Abstract:
In a multi-domain vertical alignment liquid crystal display, a pixel electrode, a common electrode and liquid crystal molecules are combined to form an LC alignment unit. In the LC alignment unit, at least two slits crossing each other at one point are created in the common electrode; and slanting slits are created in the pixel electrode, extending along diagonals of the pixel electrode. The slits in the common electrode and the slits in the pixel electrode stagger from one another.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
     This patent application is based on a U.S. provisional patent application No. 61/234,331 filed Aug. 17, 2009. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a liquid crystal display and an LC-aligning method of the same. The present invention more particularly relates to a multi-domain vertical alignment liquid crystal display (MVA-LCD) and an LC-aligning method of the same. 
     BACKGROUND OF THE INVENTION 
     Liquid crystal displays have now surpassed conventional CRT units and become a main stream in the market due to its compact appearance, energy-efficient feature, improved image quality and a wide range of applications. 
       FIG. 1  schematically shows components of a display area of an LCD panel. In the display area  212 , matrices of pixel electrodes  221 , thin film transistors (TFT)  222 , gate lines  223  and data lines  224  are formed on a lower glass substrate  211 . Above the pixel electrodes  221 , an alignment film  225  is provided. Oppositely, on almost the entire surface of an upper glass substrate  231  facing to the lower glass substrate  211 , a common electrode  233  and an alignment film  232  are formed. Furthermore, a liquid crystal (LC) layer  241  is sealed in the space between the lower alignment film  225  and the upper alignment film  232 . 
     With the alignment films  225  and  232 , liquid crystal molecules in the LC layer  241  are specifically and differentially oriented. The orientation of the alignment films is determined depending on the type of the LCD, and varies with the structures and/or material of the alignment films. For example, liquid crystal molecules arrange themselves twisted, e.g. in a helical structure, in a twisted nematic (TN) LCD before an electric field is applied. On the other hand, in a vertical alignment (VA) LCD, liquid crystal molecules naturally arrange themselves vertically. When no voltage is applied, the liquid crystal molecules of a VA LCD remain perpendicular to the substrate so as to render a black display. When a voltage is applied, the liquid crystal molecules change toward a horizontal direction, i.e. a direction parallel to the substrate, thereby allowing light to pass through and creating a white display. 
     As known, a VA LCD has good contrast when viewed vertically. However, image quality would be adversely affected if viewed at a relatively large viewing angle.  FIG. 2  illustrates viewing conditions of a VA LCD at different view positions. As shown, when liquid crystal molecules  33  tilt in response to a voltage, a gray color can be seen at a viewing position A right in front of the display. However, at viewing positions B and C, black and white colors are seen, respectively, due to different tilting angles of liquid crystal molecules  33  relative to different viewing positions. As a result, the displaying is distorted. 
     For remedying the defect, a multi-domain vertical alignment (MVA) LCD is developed, as illustrated in  FIG. 3 . As shown, a pixel is divided into a plurality of domains and liquid crystal molecules  33  are oriented differently in different domains, e.g. tilting counterclockwise in the left portion  31  and tilting clockwise in the right portion  32  of the pixel. Accordingly, under the similar condition of the grey color as illustrated in  FIG. 2 , the left portion  31  of the pixel is shown black while the right portion  32  of the pixel is shown white at the viewing position C. On the other hand, at the viewing position B, the left portion  31  of the pixel is shown white while the right portion  32  of the pixel is shown black. Therefore, the pixel is shown substantially even grey at arbitrary viewing positions. Generally, four domains are proper for wide angle viewing. 
     However, it is difficult in practice to divide a single pixel which has a size as small as 100×300 μm into four domains and control the liquid crystal molecules in the four domains to be oriented differentially. For facilitating differential orientation of liquid crystal molecules in different domains, a bump structure is provided either between the pixel electrode and its associated alignment film or the common electrode and its associated alignment film or both for automatic domain formation. Please refer to  FIG. 4 , which illustrates the principle of automatic domain formation with an example. 
     As shown in the example of  FIG. 4 , a bump structure  41  is formed between the pixel electrode  40  and the lower alignment film  42  at a boundary  43  of domains. Due to the presence of the bump structure  41 , some of the liquid crystal molecules  44  distributed above the bump structure  41  and supposed to stand vertically when no voltage is applied tilt. Then the tilting action propagates as indicated by arrows so that the liquid crystal molecules in the same domain are oriented consistently. Since the configuration of the bump structure makes the liquid crystal molecules distributed above the bump structure  41  tilt in different directions, the liquid crystal molecules in different domains are oriented differentially. 
     Alternatively or additionally, the bump structure  41  may be provided between the upper alignment film  45  and the common electrode  46 . 
     Although differential orientation of liquid crystal molecules in multi-domains can be achieved by way of bump structures as described above, the formation of the bump structure complicates the manufacturing process of the LCD panel. Therefore, slits are created in either the pixel electrode  50  or the common electrode  52  to replace the bump structure  41  to achieve the object of differential orientation of liquid crystal molecules, as illustrated in  FIG. 5A  or  FIG. 5B . The slits  51  are arranged in the pixel electrode  50  or the common electrode  52 . The shape of the slits  51  may be a circle or a “+” cross when viewed from top. In general, the use of the cross-shaped slits results in better transmittance than the use of circular slits. It is to be noted that for neat drawing, alignment films are not particularly shown in the figures. 
     However, due to the cross-shaped configuration of the slits, it takes a long response time to reach stable LC alignment since complicated movement of liquid crystal molecules is involved, including directing liquid crystal molecules  61   a  disposed above the slit  60  to a right angle, then directing liquid crystal molecules  61   b  near the slit  60  to rotate to an oblique angle and then expanding other liquid crystal molecules  61   c  to the oblique angle, as illustrated in  FIG. 6A-6C . 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention provides a MVA-LCD with slits, exhibiting improved transmittance and response time at the same time. 
     The present invention provides a multi-domain vertical alignment liquid crystal display, which comprises a first substrate and a second substrate disposed opposite to each other and having a space therebetween; a matrix of pixel electrodes formed on the first substrate, facing the second substrate, and including a plurality of alignment slits; a common electrode formed on the second substrate, facing the first substrate, and including a plurality of alignment slits; and liquid crystal molecules disposed in the space, each of which has an orientation varying with an electric field applied between the pixel electrodes and the common electrode and a position thereof relative to the alignment slits of the pixel electrodes and the common electrode; wherein the alignment slits of at least one of the pixel electrodes and the common electrode include at least two crossing slits having an included angle less than 90 degrees. 
     The present invention also provides a multi-domain vertical alignment liquid crystal display, comprising: a first substrate and a second substrate disposed opposite to each other and having a space therebetween; a matrix of pixel electrodes formed on the first substrate, facing the second substrate, and including a plurality of alignment slits; a common electrode formed on the second substrate, facing the first substrate, and including a plurality of alignment slits; and liquid crystal molecules disposed in the space, each of which has an orientation varying with an electric field applied between the pixel electrodes and the common electrode and a position thereof relative to the alignment slits of the pixel electrodes and the common electrode; wherein the alignment slits of at least one of the pixel electrodes and the common electrode include at least one slanting slit extending along a diagonal of the corresponding electrode. 
     The present invention further provides an LC-aligning method for use in a multi-domain vertical alignment liquid crystal display, wherein the multi-domain vertical alignment liquid crystal display comprises a pixel electrode, a common electrode and liquid crystal molecules forming an LC alignment unit, and the method comprises: creating at least two slits crossing each other at one point in the common electrode; and creating slanting slits in the pixel electrode, extending along diagonals of the pixel electrode; wherein the slits in the common electrode and the slits in the pixel electrode stagger from one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above contents of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
         FIG. 1  is a schematic diagram illustrating components of a display area of an LCD panel; 
         FIG. 2  is a schematic diagram illustrating viewing conditions of a VA LCD at different view positions; 
         FIG. 3  is a schematic diagram illustrating viewing conditions of a MVA LCD at different view positions; 
         FIG. 4  is a schematic diagram illustrating the principle of automatic domain formation with an example; 
         FIG. 5A  is a schematic diagram illustrating the principle of another automatic domain formation with an example; 
         FIG. 5B  is a schematic diagram illustrating the principle of a further automatic domain formation with an example; 
         FIG. 6A through 6C  are schematic diagrams illustrating the LC moving process before reaching stable LC alignment; 
         FIG. 7A  is a schematic diagram illustrating a slit configuration in a pixel electrode according to an embodiment of the present invention; 
         FIG. 7B  is a schematic diagram illustrating a slit configuration in a common electrode according to an embodiment of the present invention; 
         FIG. 8  is a schematic diagram illustrating a slit configuration in an LC alignment unit with overlapping common and pixel electrodes according to an embodiment of the present invention; 
         FIG. 9  is schematic diagram illustrating an azimuth angle (Φ) of liquid crystal molecules around a slit, which correlates to the width of the slit; and 
         FIG. 10  is a plot showing a correlation of an azimuth angle to a ratio of slit width to cell gap (Ws/d; horizontal axis). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In order to improve transmittance and response time of a MVA-LCD with slits, the slit configuration is particularly designed according to the present invention. The slit configuration to be designed includes shape, size and position of the slit, an aperture ratio of the pixel with the slit, etc., which affects performance of the display in a variety of ways. 
     Please refer to  FIGS. 7A and 7B  which illustrate slit configurations of a MVA-LCD according to an embodiment of the present invention. The LCD is exemplified to have a structure similar to that shown in  FIG. 1 . The slits are distributed in both common electrode  71  and pixel electrodes  70 . In this embodiment, each pixel is divided into a plurality of sub-pixels, e.g. four sub-pixels, by dividing each pixel electrode  70  into four sub-pixel electrodes  72  with slits  720 , as shown in  FIG. 7A . In each sub-pixel electrode  72 , slits  721  are formed. On the other hand, as shown in  FIG. 7B , slits  711  and  712  are formed in the common electrode  71 . It is to be noted that only one set of slits  711  and  712  is shown as one LC alignment unit is shown herein, but there could be a plurality of sets of slits  711  and  712  formed in the common electrode  71 .  FIG. 8  further illustrates relative configurations and positions of slits in an LC alignment unit in the view of overlapping common electrode  71  and sub-pixel electrode  72 , wherein the slits  711 / 712  and  721  do not overlap with each another. 
     It is not to be limited but it is preferred that each of the sub-pixel electrodes  72  has a substantially square shape, which is beneficial to fast response. In each LC alignment unit, a slit with a “+” cross shape  711  plus a “x” cross shape  712  is formed on the common electrode  71  of the MVA-LCD at a position opposite to a center of the sub-pixel electrode  72 . It is not to be limited but it is preferred that the slits  711  and  712  are centrally positioned for relatively fast response. Furthermore, four slanting slits  721  are formed at corners of the sub-pixel electrode  72 , as illustrated in  FIG. 7 . It is not to be limited but it is preferred that the number of slanting slits  721  is four since higher slit number, although resulting in fast response, would sacrifice transmittance. Both the slanting slit portions  712  on the common electrode  71  and the slanting corner slits  721  on the sub-pixel electrode  72  extend along diagonals of the sub-pixel electrode  72 , while staggering with each other. It is also possible to use the corner slits  721  together with just the “+” cross slit  711  without the “x” cross slit  712  or use the corner slits  721  together with just the “x” cross slit  712  without the “+” cross slit  711  to improve transmittance and response speed, but the response speed might not be as fast as the use of the corner slits  721  together with both the “+” cross slit  711  and the “x” cross slit  712 . However, modification or variation could be made based on the above descriptions in order to improve response speed. 
     On the common electrode side  71 , the width of the “+”-shaped slit portion  711  and the width of the “x”-shaped slit portion  712  are substantially equal, and substantially equal to the width of the slanting slits  721  and the pixel-dividing slits  720  on the sub-pixel electrode  72  side in view of balancing effect. The length of each of the two slanting slit portions  712  is about ⅓ of the diagonal distance of the sub-pixel area, penetrating through the center of the mark “+”. On the other hand, each of the slanting slits  721  extending along one of the diagonals on the sub-pixel electrode  72  side also has a length about ⅓ of the diagonal distance from a corner of the sub-pixel electrode  72  to a position opposite to the center of the “+”-shaped slit portion  711  on the common electrode  71 . It is not to be limited but it is preferred to design the pixel and common electrodes with the length of the slits as described above in view of balance between transmittance and response time. Smaller length of slits, e.g. ¼ the diagonal distance, is advantageous in high transmittance but results in slower response compared to the length of ⅓ the diagonal distance. Larger length of slits, e.g. ½ the diagonal distance, has comparable response time with the length of ⅓ the diagonal distance, but results in lower transmittance. 
     Preferably, the ratio of the width of the slit to the cell gap (the thickness of the liquid crystal layer) is particularly designed in order to improve liquid crystal alignment. The optimal ratio will be described with reference to  FIG. 9  and  FIG. 10 . 
     As shown in  FIG. 9 , when a slit  22  is disposed in an electrode  21 , the liquid crystal molecules  24  near the center of the slit commonly incline with the presence of the slit. As the width of the slit decreases, an azimuth angle (Φ) of a liquid crystal molecule is correspondingly reduced. The azimuth angle (Φ) of the liquid crystal molecules around the slit correlates to the width of the slit. Simulations under relative permittivity (ε)  3  and  6  are performed, and the correlation of the azimuth angle to the ratio of the width of the slit to the cell gap (Ws/d; horizontal axis) is shown in  FIG. 10 . As shown, the value of Ws/d ranged from at least 1.0 to 3.0 widens the azimuth angle of the liquid crystal molecule. More specifically, it is preferable that the value of Ws/d ranges from 1.2 to 2.5 for the liquid crystal molecule to have an azimuth angle of 45±10 degrees in view of fast response. 
     Furthermore, it is preferred that when a linear polarizer plate is used, the orientation of the “+”-shaped slit is consistent to an absorbing axis of the polarizer plate. It is also preferable that the length and the width of the sub-pixel are ranged from 30 μm to 70 μm in view of balance between transmittance and response time. If the size of an LC alignment unit, i.e. a sub-pixel, is too small, transmittance decreases. On the other hand, response time increases with the increase of the size of the LC alignment unit. 
     With the slit configuration described above, optional transmission and response time of a MVA-LCD can be achieved. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not to be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.