Patent Publication Number: US-7910928-B2

Title: TFT array substrate and method for fabricating the same

Description:
This is a divisional application of U.S. patent application Ser. No. 11/643,793, filed on Dec. 22, 2006, now U.S. Pat. No. 7,785,940 which claims the benefit of Korean Patent Application No. P2005-134994, filed on Dec. 30, 2005, all of which are hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a four-mask process thin film transistor (TFT) array substrate and method for fabricating the same that prevent a semiconductor tail from being formed. An open area is obtained and wavy noise is prevented from occurring. 
     2. Discussion of the Related Art 
     Among the various flat displays, LCD devices have attracted much attention because of their high contrast ratio, good gray scale, high-quality moving picture image, and low power consumption. 
     Various patterns of driving elements or lines are provided on a substrate to drive an LCD device. These patterns may be formed by photolithography. 
     Photolithography involves complicated steps such as coating a film layer of a substrate with photoresist which is sensitive to ultraviolet rays, performing exposure and development of the substrate after positioning an exposure mask above the substrate, etching the film layer by using the patterned photoresist as a mask, and stripping the photoresist. 
     A TFT array substrate of a related art LCD device may include a gate line layer, a gate insulation layer, a semiconductor layer, a data line layer, a passivation layer, and a pixel electrode. Typically, five to seven masks are needed in order to form the above-mentioned elements on the TFT array substrate. Increasing the number of masks used by photolithography results in a greater probability of defects. 
     Recently, to overcome this problem, low-mask technology has been actively studied, which improves productivity and improves the process margin by fabricating the substrate using the minimum number of masks and photolithography. 
     Hereinafter, a method for fabricating a related art TFT array substrate will be explained with reference to the accompanying drawings. 
       FIGS. 1A to 1G  are cross sectional views illustrating a method for fabricating a related art TFT array substrate.  FIG. 2  is a cross sectional view illustrating defects of a related art TFT array substrate. 
     To form the related art TFT array substrate of the LCD device, as shown in  FIG. 1A , aluminum neodymium (AlNd) and molybdenum (Mo) are deposited on a substrate  11 , and are patterned by photolithography using a first mask, thereby forming a multitude of gate lines (not shown), a gate electrode  12   a , a lower capacitor electrode  32 , and a gate pad electrode (G.P)  22 . The multitude of gate lines, the gate electrode  12   a , and the lower capacitor electrode  32  are formed in an active region, and the gate pad electrode (G.P)  22  is formed in a pad region. 
     Then, an inorganic material of silicon oxide (SiOx) or silicon nitride (SiNx), by way of example, is deposited on an entire surface of the substrate  11  including the gate electrode  12   a  under high temperature conditions, thereby forming a gate insulation layer  13 . Thereafter, an amorphous silicon layer (a-Si)  14   d  and a metal layer  15   d  of molybdenum (Mo) are sequentially deposited on the entire surface of the substrate  11  including the gate insulation layer  13 . Thereon, a photoresist  50  is coated and is patterned by photolithography using a second mask, thereby patterning the photoresist  50  having a step coverage. The second mask may be a diffraction exposure mask, for example, a slit mask or a half-tone mask, to form the step coverage in the photoresist. 
     Then, the exposed amorphous silicon layer  14   d  and metal layer  15   d  are etched using the photoresist  50  having the step coverage as a mask, thereby forming a multitude of data lines (not shown), source and drain electrodes  15   a  and  15   b , and a semiconductor layer  14 , as shown in  FIG. 1B . A two-step etching method is applied that includes a wet-etching step for the metal layer  15   d  and a dry-etching step for the amorphous silicon layer  14   d.    
     As shown in  FIG. 1C , the thickness of the photoresist  50  is decreased by ashing the photoresist  50 , to thereby expose the metal layer corresponding to a channel region of a thin film transistor (TFT). Then, the exposed metal layer is etched by wet-etching so that the source and drain electrodes  15   a  and  15   b  are separated from each other, thereby further defining the channel region. 
     When ashing the photoresist, the width and thickness of photoresist are decreased, whereby an active layer  14  and the source/drain electrodes  15   a / 15   b  slightly protrude out of the edge of the photoresist. When performing wet-etching to separate the source and drain electrodes from each other, the protruding portion of the metal layer is etched together, but the active layer is not etched. The protruding portion of the active layer, which is protruding out of the source and drain electrodes, is referred to as a semiconductor tail  14   a.    
     As shown in  FIG. 2 , the semiconductor tail  14   a  protrudes out from the data line  15  as well as the source and drain electrodes  15   a  and  15   b.    
     Accordingly, an active region includes a plurality of data lines (not shown), a semiconductor layer  14 , source/drain electrodes  15   a / 15   b , and a pad region with a data pad electrode (not shown). The semiconductor layer may be etched with the data line and the source/drain electrodes together, so that the semiconductor layer has substantially the same pattern as that of the data line and the source/drain electrodes. 
     Thus, the deposited gate electrode  12   a , gate insulation layer  13 , semiconductor layer  14  and source/drain electrodes  15   a / 15   b  form the thin film transistor that controls the voltage-on/off state applied to a sub-pixel. 
     As shown in  FIG. 1E , after removing the photoresist  50 , an organic insulation material of BCB or an inorganic insulation material of SiNx is formed on the entire surface of the substrate including the source and drain electrodes  15   a  and  15   b , thereby forming a passivation layer  16 . 
     After that, as shown in  FIG. 1F , the passivation layer  16  is partially removed by photolithography using a third mask, to thereby form a contact hole  71  which exposes the drain electrode  15   b . By removing the passivation layer  16  and the gate insulation layer  13 , a pad open area  81  is formed to expose the gate pad electrode  22 . Although not shown, a pad open area is also formed to expose the data pad electrode. 
     As shown in  FIG. 1G , a transparent conductive material of Indium Tin Oxide (ITO) or Indium Zinc Oxide (IZO), by way of example, is deposited on the entire surface of the substrate including the passivation layer  16 , and is then patterned by photolithography using a fourth mask, to thereby form a pixel electrode  17  connected with the drain electrode  15   b . Thus, the TFT array substrate is completed. At the same time, a transparent conductive layer  27  is formed to cover the pad open area, thereby preventing the gate and data pad electrodes from being oxidized. 
     Four exposure masks are required to form the TFT array substrate. 
     However, the related art TFT array substrate and the method for fabricating the same have the following disadvantages. 
     Forming the pattern of the TFT array substrate by the four diffraction exposure masks necessarily requires the process of patterning the data line layer and the semiconductor layer. A semiconductor tail is formed at the edge of the source/drain electrodes and data line, when defining the channel region by separating the source and drain electrodes from each other after ashing the photoresist having the step coverage. As shown in  FIG. 2 , the semiconductor tail  14   a  having a width of about 1.5 μm is formed at the edge of the data line  15 , which causes the device to malfunction. 
     The semiconductor tail causes ‘wavy noise’ when turning on/off a backlight unit (B/L). The conductive properties of the semiconductor layer are changed based on the application of light. That is, if light is applied to the semiconductor layer, the semiconductor layer becomes conductive. If light is not applied to the semiconductor layer, the semiconductor layer becomes nonconductive. 
     If the backlight unit is turned off, a parasitic capacitance occurs between the data line  15  and the pixel electrode  17  owing to the semiconductor layer having no conductivity. Meanwhile, if the backlight unit is turned on, the semiconductor layer, which is more adjacent to the pixel electrode than the data line, becomes conductive whereby the parasitic capacitance occurs between the semiconductor layer  14  and the pixel electrode  17 . 
     Because of the width of the semiconductor tail  14   a , the data line  15  differs in size from the semiconductor layer  14 . Thus, the parasitic capacitance generated between the data line  15  and the pixel electrode  17  is different from the parasitic capacitance generated between the semiconductor layer  14  and the pixel electrode  17 . That is, the parasitic capacitance increases between the semiconductor layer and the pixel electrode because the semiconductor layer is substantially larger than the data line. Because parasitic capacitance changes as a function of the voltage-on/off state of the backlight unit, a wavy noise occurs on images. 
     Also, the pixel electrode  17  is at a predetermined distance from the data line  15  and the semiconductor layer  14  (about 5.0 and 3.5 μm, respectively). Thus, the pixel electrode  17  is further from the data line  15  by the width (about 1.5 μm) of the semiconductor tail  14   a , whereby an open area decreases. Also, the area of a black matrix  90  is increased according to the semiconductor tail, wherein the black matrix  90  prevents light from leaking, so that the aperture ratio of the device is decreased. 
     Also, the turn-off current of the device increases due to the photocurrent generated by the semiconductor tail. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a TFT array substrate and a method for fabricating the same which substantially obviates one or more problems due to limitations and disadvantages of the related art. 
     An advantage of the present invention is to provide a four-mask process thin film transistor (TFT) array substrate and method for fabricating the same which prevents a semiconductor tail from being formed to thereby obtain a constant open area and to prevent wavy noise from occurring. 
     Additional features and advantages of the present invention will be set forth in the description which follows, and in part will be apparent from the description or may be learned from practice of the invention. These 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 invention, as embodied and broadly described herein, a TFT array substrate includes a gate line and a gate electrode formed on a substrate; a gate insulation layer formed on the gate line and the gate electrode; a data line formed substantially perpendicular to the gate line to define a sub-pixel, and source and drain electrodes formed above the gate electrode; a semiconductor layer formed below the data line and the source and drain electrodes, wherein the semiconductor layer is formed in substantially the same pattern as those of the data line and the source and drain electrodes; a pixel electrode directly connected with the drain electrode without a contact hole; a passivation layer formed above the pixel electrode; a gate pad electrode formed at the end of the gate line as one body; and a data pad electrode formed at the end of the data line as one body. 
     In another aspect of the present invention, a method for fabricating a TFT array substrate includes forming a gate line, a gate electrode and a pad electrode on a substrate; sequentially depositing a gate insulation layer, a semiconductor layer and a metal layer on an entire surface of the substrate including the gate line; forming an open area in the pad electrode; forming a data line and source/drain electrodes by patterning the semiconductor layer and the metal layer; forming a pixel electrode connected with the drain electrode and a transparent conductive layer connected with the pad electrode by depositing and patterning a transparent conductive material on the entire surface of the substrate including the data line, and simultaneously defining a channel region by separating the source and drain electrodes from each other; and forming a passivation layer on the remaining surface of the substrate except the portion having the transparent conductive layer. 
     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 
       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 principle of the invention. 
       In the drawings: 
         FIGS. 1A through 1G  are cross sectional views illustrating a method for fabricating a related art TFT array substrate; 
         FIG. 2  is a cross sectional view illustrating defects of a related art TFT array substrate; 
         FIG. 3  is a plan view illustrating a TFT array substrate according to the first embodiment of the present invention; 
         FIG. 4  is a cross sectional view illustrating a TFT array substrate according to the first embodiment of the present invention; 
         FIG. 5  is a cross sectional view illustrating an LCD device according to the present invention; 
         FIGS. 6A through 6K  are cross sectional views illustrating a method for fabricating a TFT array substrate according to the first embodiment of the present invention; 
         FIG. 7  is a plan view illustrating a TFT array substrate according to the second embodiment of the present invention; 
         FIG. 8  is a cross sectional view illustrating a TFT array substrate according to the second embodiment of the present invention; 
         FIGS. 9A through 9D  are plan views illustrating a TFT array substrate according to the second embodiment of the present invention; 
         FIGS. 10A through 10K  are cross sectional views illustrating a method for fabricating a TFT array substrate according to the second embodiment of the present invention; 
         FIG. 11  is a plan view illustrating a TFT array substrate according to the third embodiment of the present invention; 
         FIG. 12  is a cross sectional view illustrating a TFT array substrate according to the third embodiment of the present invention; 
         FIG. 13  is a plan view illustrating a TFT array substrate according to the fourth embodiment of the present invention; and 
         FIG. 14  is a cross sectional view illustrating a TFT array substrate according to the fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     Hereinafter, a TFT array substrate according to the present invention and a method for fabricating the same will be described with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 3  is a plan view illustrating a TFT array substrate according to the first embodiment of the present invention.  FIG. 4  is a cross sectional view illustrating a TFT array substrate according to the first embodiment of the present invention.  FIG. 5  is a cross sectional view illustrating an LCD device according to the present invention.  FIGS. 6A  to  6 K are cross sectional views illustrating a method for fabricating a TFT array substrate according to the first embodiment of the present invention. 
     The TFT array substrate of the LCD device according to the first embodiment of the present invention is for to a TN mode device. As shown in  FIGS. 3 and 4 , the TFT array substrate includes an active region with a pixel electrode  117  and a thin film transistor (TFT) and gate and data pad regions (G.P, D.P) with a pad electrode  122  connected with an external driving circuit (not shown). 
     The active region includes gate and data lines  112  and  115  formed substantially perpendicular to one another to define a sub-pixel, a gate insulation layer  113  formed on an entire surface of a substrate corresponding to a portion between the gate and data lines  112  and  115 , a thin film transistor (TFT) formed adjacent to a crossing portion of the gate and data lines  112  and  115 , a pixel electrode  117  formed on an entire area inside the sub-pixel connected to a drain electrode  115   b  of the thin film transistor (TFT), and a passivation layer  116  formed on portions of the pixel electrode  117  except an open area of the sub-pixel. For reference, a dotted area provided above the pixel electrode corresponds to the portion formed by removing the passivation layer  116   a  from the open area of the sub-pixel. 
     The insulation layer is not formed between the pixel electrode  117  and a data line layer. Thus, the pixel electrode is connected with the drain electrode  115   b  without using a contact hole. 
     The thin film transistor (TFT) includes a gate electrode  112   a , the gate insulation layer  113 , a semiconductor layer  114 , and source and drain electrodes  115   a  and  115   b . A channel region of the semiconductor layer exposed between the source and drain electrodes is treated with oxygen plasma, thereby preventing the semiconductor layer from deteriorating. In addition, the passivation layer is formed in the channel region. At this time, a predetermined portion of the gate line  112  serves as the gate electrode  112   a.    
     The semiconductor layer and the data line layer are fabricated by the same mask process. Thus, the semiconductor layer and the data line layer are formed in substantially the same pattern, wherein the semiconductor layer is provided below the data line and the source/drain electrodes. 
     At this time, a storage capacitor (Cst) is formed in the active region. The storage capacitor (Cst) includes a lower capacitor electrode  132  formed on the same layer as the gate line  112 , an upper capacitor electrode  135  formed on the same layer as the pixel electrode  117  and overlapped with the lower capacitor electrode  132 , and the gate insulation layer  113  formed between the lower and upper capacitor electrodes  132  and  135 . 
     For one preferred embodiment, the lower capacitor electrode  132  may be formed of a predetermined portion of the gate line, and the upper capacitor electrode  135  may be formed by extending the pixel electrode to the gate line. 
     The pad region includes the gate pad electrode (G.P) and the data pad electrode (D.P). The gate pad electrode (G.P) is formed at the end of the gate line  112 , wherein the gate pad electrode (G.P) transmits a scanning signal. The data pad electrode (D.P) is formed at the end of the data line  115 , wherein the data pad electrode (D.P) transmits a video signal. The gate and data pad electrodes are covered with a transparent conductive layer  127 , so that it may be possible to prevent the gate and data pad electrodes from being oxidized by external oxygen. In  FIG. 3 , the dotted area provided above the pad electrode  122  corresponds to the portion from which the passivation layer  116   b  is removed. 
     The transparent conductive layer  127  is electrically connected with the gate and data pad electrodes  122  through an open area  150 . The open area  150  is formed by removing the gate insulation layer  113  from the portion between the transparent conductive layer and the pad electrode. Then, the passivation layer  116  is formed in the pad region except the portion including the transparent conductive layer  127 . 
     The gate pad electrode (G.P) extends from the gate line  112 , that is, the gate pad electrode (G.P) is formed as one body with the gate line  112 . Also, the data pad electrode (D.P) is formed on the same layer as the gate line  112 , whereby the transparent conductive layer provided above the data pad electrode is connected with the data line. 
     In the TFT array substrate, as shown in  FIG. 5 , the semiconductor layer  114  is not exposed to the outside of the data line  115 , so that it may be possible to prevent the aperture ratio from being lowered by a portion (about 1.5 μm of  FIG. 2 ) of a semiconductor tail. Thus, a black matrix (for preventing light leakage) decreases in size. For reference, a gap between the data line  115  and the pixel electrode  117  corresponds to about 3.5 μm, which may suffer light leakage. Accordingly, the black matrix is provided in the gap between the data line  115  and the pixel electrode  117 , to thereby prevent light from leaking in the gap. 
     Although not shown, the TFT array substrate including the pixel electrode and the thin film transistor may be provided opposite to another substrate with a common electrode and a color filter layer. Then, the two substrates are bonded to each other, and a liquid crystal layer is formed between the two substrates, thereby completing the LCD device. 
     To form the TFT array substrate of the LCD device, as shown in  FIG. 6A , a metal layer is deposited by sputtering on the transparent substrate  111  having good heat-resistance and is then patterned by photolithography using a first exposure mask, thereby forming the gate line ( 112  of  FIG. 3 ), the gate electrode  112   a  of the TFT, the lower capacitor electrode  132  of the storage capacitor region (Cst), the gate pad electrode  122  of the gate pad region (G.P), and the data pad electrode  122  of the data pad region (D.P). For example, the metal layer may be formed of copper (Cu), copper alloy (Cu alloy), aluminum (Al), aluminum neodymium (AlNd), molybdenum (Mo), molybdenum alloy (Mo alloy), chrome (Cr), chrome alloy (Cr alloy), titanium (Ti), titanium alloy (Ti alloy), silver (Ag), or silver alloy (Ag alloy). Preferably, the metal layer is formed of a deposition layer of Mo/AlNd. 
     The lower capacitor electrode  132  may be formed of a predetermined pattern that is separated in parallel from the gate line. In another aspect, a predetermined portion of the gate line serves as the lower capacitor electrode  132 . The gate electrode  112   a  is formed as one body with the gate line, and the gate pad electrode  122  is formed as one body with the end of the gate line. The data pad electrode  122  is formed at the end of the data line that is fabricated by the following process. 
     An inorganic insulation layer of silicon nitride (SiNx) or silicon oxide (SiOx) is deposited on an entire surface of the substrate including the gate electrode  112   a  by CVD (Chemical Vapor Deposition) under high temperature conditions. After that, an amorphous silicon layer  114   d  is deposited on the inorganic insulation layer. Then, a metal layer  115   d  is deposited by sputtering, for example, copper (Cu), copper alloy (Cu alloy), aluminum (Al), aluminum neodymium (AlNd), molybdenum (Mo), molybdenum alloy (Mo alloy), chrome (Cr), chrome alloy (Cr alloy), titanium (Ti), titanium alloy (Ti alloy), silver (Ag), or silver alloy (Ag alloy). Preferably, the metal layer  115   d  is formed of molybdenum (Mo) by sputtering. 
     After that, as shown in  FIG. 6B , a photoresist  155  of UV-curing resin is coated onto an entire surface of the metal layer  115   d  by a spinning method or a roll-coating method. After a second exposure mask (not shown) having a predetermined pattern is positioned substantially above the photoresist, the photoresist is exposed to UV or X-rays, and then the exposed photoresist is developed, thereby forming the photoresist pattern having the step coverage. 
     The second exposure mask is a diffraction exposure mask. To form the diffraction exposure mask, a light-shielding layer of a metal material and a semi-transparent layer are formed on a transparent substrate. The diffraction exposure mask may include three regions corresponding to a transparent region, a semi-transparent region, and a light-shielding region. The transparent region has light transmittance of 100%, the light-shielding region has light transmittance of 0%, and the semi-transparent region has light transmittance between about 0% and about 100%. After applying the diffraction exposure to the photoresist, the photoresist is divided into three regions: a first region corresponding to the transparent region of the diffraction exposure mask, wherein the photoresist is completely removed by development, a second region corresponding to the light-shielding region of the diffraction exposure mask, wherein the photoresist has a first thickness without being removed, and a third region corresponding to the semi-transparent region of the diffraction exposure mask, wherein the photoresist has a second thickness according as the photoresist is partially removed. Thus, the second thickness is substantially less than the first thickness. The photoresist may be a positive type or a negative type photoresist. For the positive type photoresist, if the predetermined portion of photoresist is exposed to light, it is removed. Meanwhile, for the negative type photoresist, if the predetermined portion of photoresist is not exposed to light, it is removed. 
     After the diffraction exposure process, the photoresist  155  has the step coverage. That is, the predetermined portions of the photoresist having the first thickness correspond to the data line and the TFT region of the source and drain electrodes. Also, the photoresist is completely removed above the gate pad electrode and the data pad electrode. Then, the other portions of the photoresist have a second thickness. 
     As shown in  FIG. 6C , the metal layer  115   d , the amorphous silicon layer  114   d , and the gate insulation layer  113  are etched by the photoresist  155  as a mask, to hereby form the open area  150  above the gate and data pad electrodes. After the pad electrode  122  is formed of Mo/AlNd, the molybdenum (Mo) is removed from the pad electrode when forming the open area  150 . 
     The process of etching the metal layer, the semiconductor layer, and the gate insulation layer above the gate and data pad electrodes  122  is comprised of a first step of etching the metal layer provided above the gate and data pad electrodes by a wet-etching method, and a second step of etching the semiconductor layer and the gate insulation layer provided above the gate and data pad electrodes by a dry-etching method. The wet-etching method uses a dipping or spraying technique with an etchant of HNO 3 /H 3 PO 4 /CH 3 COOH/H 2 O. The dry-etching method uses plasma gas or radical. 
     The process of etching the metal layer, the semiconductor layer, and the gate insulation layer may be performed in one dry-etching chamber. In this case, the metal layer, the semiconductor layer, and the gate insulation layer are etched with different etching gases. If etching the metal material (Mo), SF 6 , Cl 2  or O 2  may be used as the etching gas. If using amorphous silicon, SF 6 , Cl 2  or H 2  may be used as the etching gas. If etching the gate insulation layer, SF 6 , O 2  or He may be used as the etching gas. 
     Then, as shown in  FIG. 6D , the predetermined portion of the photoresist having the second thickness is completely removed by ashing the photoresist  155 . As shown in  FIG. 6E , the metal layer  115   d  and the amorphous silicon layer  114   d  are sequentially etched by using the photoresist  155  as a mask, thereby forming the data line ( 115  of  FIG. 3 ), the source and drain electrodes  115   a  and  115   b , and the semiconductor layer  114  below the source and drain electrodes. Because the data line layer and the semiconductor layer are fabricated by the same mask process, the pattern of the semiconductor layer is substantially the same as those of the data line and the source and drain electrodes, wherein the semiconductor layer is formed below the data line and the source/drain electrodes. 
     By one diffraction exposure process, it is possible to form the data line, the source and drain electrodes and the semiconductor layer  114  at the same time. The data line is formed substantially perpendicular to the gate line, whereby the sub-pixel is defined. Also, the semiconductor layer  114  and the source/drain electrodes  115   a / 115   b  are sequentially formed on the gate electrode  112   a , thereby completing the thin film transistor (TFT). 
     As shown in  FIG. 6F , after removing the photoresist  155 , a transparent conductive layer  117   d  is deposited on the entire surface of the substrate including the source and drain electrodes  115   a  and  115   b , wherein the transparent conductive layer  117   d  is formed of Indium Tin Oxide (ITO) or Indium Zinc Oxide (IZO). After that, as shown in  FIG. 6G , a photoresist  151  is coated onto the transparent conductive layer  117   d , and is then patterned by photolithography using a third exposure mask. 
     As shown in  FIG. 6H , the transparent conductive layer  117   d  is etched by using the photoresist  151  as a mask, thereby forming the pixel electrode  117 , the upper capacitor electrode  135 , and the transparent conductive layer  127 . Then, the channel region of the thin film transistor is defined by etching the space between the source and drain electrodes  115   a  and  115   b . When wet-etching the space between the source and drain electrodes  115   a  and  115   b , an etchant of Hydrofluoric Acid (HF), Buffered Oxide Etchant (BOE), NJ 4 F, or a mixed solution thereof may be used. An etchant which can etch ITO is available for the etching of molybdenum (Mo). 
     The steps of defining the channel region and forming the pixel electrode are performed at the same time, whereby it is possible to prevent a semiconductor tail from being formed in the channel region. 
     The pixel electrode  117  is formed in the entire area of the sub-pixel. The pixel electrode  117  is directly connected with the drain electrode  115   b  without a contact hole. Also, the upper capacitor electrode  135  extends from the pixel electrode, that is, the upper capacitor electrode  135  is formed as one body with the pixel electrode. 
     The transparent conductive layer  127  covers the gate pad electrode  122  and the data pad electrode  122 , so that it is possible to prevent the gate and data pad electrodes from being oxidized. The transparent conductive layer positioned above the data pad electrode extends to the data line of the active region, whereby the transparent conductive layer is electrically connected with the data line. 
     The upper capacitor electrode  135  is overlapped with the lower capacitor electrode  132  with the gate insulation layer therebetween, thereby completing the storage capacitor (Cst). Then, after removing the photoresist  151 , as shown in  FIG. 6I , n + -type impurity ions are implanted to the channel region of the semiconductor layer  114  exposed between the source and drain electrodes  115   a  and  115   b , and then are treated with O 2  plasma, thereby preventing the semiconductor layer from deteriorating due to ambient light. 
     As shown in  FIG. 6J , an inorganic insulation layer of silicon oxide (SiO 2 ) or silicon nitride (SiNx) is formed on the entire surface of the substrate including the pixel electrode  117 , wherein the inorganic insulation layer is formed at a thickness of about 1000 Angstroms or less, thereby forming the passivation layer  116 . Then, the photoresist  152  is coated onto the passivation layer  116 , and is then patterned by photolithography using a fourth mask. Then, the passivation layer exposed by the photoresist is etched. The passivation layer is formed in the channel region between the source and drain electrodes. 
     After removing the photoresist, as shown in  FIG. 6K , the passivation layer  116  is left in the remaining regions except the portion of the open area of the sub-pixel and the pad electrode (or transparent conductive layer). That is, an image is displayed through the open area of the sub-pixel from which the passivation layer is removed. Also, the pad electrode is electrically connected with an external driving circuit through the transparent conductive layer from which the passivation layer is removed. 
     The TFT array substrate according to the first embodiment of the present invention is fabricated by four exposure masks that is appropriate for low-mask technology. Because the amorphous silicon layer and the metal layer are etched at the same time, it is possible to prevent a semiconductor tail from being formed as the result of over-etching of the metal layer. As a result, aperture ratio is improved by removing the passivation layer from the sub-pixel. 
     Second, Third and Fourth Embodiments 
     The first embodiment of the present invention focused on a TN mode LCD device. However, the present invention may be applicable to a TFT array substrate of an IPS mode LCD device which will be explained as follows. 
       FIG. 7  is a plan view illustrating a TFT array substrate according to the second embodiment of the present invention.  FIG. 8  is a cross sectional view illustrating a TFT array substrate according to the second embodiment of the present invention.  FIGS. 9A to 9D  are plan views illustrating a TFT array substrate according to the second embodiment of the present invention.  FIGS. 10A to 10K  are cross sectional views illustrating a method for fabricating a TFT array substrate according to the second embodiment of the present invention. 
       FIG. 11  is a plan view illustrating a TFT array substrate according to the third embodiment of the present invention.  FIG. 12  is a cross sectional view illustrating a TFT array substrate according to the third embodiment of the present invention.  FIG. 13  is a plan view illustrating a TFT array substrate according to the fourth embodiment of the present invention.  FIG. 14  is a cross sectional view illustrating a TFT array substrate according to the fourth embodiment of the present invention. 
     As shown in  FIGS. 7 and 8 , the TFT array substrate of the IPS mode LCD device includes an active region with a common electrode  224 , a pixel electrode  217  and a thin film transistor (TFT), and gate and data pad regions (G.P, D.P) with a pad electrode  222  connected with an external driving circuit (not shown). 
     The active region includes gate and data lines  212  and  215  formed substantially perpendicular to one another to define a sub-pixel, a gate insulation layer  213  formed on an entire surface of a substrate corresponding to a portion between the gate and data lines, a thin film transistor (TFT) formed adjacent to a crossing portion of the gate and data lines  212  and  215 , a pixel electrode  217  overlapped with a drain electrode  215   b  of the thin film transistor (TFT), a common line  225  that transmits a Vcom signal, the common line formed in parallel to the gate line  212 , a common electrode  224  that diverges from the common line  225  and is formed in parallel to the pixel electrode  217  to generate a horizontal electric field, and a passivation layer  216  formed on portions of the pixel electrode  217  except an open area of the sub-pixel. 
     The pixel electrode  217  is formed directly on a data line layer without forming an insulation layer, whereby the pixel electrode  217  is electrically connected with a drain electrode without a contact hole. Also, a thin gate insulation layer  213  is interposed between the pixel electrode  217  and the common electrode  224 , thereby improving the intensity of the horizontal electric field. 
     The thin film transistor (TFT) is comprised of a gate electrode  212   a , the gate insulation layer  213 , a semiconductor layer  214 , and source and drain electrodes  215   a  and  215   b . A channel region of the semiconductor layer exposed between the source and drain electrodes is treated with oxygen plasma, thereby preventing the semiconductor layer from deteriorating by the application of ambient light. In addition, the passivation layer is formed in the channel region. A predetermined portion of the gate line  212  serves as the gate electrode  212   a.    
     To decrease the number of masks used, the semiconductor layer and the data line layer are fabricated by the same mask process, whereby they are formed in substantially the same pattern. In this case, the semiconductor layer is provided below the data line and the source/drain electrodes. 
     A storage capacitor (Cst) is formed in the active region. The storage capacitor (Cst) includes a lower capacitor electrode  132  formed on the same layer as the gate line  112 , an upper capacitor electrode  135  formed on the same layer as the pixel electrode  117  and overlapped with the lower capacitor electrode  132 , and the gate insulation layer  113  formed between the lower and upper capacitor electrodes  132  and  135 . 
     The pad region includes the gate pad electrode (G.P) and the data pad electrode (D.P). The gate pad electrode (G.P) is formed at the end of the gate line  212 , wherein the gate pad electrode (G.P) transmits a scanning signal. The data pad electrode (D.P) is formed at the end of the data line  215 , wherein the data pad electrode (D.P) transmits a video signal. The gate and data pad electrodes are covered with a transparent conductive layer  227  ( FIG. 7 ), so that it is possible to prevent the gate and data pad electrodes from being oxidized by external oxygen. 
     The transparent conductive layer  227  is electrically connected with the gate and data pad electrodes  222  through an open area  250 . The open area  250  is formed by removing the gate insulation layer  213  between the transparent conductive layer and the pad electrode. Then, the passivation layer  216  is formed in the pad region except the portion including the transparent conductive layer  227 . 
     The gate pad electrode (G.P) extends from the gate line  212 , that is, the gate pad electrode (G.P) is formed as one body with the gate line  212 . Also, the data pad electrode (D.P) is formed on the same layer as the gate line  212 , whereby the transparent conductive layer provided above the data pad electrode is connected with the data line. 
     In the TFT array substrate, as shown in  FIG. 5 , the semiconductor layer  114  is not exposed to the outside of the data line  115 , so that it is possible to prevent aperture ratio from being lowered by a portion (about 1.5 μm of  FIG. 2 ) of a semiconductor tail. Thus, a black matrix for preventing light leakage decreases in size. For reference, a gap between the data line  115  and the pixel electrode  117  corresponds to about 3.5 μm, which may suffer light leakage. Accordingly, the black matrix is provided in the gap between the data line  115  and the pixel electrode  117 , to thereby prevent light from leaking in the gap. 
     In the TFT array substrate, the semiconductor layer  214  is not exposed to the outside of the data line  215 , so that it is possible to prevent aperture ratio from being lowered by a portion of a semiconductor tail. 
     Although not shown, the TFT array substrate including the common electrode, the pixel electrode and the thin film transistor may be provided opposite to another substrate provided with a color filter layer. Then, the two substrates are bonded to each other, and a liquid crystal layer is formed between the two substrates, thereby completing the LCD device. 
     To form the TFT array substrate of the IPS mode LCD device, as shown in  FIGS. 9A and 10A , a metal layer is deposited by sputtering on a transparent substrate having good heat-resistance and is then patterned by photolithography using a first exposure mask, thereby forming the gate line  212 , the common line  225 , the common electrode  224 , the gate electrode  212   a  of the TFT, the lower capacitor electrode  232  of the storage capacitor region (Cst), the gate pad electrode  222  of the gate pad region (G.P), and the data pad electrode  222  of the data pad region (D.P). 
     For example, the metal layer may be formed of copper (Cu), copper alloy (Cu alloy), aluminum (Al), aluminum neodymium (AlNd), molybdenum (Mo), molybdenum alloy (Mo alloy), chrome (Cr), chrome alloy (Cr alloy), titanium (Ti), titanium alloy (Ti alloy), silver (Ag), or silver alloy (Ag alloy). Preferably, the metal layer is formed of a deposition layer of Mo/AlNd. The common line  225  is formed substantially in parallel to the gate line  212 , and the common electrode  224  diverges from the common line  225 . 
     The lower capacitor electrode  232  is formed as one body with the common line  225  and the common electrode  224 . The gate electrode  212   a  is formed of a predetermined portion of the gate line  212 . Also, the gate pad electrode  222  is formed at the end of the gate line  212 , that is, the gate pad electrode  222  is formed as one body with the gate line  212 . The data pad electrode  222  is formed at the end of the data line which is formed by the following process. 
     As shown in  FIG. 10B , an inorganic insulation layer of silicon nitride (SiNx) or silicon oxide (SiOx) is deposited on an entire surface of the substrate including the gate electrode  212   a  by CVD (Chemical Vapor Deposition) under high temperature conditions. After that, an amorphous silicon layer  214   d  is deposited on the inorganic insulation layer. Then, a metal layer  215   d  is deposited by sputtering, for example, copper (Cu), copper alloy (Cu alloy), aluminum (Al), aluminum neodymium (AlNd), molybdenum (Mo), molybdenum alloy (Mo alloy), chrome (Cr), chrome alloy (Cr alloy), titanium (Ti), titanium alloy (Ti alloy), silver (Ag), or silver alloy (Ag alloy). Preferably, the metal layer  215   d  is formed of molybdenum (Mo). 
     As shown in  FIG. 10C , a photoresist  255  of UV-curing resin is coated onto an entire surface of the metal layer  215   d  by a spinning method or a roll-coating method. After a second exposure mask (not shown) having a predetermined pattern is positioned substantially above the photoresist, the photoresist is exposed to UV or X-rays, and then the exposed photoresist is developed, thereby forming the photoresist pattern having the step coverage. 
     The second exposure mask is a diffraction exposure mask. After the diffraction exposure process, the photoresist  255  has the step coverage. That is, the predetermined portions of the photoresist are not removed, so that they have a first thickness which correspond to the data line and the TFT region of the source and drain electrodes. Also, the photoresist is completely removed above the gate pad electrode and the data pad electrode. Then, the other portions of the photoresist have a second thickness, wherein the second thickness is substantially less than the first thickness. 
     As shown in  FIGS. 9B and 10D , the metal layer  215   d , the amorphous silicon layer  214   d , and the gate insulation layer  213  are etched by the photoresist  255  as a mask, to thereby form the open area  250  above the gate and data pad electrodes (G.P, D.P). The etching process is performed by a first step of etching the metal layer  215   d  provided above the pad electrode  222  by wet-etching method and a second step of etching the amorphous silicon layer  214   d  and the gate insulation layer  213  provided above the pad electrode  222  by a dry-etching method. In the meantime, the process of etching the metal layer, the amorphous silicon layer, and the gate insulation layer may be performed in one dry-etching chamber. If the pad electrode  222  is formed of Mo/AlNd, the molybdenum (Mo) is removed from the pad electrode when forming the open area  250 . 
     As shown in  FIG. 10E , the predetermined portions of the photoresist  255  having the second thickness is completely removed by ashing the photoresist  255 . Then, the metal layer  215   d  and the amorphous silicon layer  214   d  are sequentially etched using the remaining photoresist  255  as a mask, thereby forming the data line, the source and drain electrodes  215   a  and  215   b , the upper capacitor electrode  235 , and the semiconductor layer  214 . The semiconductor layer  214  is positioned below the source and drain electrodes. 
     The upper capacitor electrode  235  is overlapped with the lower capacitor electrode  232  with the gate insulation layer  213  therebetween, thereby completing the storage capacitor (Cst). The upper capacitor electrode  235  is formed as one body with the drain electrode  215   b.    
     As shown in  FIGS. 9B and 10E , the data line is formed substantially perpendicular to the gate line  212 , to thereby define the sub-pixel. Then, the semiconductor layer  214  and the source/drain electrodes  215   a  and  215   b  are formed above and overlapped with the gate electrode  212   a , wherein the semiconductor layer  214  and the source/drain electrodes  215   a  and  215   b  form the thin film transistor. The data line layer and the semiconductor layer are formed by the same mask process. Thus, the semiconductor layer and the data line layer are formed in substantially the same pattern, wherein the semiconductor layer is provided below the data line and the source/drain electrodes. 
     As shown in  FIG. 10F , after removing the photoresist  255 , a transparent conductive layer  217   d  is deposited on the entire surface of the substrate including the source and drain electrodes  215   a  and  215   b , wherein the transparent conductive layer  217   d  is formed of Indium Tin Oxide (ITO) or Indium Zinc Oxide (IZO), by way of example. Then, as shown in  FIG. 10G , a photoresist  251  is coated onto the transparent conductive layer  217   d , and is then patterned by photolithography using a third exposure mask. 
     Then, the transparent conductive layer  217   d  is etched using the patterned photoresist  251  as a mask, thereby forming the pixel electrode  217  and the transparent conductive layer  227 . Also, the channel region of the thin film transistor is defined by etching the space between the source and drain electrodes  215   a  and  215   b . It is necessary to wet-etch together the transparent conductive layer and the metal layer using Hydrofluoric Acid (HF), Buffered Oxide Etchant (BOE), NJ 4 F, or a mixed solution thereof, for example. The etchant used to etch ITO is available for the etching of molybdenum (Mo) of the source and drain electrodes. 
     The steps of defining the channel region and forming the pixel electrode are performed at the same time, whereby it is possible to prevent a semiconductor tail from being formed in the channel region. 
     The pixel electrode  217  is directly connected with the drain electrode  215   b  without a contact hole, and the pixel electrode  217  is formed substantially in parallel to the common electrode  224 , to thereby generate the horizontal electric field. The transparent conductive layer  227  covers the pad electrode  222 , to prevent the pad electrode  222  from being oxidized. The transparent conductive layer  227  provided above the data pad electrode extends to the data line of the active region, whereby the transparent conductive layer  227  is electrically connected with the data line  215 . Then, after removing the photoresist  251 , as shown in  FIG. 10H , the channel region of the semiconductor layer exposed between the source and drain electrodes is treated with O 2  plasma, thereby preventing the semiconductor layer from deteriorating by the application of ambient light. 
     As shown in  FIG. 10I , the passivation layer  216  is formed by depositing an inorganic insulation material of silicon nitride (SiNx) or silicon oxide (SiO 2 ), for example, on the entire surface of the substrate including the pixel electrode  217 , or by coating an organic insulation material of BCB or acrylic resin onto the entire surface of the substrate including the pixel electrode  217 . Thereon, a photoresist  252  is coated as shown in  FIG. 10J , and is then patterned by photolithography using a fourth mask. Then, the passivation layer  216  exposed by the photoresist  252  is etched by a dry-etching process. 
     Referring to  FIGS. 9D and 10K , after removing the photoresist, the passivation layer  216  is left in the remaining portions except the open area of the sub-pixel and the pad electrode. That is, the passivation layer  216   a  is removed from the open area of the sub-pixel, and the passivation layer  216   b  is removed from the pad electrode. Thus, an image is displayed through the open area of the sub-pixel from which the passivation layer is removed. Also, the pad electrode is electrically connected with an external driving circuit through the transparent conductive layer from which the passivation layer is removed. The passivation layer is formed in the channel region between the source and drain electrodes. 
     If performing dry-etching to the passivation layer of the opening area of the sub-pixel, the gate insulation layer  113  adjacent to the pixel electrode  117  is also etched together, whereby the under-cut phenomenon occurs at the edge of the pixel electrode. 
     As shown in  FIGS. 11 and 12 , because the passivation layer  216  is left in the active region, and the passivation layer  216   b  is removed from the pad electrode  222  of the pad region, it is possible to prevent the under-cut phenomenon from occurring at the edge of the pixel electrode of the active region. That is, the etching process is not applied to the passivation layer of the open area of the sub-pixel, so that it is possible to prevent the gate insulation layer positioned below the pixel electrode from being etched. 
     Light is absorbed by the passivation layer of the open area of the sub-pixel, whereby the transmittance of the device may be lowered. In addition to the passivation layer  216   a  of the open area of the sub-pixel and the passivation layer  216   b  of the pad electrode  222 , the gate insulation layer  213   a  is also removed from the open area of the sub-pixel, as shown in  FIGS. 13 and 14 , thereby improving the transmittance of the device and preventing the under-cut phenomenon from occurring in the edge of the pixel electrode  217 . 
     When dry-etching the gate insulation layer  213  to form the open area  250  that exposes the pad electrode  222 , the gate insulation layer  213   a  is also removed from the open area of the sub-pixel. The common electrode  224  of the open area of the sub-pixel is also exposed to the outside. 
     After forming the transparent conductive layer  227  connected with the pad electrode through the open area and the pixel electrode  217  formed in parallel to the common electrode, the passivation layer  216  is formed on the entire surface of the substrate including the pixel electrode. The passivation layer  216   a  is removed from the open area of the sub-pixel, and the passivation layer  216   b  is removed from the pad electrode, whereby the pixel electrode and the transparent conductive layer are exposed to the outside. 
     The gate insulation layer is not formed in the sub-pixel when etching the passivation layer  216   a  of the open area of the sub-pixel. Thus, it is possible to prevent the under-cut phenomenon from occurring by the etching of the gate insulation layer. 
     Also, the gate insulation layer and the passivation layer, which absorbs light, are not formed in the sub-pixel, so that the transmittance of the device improves. 
     The TFT array substrate of the LCD device according to the present invention is fabricated using four exposure masks, which is appropriate for low-mask technology. Because the process of defining the channel region is performed together with the process of forming the pixel electrode, it is possible to prevent the semiconductor tail from being formed when defining the channel region. 
     In a preferred embodiment of the present invention, the data pad electrode and the gate pad electrode are formed on the same layer, and the data pad electrode is connected with the data line by etching the transparent conductive layer of the pad region to the data line of the active region. However, the present invention is not limited to this structure. The data pad electrode and the data line may be formed on the same layer, to obtain a simplified structure. 
     As mentioned above, the TFT array substrate according to the present invention and the method for fabricating the same have the following advantages. 
     First, the process of defining the channel region is performed together with the process of forming the pixel electrode, so that it is possible to prevent a semiconductor tail from being formed in the channel region. The semiconductor tail may cause wavy noise whenever the backlight unit (B/L) is turned-on/off. 
     Also, because the semiconductor tail is not formed outside of the data line, it is possible to prevent the aperture ratio from being lowered by the area of the semiconductor tail. Thus, the black matrix (which prevents light leakage) decreases in size. 
     Also, the turn-off current may be increased due to the photocurrent of the semiconductor tail. By removing the semiconductor tail, it is possible to prevent the turn-off current of the device from being increased. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.