Abstract:
An in-plane switching mode liquid crystal display (IPS-LCD) A plurality of lengthwise-extending common electrodes is formed in each pixel area, and each common electrode has a curved profile. A plurality of lengthwise-extending pixel electrodes are formed in each pixel area, and each pixel electrode has a curved profile. The pixel electrodes are disposed in the intervals of the common electrodes, and two adjacent pixel electrode and common electrode form a sub-pixel area. Liquid crystal molecules are positioned within each sub-pixel area where is defined as at least two single-domain areas according to the rotating direction of the liquid crystal molecules. The two adjacent pixel electrode and common electrode within each sub-pixel area are not parallel to each other, thus a gradient in-plane electric field is generated therebetween.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a two-domain in-plane switching mode liquid crystal display (IPS-LCD) and, more particularly, to an electrode array of the IPS-LCD to generate a gradient electric field between pixel electrode and common electrode.  
           [0003]    2. Description of the Related Art  
           [0004]    In-Plane Switching mode liquid crystal display (IPS-LCD) has been used or suggested in wide view angle display technology to improve a conventional twisted nematic (TN)-LCD. The IPS-LCD has common electrodes and pixel electrodes formed on a lower glass substrate (TFT substrate) and an in-plane electrode field therebetween is generated to rearrange the LC molecules along the electrode field. Accordingly, the IPS-LCD device can improve viewing angle, contrast ratio and color shift over the conventional twisted nematic device.  
           [0005]    Depending on the electrode array of the common electrodes and the pixel electrodes, the IPS-LCD is classified into a single-domain type and a two-domain type. FIG. 1 is a top view showing an electrode array of a conventional single-domain IPS-LCD  10 . Two parallel gate lines  2  are orthogonal to two parallel data lines  4  to define a rectangular-shaped pixel area, in which a TFT device  5 , a comb-shaped pixel electrode  6  and a herringbone-shaped common electrode  8  are formed. The center wiring  8   a  of the common electrode  8  transversely extends along the center of the pixel area, and the bones  8   b  of the common electrode  8  lengthwise extend form the center wiring  8   a . The teeth  6   a  of the pixel electrode  6  are disposed in the intervals of the bones  8   b . When an outer voltage is applied to the IPS-LCD, an in-plane electric field is generated between the adjacent bone  8   b  and tooth  6   a.    
           [0006]    The strip-shaped teeth  6   a  are parallel to the strip-shaped bones  8   b , thus the in-plane electric field generated between the adjacent bone  8   b  and tooth  6   a  is uniform. The uniform electric field indicates that the differential (or gradient) of the strength (including direction) of the electric field at each point to the corresponding planar space is zero. This can make LC molecules have a uniform driving state at the same time, but this needs a higher driving voltage. A way to solve this problem is to reduce the distance between the adjacent bone  8   b  and tooth  6   a  by increasing the number of the teeth  6   a  and the bones  8   b  in the pixel area to lower the driving voltage of the IPS-LCD  10 . However, the line width of the teeth  6   a  and bones  8   b  cannot be narrower than about 3 to 4 microns, thus the aperture ratio of the pixel area is decreasing as the number of the teeth  6   a  and the bones  8   b  is increasing. The single-domain IPS-LCD  10  with the above-described array design of a uniform electric field cannot simultaneously give consideration to the requirements of high aperture ratio and low driving voltage.  
           [0007]    A two-domain IPS-LCD is developed for solving a problem of color shift of the single-domain IPS-LCD  10 . FIG. 2 is a top view showing an electrode array of a conventional two-domain IPS-LCD  20 . Two transverse-extending gate lines  12  and two lengthwise-extending data lines  14  define a pixel area  11 , in which a TFT device  15 , a comb-shaped pixel electrode  16  and a herring bone-shaped common electrode  18  are formed. Each of the teeth  16   a  of the pixel electrode  16  and the bones  18   b  of the common electrode  18  has a chevron-shaped profile and the inclined portion of the chevron-shaped profile is parallel to the adjacent electrode. When an outer voltage is applied to the IPS-LCD  20 , the directions of the electric fields at two sides of the center wiring  18   a  are different to make a part of LC molecules rotate in counterclockwise direction and another part of LC molecules rotate in clockwise direction. Therefore, the center wiring  18   a  demarcates the pixel area  11  as an upper single-domain area  11   a  and a lower single-domain area  11   b . Within each single-domain area  11   a  or  11   b , the inclined portion of the tooth  16   a  is parallel to the inclined portion of the bone  18   b  and thus a uniform in-plane electric field is generated therebetween. This still encounters the problems of high driving voltage and low aperture ratio.  
           [0008]    [0008]FIG. 3 is a top view showing the rotation of LC molecules in a uniform electric field of a conventional multi-domain IPS-LCD  30 . In a pixel area, three pixel electrodes  24   a ,  24   b  and  24   c  are disposed in the intervals of four common electrodes  26   a ,  26   b ,  26   c  and  26   d  between two adjacent data lines  22 . Each of the data lines  22 , the pixel electrodes  24  and the common electrodes  26  has a specific profile that is formed by lengthwise connecting at least two “&lt;” shape. In addition, an orientation layer is formed to cover the pixel area and rubbed in a direction shown as arrow A. When no outer voltage or an outer voltage below the threshold voltage is applied to the IPS-LCD  30 , LC molecules  28  are aligned along the direction shown as arrow A.  
           [0009]    Hereinafter, a single-domain area between the pixel electrode  24   b  and the common electrode  26   b  is an example of how the first LC molecule  28 I adjacent to the tip of the “&lt;” shape and the second LC molecule  28 II far away from the tip of the “&lt;” shape rotate. When an outer voltage is applied to generate an in-plane electric field that is lager than a threshold electric field, the directors of the first LC molecule  28 I and the second LC molecule  28 II rotate uniformly to become the LC molecules  28 I′ and  28 II′, respectively. Since the rotating angles, depending on the degree of the applied voltage, of the first LC molecule  28 I and the second LC molecule  28 II are the same, the rotated LC molecules  28 I′ and  28 II′ are parallel to each other. That is, the first LC molecule  28 I and the second LC molecule  28 II simultaneously start rotating, simultaneously stop rotating, and have the same rotating angle. There is no rotating moment generated between the first LC molecule  28 I and the second LC molecule  28 II, and no elastic distorted energy existed between the first LC molecule  28 I and the second LC molecule  28 II. Further, if the applied voltage is smaller than the threshold electric field, all the LC molecules  28  positioned in the single-domain area cannot rotate because they have the same threshold electric field. This still encounters the problems of high driving voltage and low aperture ratio.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is to provide an electrode array of a two-domain IPS-LCD to solve the problems caused by prior art.  
           [0011]    In the IPS-LCD, a plurality of lengthwise-extending common electrodes is formed in each pixel area, and each common electrode has a curved profile. A plurality of lengthwise-extending pixel electrodes are formed in each pixel area, and each pixel electrode has a curved profile. The pixel electrodes are disposed in the intervals of the common electrodes, and two adjacent pixel electrode and common electrode form a sub-pixel area. Liquid crystal molecules are positioned within each sub-pixel area where is defined as at least two single-domain areas according to the rotating direction of the liquid crystal molecules. The two adjacent pixel electrode and common electrode within each sub-pixel area are not parallel to each other, thus a gradient in-plane electric field is generated therebetween.  
           [0012]    Accordingly, it is a principal object of the invention to provide an IPS-LCD with a lower driving voltage without reducing aperture ratio.  
           [0013]    Yet another object of the invention is to provide an IPS-LCD with a gradient electric field within a single-domain area.  
           [0014]    It is a further object of the invention to provide an IPS-LCD to decrease the turn-on time.  
           [0015]    These and other objects of the present invention will become readily apparent upon further review of the following specification and drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a top view showing an electrode array of a conventional single-domain IPS-LCD.  
         [0017]    [0017]FIG. 2 is a top view showing an electrode array of a conventional two-domain IPS-LCD.  
         [0018]    [0018]FIG. 3 is a top view showing the rotation of LC molecules in a uniform electric field of a conventional multi-domain IPS-LCD.  
         [0019]    [0019]FIG. 4 is a top view showing a electrode array of a two-domain IPS-LCD according to the first embodiment of the present invention.  
         [0020]    [0020]FIG. 5 is a top view showing an electrode array of a two-domain IPS-LCD according to the second embodiment of the present invention. 
     
    
       [0021]    Similar reference characters denote corresponding features consistently throughout the attached drawings.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    The present invention provides an electrode array of a two-domain IPS-LCD to generate a gradient electric field between pixel electrode and common electrode by modifying the profile and width of the pixel electrode and the common electrode, such as fabricating a specific profile with at least a “&lt;” shape, increasing the line width near the tip of the “&lt;” shape, and adjusting the distance between adjacent tips of the “&lt;” shape. This forms a larger distance and a smaller distance between the adjacent pixel electrode and common electrode within a single-domain area, thus a gradient electric field is generated therebetween after applying an outer voltage. The gradient electric field indicates that the differential of the strength (including direction) of the in-plane electric field at each point to the corresponding planar area is not zero. Accordingly, a larger electric field is generated on a first area where the distance between the adjacent pixel electrode and common electrode is smaller, and a smaller electric field is generated on a second area where the distance between the adjacent pixel electrode and common electrode is larger. Since the effect of threshold electric field is defined on the first area where the largest electric field is generated, the first LC molecule positioned on the first area starts to rotate when the applied voltage is to generate a field larger than the threshold electric field. Also, as time is increasing, the rotating angle of the first LC molecule is increasing to convey elastic distorted energy to adjacent LC molecules positioned far away from the first area and drive the adjacent LC molecules to rotate. Furthermore, the angle θ between the alignment direction of the LC molecule and the in-plane electric field is not uniform. When the LC molecule is composed of a positive dielectric anisotropy material, the rotating range of angle θ is about 0°-89° (from 0°-60° or up to 89°, preferably about 890°˜60°). When the LC molecule is composed of a negative dielectric anisotropy material, the LC molecule initial alignment is at θ=90°, and the rotating range of angle θ is about 90°˜179° (from 90°˜150° or up to 179°). Moreover, compared with the conventional design of uniform electric field in an IPS-LCD, the LC molecules positioned far away from the first area has a lower effective threshold electric field in the present invention. Therefore, the two-domain IPS-LCD of the present invention has a lower driving voltage without decreasing the aperture ratio.  
         [0023]    Hereinafter, two preferred electrode arrays are shown to describe how to modify the profile of the pixel electrode and the common electrode to achieve the gradient electric field within a single-domain area. No matter how many curves are fabricated on the sidewall of the electrode within a sub pixel area, the smallest distance W between a pixel electrode and a common electrode in one single-domain area is the same with that in another single-domain area. Also, the smallest distance W in one single-domain area of the sub-pixel area is the same with that in one single-domain area of the adjacent sub-pixel area.  
         [0024]    [First Embodiment] 
         [0025]    [0025]FIG. 4 is a top view showing a electrode array of a two-domain IPS-LCD  40  according to the first embodiment of the present invention. Two transverse-extending gate lines  31  and two lengthwise-extending data lines  32  define a pixel area  42 , in which a TFT device  33 , two pixel electrodes  34   a  and  34   b  and three common electrodes  36   a ,  36   b  and  36   c  are formed. The pixel electrodes  34   a  and  34   b  are formed in the intervals of the common electrodes  36   a ,  36   b  and  36   c . Each of the data line  32 , the pixel electrodes  34   a  and  34   b  and the common electrodes  36   a ,  36   b  and  36   c  has the same profile that is formed by lengthwise connecting at least two “&lt;” shape. Also, an orientation layer is formed to cover the pixel area  42  and rubbed along a direction as shown by arrow A. Before applying an outer voltage to the IPS-LCD  40 , LC molecules  38  are aligned along the direction shown by arrow A. Furthermore, the light leak area between the data line  32  and the common electrode  36  is covered by a black matrix formed on a color filter substrate.  
         [0026]    Compared with the conventional common electrodes  26   a ,  26   b  and  26   c  having the same profile shown in FIG. 3, the three common electrodes  36   a ,  36   b  and  36   c  have different profiles. The first common electrode  36   a  is modified by widening the indentation of the &lt;-shaped profile of the common electrode  26   a , thus a smallest distance W is formed between the planarization of the first common electrode  36   a  and the tip of the &lt;-shaped pixel electrode  34   a . The second common electrode  36   b  is modified by widening the indentation of the &lt;-shaped profile of the common electrode  26   b  and widening the protrusion of the &lt;-shaped profile of the common electrode  26   b . Thus, a smallest distance W is formed between the indentation of the &lt;-shaped pixel electrode  34   a  and the protrusion of the second common electrode  36   b . Also, a smallest distance W is formed between the protrusion of the &lt;-shaped pixel electrode  34   b  and the planarization of the second common electrode  36   b . The third common electrode  36   c  is modified by widening the protrusion of the &lt;-shaped profile of the common electrode  26   c , thus a smallest distance W is formed between the indentation of the pixel electrode  34   b  and the protrusion of the third common electrode  36   c.    
         [0027]    Hereinafter, a single-domain area between the pixel electrode  34   b  and the second common electrode  36   b  is an example to explain the rotation of the first LC molecule  38 I positioned in a first area where the smallest distance W is formed and the second LC molecule  28 II positioned in a second area that is far away from the first area. After an outer voltage is applied to the IPS-LCD  40 , the highest electric field is generated at the tip of the &lt;-shaped pixel electrode  34   b , and the lowest electric field is generated at the top or bottom of the &lt;-shaped pixel electrode  34   b . This forms a gradient electric field between the pixel electrode  34   b  and the common electrode  36   b . When the applied voltage is increased to generate an electric field larger than the threshold electric field, the first LC molecule  38 I in the first area starts to rotate, but the second LC molecule  38 II still keeps in a static state because the local electric field in the second area is low. As the driving time is increasing, the rotating angle θ 1  of the first LC molecule  38 I is increasing to convey elastic distorted energy to the second LC molecule  38 II. In the meanwhile, the combination of the elastic distorted energy and the local electric field can drive the second LC molecule  38 II to rotate, wherein the rotation angle θ 2  of the second LC molecule  38 II is smaller than the rotating angle θ 1 .  
         [0028]    In a condition of generating a gradient electric field within a single-domain area, the first LC molecule  38 I and the second LC molecule  38 II do not simultaneously start rotating, nor have the same rotating angle. A rotating moment is generated between the first LC molecule  38 I and the second LC molecule  38 II to provide elastic distorted energy existed to rotate the second LC molecule  381 I even though the applied voltage is not enough. Thus, the effective threshold electric field of the second LC molecule  38 II is smaller than that in a uniform electric field. This achieves a lower driving voltage without decreasing the aperture ratio of the IPS-LCD  40 . Also, when a conventional driving voltage is applied to the IPS-LCD  40 , the turn-on time is faster than that in the conventional device.  
         [0029]    [Second Embodiment] 
         [0030]    [0030]FIG. 5 is a top view showing an electrode array of a two-domain IPS-LCD  50  according to the second embodiment of the present invention. In the second embodiment, the first pixel electrode  34   a  and the second common electrode  36   b  described in the first embodiment are further modified. Compared with the first embodiment shown in FIG. 4, the first pixel electrode  34  is modified by widening the indentation of the &lt;-shaped profile, and the second common electrode  36   b  is modified by removing the widened portion on the protrusion of the &lt;-shaped profile. Thus, the smallest distance W is formed between the planarization of the first pixel electrode  34   a  and the tip of the second common electrode  36   b.    
         [0031]    [Third Embodiment] 
         [0032]    With regard to the fabrication of the electrode array shown in FIG. 4, the third embodiment provides three methods of forming tri-layered conductive structure on a TFT substrate. The first method employs a first conductive layer of MoW to form the patterns of the gate line and the common electrode, and then employs a second conductive layer of Mo/Al/Mo to form the patterns of the data line and the pixel electrode. The second method employs a first conductive layer of MoW to form the patterns of the gate line and the common electrode, and then employs a second conductive layer of Mo/Al/Mo to form the pattern of the data line, and finally employs a third conductive layer of ITO to form the pattern of the pixel electrode. The third method employs a first conductive layer of MoW to form the patterns of the gate line and the common electrode with a double-&lt;profile as shown in FIG. 3. Then, a second conductive layer of Mo/Al/Mo is employed to form the patterns of the data line and the pixel electrode with a double-&lt;profile as shown in FIG. 3. Finally, a third conductive layer of ITO is employed to complete the profiles of the common electrode and the pixel electrode shown in FIG. 4, and the third conductive layer is electrically connected to the second conductive layer within each pattern.  
         [0033]    It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.