Patent Publication Number: US-6906770-B2

Title: Array substrate for in-plane switching liquid crystal display device and method of fabricating the same

Description:
The present invention claims the benefit of Korean Patent Application No. 2002-88083, filed in Korea on Dec. 31, 2002, which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display (LCD) device and a method of fabricating an LCD device, and more particularly, to an array substrate for an in-plane switching (IPS) liquid crystal display device and a method of fabricating an in-plane switching (IPS) liquid crystal display device. 
     2. Discussion of the Related Art 
     In general, a liquid crystal display (LCD) device is driven by making use of optical anisotropy and polarization characteristics of a liquid crystal material. The LCD device commonly includes two substrates that are spaced apart and face each other, and a liquid crystal layer interposed between the two substrates. Each of the substrates includes electrodes that face each other, wherein a voltage supplied to each electrode induces an electric field perpendicular to the substrates between the electrodes. An alignment of liquid crystal molecules of the liquid crystal layer is changed by varying an intensity or direction of the applied electric field. Accordingly, the LCD device displays an image by varying light transmittance through the liquid crystal layer in accordance with the arrangement of the liquid crystal molecules. Thus, the LCD device has superior light transmittance and aperture ratio. 
     An LCD device that includes thin film transistors and a matrix configuration of pixel electrodes, which is referred to as an active matrix LCD device, is commonly used because of its high resolution and ability to quickly display moving images. However, the LCD device is disadvantageous due to its narrow viewing angle. To overcome the narrow viewing angle, an in-plane switching (IPS) LCD device has been developed that implements an electric field that is parallel to surfaces of the substrates. 
       FIG. 1  is a plan view of an array substrate for an in-plane switching (IPS) liquid crystal display (LCD) device according to the related art. In  FIG. 1 , a gate line  12  is formed along a first direction on a substrate  10 , and a common line  16  is formed along the first direction parallel to the gate line  12 . In addition, a data line  24  is formed along a second direction perpendicular to the first direction to cross the gate line  12  and the common line  16 . Accordingly, the data line  24  and the gate line  12  define a pixel area P. 
     A thin film transistor T is formed at the crossing of the gate line  12  and the data line  24  to function as a switching element. The thin film transistor T is composed of a gate electrode  14  that is connected to the gate line  12 , a source electrode  26  that is connected to the data line  24 , a drain electrode  28  that is spaced apart from the source electrode  26 , and a semiconductor layer  20  that is disposed between the gate electrode  14  and the source and drain electrodes  26  and  28 . The source electrode  26  may have a U-shape, and may surround a part of the drain electrode  28 , which may have a rod shape. 
     In the pixel area P, a pixel electrode  30  and a common electrode  17  are formed, wherein the pixel electrode  30  is connected to the drain electrode  28  and the common electrode  17  is connected to the common line  16 . The pixel electrode  30  is composed of an extension part  30   a , a plurality of vertical parts  30   b , and a horizontal part  30   c . The extension part  30   a  is connected to the drain electrode  28 , and the plurality of vertical parts  30   b , which are spaced apart from each other, vertically extend from the extension part  30   a . The horizontal part  30   c  overlaps the common line  16  and is connected to the plurality of vertical parts  30   b.    
     The common electrode  17  includes a horizontal portion  17   a  and a plurality of vertical portions  17   b , wherein the horizontal portion  17   a  overlaps the extension part  30   a  of the pixel electrode  30 . The plurality of vertical portions  17   b  vertically extend from the common line  16  and are alternately arranged with the plurality of vertical parts  30   b  of the pixel electrode  30 . The plurality of vertical portions  17   b  are connected to the horizontal portion  17   a  of the common electrode  17 . 
     The common line  16  and the horizontal part  30   c  of the pixel electrode  30  form a storage capacitor C that is parallel to a liquid crystal capacitor. Accordingly, the common line  16  functions as a first storage electrode and the horizontal part  30   c  functions as a second storage electrode. 
     Although not shown in the figure, the array substrate may further include an alignment layer, which may be rubbed at an angle of about 110 degrees with respect to the gate line  12 . 
       FIG. 2  shows voltage waveforms of a liquid crystal display device according to the related art. In  FIG. 2 , when a gate voltage Vg in an ON-state, a gate high voltage Vgh of about +18V is applied. When the gate voltage Vg is in an OFF-state, a gate low voltage Vgl of about −5V is applied. In addition, a data voltage Vd has an alternating polarity between Vdh to Vdl. When a thin film transistor is turned ON by the gate high voltage Vgh, a value of the data voltage Vd is transmitted to a pixel electrode, i.e., a liquid crystal capacitor, and the value of the data voltage Vg is maintained until the next gate high voltage Vgh is supplied. A common voltage Vcom may have a value within a range of about 3V to about 5V. 
     In general, in a normally black (NB) mode, when no voltage is supplied, a black image is displayed. However, in the liquid crystal display device including the array substrate of  FIG. 1 , light leakage may occur at an initial time. For example, in  FIG. 1 , the gate low voltage Vgl is initially supplied to the gate electrode, while the common voltage Vcom of about 3V to about 5V is also supplied to the common electrode. Thus, there is a voltage difference between the gate electrode and the common electrode, whereby light may leak between the gate electrode and the common electrode. 
       FIGS. 3 and 4  are enlarged plan views of region A of  FIG. 1  according to the related art and show a light leakage area.  FIG. 3  shows an original arrangement of liquid crystal molecules, and  FIG. 4  shows an initial arrangement of the liquid crystal molecules when gate voltage is initial in an OFF-state. In  FIG. 3 , the liquid crystal molecules  50  are originally arranged along a rubbing direction B that is at an angle of about 110 degrees with respect to the gate line  12 . In  FIG. 4 , when the gate voltage is initially in an OFF-state, there is a voltage difference between the gate electrode  14  and the common electrode  17  due to the gate low voltage and the common voltage. Thus, an electric field is induced between the gate electrode  14  and the common electrode  17 . The gate electrode  14  and the common electrode  17  have inclined sides that are parallel to the rubbing direction B. Accordingly, the liquid crystal molecules  50  between the gate electrode  14  and the common electrode  17  are arranged along the electric field E perpendicular to the rubbing direction B, while other liquid crystal molecules  50  are arranged along the rubbing direction B. Thus, light leakage occurs between the gate electrode  14  and the common electrode  17 . In order to prevent the light leakage, a black matrix may shield the portion between the gate electrode  14  and the common electrode  17 . However, light may be transmitted within the portion of the gate electrode  14  and the common electrode  17  due to misalignment between the two substrates of the liquid crystal display device, thereby making it difficult to achieve a display device displaying high quality images. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an array substrate for an in-plane switching (IPS) liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an array substrate for an in-plane switching (IPS) liquid crystal display device having improved light leakage prevention. 
     Another object of the present invention is provide a method of fabricating an array substrate for an in-plane switching (IPS) liquid crystal display device having improved light leakage prevention. 
     Another object of the present invention is to provide an array substrate for an in-plane switching (IPS) liquid crystal display device having improved image quality. 
     Another object of the present invention is to provide a method of fabricating an array substrate for an in-plane switching (IPS) liquid crystal display device having improved image quality. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an array substrate of an in-plane switching liquid crystal display device includes a substrate, a gate line and a data line on the substrate crossing each other to define a pixel area, a thin film transistor at the crossing of the gate line and the data line, the thin film transistor including a gate electrode having a first inclined side with respect to the gate line, a semiconductor layer, a source electrode, and a drain electrode, a common line parallel to the gate line, a common electrode having a second inclined side parallel to the first inclined side of the gate electrode within the pixel area, and a pixel electrode alternating with the common electrode within the pixel area, wherein the first inclined side of the gate electrode and the second inclined side of the common electrode are perpendicular to a rubbing direction of the array substrate. 
     In another aspect, a method of fabricating an array substrate of an in-plane switching liquid crystal display device includes forming a gate line and a data line on a substrate crossing each other to define a pixel area, forming a thin film transistor at the crossing of the gate line and the data line, the thin film transistor including a gate electrode having a first inclined side with respect to the gate line, a semiconductor layer, a source electrode, and a drain electrode, forming a common line parallel to the gate line, forming a common electrode having a second inclined side parallel to the first inclined side of the gate electrode within the pixel area, forming a pixel electrode alternating with the common electrode within the pixel area, and forming an alignment direction of the array substrate, the alignment direction being perpendicular to the first inclined side of the gate electrode and the second inclined side of the common electrode. 
     In another aspect, an array substrate of a liquid crystal display device includes a gate line and a data line on a substrate crossing each other to define a pixel area, a thin film transistor at the crossing of the gate line and the data line including a gate electrode having a first inclined side extending along a first direction with respect to the gate line, a common line parallel to the gate line, a common electrode having a horizontal portion connected to a plurality of vertical portions, the horizontal portion having a second inclined side extending along the first direction, and a pixel electrode having a plurality of vertical portions alternating with the plurality of vertical portions of the common electrode, wherein the array substrate includes a second direction perpendicular to the first direction. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a plan view of an array substrate for an in-plane switching (EPS) liquid crystal display (LCD) device according to the related art; 
         FIG. 2  is a schematic view showing voltage waveforms of a liquid crystal display device according to the related art; 
         FIGS. 3 and 4  are enlarged plan views of region A of  FIG. 1  according to the related art; 
         FIG. 5  is a plan view of an exemplary array substrate for an IPS LCD device according to the present invention; and 
         FIG. 6  is an enlarged plan view of region I of  FIG. 5  according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Reference will now be made in detail to the illustrated embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 5  is a plan view of an exemplary array substrate for an IPS LCD device according to the present invention. In  FIG. 5 , a gate line  112  may be formed along a first direction on a substrate  100 , and a common line  116  may be formed along the first direction parallel to the gate line  112  on the substrate  100 . In addition, a data line  124  may be formed on the substrate  100  along a second direction perpendicular to the first direction to cross the gate line  112  and the common line  116 , wherein the data line  124  and the gate line  112  may define a pixel area P. 
     A thin film transistor T may be formed at the crossing of the gate and data lines  112  and  124  to function as a switching element. The thin film transistor T may include a gate electrode  114  that may be connected to the gate line  112 , a source electrode  126  that may be connected to the data line  124 , a drain electrode  128  that may be spaced apart from the source electrode  126 , and a semiconductor layer  120  between the gate electrode  114  and the source and drain electrodes  126  and  128 . 
     In the array substrate, the source electrode  126  may have a U-shape, and the drain electrode  128  may have a rod shape that may be surrounded by the source electrode  126 . Accordingly, a channel of the thin film transistor T, which corresponds to the semiconductor layer  120  between the source electrode  126  and the drain electrode  128 , may also have a U-shape. Since a length of the channel decreases and a width of the channel increases, current flowing through the channel may increase. 
     In  FIG. 5 , the gate electrode  114  may have a first inclined side extending along a first direction with respect to the gate line  112 . The first inclined side (i.e., the first direction) of the gate-electrode  114  may form an angle θ of about 160 degrees with the gate line  112 , wherein the thin film transistor T may be positioned in an left-upper portion of the pixel area P. In addition, a pixel electrode  130  and a common electrode  117  may be formed in the pixel area P, wherein the common electrode  117  may include a horizontal portion  117   a  and a plurality of vertical portions  117   b . First end portions of the vertical portions  117   b  may be connected to the common line  116 , and second end portions of the vertical portions  117   b  may be connected to the horizontal portion  117   a.    
     The pixel electrode  130  may include an extension portion  130   a , a plurality of vertical portions  130   b , and a horizontal portion  130   c . The extension portion  130   a  may extend from the drain electrode  128 , and may overlap the horizontal portion  117   a  of the common electrode  117 . The plurality of vertical potions  130   b  may be alternatingly arranged with the plurality of vertical portions  117   b  of the common electrode  117 . First end portions of the vertical portions  130   b  may be connected to the extension portion  130   a  of the pixel electrode  130 , and second end portions of the vertical portions  130   b  may be connected to the horizontal portion  130   c  of the pixel electrode  130 . The horizontal portion  130   c  of the pixel electrode  130  may overlap the common line  116 , thereby forming a storage capacitor. 
     The horizontal portion  117   a  of the common electrode  117  may have a second inclined side extending along the first direction with respect to the gate line  112  that may face and be parallel to the first inclined side of the gate electrode  114 . The first inclined side and the second inclined side (i.e., the first direction) may be perpendicular to a rubbing direction G (i.e., second direction) that may form an angle of about 110 degrees with respect to the gate line  112 . Thus, when a gate low voltage is supplied at an initial time, an electric field induced between the gate electrode  114  and the horizontal portion  117   a  of the common electrode  117  may be parallel to the rubbing direction G 
       FIG. 6  is an enlarged plan view of region I of  FIG. 5  according to the present invention. In  FIG. 6 , only the gate line  112  (in FIG.  5 ), the gate electrode  114 , and the common electrode  117  are shown for the sake of brevity. As shown in  FIG. 6 , the gate electrode  114  and the horizontal portion  117   a  of the common electrode  117  may have inclined sides forming an angle of about 160 degrees with respect to the gate line  112 , and the gate electrode  114  and the horizontal portion  117   a  of the common electrode  117  may face each other. Thus, an electric field H induced between the gate electrode  114  and the horizontal portion  117   a  of the common electrode  117  may be parallel to the rubbing direction G, which may form an angle of about  110  degrees with respect to the gate line  112 . Accordingly, liquid crystal molecules  150  between the gate electrode  114  and the horizontal portion  117   a  of the common electrode  117  may be arranged parallel to the rubbing direction G similarly to other liquid crystal molecules  150 . 
     According to the present invention, although the gate low voltage may be initially supplied, light leakage may not occur and a clear black image may be displayed. Thus, image quality of the LCD device may be improved. In addition, an aligning margin for attaching two substrates of the liquid crystal display device may be reduced. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the array substrate for in-plane switching liquid crystal display device of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.