Patent Publication Number: US-6700635-B2

Title: Liquid crystal display panel

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a liquid crystal display (LCD) panel, and, in particular, to a thin film transistor (TFT) array panel for an LCD. 
     (b) Description of the Related Art 
     Generally, an LCD includes an upper panel having a common electrode and a plurality of color filters, a lower panel having pluralities of TFTs and pixel electrodes, and a liquid crystal layer between the two panels. The pixel electrodes and the common electrode are applied with different electric potentials to generate electric fields which change the alignment of liquid crystal molecules to control the light transmittance, thereby displaying images. 
     However, an LCD has a serious disadvantage of its narrow viewing angle. To overcome this disadvantage, various techniques for widening the viewing angle are suggested. Among them, there is a technique that the liquid crystal molecules are aligned perpendicular to the upper and the lower panels and pluralities of openings and/or projections are provided in the pixel electrodes and a common electrode facing the pixel electrodes. 
     In a conventional technique directed to the openings, the openings are provided in both the pixel electrodes and the common electrode to generate fringe field. The tilt directions of the liquid crystal molecules are adjusted by using the fringe field. A conventional technique related to the projections adjusts the tilt directions of the liquid crystal molecules by using electric fields which are distorted by the projections. In a European Patent EP 0 884 626 A2, various features of the projections are suggested. 
     Although the viewing angle of the LCD is improved as suggested by the above conventional techniques, the response time of the liquid crystal molecules is not still improved so that it is difficult to display dynamic images due to afterimages. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to improve the response time as well as the viewing angle of an LCD. 
     The LCD panel according to the present invention includes an insulating substrate and a pixel electrode formed on the substrate. The pixel electrode has pluralities of openings and X-shaped projections protruding from surface of the pixel electrode. The openings are disposed between the projections. An area enclosed by the X-shaped projections, the openings, and boundary of the pixel electrode has a planar shape of equilateral trapezoid or acute angle. A long side of the equilateral trapezoid is convex or curved. 
     Furthermore, the LCD panel according to the present invention includes a plurality of X-shaped members located under the respective projections and an insulating layer between the projections and the X-shaped members, thereby causing the projections to protrude. The insulating layer has a double-layered structure. The pixel electrode except for the projections is in direct contact with the substrate. The X-shaped members are made of metal and at least two thereof are connected to each other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a layout view of an LCD panel according to a first embodiment of the present invention; 
     FIG. 2 is a sectional view taken along line II-II′ in FIG. 1; 
     FIG. 3 is a layout view of an LCD panel according to a second embodiment of the present invention; 
     FIG. 4 is a sectional view taken along line IV-IV′ in FIG. 3; 
     FIG. 5 is a layout view of an LCD panel according to a third embodiment of the present invention; 
     FIG. 6 is a layout view of an LCD panel according to a fourth embodiment of the present invention; 
     FIG. 7 is a graph showing voltage-transmittance characteristics of the LCDs according to a conventional technique and the first and the second embodiments of the present invention; 
     FIG. 8 is a graph showing time-transmittance characteristics of the LCDs according to a conventional technique and the first and the second embodiments of the present invention; 
     FIG. 9A shows a matrix of pixels according to a preferred embodiment of the present invention when the distance between a data line and a pixel electrode is 5 μm; and 
     FIG. 9B shows a matrix of pixels according to a preferred embodiment of the present invention when the distance between a data line and a pixel electrode is 10 μm. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numerals refer to like elements throughout. It will be understood that when an element such as a layer, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
     FIG. 1 is a layout view of an LCD panel according to a first embodiment of the present invention and FIG. 2 is a sectional view taken along line II-II′ in FIG.  1 . 
     A plurality of gate lines  20  transmitting scanning signals and extending in a transverse direction and three X-shaped members  21  made up of the same material as the gate lines  20  are formed on an insulating substrate  10 , and the members  21  are arranged in a longitudinal direction. A gate insulating film  30  covers both the gate lines  20  and the X-shaped members  21 , and portions of the gate insulating film  30  near the center of the X-shaped member  21  have rectangular shapes. A plurality of data lines  60  transmitting image signals and extending in the longitudinal direction are formed on the gate insulating film  30 . A passivation film  70  is formed on the data lines  60 , and a pixel electrode  80  is formed on the passivation film  70  and the substrate  10 . The planar shapes of the gate insulating film  30  and the passivation film  70  are the same. 
     One pixel electrode  80  is located in a pixel area defined by the intersections of the two adjacent gate lines  60  and the two adjacent data lines  20 , and has two linear openings  81  extending in the transverse direction, which divide the pixel electrodes  80  into three rectangular portions arranged in the longitudinal direction. The portions are connected in turn, and each portion of the pixel electrode  80  has an X-shaped projection  71  formed by the X-shaped member  21  thereunder, and portions of the gate insulating film  30  and the passivation film  70  on the member  21 . Each projection  71 , which lies in the respective portion, defines four areas for one portion. Since the gate insulating film  30  and the passivation film  70  are also located on the gate lines  20  and the data lines  60 , the layered structure on the wires  20  and  60  acts as peripheral projections of the pixel electrode  80 . Each area enclosed by the projections  71 , the openings  81  and the peripheral projections is in a shape of equilateral trapezoid. The areas may be defined as the areas where the pixel electrode  80  is in direct contact with the substrate  10 . That is, each area has a planar shape of triangle, of which corner at the center of X-shape is chamfered. 
     This structure causes a splay arrangement or a bend arrangement of the liquid crystal molecules in each domain, which is defined as a portion of the liquid crystal layer over each divided area, to be reinforced to improve the response time. The response times τ on , which is a transition time from the state with 10% transmittance to the state with 90% transmittance, and τ off , the transition time from the state with 90% transmittance to the state with 10% transmittance are expressed as follows:                  τ   on     =     γ         Δɛ                   E   2         4      π       -         π   2       d   2          K                
            τ   off     =       γ                   d   2           π   2        K                 &lt;Equation  1&gt;                         
     where Δ∈ is dielectric anisotropy, γ is a coefficient of rotary viscosity, d is a cell gap, E is applied voltage, and K is a coefficient related to elasticity. The coefficient K is defined as follows:              K   =       K   11     +         K   33     -     2        K   22         4               &lt;Equation  2&gt;                         
     where K 11 , K 22  and K 33  are coefficients of splay elasticity, twist elasticity and bend elasticity of the liquid crystal molecules, respectively. As shown Equations 1 and 2, the response time τ on  or τ off  gets smaller as the coefficient K increases, that is, K 11  or K 33  increases, thereby improving the response time. 
     FIG. 3 is a layout view of an LCD panel according to a second embodiment of the present invention, and FIG. 4 is a sectional view taken along line IV-IV′ in FIG.  3 . 
     The layered structure of an LCD panel of the second embodiment is almost the same as that of the first embodiment. 
     The important features of the second embodiment different from the first embodiment are the shapes of X-shaped projections  71  of a pixel electrode  80  and a passivation film  70  and a gate insulating film  30  thereunder. That is, the X-shaped projection  71  has the width increasing as goes from the center toward ends, and does not include the rectangular shape which the first embodiment includes. Accordingly, each area enclosed by the projections  71 , the openings  81  and the boundaries of the pixel electrode  80  is in a shape of acute triangle, thereby reinforcing the splay arrangement or the bend arrangement near the projections. 
     FIG. 5 is a layout view of an LCD panel according to a third embodiment of the present invention. 
     The layered structure of an LCD panel of the third embodiment is almost the same as that of the first embodiment. 
     The primary features of the third embodiment different from the first embodiment are that common signal lines  22  connected to X-shaped members  21  are provided, and the outlines of a pixel electrode  80  are convexly curved to make the boundary lines of the divided areas to be curved. 
     In detail, the shape of each portion of the pixel electrode  80  divided by openings  81  is a curved rectangle so that each area, which is in a shape of equilateral trapezoid in FIG. 1, is in a shape of equilateral trapezoid with a curved long side. In addition, X-shaped members  21  protrude out of the apexes of the curved rectangle and so the pixel electrode  80 . The long sides of each opening  81  are concave, and the short sides are convex. At least two of the three X-shaped members  21  are connected to each other at the ends thereof, and the common signal line  22  is connected to the connection of the X-shaped members  21 . The common signal line  22  extends from the connection toward data lines  60  and is connected to the X-shaped members  21  of the adjacent pixel after intersecting the data lines  60 . 
     Furthermore, FIG. 5 shows a TFT located near the intersections of the gate lines  20  and the data lines  60 , and a connection structure of the TFT and the pixel electrode  80 . The TFT includes a gate electrode  26  which is a branch of the gate line  20 , an I-shaped source electrode  65  which is a branch of the data line  60 , and a drain electrode  66  which is spaced apart from the source electrode  65  and formed of the same layer as the data lines  60 . The drain electrode  66  is covered by a passivation film (reference numeral  70  in FIG.  1 ), and a contact hole  72  is provided on a portion of the passivation film  70  on the drain electrode  66 . The portion of the pixel electrode  80  adjacent to the TFT extends downward to be connected to the drain electrode  66  via the contact hole  72 . 
     FIG. 6 shows an LCD panel according to a fourth embodiment of the present invention. 
     The layered structure of an LCD panel of the fourth embodiment is almost the same as that of the third embodiment. However, the outlines of a pixel electrode  80  in this embodiment are much curved to make the boundary lines of the divided areas to be circled. 
     In the above embodiments, both the projections and the openings are provided on the TFT array panel. However, the projections may be provided on a color filter panel opposite to the TFT array panel, and the openings alone may be provided on the TFT array panel. Furthermore, the openings alone may be provided on both the panels. In this case, the planer shapes of the projections or the openings may be the same as those of the first to the fourth embodiments. 
     FIGS. 7 and 8 show transmittance as function of applied voltage and time lapse, respectively, for LCDs according to a conventional technique and the first and the second embodiments of the present invention. The curves A and E, B and F, C and G, and D and H are those of a conventional LCD, the first embodiment LCD, the second embodiment LCD, and a modified embodiment LCD where the interior angles of the triangular areas near the center of the X-shaped members are smaller than those of the second embodiment, respectively. 
     As shown in FIGS. 7 and 8, although the transmittances of the LCDs according to the present invention are slightly smaller than that of the conventional LCD, they are still satisfactory to displaying images with gray scale. In addition, the curves F, G and H of the present invention are flatter than the curve E of the conventional technique as shown in FIG.  8 . 
     The following table 1 shows the response time of the above-described LCDs of the present invention compared with the conventional technique. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 T on   
                 T off   
                 T total   
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 The Conventional Technique 
                 31.6 
                 13.8 
                 45.4 
               
               
                   
                 The First Embodiment 
                 21.3 
                 14.4 
                 35.7 
               
               
                   
                 The Second Embodiment 
                 27.2 
                 14.7 
                 41.9 
               
               
                   
                 The Modified Second Embodiment 
                 29.7 
                 14.3 
                 44.0 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 1, the response time of the LCDs of the present invention is shorter than that of the conventional technique. As described above, the improved response time of the present invention results from the reinforcement of the splay and the bend arrangements. 
     FIGS. 9A and 9B show a matrix of pixels when the distance between a data line and a pixel electrode is 5 μm and 10 μm, respectively. 
     As shown in the figures, an amount of brush or texture in the case of 10 μm distance is smaller than that in the case of 5 μm distance. That is, as the distance between the pixel electrodes increases, the alignment of the liquid crystal molecules becomes stable, thereby diminishing brush or texture. 
     The present invention reinforces the bend and the splay arrangement of the liquid crystal molecules so as to improve the response time. 
     While the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and substitutions can be made thereto without departing from the spirit and scope of the present invention as set forth in the appended claims.