Abstract:
Disclosed is a liquid crystal display comprising a first substrate including wiring, which intersects to define unit pixels, and a first electrode formed in each unit pixel; a second substrate provided opposing the first substrate at a predetermined distance and including a second electrode formed over an entire surface of the second substrate, the second electrode generating an electric field with the first electrode; and a liquid crystal layer injected between the first substrate and the second substrate and including liquid crystal molecules that are horizontally oriented in one direction, the liquid crystal molecules, as a result of the electric field generated between the first and second substrates, having a symmetrically bent alignment about an imaginary center plane parallel to the first and second substrates at a center position therebetween, wherein the first electrode is protruded in a direction toward the second substrate at edges where orientation for the liquid crystal molecules starts.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (a) Field of the Invention  
         [0002]     The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display having a bent alignment of liquid crystal molecules to obtain a wide viewing angle and a fast response time.  
         [0003]     (b) Description of the Related Art  
         [0004]     Liquid crystal displays typically include a first substrate having common electrodes and a color filter, and a second substrate having thin film transistors and pixel electrodes. The first substrate and the second substrate are provided substantially in parallel with a predetermined gap therebetween, and liquid crystal is injected between the two opposing substrates. An electric field is formed between the substrates by applying different voltages to the pixel electrodes and common electrodes. Accordingly, the alignment of liquid crystal molecules of the liquid crystal material is varied to thereby control the transmittance of incident light.  
         [0005]     Various types of liquid crystal displays have been developed to improve response times and viewing angle. They include the HAN (hybrid aligned nematic) mode liquid crystal display and the OCB (optically compensated bend) mode liquid crystal display. The OCB mode LCD includes an electrode formed on each opposing substrate, the electrodes acting to form an electric field that is perpendicular to the two substrates; liquid crystal injected between the two substrates; and an alignment layer formed on each substrate, the alignment layers providing a force to align the liquid crystal molecules in a direction substantially parallel to the two substrates.  
         [0006]     In the OCB mode LCD, a symmetrical arrangement is realized about an imaginary center plane between the two substrates and parallel to the same. That is, the liquid crystal molecules are aligned substantially parallel to the substrates, then are increasingly slanted until reaching this center plane where the liquid crystal molecules are substantially perpendicular to the two substrates. A wide viewing angle is achieved as a result. To obtain such a bent alignment of the liquid crystal molecules, a horizontal alignment agent that is oriented in the same direction is used and a high voltage is initially applied. Also, since the liquid crystal molecules move in the same orientation when the LCD is operated, a wide viewing angle and a fast response time are realized.  
         [0007]     However, in such a LCD, in areas where unit pixels begin, a smooth bending alignment of the liquid crystal molecules cannot be aligned in a smooth bending pattern, thereby limiting the display characteristics. This is a result of the opposing directions of the bending alignment of the liquid crystal molecules and the bending direction of the electric field at edges of the pixel electrodes. That is, unlike the common electrode, which is formed over an entire surface of the substrate, the pixel electrodes are divided for each pixel region such that this opposing direction of LC molecule alignment and electric field occurs at the edges of the pixel electrodes.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been made in an effort to solve the above problems.  
         [0009]     It is an object of the present invention to provide a liquid crystal display that realizes a stable bent alignment of liquid crystal molecules such that a wide viewing angle and fast response times are obtained.  
         [0010]     To achieve the above object, the present invention provides a liquid crystal display comprising a first substrate including wiring, which intersects to define unit pixels, and a first electrode formed in each unit pixel; a second substrate provided opposing the first substrate at a predetermined distance from the same, and including a second electrode formed over an entire surface of the second substrate, the second electrode generating an electric field with the first electrode; and a liquid crystal layer injected between the first and second substrates and including liquid crystal molecules that are horizontally oriented in one direction, the liquid crystal molecules, as a result of the electric field generated between the first and second substrates, having a symmetrically bent alignment about an imaginary center plane parallel to the first and second substrates at a center position therebetween, wherein the first electrode is protruded in a direction toward the second substrate at edges where orientation for the liquid crystal molecules starts.  
         [0011]     According to a feature of the present invention, a protrusion pattern is formed under the first electrode.  
         [0012]     According to another feature of the present invention, a thickness of the protrusion pattern is 1-4 μm.  
         [0013]     According to yet another feature of the present invention, the wiring includes gate lines for transmitting gate signals, and data lines intersecting the gate lines and transmitting image signals.  
         [0014]     According to still yet another feature of the present invention, the liquid crystal display further comprises storage capacitance wiring formed on a same layer as the gate lines but separated from the same, the storage capacitance wiring overlapping the first electrode.  
         [0015]     According to still yet another feature of the present invention, the first substrate includes a thin film transistor having a gate electrode formed at areas where the gate lines intersect the data lines and connected to the gate lines, a source electrode connected to the data lines, a drain electrode opposing the source electrode with respect the gate electrode, and a semiconductor layer.  
         [0016]     According to still yet another feature of the present invention, the protrusion pattern is formed on a same layer as organic insulation material, the gate lines, or the semiconductor layer.  
         [0017]     According to still yet another feature of the present invention, first and second orientation layers are formed respectively on the first and second substrates, the orientation layers providing an orienting force to the liquid crystal molecules in an identical direction horizontal to the substrates.  
         [0018]     According to still yet another feature of the present invention, the liquid crystal layer has a positive anisotropic dielectricity.  
         [0019]     In another aspect, the present invention provides a liquid crystal display comprising a first substrate including wiring, which intersects to define unit pixels, and a first electrode formed in each unit pixel; a second substrate provided opposing the first substrate at a predetermined distance from the same, and including a second electrode formed over an entire surface of the second substrate, the second electrode generating an electric field with the first electrode; a liquid crystal layer injected between the first and second substrates and including liquid crystal molecules that are horizontally oriented in one direction, the liquid crystal molecules, as a result of the electric field generated between the first and second substrates, having a symmetrically bent alignment about an imaginary center plane parallel to the first and second substrates at a center position therebetween; and a protrusion pattern formed on the first substrate or second substrate, the protrusion pattern forming a slanted surface that opposes an orientation direction at edges of the unit pixels where orientation of the liquid crystal molecules begins or ends.  
         [0020]     According to a feature of the present invention, the liquid crystal molecules have a larger pretilt angle with respect to the first and second substrates at the edges of the unit pixels than at other areas.  
         [0021]     According to another feature of the present invention, the wiring includes gate lines for transmitting gate signals, and data lines intersecting the gate lines and transmitting image signals.  
         [0022]     According to yet another feature of the present invention, the first substrate includes a thin film transistor having a gate electrode formed at areas where the gate lines intersect the data lines and connected to the gate lines, a gate insulation layer covering the gate electrode, a semiconductor layer formed over the gate insulation layer, a source electrode connected to the data lines, and a drain electrode opposing the source electrode with respect the gate electrode.  
         [0023]     According to still yet another feature of the present invention, the protrusion pattern is formed on a same layer as organic insulation material, the gate lines, the semiconductor layer, or the data lines.  
         [0024]     According to still yet another feature of the present invention, wherein the liquid crystal display further comprises storage capacitance wiring formed on a same layer as the gate lines but separated from the same, the storage capacitance wiring overlapping the first electrode.  
         [0025]     According to still yet another feature of the present invention, the protrusion pattern forms a depression on the first and second substrates at edges of the unit pixels where orientation of the liquid crystal molecules starts.  
         [0026]     According to still yet another feature of the present invention, the protrusion pattern is formed on the first and second substrates at edges of the unit pixels where orientation of the liquid crystal molecules ends. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention:  
         [0028]      FIG. 1  is a schematic plan view of a TFT substrate used in a liquid crystal display according to a first preferred embodiment of the present invention;  
         [0029]      FIG. 2  is a sectional view taken along line II-II′ of  FIG. 1 ;  
         [0030]      FIG. 3  is a schematic plan view of a TFT substrate used in a liquid crystal display according to a second preferred embodiment of the present invention;  
         [0031]      FIG. 4  is a sectional view taken along line IV-IV′ of  FIG. 3 ;  
         [0032]      FIG. 5  is a sectional view of a liquid crystal display according to a third preferred embodiment of the present invention;  
         [0033]      FIG. 6  is a sectional view of a liquid crystal display according to a fourth preferred embodiment of the present invention;  
         [0034]      FIG. 7  is a schematic plan view of a TFT substrate used in a liquid crystal display having independent storage capacitance wiring according to a fifth preferred embodiment of the present invention; and  
         [0035]      FIG. 8  is a sectional view taken along line VIII-VIII′ of  FIG. 7 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  
         [0037]     In the present invention, in order to obtain a stable bent alignment of liquid crystal molecules, a structure of electrodes is altered such that a direction of an electric field corresponds to a bent alignment direction or orientation direction of the liquid crystal molecules, or a pretilt angle of the liquid crystal molecules is raised by the formation of grooves or protrusions.  
         [0038]      FIG. 1  shows a schematic plan view of a TFT substrate used in a liquid crystal display according to a first preferred embodiment of the present invention, and  FIG. 2  shows a sectional view taken along line II-II′ of  FIG. 1 . A color filter substrate is shown together with the TFT substrate in  FIG. 2 .  
         [0039]     The liquid crystal display includes a lower substrate  100  (the TFT substrate) and an upper substrate  200  (the color filter substrate). The lower and upper substrates  100  and  200  are positioned substantially in parallel with a predetermined gap therebetween. The lower substrate  100  includes a lower insulation substrate  10 , and gate wiring formed of a low-resistance conductive material over the insulation substrate  10 . The gate wiring includes gate lines  22  extended horizontally (in  FIG. 1 ) and gate electrodes  26  of a thin film transistor, the gate electrodes  26  being connected to the gate lines  22 . The gate wiring also includes gate pads (not shown) connected to ends of the gate lines  22  for transmitting gate signals thereto.  
         [0040]     A gate insulation layer  30  made of SiNx is formed on top of the lower insulation substrate  10  to cover the gate lines  22  and the gate electrodes  26 . A semiconductor layer  40  made of a semiconductor material such as amorphous silicon is formed over the gate insulation layer  30 , and ohmic contact layers  55  and  56  are formed over the semiconductor layer  40 . The ohmic contact layers  55  and  56  are made of a material such as n+ hydrogenated amorphous silicon, on which is n-type impurities at a high concentration, or doped silicide.  
         [0041]     Data wiring made of a conductive material such as metal is formed over the ohmic contact layers  55  and  56  and gate insulation layer  30 . The data wiring includes data lines  62  formed vertically (in  FIG. 1 ) to intersect the gate lines  22 , thereby defining unit pixels P; source electrodes  65  branched from the data lines  62  and extending to partially cover the ohmic contact layer  55 ; and drain electrodes  66  separated from the source electrodes  65  and formed on top of the ohmic contact layer  56  extending in a direction opposite the drain electrodes  66  with respect to the gate electrodes  26 . The data wiring also includes data pads (not shown) connected to one end of the data lines  62 . The data pads receive image signals.  
         [0042]     A protection layer  70 ′ is formed over the semiconductor layer  40  and portions of the data wiring that the semiconductor layer  40  does not cover. Contact holes  76  are formed in the protection layer  70  to expose the drain electrodes  66 . Additional contact holes may be formed in the protection layer  70  to expose the data pads, gate pads, and the gate insulation layer  30 .  
         [0043]     A protrusion pattern  92  that is lined up vertically (in  FIG. 1 ) with the data lines  62  is formed over the protection layer  70 . To minimize signal interference between the data lines  62  and pixel electrodes  82  (to be described hereinafter), the protrusion pattern  92  is preferably made of an organic insulation material having a low anisotropy, and is between 1-4 μm in height. The protrusion pattern  92  can be made of an organic insulation material that is sensitive to light. Also, the protrusion pattern  92  can be formed on the same layer as the gate wiring, semiconductor layer  40 , and the ohmic contact layers  55  and  56 .  
         [0044]     The pixel electrodes  82 , which are made of a clear conductive material such as IZO (indium zinc oxide) or ITO (indium tin oxide), are formed on the protection layer  70 . The pixel electrodes  82  are electrically connected to the drain electrodes  66  via the contact holes  76  of the protection layer  70 . It is possible to form auxiliary gate pads and auxiliary data pads on the same layer as the pixel electrodes  82 , with the auxiliary gate pads and auxiliary data pads contacting the gate pads and data pads, respectively, through the contact holes  76  of the protection layer  70 . With reference to  FIG. 2 , edges of the pixel electrodes  82  where orientation starts are extended to partially overlap the protrusion pattern  92  in each unit pixel P. As a result, a bent fringe field at an area A in each pixel unit P where orientation starts is identically formed to a bent alignment direction of liquid crystal molecules  310  of a liquid crystal layer  300 .  
         [0045]     The pixel electrodes  82 , with reference to  FIG. 1 , overlap the gate lines  22  to realize a storage capacitor, and when a storage capacitance is insufficient, it is possible to add wiring for storage capacitance on the same layer as the gate wiring and adjacent to the data lines  62 . This will be described in more detail hereinbelow.  
         [0046]     An upper insulation substrate  201  forms the upper substrate  200  and is located opposite the lower insulation substrate  10 . A black matrix  202  having an opening pattern is formed on the upper insulation substrate  201 , and a color filter  203  is formed in the openings of the black matrix  202  partially overlapping the black matrix  202 . Areas where the color filter  203  is formed correspond to the unit pixels P. A common electrode  204  is formed over the black matrix  202  and the color filter  203 .  
         [0047]     Formed as innermost layers of the lower insulation substrate  10  and the upper insulation substrate  201  are alignment layers  401  and  402 , respectively. The alignment layers  401  and  402  provide a force to orient the liquid crystal molecules  310  of the liquid crystal layer  300  in nearly a direction parallel to the lower and upper insulation substrates  10  and  201 . Here, liquid crystal of the liquid crystal layer  300  has a positive anisotropic dielectricity.  
         [0048]     In a state where a critical voltage is applied to the pixel electrodes  82  and the common electrode  204 , the liquid crystal molecules  310  of the liquid crystal layer  300  adjacent to the lower and upper insulation substrates  10  and  201  are aligned at a specific angle with respect to the substrates  10  and  201  due to the force of the orientation layers  401  and  402 , as well as the characteristics of the liquid crystal molecules  310 . However, as approaching an imaginary center plane, which is parallel to the substrates  10  and  201  at a center position therebetween the liquid crystal molecules  310  are increasingly bent as a result of the electric field formed between the substrates  10  and  201  (and the decreasing influence of the orienting force of the alignment layers  401  and  402 ) until becoming substantially perpendicular to the substrates  10  and  201  at the center plane. Accordingly, the liquid crystal molecules  310  form two symmetrical regions about the center plane to compensate for a phase retardation of light passing through the liquid crystal layer  300 . A wide viewing angle is obtained as a result.  
         [0049]     Since the protrusion pattern  92  is formed at edges of the unit pixels P where orientation starts (areas A), the pixel electrodes  82  at these regions are raised since they are formed partially over the protrusion pattern  92 . As a result, the electric field at areas A is bent identically to the bent alignment direction of the liquid crystal molecules  310  such that a smooth bent alignment of the liquid crystal molecules  310  occurs. That is, it can prevent the broken bent alignment of the liquid crystal molecules  310  of the prior art resulting from opposing directions of the electric field and alignment of the liquid crystal molecules.  
         [0050]     In the first preferred embodiment of the present invention described above, liquid crystal molecules are oriented in the horizontal direction (in  FIG. 1 ) and the protrusion pattern  92  is formed in the vertical direction (in  FIG. 1 ). However, if the liquid crystal molecules are set to be oriented in the vertical direction, the protrusion pattern  92  can be formed at areas where orientation starts in the unit pixels P in parallel with the gate wiring.  
         [0051]      FIG. 3  shows a schematic plan view of a TFT substrate used in a liquid crystal display according to a second preferred embodiment of the present invention.  FIG. 4  is a sectional view taken along line IV-IV′ of  FIG. 3 . Since much of the structure is similar to the first preferred embodiment, only differences in structure from the first preferred embodiment will be described. Like reference numerals will be used for like elements.  
         [0052]     An organic insulation layer pattern  92  is formed over the protection layer  70 . The organic insulation layer pattern  92  is formed in parallel with the data lines  62  and has a slanted region  94  that opposes an orientation direction (shown by the arrow) at areas A. Here, if the orientation direction is perpendicular to the direction shown in  FIG. 4 , the organic insulation layer pattern  92  may be formed in parallel with the gate lines  22 .  
         [0053]     Since the alignment layer  401  is formed over the organic insulation layer pattern  92 , and therefore following the slanted region  94  of the organic insulation layer pattern  92 , a depression is formed at the areas A. As a result, the liquid crystal molecules  310  adjacent to the lower insulation substrate  10  are aligned having a pretit angle with respect to the surface of the lower insulation substrate  10 . That is, at areas A, the liquid crystal molecules  310  are aligned having a larger pretilt angle than at other areas. Accordingly, the liquid crystal molecules  310  are minimally influenced by the electric field at areas A and are stably aligned in a bent formation even with the generation of a bent fringe field at edges of the unit pixels P. Therefore, it can prevent the broken bent alignment of the liquid crystal molecules  310  of the prior art resulting from opposing directions of the electric field and alignment of the liquid crystal molecules.  
         [0054]     Although the organic insulation layer pattern  92  having the slanted region  94  is described as formed on a separate layer as the other elements formed on the lower insulation substrate  10 , it is possible for the organic insulation layer pattern  92  to be formed on the same layer as the gate wiring, gate insulation layer  30 , semiconductor layer  40 , ohmic contact layers  55  and  56 , data wiring, or protection layer  70 . Further, a protrusion pattern can be formed to generate the slanted region  94  at the areas A rather than elevating an area between the areas A as in the second embodiment described above. Also, a protrusion or depression pattern can be formed at the areas A on the upper insulation substrate  201  to better achieve the stable bent alignment of the liquid crystal molecules  310 .  
         [0055]      FIG. 5  shows a sectional view of a liquid crystal display according to a third preferred embodiment of the present invention. Since much of the structure is similar to the first preferred embodiment, only differences in structure from the first preferred embodiment will be described. Like reference numerals will be used for like elements.  
         [0056]     As shown in the drawing, a protrusion pattern  90  is formed at areas B where orientation ends. The protrusion pattern  90  results in a slanted surface opposing an orientation direction (→). The protrusion pattern  90  includes a first pattern  25  formed on the same layer as the gate wiring, a second pattern  42  formed on the same layer as the semiconductor layer  40 , and a third pattern  52  formed on the same layer as the ohmic contact layers  55  and  56 . The data lines  62  are formed over the third pattern  52 . It is possible to form the protrusion pattern  90  one, two or more of the patterns  25 ,  42 , and  52 , or can be formed using a separate organic insulation layer.  
         [0057]     In the above structure, the alignment layer  401  is formed over the provided pattern (i.e., the protrusion pattern  90 ) as with the second embodiment. Accordingly, the liquid crystal molecules  310  aligned over the slanted surface of the protrusion pattern  90  have a pretilt angle with respect to the substrate  10 . That is, the liquid crystal molecules  310  are aligned having a larger pretilt angle with respect to the substrate  10  at the areas B than at other areas. As a result, a stable bent alignment of the liquid crystal molecules  310  is obtained. Also, a protrusion or depression pattern can be formed on the upper insulation substrate  201  to better achieve the stable bent alignment of the liquid crystal molecules  310 .  
         [0058]      FIG. 6  shows a sectional view of a liquid crystal display according to a fourth preferred embodiment of the present invention. Since much of the structure is similar to the first preferred embodiment, only differences in structure from the first preferred embodiment will be described. Like reference numerals will be used for like elements.  
         [0059]     As shown in the drawing, the color filter  203  of the upper insulation substrate  201  is formed having a slanted surface  213  that opposes an orientation direction (→) at areas A where orientation begins. Also, a protrusion pattern  206  is formed over the color filter  203  and the black matrix  202  is a protrusion pattern  206  having a slanted surface  216  that opposes the orientation direction (→) at the areas B where orientation ends.  
         [0060]     In the above structure, the orientation layer  402  is formed over the provided pattern as with the above second and third embodiments. Accordingly, the liquid crystal molecules  310  aligned over the slanted surface of the protrusion pattern  90  have a pretilt angle with respect to the substrate  10 . That is, the liquid crystal molecules  310  are aligned having a larger pretilt angle with respect to the substrate  10  at the areas A and B than at other areas. As a result, a stable bent alignment of the liquid crystal molecules  310  is obtained both where orientation starts and ends.  
         [0061]      FIG. 7  shows a schematic plan view of a TFT substrate used in a liquid crystal display having independent storage capcitance wiring according to a fifth preferred embodiment of the present invention.  FIG. 8  is a sectional view taken along line VII-VII′ of  FIG. 7 . Since much of the structure is similar to the first preferred embodiment, only differences in structure from the first preferred embodiment will be described. Like reference numerals will be used for like elements.  
         [0062]     Storage capacitance wiring is formed on the same layer as the gate wiring but separated from it. The storage capacitance wiring overlaps the pixel electrode  82 . The storage capacitance wiring includes storage electrode lines  25  formed extending in the same direction as the gate line  22  (i.e., horizontally in  FIG. 7 ) and at two ends of a unit pixel P, and a storage electrode  27  formed vertically (in  FIG. 7 ) to interconnect the storage electrode lines  25 . The drain electrode  66  is extended horizontally (in  FIG. 7 ) to overlap one of the storage electrode lines  25 , thereby ensuring a sufficient storage capacitance. In the fifth embodiment, a boundary of the protrusion pattern  92  is positioned between the data lines  62  and storage electrodes  27 . However, it is possible that the boundary of the protrusion pattern  92  extend over the storage electrodes  27 , and a boundary of the pixel electrode  82  can be formed over the data lines  62  as in the first embodiment.  
         [0063]     In the fifth embodiment, with the addition of the independent storage capacitance wiring to the basic structure of the first embodiment, a stable bent alignment of the liquid crystal molecules  310  is obtained. This can also be achieved through configurations outlined in the second, third and fourth embodiments. That is, the independent storage capacitance wiring can be added also to the basic structures of the second, third and fourth embodiments.  
         [0064]     Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught that may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.