on the substrate  10  in the form of comb. A pixel electrode  13  is also formed on the substrate  10  in the form of comb, the comb-like electrodes  11  and  13  form a teeth shape. When an external voltage is applied between the common electrode  11  and the pixel electrode  13 , an electric field F 1  is formed therebetween and the liquid crystal molecules  15  are moved such that their long axes are parallel to the electric field F 1 . 
     There is a color shift in the direction of long and short axes due to the optical anisotropy since the long and short axes of liquid crystal molecule are different from each other in length. 
     As well known, the refractive anisotropy(or birefringence, Δn) has occurred due to the difference of the lengths of the long and the short axes. The refractive anisotropy(Δn) is also varied from the observer&#39;s viewing directions. Therefore a predetermined color is appeared on the region where the polar angle if 0 degree and the azimuth angles range of degrees 0, 90, 180 and 270 in spite of the white state. This results in color shift and more detailed description thereof is attached with reference to the equation 1. 
     
       
         T≈T 0  sin 2 (2χ)·sin 2 (π·Δnd/λ)  equation 1 
       
     
     wherein, 
     T : transmittance; 
     T 0 : transmittance to the reference light; 
     χ: angle between an optical axis of liquid crystal molecule and a polarizing axis of the polarizing plate; 
     Δn: birefringence; 
     d: distance or gap between the upper and lower substrates(thickness of the liquid crystal layer); and 
     λ: wavelength of the incident light. 
     So as to obtain the maximum transmittance T, the χ should be π/4 or the Δnd/λ should be π/2 according to the equation 1. As the Δnd varies with the birefringence difference of the LC molecules from viewing directions, the λ value is varied in order to make Δnd/λ to be π/2. According to this condition, the color corresponding to the varied wavelength λ appears. 
     Accordingly, as the value of Δn relatively decreases at the viewing direction “a” toward the short axes of the LC molecules, the wavelength of the incident light for obtaining the maximum transmittance relatively decreases. Consequently a blue color having shorter wavelength than a white color is emerged. 
     On the other hand, as the value of An relatively increases at the viewing direction “b” toward the short axes of the LC molecules, the wavelength of incident light relatively increases. Consequently a yellow color having a longer wavelength than the white color is emerged. This causes deterioration to the resolution of IPS-LCDs. 
     To prevent the above disadvantages caused by color shift, a method for forming multi-domain within a unit pixel has been suggested. According to this method, one portion of an alignment layer is rubbed to a first direction, the other portion of the alignment layer is rubbed to second direction symmetrized the first direction. This method, however, is required additional masks compared to the conventional process of liquid crystal molecules alignment. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the invention is to provide a LCD having wide viewing angle without color shift occurrence. 
     Another object of the present invention is to provide a method for fabricating such LCD mentioned above. 
     So as to accomplish the objects for the present invention, an LCD, comprising: a substrate; and a common electrode and a pixel electrode having same spiral shape formed on the substrate and being opposed each other, wherein an electric field formed between the common electrode and the pixel electrode is radial shape. 
     Further, the present invention provides an LCD having an electric field formed in parallel to surface of a substrate, comprising; a common electrode formed on the substrate with annular shape whose both edges are spaced each other; and pixel electrode formed on the substrate with annular shape whose both edges are spaced each other corresponding to the shape of the common electrode, wherein an electric field formed between the common electrode and the pixel electrode is radial shape and symmetric shape which is based on the kernel of respective electrode. 
     Also, to accomplish the above objects, the present invention provides a method for fabricating an LCD comprising the step of: forming a common electrode of spiral shape on a substrate; forming a pixel electrode of spiral shape to be opposed the common electrode; and coating an alignment layer on the surface of the substrate where the common electrode and the pixel electrode are disposed thereon, wherein a radial electric field is formed when a voltage is applied between the common electrode and the pixel electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a plane view showing a general IPS mode LCD. 
     FIG. 2 a  is a plane view showing an electrode structure and an arrangement of liquid crystal molecules in LCD according to one embodiment of the present invention. 
     FIG. 2 b  is a cross sectional view showing an electrode structure of LCD according to another embodiment of the present invention. 
     FIG. 3 is a plane view showing a liquid crystal molecules of negative dielectric anisotropy according to one embodiment of the present invention. 
     FIG. 4 is a plane view showing an electrode structure of an LCD according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, preferred embodiments of the invention will be described in detail with reference to attached drawings. 
     FIG. 2 a  and FIG. 2 b  are drawings for showing liquid crystal molecule of positive dielectric anisotropy when it is vertically aligned. A common electrode  31  and pixel electrode  33  of squared spiral structure are opposed. 
     An electric field is formed across the common electrode  31  and the pixel electrode  33  when an external voltage is applied therebetween. At this time, the electric field has a radial shape since the common electrode  31  and the pixel electrode  33  are spiral shaped, then liquid crystal molecules  35  are aligned along the electric field of the radial shape. Accordingly, long axes and short axes of liquid crystal molecules are observed at the same time by the user in every azimuth angle of the screen. Therefore, the color shift is not occurred, since optical anisotropy is compensated. 
     FIG. 2 b  is a cross-sectional view taken along the line a-b of FIG. 2 a  and the electric field is indicated as arrows. As shown in the drawings, liquid crystal molecules  35  are symmetrical due to the electric field&#39;s being symmetrical. A common electrode  31  is formed on the substrate used as an upper or a lower plate for LCD(not shown) and an insulating layer  30  is deposited thereon. A pixel electrode  33  is formed on the insulating layer  30  not to overlap the common electrode  31 . The height difference referenced as “h” between the upper of pixel electrode and the lower of the common electrode is decided according to the thickness of insulating layer  30 . Although, FIG. 2 b  shows the formation of pixel electrode  33  on the common electrode  31  by an insulating layer  30 , it is also available that the common electrode  31  and the pixel electrode  33  are formed on the same plane without the height difference “h”. Thereafter an alignment layer  36  is formed on the pixel electrode  33  and the insulating layer  30 . The alignment layer  36  is a homeotropic layer which needs no rubbing treatment. 
     Therefore according to this embodiment, liquid crystal molecules  35  are aligned radially in accordance with electrode  31 , 33  structure without some times rubbing process. 
     FIG. 3 shows liquid crystal molecules  45  of negative dielectric anisotropy used in the electrode structure of FIG. 2 a . The liquid crystal molecules  45  are aligned substantially in a same direction of the liquid crystal molecules  35  of FIG. 2 a  except the liquid crystal molecules  45  aligned such that their short axes are perpendicular to the direction of electric field. Namely, the liquid crystal molecules  45  are aligned in symmetric configuration based on kernel of the pixel electrode  43  of spiral shape. Accordingly, the optical anisotropy which depends upon the viewing angles III and IV is equal to each other. 
     Although FIG. 2 a  and FIG. 3 a  show a square type spiral structure, the square can be replaced with a circle or a triangle. 
     FIG. 4 shows another embodiment of the invention, where a common electrode  51  and a pixel electrode  53  are shaped of annular whose both edges are separated each other such as the letter “C”. Those electrodes include withdrawal rods  51 - 1  and  53 - 1  which are withdrawn to the exterior of the band so as to be applied an external voltage. When an external voltage is applied to the common electrode  51  and the pixel electrode  53  through those rods  51 - 1  and  53 - 1 , a radial electric field is formed and then the liquid crystal molecules  55  are aligned along the direction of the electric field. The optical isotropy of liquid crystal molecule  55  equals each other. That means, the equal value of optical anisotropy in a viewing angle (V) and in a viewing angle (VI) prevents color shift and enhances the characteristics of viewing angle. 
     The electrode structure of FIG. 4 may be applied to the liquid crystal molecules of positive dielectric anisotropy as well as those of negative dielectric anisotropy. The electrode are also changeable as long as electric field is symmetrical from all directions based on central portion of the electrode. 
     Further, the pixel and the common electrodes can be disposed on the same plane and can be disposed by insulating layer upwardly with a predetermined height. 
     As described above, the liquid crystal molecules are symmetrical against all direction of views since the common electrode and the pixel electrode are arranged for forming the electric field to be radial shape. Therefore the liquid crystal molecules have the same optical isotropy which prevents color shift of the conventional IPS mode devices and improving the characteristics of viewing angle. 
     Also this invention can reduce manufacturing steps and masks since additional rubbing processes are not required. 
     While only preferred embodiments have been discussed and described, various changes and modifications can be made without departing from the spirit and scope of the invention as defined by the following claims.