Abstract:
A method of manufacturing an active matrix substrate is presented. The method includes forming a transistor having a gate line, a semiconductor layer, an insulating layer between the gate line and the semiconductor layer, a source electrode, and a drain electrode; forming a pixel electrode comprising a first sub-pixel electrode and a second sub-pixel electrode; forming an auxiliary coupling electrode connected to the second sub-pixel electrode through a first contact hole; and forming the first sub-pixel electrode through a second contact hole connected to the drain electrode of the transistor. The auxiliary coupling electrode and the first sub-pixel electrode overlap each other such that the second sub-pixel electrode is capacitively coupled to the first sub-pixel electrode and the auxiliary coupling electrode and the electrode part form a capacitor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a division of U.S. patent application Ser. No. 11/145,303 filed Jun. 3, 2005, which claims priority of Korean Patent Application No. 10-2004-0040383 filed Jun. 3, 2004, the contents of which are herein incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a liquid crystal display, and more particularly to an LCD having a wide viewing angle. 
         [0004]    2. Description of the Related Art 
         [0005]    A liquid crystal display (LCD), which is one of the most widely used flat panel displays, typically includes two substrates having field-generating electrodes thereon and a liquid crystal (LC) layer interposed between the two substrates. To produce an image on the LCD, voltage signals are applied to the field-generating electrodes of the substrates to generate an electric field across the LC layer and thus control the orientation of LC molecules of the LC layer. The controlled orientation of the LC molecules creates an image by adjusting the polarization of incident light. 
         [0006]    In comparison with conventional cathode ray tube (CRT) displays, an LCD has a narrower viewing angle. A number of techniques have been proposed to overcome this drawback, one of the techniques being known as Vertically Aligned (VA) mode LCD. In the VA mode LCD, the liquid crystals are vertically aligned to the substrate surface plane at the off-state where no voltage or off-voltage is applied, so that the incident light leakage through the LCD is almost zero in the black state (off state with Cross-Nicole state). Due to this minimized light leakage, the Contrast Ratio (CR) of the VA mode LCD, which represents the ratio of white state luminance to black state luminance, is higher than that of any other mode LCDs, such as Twisted-Nematic (TN) mode LCD and In-Plane-Switching (IPS) mode LCD, which employs another enhanced viewing angle technique. 
         [0007]    As described above, the VA mode LCD provides an improved viewing angle from the standpoint of Contrast Ratio. However, in the VA mode LCD, like other mode LCDs, the color patterns viewed from the vertical direction to the LCD and from the slanted direction are recognized somewhat differently. This phenomenon (“Color Shift”) comes from the fact that the light path changes depending on the viewing angle. Accordingly, the voltage-transmittance curve (the V-T curve) or the gamma curve also change relative to the viewing angle. 
       SUMMARY 
       [0008]    The present invention is directed to the structure of an LCD panel that can produce an improved visual image. 
         [0009]    In accordance with an embodiment of the present invention, an active matrix substrate of the LCD panel includes a transistor, a pixel electrode having a first sub-pixel electrode and a second sub-pixel electrode, a first electrode connected to the second sub-pixel electrode, and a second electrode connected to the source electrode of the transistor and the first sub-pixel electrode. The second electrode is coupled to the first electrode so as to form a capacitor. The active matrix substrate can include a protective insulating layer between the second electrode and the pixel electrode, and the protective insulating layer can include a color filter layer. The semiconductor layer of the transistor can extend such that a portion of the semiconductor layer has the same boundary as the second electrode. 
         [0010]    In accordance with another embodiment of the present invention, a LCD panel includes an active matrix substrate, a patterned substrate disposed opposite to the active matrix substrate, and a liquid crystal layer interposed between the active matrix substrate and the patterned substrate. The active matrix substrate includes a transistor, a pixel electrode having a first sub-pixel electrode and a second sub-pixel electrode, a first electrode connected to the second sub-pixel electrode, and a second electrode connected to the source electrode of the transistor and the first sub-pixel electrode. The second electrode is coupled to the first electrode so as to form a capacitor. The patterned substrate has an aperture that divides the liquid crystal layer into a plurality of domains. The first and second electrodes are formed along a portion of the aperture of the patterned substrate. 
         [0011]    In accordance with another embodiment of the present invention, a method is provided for operating a liquid crystal display comprising a first substrate, a second substrate, and a liquid crystal layer interposed between the first and second substrates. The method comprises: applying a data voltage signal via a data line to a pixel electrode, the pixel electrode comprising a first sub-pixel electrode and a second sub-pixel electrode; applying the data voltage signal to the first sub-pixel electrode; reducing the data voltage signal to a reduced data voltage signal; and applying the reduced data voltage signal to the second sub-pixel electrode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a layout of an active matrix TFT (Thin-Film-Transistor) substrate according to an embodiment of the invention. 
           [0013]      FIG. 2  shows a layout of a patterned substrate matched to the active matrix TFT substrate of  FIG. 1 . 
           [0014]      FIG. 3  shows a layout of a liquid crystal display (LCD) panel, in which the active matrix TFT substrate of  FIG. 1  and the patterned substrate of  FIG. 2  are overlapped. 
           [0015]      FIG. 4  is a cross-sectional view of  FIG. 3  along the line IV-IV′. 
           [0016]      FIG. 5  is a circuit diagram of the LCD panel of  FIG. 3 . 
           [0017]      FIG. 6  shows a layout of a liquid crystal panel according to another embodiment of the invention. 
           [0018]      FIG. 7  is a cross-sectional view of  FIG. 6  along the line VII-VII′. 
           [0019]      FIG. 8  shows a layout of an LCD panel according to another embodiment of the invention. 
           [0020]      FIG. 9  shows a layout of an active matrix TFT substrate according to another embodiment of the invention. 
           [0021]      FIG. 10  shows a layout of a patterned substrate matched to the active matrix TFT substrate of  FIG. 9 . 
           [0022]      FIG. 11  shows a layout of an LCD panel, in which the active matrix TFT substrate of  FIG. 9  and the patterned substrate of  FIG. 10  are overlapped. 
       
    
    
       [0023]    Use of the same reference symbols in different figures indicates similar or identical items. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]      FIGS. 1 to 4  illustrate portions of an LCD panel  500  in accordance with an embodiment of the invention.  FIG. 3  shows the layout of LCD panel  500 .  FIG. 1  shows the layout of an active matrix TFT substrate  100  for LCD panel  500 , and  FIG. 2  shows the layout of a patterned substrate  200  matched to active matrix TFT substrate  100  of  FIG. 1 .  FIG. 4  is the cross sectional view of  FIG. 3  along the line IV-IV′. 
         [0025]    Referring to  FIG. 4 , LCD panel  500  comprises an active matrix TFT substrate  100 , a patterned substrate  200 , and liquid crystals  310  interposed between substrates  100  and  200 . Aligning layers  11  and  21  of substrates  100  and  200 , respectively, face each other such that liquid crystals  310  can be vertically aligned to substrates  100  and  200 . In addition, polarizers  12  and  22  are provided on LCD panel  500  as shown in  FIG. 4 . 
         [0026]    Referring to  FIGS. 1 ,  4 , and  5 , active matrix TFT substrate  100  comprises a number of pixels  191 . Each pixel  191  includes a pixel electrode  190 , which is divided into sub-pixel electrodes  190   a  and  190   b . Between sub-pixel electrodes  190   a  and  190   b  is an aperture  193 . 
         [0027]    In the operation of LCD panel  500  in accordance with an embodiment of the present invention, when a data voltage signal Va is applied from a data line  171 , data voltage signal Va is applied to sub-pixel electrode  190   a  through a switching transistor Q, shown in  FIG. 5 . On the other hand, while voltage level Va is applied to sub-pixel electrode  190   a , a voltage level Vb reduced by a coupling capacitor Cpp is applied to sub-pixel electrode  190   b  that is connected to an auxiliary coupling electrode  136  through a contact hole  186 . Sub-pixel electrode  190   a  is connected to a coupling electrode  176  through a contact hole  185 . 
         [0028]    Consequently, when data voltage signal Va is applied to pixel  191  through switching transistor Q, two different voltage levels are respectively applied to sub-pixel electrodes  190   a  and  190   b . Thus, pixel  191  comes to have two sub-pixel areas having different light transmittance from each other (sometimes referred to as the gamma curve mixing effect), so that the color shift in wide viewing angle dramatically reduces. In other words, the gamma curve (gray-luminance curve) formed by merging the gamma curve of a lower voltage area and the gamma curve of a higher voltage area is less distorted than the gamma curve of a single average voltage area, when viewed at an angle. The off-axis image quality can be improved by providing sub-pixels having slightly different LC molecule tilt angles produced by the sub-pixel voltage level differential. 
         [0029]    According to an embodiment of the present invention, a coupling capacitor Cpp (shown in  FIG. 5 ) is formed by auxiliary coupling electrode  136 , coupling electrode  176 , and a gate insulator  140  (shown in  FIG. 4 ). 
         [0030]    Referring to  FIGS. 1 ,  3  and  4 , the structure of active matrix substrate  100  is explained in detail. Active matrix TFT substrate  100  includes a number of gate lines  121  on a substrate  110 , which deliver scanning (or gate) signals. Each gate line  121  extends to a gate electrode  124  of switching transistor Q. According to this embodiment, at the end of gate lines  121 , gate pads  129  are formed to connect gate lines  121  to an external driving circuit. The external driving circuit can be formed on a separate chip or on active matrix TFT substrate  100 . When the driving circuit is integrated on active matrix TFT substrate  100 , gate pads  129  may be omitted. On the same layer as gate line  121 , a storage electrode  133  is formed so as to form a storage capacitor Cst. Storage capacitor  133  connects to an adjacent storage capacitor through a storage line  131 . Auxiliary coupling electrode  136  is also on the same layer as gate line  121 . Sputtering processes can be used to form gate lines  121 , storage lines  131 , and auxiliary coupling electrode  136  so as to have a single-layer structure or a multi-layer structure comprising Al (or Al alloy), Mo (or Mo alloy), Cr (or Cr alloy), Ti (or Ti alloy), Ta (or Ta alloy), Ag (or Ag alloy), or Cu (or Cu alloy). 
         [0031]    For example, gate lines  121 , storage lines  131 , and auxiliary coupling electrode  136  can have the structure of two layers including a lower layer composed of Al—Nd alloy and an upper layer composed of Mo. 
         [0032]    Gate insulator  140  comprising silicon nitride is formed over gate lines  121 , storage lines  131 , and auxiliary coupling electrode  136  by chemical vapor deposition (CVD). An exemplary thickness of gate insulator  140  is 1000-5000 Å. Gate insulator  140  is thinner than protective layer  180 , which will be discussed later. 
         [0033]    The capacitance of coupling capacitor Cpp is inversely proportional to the thickness of gate insulator  140 , and is proportional to the overlapping area between auxiliary coupling electrode  136  and coupling electrode  176 . Accordingly, since the thickness of gate insulator  140  is relatively small, it is possible to obtain a sufficient coupling capacitance with a relatively small overlapping area between auxiliary coupling electrode  136  and coupling electrode  176 . The reduced overlapping area improves the transmittance of LCD panel  500 . 
         [0034]    A semiconductor layer, such as an amorphous silicon (a-Si) layer is formed by CVD over gate insulator  140 . The a-Si layer, by patterning, forms a channel area  154  in switching transistor Q and a semiconductor layer  151  under data line  171 . 
         [0035]    An a-Si layer highly doped with n-type impurity is formed by CVD and is patterned so as to form a source ohmic contact layer  163  and a drain ohmic contact layer  165 . The patterning of the a-Si layer highly doped with n-type impurity also produces a buffer layer  161  between semiconductor layer  151  and data line  171 . In general, the a-Si layer for channel area  154  and semiconductor layer  151  and the a-Si layer highly doped with n-type impurity for ohmic contact layers  163 ,  165  may be sequentially formed by CVD and are simultaneously patterned. 
         [0036]    Over gate insulator  140 , semiconductor layer  151 , channel area  154 , and ohmic contact layers  163  and  165  are formed data line  171 , a drain electrode  175 , and a source electrode  173  of switching transistor Q. Source electrode  173  extends from data line  171  so that data signals are supplied to source electrode  173  through data line  171 . At an end portion of data line  171 , data pad  179  is formed to connect data line  171  to an external data driving circuit. Alternatively, the data driving circuit can be integrated on active matrix TFT substrate  100  and directly connected to data line  171 . 
         [0037]    Coupling electrode  176  which extends from drain electrode  175  is formed when data line  171  is formed. Data line  171 , drain electrode  175 , and coupling electrode  176  may be formed by sputtering and patterning of a metal layer comprising, e.g., Al (Al alloy), Mo (Mo alloy), Cr (Cr alloy), Ti (Ti alloy), Ta (Ta alloy), Ag (Ag alloy), or Cu (Cu alloy). Data line  171 , drain electrode  175 , and coupling electrode  176  can have a single layer structure or a multi-layer structure. An exemplary three-layer structure of data line  171 , drain electrode  175 , and coupling electrode  176  can have an Al middle layer and upper and lower layers composed of Mo nitride or Mo—Nb alloy. 
         [0038]    A protective layer  180  comprising a first protective layer  801  and a second protective layer  802  is formed over active matrix TFT substrate  100  after the formation of the data line  171 , drain electrode  175 , and coupling electrode  176 . 
         [0039]    First protective layer  801  is formed of silicon nitride with a thickness of 1000-5000 Å by CVD. Second protective layer  802  is formed of an organic material with thickness of 1.0-5.0 μm by a slit or spin coating method. 
         [0040]    Second protective layer  802  has a relatively low dielectric constant, which can be 1.0-5.0, and has a large thickness, which can be above 1.0 μm (preferably 1.0-5.0 μm). Accordingly, because the capacitance between pixel electrode  190  and data line  171  is minimized, the area of pixel electrode  190  can be increased. The increase of the area of pixel electrode  190  increases the transmittance of LCD panel  500 . In other embodiments, first protective layer  801  can be omitted. In accordance with another embodiment of the present invention, second protective layer  802  can include a color filter layer. In this case, a color filter layer  230  of patterned substrate  200  is removed. 
         [0041]    Protective layer  180  includes contact holes  182  and  185 , which expose an end of data line  171  and a portion of drain electrode  175 , respectively. Protective layer  180  also includes contact holes  181  and  186 , which expose an end of gate line  121  and a portion of auxiliary coupling electrode  136 , respectively. Contact holes  181  and  186  extend through gate insulator  140 . 
         [0042]    Over protective layer  180 , pixel electrode  190  including a number of sub-pixels electrodes  190   a  and  190   b  and redundant pads  81  and  82  is formed by sputtering of a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO). 
         [0043]    Pixel electrode  190  is divided into sub-pixel electrodes  190   a  and  190   b  by aperture  193 . Sub-pixel electrode  190   a  connects through contact hole  185  to drain electrode  175  of switching transistor Q. Sub-pixel electrode  190   b  connects to auxiliary coupling electrode  136  through contact hole  186 , and auxiliary coupling electrode  136  is coupled to coupling electrode  176  to form a capacitor. 
         [0044]    In LCD panel  500 , voltage level Vb reduced by coupling capacitor Cpp is applied to the sub-pixel electrode  190   b  while data voltage level Va is applied to sub-pixel electrode  190   a.    
         [0045]    Connecting pads  81  and  82 , which are often made of ITO or IZO, are formed to connect gate pad  129  and data pad  179 , respectively, to external driving circuits. Contact holes  181  and  182  connect pads  81  and  82  to gate pad  129  and data pad  179 , respectively. In case that the external driving circuits are formed on active matrix TFT substrate  100 , the external driving circuits may directly connect to gate and data lines  121  and  171 . Pads  81 ,  82 ,  129 , and  179  and contact holes  181  and  182  are omitted. 
         [0046]    Over pixel electrode  190 , aligning layer  11  for aligning liquid crystal molecules  310  in the direction perpendicular to active matrix TFT substrate  100  is formed of a polymeric material such as polyimide. 
         [0047]    Referring to  FIGS. 2 ,  3  and  4 , the structure of patterned substrate  200  will be explained. A black matrix  220  is formed on a base substrate  210  so as to prevent light leakage caused by the electric field interference by date line  171  or gate line  121 . 
         [0048]    A color filter layer  230 , which includes red, green, and blue elements, is formed on black matrix  220  and substrate  210  so as to express various combinations of colors. 
         [0049]    An overcoat layer  250  is formed over color filter layer  230  so that the surface of overcoat layer  250  is substantially flat. Then, a common electrode  270 , which is made of a transparent conductive material such as ITO or IZO, is formed by sputtering on overcoat layer  250 . Common electrode  270  includes a number of apertures  271 . 
         [0050]    The arrangement of apertures  193  and  271  are designed so as to control the liquid crystal domain by directing liquid crystals  310  into pre-determined orientations. The average orientation of liquid crystals is preferably at a 45° angle relative to the polarizing axes of the polarizing films of the LCD display. Generally, the polarizing axes of polarizing films are parallel or perpendicular to the data line and the average orientation of liquid crystals is perpendicular to that of the aperture. Accordingly, the apertures may be formed to be diagonally oriented. Depending on the layout of apertures  193  and  271 , the liquid crystal texture can be reduced, and light-transmittance can be improved. For example, notches  262  of aperture  271  can provide more precise control of liquid crystals  310  in a certain region. 
         [0051]    In another embodiment according to the present invention, alternative means such as protrusions can be used for the domain control. These protrusions are generally formed on the pixel electrode and/or the common electrode and are made of an organic material. It is also possible to mix protrusions and apertures as domain controlling means. For example, the apertures are formed on the active matrix TFT substrate  100  and the protrusions are formed on patterned substrate  200 , or the protrusions are formed on active matrix TFT substrate  100  and the apertures are formed on patterned substrate  200 . 
         [0052]    After being fabricated as described above, active matrix TFT substrate  100  and patterned substrate  200  are assembled with each other, and liquid crystals  310  are interposed between active matrix TFT substrate  100  and patterned substrate  200 . 
         [0053]      FIGS. 6 and 7  illustrate LCD panel  500  in accordance with another embodiment of the present invention.  FIG. 6  is the layout of LCD panel  500 , and  FIG. 7  is the cross sectional view of LCD panel  500  along the line VII-VII′ of  FIG. 6 . 
         [0054]    LCD panel  500  of  FIGS. 6 and 7  is basically the same as LCD panel  500  of  FIGS. 3 and 4 , except with respect to the layout of the semiconductor channel area and ohmic contact layer of the switching transistor Q. Accordingly, detailed explanation on the common structure will be omitted. 
         [0055]    Referring to  FIG. 7 , semiconductor channel area  154 , a semiconductor layer  154 ′ extending from semiconductor channel area  154 , and semiconductor layer  151  have substantially the same boundary as data line  171 , source electrode  173 , drain electrode  175 , and coupling electrode  176 . This same boundary profile results in when the a-Si layer for semiconductor channel area  154  and semiconductor layers  151  and  154 ′ and the metal layer for data line  171 , source electrode  173 , drain electrode  175 , and coupling electrode  176  are simultaneously patterned. 
         [0056]    The simultaneous patterning, after the deposition of an a-Si layer, an a-Si layer highly doped with n-type impurity, and a metal layer, uses a slit-mask photo resist pattern. In the slit-mask photo resist pattern, a slit with a half-tone exposure is formed at a region corresponding to channel area  154  so as to control the depth of patterning. This simultaneous patterning of  FIG. 7 , in comparison to the two-step patterning of  FIG. 4 , reduces fabrication cost and time. 
         [0057]      FIG. 8  is a layout of an LCD panel  500  according to another embodiment of the invention. LCD panel  500  of  FIG. 8  is basically the same as LCD panel  500  of  FIGS. 3 and 4 , except with respect to the layout of coupling electrode  176  and auxiliary coupling electrode  136 . Therefore, detailed explanation on the common structure will be omitted. 
         [0058]    Referring to  FIG. 8 , coupling electrode  176  and auxiliary coupling electrode  136  are formed along a portion of apertures  271  of common electrode  270  of  FIG. 2 , so that light leakage through aperture  271  is reduced. Accordingly, light transmittance of LCD panel  500  can be increased. 
         [0059]      FIGS. 9 ,  10 , and  11  illustrate an LCD panel  500  in accordance with another embodiment of the invention.  FIG. 9  is the layout of active matrix TFT substrate  100 ,  FIG. 10  is the layout of patterned substrate  200 , and  FIG. 11  is the layout of LCD panel  500 . LCD panel  500  of  FIG. 11  is basically the same as LCD panel  500  of  FIGS. 3 and 4 , except with respect to the layout of domains and apertures  196  of pixel electrode  190  and common electrode  270 . Therefore, detailed explanation on the common structure will be omitted. LCD panel  500  of  FIG. 11  has more apertures  196  and domains than LCD panel  500  of  FIGS. 3 and 4 . The increased number of apertures  196  and domains can prevent color shifts in viewing angle while effectively controlling liquid crystals in a relatively large size pixel area. 
         [0060]    As described above, according to the present invention, in case of using gate insulator  140  as an interposing dielectric layer of a coupling capacitor, it can be possible to obtain sufficient coupling capacitance with a relatively small overlapping area of opposing electrodes, so as to minimize the reduction of transmittance due to overlapping area of opposed electrodes and simultaneously to prevent color shifts in viewing angle. 
         [0061]    Although the illustrative embodiments have been described herein with reference to the accompanying drawings, it is to be understood that the present invention is not limited to those specific embodiments, and that various changes and modifications may be affected therein by one of ordinary skill in the related art without departing from the spirit and scope of the present invention. All such changes and modifications are intended to be included within the scope of the invention as defined in the appended claims.