Abstract:
Disclosed are a liquid crystal display device without a black matrix capable of eliminating light leakage while not decreasing opening degree and reducing production costs and, in addition, a method for fabricating the liquid crystal display device described above. The liquid crystal display device includes: a thin film transistor formed on a first substrate; a first passivation layer formed on the first substrate including the thin film transistor; a color filter layer formed on the first passivation layer; a second passivation layer formed on the first substrate including the color filter layer; a pixel electrode which passes through the second and the first passivation layers, is electrically connected to a part of the thin film transistor and has a lamination structure of transparent metal and opacity metal, wherein the transparent metal part has a width wider than that of the opacity metal part; and a second substrate corresponding to the first substrate.

Description:
[0001]    This application claims the benefit of Korean Patent Application No. P10-2008-0093742, filed on Sep. 24, 2008, which is hereby incorporated by reference as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to liquid crystal display devices and a method for manufacturing the same and, more particularly, to a liquid crystal display device with improved luminescence and a method for manufacturing the same. 
         [0004]    2. Discussion of the Related Art 
         [0005]    With progress towards an advanced information society, there is a strong need for development of high quality flat panel display devices with excellent characteristics such as thinness, lightweight and low power consumption. Among those, a liquid crystal display device with superior resolution, color display, image quality, etc. is widely used in various applications including a notebook type computer, a laptop monitor, and the like. 
         [0006]    In general, a liquid crystal display device has a structure wherein two substrates have two sides facing each other, each of which has an electrode thereon, and a liquid crystal material is introduced between the substrates. Therefore, when a certain voltage is applied to both of the electrodes to generate an electric field, liquid crystal molecules become movable by the electric field to vary light transmittance so that the liquid crystal display device may display images by the varied light transmittance. 
         [0007]    A bottom substrate of the liquid crystal display device is fabricated by using an array substrate having a thin film transistor, which applies signals to a pixel electrode, so as to form a thin layer, lithographically etching the formed thin layer, and repeating these processes. A top substrate of the liquid crystal display device comprises a common electrode and a color filter, which has three colors of red (R), green (G) and blue (B) arranged in sequence, and this top substrate is fabricated by pigment dispersion, dyeing, electro-deposition, and so forth. Among these, the pigment dispersion has superior precision and excellent reproduction, thus being widely applicable. 
         [0008]    Such a liquid crystal display device is normally fabricated by forming an array substrate and a color filter substrate, and arranging a pixel electrode on a bottom substrate to correspond to a color filter on a top substrate. During the arrangement step, misalignment problems may occur to cause failures such as light leakage. 
         [0009]    In order to solve problems described above, the top substrate may have a wider black matrix and, in this case, a degree of opening of the liquid crystal display device may be reduced. 
         [0010]    Therefore, a method has recently been proposed to form a color filter on an array substrate in order to prevent misalignment and improve degree of opening of the liquid crystal display device. Such a structure of a color filter formed on a top side of a thin film transistor refers to a Color Filter on Thin Film Transistor (COT) structure. 
         [0011]      FIG. 1  is a cross-sectional view illustrating a typical liquid crystal display device having a COT structure. 
         [0012]    Referred to  FIG. 1 , a gate electrode  12  made of a conductive substance such as metal is formed on a transparent first substrate  11 , and a gate insulating film  13  consisting of a silicon nitride (SiNx) or silicon oxide (SiO 2 ) film covers the gate substrate  12 . 
         [0013]    On the gate insulating film  13  formed on the top side of the gate electrode  12 , an active layer  14  made of amorphous silicon may be formed, followed by additionally forming an ohmic contact layer  15 , which comprises amorphous silicon and is doped with foreign materials (or impurities), on the active layer  14 . 
         [0014]    A source electrode  16   a  made of conductive substance such as metal as well as a drain electrode  16   b  are formed on a top side of the ohmic contact layer  15 , wherein the source and drain electrodes  16   a  and  16   b  are used to fabricate a thin film transistor T together with the gate electrode  12 . 
         [0015]    Although not illustrated in the drawings, the gate electrode  12  is connected to a gate wiring while the source electrode  16   a  is connected to a data wiring. Both the gate wiring and the data wiring cross to each other at right angles to define a pixel region. 
         [0016]    A first substrate  11  including the source and drain electrode  16   a  and  16   b  may have a protective film  17  which comprises a silicon nitride film, a silicon oxide film or an organic insulating film in order to protect the thin film transistor T. 
         [0017]    In the pixel region on the top side of the protective film  17 , a color filter  18  is formed wherein R, G and B colors are aligned in sequence and each color corresponds to each pixel region. The color filter  18  may include a contact hole  19  exposing the drain electrode  16   b  in addition to the protective film  17 . 
         [0018]    A pixel electrode  20  made of a transparent conductive substance is formed on a top side of the color filter  18  to be electrically connected to the drain electrode  16   b  through the contact hole  19 . 
         [0019]    Further, a second transparent substrate  21  is located a certain distance above the first substrate  11  and a black matrix  22  is placed on an inner side of the second substrate  21  at a position corresponding to the thin transistor T. Although not illustrated, the black matrix  22  has an opening at a position corresponding to the pixel electrode  20  and is formed on a bottom surface of the substrate. 
         [0020]    Therefore, the black matrix  22  may prevent light leakage since liquid molecules are tilted on other parts except the pixel electrode  20 , and may shield light incident on a channel part, thereby inhibiting generation of light leakage current. 
         [0021]    In addition, an over-coat layer  23  is entirely formed on the bottom surface of the second substrate  21  having the black matrix  22 . 
         [0022]    A liquid crystal layer  30  is formed between the first substrate  11  and the second substrate  21 . 
         [0023]    As for the liquid crystal display device with a COT structure described above, the color filter  18  is formed on the first substrate  11  so that misalignment of the color filter and the pixel electrode  20  does not occur when the first substrate  11  is combined with the second substrate  21 . 
         [0024]    Therefore, alignment margin of the black matrix  22  in the second substrate  21  may be reduced and, if a black matrix substance with light penetration inhibitory effects is used to form a barrier pattern, the black matrix  22  on the second substrate  22  may be omitted, thereby improving opening degree of the liquid crystal display device. 
         [0025]      FIGS. 2A to 2D  are cross-sectional views illustrating a conventional method for manufacturing a liquid crystal display device having a COT structure. 
         [0026]    As illustrated in  FIG. 2A , a metal substance is deposited on a first transparent substrate  31  and is selectively removed through photolithography so as to form a gate electrode  32  and a common wiring  33 . 
         [0027]    A gate wiring (not shown) which is connected to the gate electrode  32  and extends in one direction may be formed during formation of the gate electrode. 
         [0028]    Continuously, an insulating substance such as a silicon nitride film or a silicon oxide film is thoroughly deposited on a top surface of the first substrate  31  including the gate electrode  32  so as to form a gate insulating film  34 . 
         [0029]    As illustrated in  FIG. 2B , an amorphous silicon layer and another amorphous silicon layer doped with impurities are doped on the gate insulating film  34  in this order. 
         [0030]    After that, the amorphous silicon layer doped with impurities and the other amorphous silicon layer located under the doped silicon layer are selectively removed to form an active layer  35  and an ohmic contact layer  36 . 
         [0031]    Following this, a metal substance is entirely deposited on the first substrate  31  and selectively removed through photolithography to form a source electrode  37   a  and a drain electrode  37   b.    
         [0032]    While forming the source electrode  37   a  and the drain electrode  37   b,  a data wiring (not shown) which is extended from the source electrode  37   a  and crosses the gate wiring at right angles to define a pixel region may also be formed. 
         [0033]    The ohmic contact layers  36  exposed by the source electrode  37   a  and the drain electrode  37   b  are selectively removed. Herein, the source electrode  37   a  and the drain electrode  37   b  may be formed a certain distance apart from each other in order to form a channel in a following process. 
         [0034]    Subsequently, a first passivation layer  38  is entirely formed on the top surface of the first substrate  31 . 
         [0035]    As illustrated in  FIG. 2C , a photosensitive material is applied to a top side of the first passivation layer  38 , followed by exposing and patterning the same to form a color filter layer  39  in the pixel region. 
         [0036]    Since the color filter layer  39  normally comprises R, G and B colors, applying, exposing and developing processes may be repeated three times so as to produce the color filter layer capable of embodying the colors, respectively. 
         [0037]    After that, a second passivation layer  40  is entirely formed on the top surface of the first substrate  31  having the color filter layer  39 , and then, the second and the first passivation layers  40  and  38  are selectively removed by a photolithographic process, so as to form a contact hole through which the drain electrode  37   b  is partially exposed. 
         [0038]    Next, a transparent conductive material is deposited on the entire portion of the top surface of the first substrate  31  having the contact hole, and is selectively removed by a photolithographic process so that a pixel electrode  41  connected to the drain electrode  37   b  through the contact hole and a common electrode  42  spaced from the pixel electrode  41  at a certain distance may be formed. 
         [0039]    As illustrated in  FIG. 2D , a black matrix  52  which is arranged at a certain interval corresponding to a thin film transistor except the pixel region may be formed on a bottom surface of a second substrate  51  corresponding to the first substrate  31 , and then, an over-coat layer  53  may be entirely formed on the bottom surface of the second substrate  51  having the black matrix  52 . 
         [0040]    However, the conventional method for manufacturing a liquid crystal display device as described above has problems as follows. 
         [0041]    That is, a black matrix typically formed on a top substrate corresponds to a thin transistor formed on a bottom substrate in order to prevent light leakage at stepped parts due to color overlap. Therefore, formation of such a black matrix may cause an increase in production costs and light leakage due to misalignment during formation of the black matrix. 
       SUMMARY OF THE INVENTION 
       [0042]    Accordingly, the present invention is directed to solve the problems described above in regard to conventional techniques, and an object of the present invention is to provide a liquid crystal display device without a black matrix, capable of preventing light leakage while not decreasing opening degree and, in addition, reducing production costs and, as well as a method for fabricating the liquid crystal display device described above. 
         [0043]    To achieve this object and other advantages and in accordance with the purpose of the invention, there is provided a liquid crystal display device according to the present invention comprising: a thin film transistor formed on a first substrate; a first passivation layer formed on the first substrate including the thin film transistor; a color filter layer formed on the first passivation layer; a second passivation layer formed on the first substrate including the color filter layer; a pixel electrode which passes through the second and the first passivation layers, is electrically connected to a part of the thin film transistor and has a lamination structure of transparent metal and opacity metal, wherein the transparent metal part has a width wider than that of the opacity metal part; and a second substrate corresponding to the first substrate. 
         [0044]    In addition, a method for fabrication of a liquid crystal display device according to the present invention comprises: forming a thin film transistor on a first substrate; forming a first passivation layer on the first substrate including the thin film transistor; forming a color filter layer on the first passivation layer; forming a second passivation layer on the first substrate including the color filter layer; selectively removing the second and the first passivation layers to form a contact hole through which the thin film transistor is partially exposed; laminating a transparent metal and an opacity metal on the entire portion of the top surface of the first substrate including the contact hole in sequence; selectively removing the opacity metal and the transparent metal to form a pixel electrode which is electrically connected to a part of the thin film transistor through the contact hole; selectively removing the opacity metal part to expose an edge portion of the transparent metal part; and forming a second substrate corresponding to the first substrate. 
         [0045]    The liquid crystal display device and the method for fabricating the same according to the present invention are advantageous in that: 
         [0046]    First, a top substrate has no black matrix and a pixel electrode has a lamination structure of transparent metal and opacity metal substances so that a top side of the pixel electrode is covered with the opacity metal part so as to reduce black luminescence while an edge portion of the pixel electrode is formed of transparent metal part to open the electrode part, thus improving luminescence. 
         [0047]    Second, neither a black matrix nor over-coat layer is formed, thereby simplifying production processes and decreasing production costs. 
         [0048]    It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0049]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
           [0050]      FIG. 1  is a cross-sectional view illustrating a liquid crystal display device having a typical COT structure; 
           [0051]      FIGS. 2A to 2D  are cross-sectional views illustrating a conventional method for manufacturing a liquid crystal display device having a COT structure; 
           [0052]      FIG. 3  is a cross-sectional view illustrating a liquid crystal display device according to the present invention; and 
           [0053]      FIGS. 4A to 4G  are cross-sectional views illustrating a method for fabrication of a liquid crystal display device according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0054]    Hereinafter, other purposes, characteristics and other beneficial features of the present invention will become apparent from the following detailed description with reference to illustrative examples, taken in conjunction with the accompanying drawings. 
         [0055]    A liquid crystal display device and a method for fabricating the same according to the present invention to achieve the above objects will be described in detail by the following description with reference to the accompanying drawings. 
         [0056]      FIG. 3  is a cross-sectional view illustrating a liquid crystal display device of the present invention. 
         [0057]    As illustrated in  FIG. 3 , the liquid crystal display device includes: a gate electrode  102  and a common wiring  103  formed on a first substrate  101  at a certain interval; a gate insulating film  104  formed on a top surface of the first substrate including the gate electrode  102 ; an active layer  105  formed on the gate insulating film  104  to correspond to the gate electrode  102 ; a source electrode  107   a  and a drain electrode  107   b  which are formed a certain distance from both ends of the active layer  105  by interposing ohmic contact layers  106 , respectively; a first passivation layer  108  formed on the entire portion of the top surface of the first substrate  101  including the source electrode  107   a  and the drain electrode  107   b;  a color filter layer  109  formed on a top side of the first passivation layer  108 ; a second passivation layer  110  formed on the entire portion of the top surface of the first substrate  101  including the color filter layer  109 ; a pixel electrode  105  which passes through the second and the first passivation layers  110  and  108 , and then, is electrically connected to the drain electrode  107   b;  a common electrode  116  which is formed on a top side of the second passivation layer  110  and spaced from the pixel electrode  115  at a certain interval; and a second substrate  201  formed to correspond to the first substrate  101 . 
         [0058]    Each of the pixel electrode  115  and the common electrode  116  has a lamination structure of transparent metal  112  and opacity metal  113 , wherein the opacity metal part  113  has a width narrower than that of the transparent metal part  112  and an edge portion of the transparent metal part  112  is exposed. That is, in order to expose both edge portions of the transparent metal part  112 , the opacity metal part  113  has a narrower width compared to the transparent metal part  112 . 
         [0059]    Meanwhile, the gate electrode  102 , the active layer  106 , the source electrode  107   a  and the drain electrode  107   b  constitute the thin film transistor. 
         [0060]      FIGS. 4A to 4G  are cross-sectional views illustrating a method for fabrication of a liquid crystal display device according to the present invention. 
         [0061]    As illustrated in  FIG. 4A , a metal substance is deposited on a top surface of a first transparent substrate  101 , and then, is selectively removed by a photolithographic process so as to form a gate electrode  102  and a common electrode  103 . 
         [0062]    In this case, a gate wiring (not shown) which is connected to the gate electrode  102  and extends in one direction may be formed while forming the gate electrode  102 . 
         [0063]    The metal substance used herein may be formed by depositing one metal having low resistance selected from copper (Cu), aluminum (Al), aluminum alloy such as aluminum neodymium (AlNd), molybdenum (Mo), chromium (Cr), titanium (Ti), tantalum (Ta), molybdenum-tungsten (MoW), and so forth to form a single layer or continuously depositing two or more of the above metals to form a double layer. 
         [0064]    Following this, an inorganic insulating material such as silicon oxide (SiOx) or silicon nitride (SiNx) is deposited on the top surface of the first substrate  101  including the gate electrode  102  by a commonly known plasma enhanced chemical vapor deposition (PECVD) method so as to form a gate insulating film  103 . 
         [0065]    As illustrated in  FIG. 4B , an amorphous silicon layer and another amorphous silicon layer doped with impurities are sequentially deposited on the gate insulating film  103 . 
         [0066]    Continuously, the doped silicon layer and the silicon layer placed under the doped silicon layer are selectively removed through photolithography so as to form an active layer  105  and an ohmic contact layer  106 . 
         [0067]    Next, a metal substance is deposited on the entire portion of the top surface of the first substrate  101 , and then, selectively removed through photolithography so as to form a source electrode  107   a  and a drain electrode  107   b.    
         [0068]    During formation of the source electrode  107   a  and the drain electrode  107   b,  a data wiring (not shown) extending from the source electrode  107   a  and crossing the gate wiring at right angles so as to define a pixel region. 
         [0069]    The metal substance used herein may be formed by depositing one metal having low resistance selected from copper (Cu), aluminum (Al), aluminum alloy such as aluminum neodymium (AlNd), molybdenum (Mo), chromium (Cr), titanium (Ti), tantalum (Ta), molybdenum-tungsten (MoW), and so forth to form a single layer or continuously depositing two or more of the above metals to form a double layer. 
         [0070]    The ohmic contact layer  106  exposed by the source electrode  107   a  and the drain electrode  107   b  may be selectively removed. Herein, the source electrode  107   a  and the drain electrode  107   b  are spaced from each other at a certain interval to form a channel in a following process. 
         [0071]    In an exemplary embodiment of the present invention, different masking processes are adopted to form the active layer  105 , the source electrode  107   a  and the drain electrode  107   b,  however, the present invention is not particularly limited thereto. Preferably, in order to reduce the number of masks to be used, an amorphous silicon layer and another amorphous silicon layer doped with impurities are formed in sequence, followed by depositing a metal substance and etching all of these with only one mask. 
         [0072]    Following this, the first passivation layer  108  is formed on the entire portion of the top surface of the first substrate  101  including the source electrode  107   a  and the drain electrode  107   b.    
         [0073]    As illustrated in  FIG. 4C , a photosensitive material is applied to the first passivation layer  108 , and then, is exposed and patterned to form a color filter layer  109  in the pixel region. 
         [0074]    The color filer layer  109  comprises R, G and B colors and these applying, exposing and developing processes may be repeated three times in order to embody all of these colors. 
         [0075]    Subsequently, a second passivation layer  110  is formed on the entire portion of the top surface of the first substrate  101  including the color filter layer  109 , and the second and the first passivation layers  110  and  108  are selectively removed through photolithography, so as to form a contact hole  111 . 
         [0076]    As illustrated in  FIG. 4D , a transparent metal  112  and an opacity metal  113  are deposited in sequence on the entire portion of the top surface of the first substrate  101  including the contact hole  111 . 
         [0077]    The transparent metal  112  may include indium tin oxide (ITO), tin oxide (TO), indium zinc oxide (IZO), indium tin zinc oxide (ITZO), etc. 
         [0078]    The opacity metal  113  may include Cu, Al, AlNd, Mo, Cr, Ti, Ta, etc. 
         [0079]    In an exemplary embodiment of the present invention, the transparent metal is ITO while the opacity metal is MoTi. 
         [0080]    Next, after a photoresist  114  is applied to the opacity metal  113 , the photoresist  114  is selectively patterned by exposing and developing processes. 
         [0081]    As illustrated in  FIG. 4E , using the patterned photoresist  114  as a mask, the opacity metal  113  and the transparent metal  112  are selectively removed so as to form a pixel electrode  115  and a common electrode  116 . 
         [0082]    As illustrated in  FIG. 4F , after O2 ashing the patterned photoresist, a thickness and a width of the photoresist  114  are decreased. 
         [0083]    Using the ashed photoresist  114   a  as a mask, the opacity metal  113  is selectively removed. 
         [0084]    The selectively removed opacity metal  113  may have a width narrower than that of the transparent metal  112 , so that an edge portion of the transparent metal  112  is exposed. 
         [0085]    As illustrated in  FIG. 4G , after removing the photoresist  114   a,  a second substrate  201  is prepared to correspond to the first substrate  101 . 
         [0086]    Although not illustrated in the drawings, after a column spacer is placed on the second substrate  201 , the first substrate  101  is combined with the substrate  201 , followed by injecting liquid crystal therein. 
         [0087]    In an exemplary embodiment of the present invention, liquid crystal is introduced to the combined first and second substrates. However, liquid crystal is firstly dropped into the center of the first substrate  101  after forming a sealant around a peripheral side of the first substrate  101 , followed by combination of the first substrate  101  with the second substrate  201 . 
         [0088]    The following Table 1 shows luminescence and CR characteristics of a pixel electrode having a lamination structure of ITO and MoTi according to an exemplary embodiment of the present invention. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 ITO 
                 MoTi 
                 ITO + MoTi 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 White luminescence (nit) 
                 600 
                 550 
                 597 
               
               
                   
                 C/R 
                 1000 
                 1200 
                 1147 
               
               
                   
                 Black luminescence (nit) 
                 0.6 
                 0.45 
                 0.52 
               
               
                   
                   
               
             
          
         
       
     
         [0089]    As shown in Table 1, ITO has relatively high White luminescence and Black luminescence compared to MoTi, whereas MoTi exhibits C/R higher than that of ITO. 
         [0090]    Accordingly, the present inventive method may fabricate a pixel electrode having a lamination structure of ITO+MoTi with favorable features described above, thus improving C/R as well as White or Black luminescence. In other words, the present invention may attain increased transmittance while reducing Black luminescence of a liquid crystal display device. 
         [0091]    Although the exemplary embodiment of the present invention describes a liquid crystal display device having a COT structure using first and second passivation layers and a method for fabrication thereof, the present invention is not particularly restricted thereto. Preferably, the present invention may be applied to a typical IPS mode liquid crystal display device wherein a color filter layer is formed on a second substrate, as well as a method for fabrication of the same. 
         [0092]    More particularly, as described above, a thin film transistor is formed on a first substrate  101 , a passivation layer is formed on a top surface of the first substrate including the thin film transistor, and the passivation layer is selectively removed to expose a drain electrode of the thin film transistor, resulting in formation of a contact hole. Following this, a transparent metal and an opacity metal are deposited in this order on the entire portion of the top surface of the first substrate, and then, are treated by a photolithographic process to form a pixel electrode and a common electrode and, in addition, to form a color filter layer on a second substrate corresponding to the first substrate. 
         [0093]    Although technical constructions and other features of the present invention have been described, it will be apparent to those skilled in the art that the present invention is not limited to the exemplary embodiments and accompanying drawings described above but may cover substitutions, variations and/or modifications thereof without departing from the scope of the invention defined in the appended claims.