Abstract:
An in-plane switching mode liquid crystal display device includes first and second substrates. A plurality of gate and data bus lines define pixel regions and arranged on the first substrate. A plurality of thin film transistors are adjacent respective cross points of the gate and data bus lines. A plurality of gate electrodes are connected to said gate bus lines. A gate insulator is on the gate electrodes and a first metal layer includes a plurality of first electrodes on the gate insulator. A passivation layer is on the first metal layer. A transparent second metal layer includes a plurality of second electrodes on the passivation layer, the first and second electrodes applying plane electric fields.

Description:
This application is a division of application Ser. No. 09/079,894 filed May 15, 1998, and claims the benefit of Korean Application No. 1997-19200, filed on May 19, 1997, which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display device, and more particularly, an in-plane switching mode liquid crystal display device. 
     2. Discussion of the Related Art 
     Twisted nematic liquid crystal display devices (hereinafter TN LCDs) having high image quality and low power consumption are widely applied to flat panel display devices. TN LCDs, however, have a narrow viewing angle due to refractive anisotropy of liquid crystal molecules. This is because horizontally aligned liquid crystal molecules prior to applying a voltage become nearly vertically aligned with respect to the substrate when voltage is applied to the liquid crystal panel. 
     Recently, in-plane switching mode liquid crystal display devices (hereinafter IPS-LCDs) have been widely studied for improving the viewing angle characteristic. These liquid crystal molecules are nearly horizontally aligned. 
     FIG. 1 a  is a plan view of a unit pixel of a conventional active matrix LCD. As shown in FIG. 1 a , unit pixel region is defined by a gate bus line  1  and a data bus line  2  in which the lines  1 ,  2  are arranged perpendicularly and/or horizontally in a matrix on a transparent substrate (first substrate)  10 . A common line  3  is arranged parallel to the gate bus line  1  in the pixel region. A thin film transistor (TFT) is formed adjacent a cross point of the data bus line  2  and the gate bus line  1 . The TFT, as shown in FIG. 1 b  which is a sectional view according to line I—I of FIG. 1 a , includes a gate electrode  5 , a gate insulator  12 , a semiconductor layer  15 , a channel layer  16 , and source/drain electrode  6 . The gate electrode S is connected to the gate bus line  1  and source/drain electrode  6  is connected to the data bus line  2 . The gate insulator  12  is formed on the whole surface of the first substrate of the first substrate  10 . 
     A common electrode  9  and a data electrode  8  are formed in the pixel region. The common electrode  9  is formed with the gate electrode  5  and connected to the common line  3 . The date electrode  8  is formed with the source/drain electrode  6  and electrically connected to the source/drain electrode  6 . Further, a passivation layer  20  and a first alignment layer  23   a  are deposited on the whole surface of the first substrate  10 . 
     On a second substrate  11 , a black matrix  28  is formed to prevent a light leakage which is generated around the TFT, the gate bus line  1 , and the data bus line  2 . A color filter layer  29 , and a second alignment layer  23   b  is formed on the black matrix  28  in sequence. Also, a liquid crystal layer  30  is formed between the first and second substrates  10 ,  11 . 
     When voltage is not applied to the LCD having the above structure, liquid crystal molecules in the liquid crystal layer  30  are aligned according to alignment directions of the first and second alignment layers  23   a ,  23   b . However, when voltage is applied between the common electrode  9  and the data electrode  8 , the liquid crystal molecules are vertically aligned to the extending directions of the common and data electrodes. In the foregoing, since liquid crystal molecules in the liquid crystal layer  30  are switched on the same plane at all times, grey inversion is not created in the up and down direction, and right and left direction of the viewing angle. 
     In the conventional LCD having the above structure, however, the aperture ratio is decreased by the data electrode  8  and the common electrode  9 , which are opaque. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an in-plane switching mode liquid crystal display that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an LCD having a high aperture ratio. 
     Another object of the present invention is to provide an LCD having a light transmissive common and data electrodes. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an in-plane switching mode liquid crystal display device comprises first and second substrates; a plurality of gate and data bus lines defining pixel regions and arranged on said first substrate; a plurality of thin film transistors adjacent respective cross points of said gate and data bus lines; a plurality of gate electrodes connected to said gate bus lines; a gate insulator on said gate electrodes; a first metal layer including a plurality of first electrodes on said gate insulator; a passivation layer on said first metal layer; and a transparent second metal layer including a plurality of second electrodes on said passivation layer, wherein said first and second electrodes apply plane electric fields. 
     In another aspect of the present invention, an in-plane switching mode liquid crystal display device comprises a thin film transistor on a substrate including source, drain, and gate electrodes; a data electrode connected to one of the source and drain electrodes; and a common electrode, wherein the data electrode, the common electrode, and the gate electrode are each on different layers over the substrate. 
     In another aspect of the present invention, an in-plane switching mode liquid crystal display device comprises a substrate; a thin film transistor including source, drain, and gate electrodes on a portion of the substrate; a first insulating layer on the substrate; a first electrode including a data electrode on the insulating layer; a second insulating layer covering the first electrode; and a second electrode including a common electrode on the second insulating layer, wherein the gate electrode, the first electrode, and the second electrode are on different layers. 
     In another aspect of the present invention, a method of forming an in-plane switching mode liquid crystal display device comprises the steps of forming a thin film transistor on a substrate including source, drain, and gate electrodes; forming a data electrode connected to one of the source and drain electrodes; and forming a common electrode, wherein the data electrode, the common electrode, and the gate electrode are each on different layers over the substrate. 
     In a further aspect of the present invention, a method for forming an in-plane switching mode liquid crystal display device comprises the steps of forming a thin film transistor including source, drain, and gate electrodes on a portion of a substrate; forming a first insulating layer on the substrate; forming a first electrode including a data electrode on the insulating layer; forming a second insulating layer covering the first electrode; and forming a second electrode including a common electrode on the second insulating layer, wherein the gate electrode, the first electrode, and the second electrode are on different layers. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
     In the drawings: 
     FIG. 1 a  is a plan view of a unit pixel of a conventional in-plane switching mode LCD; 
     FIG. 1 b  is a sectional view according to line I—I of FIG. 1 a;    
     FIG. 2 a  is a plan view of a unit pixel of an LCD according to one embodiment of the present invention; 
     FIG. 2 b  is a sectional view according to line II—II of FIG. 2 a;    
     FIG. 3 a  is a drawing showing an embodiment of a gate pad region according to the present invention; 
     FIG. 3 b  is a drawing showing an embodiment of a data pad region according to the present invention; and 
     FIG. 4 is a sectional view of an in-plane switching mode LCD according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     An LCD according to the present invention comprises first and second substrates, data and gate bus lines defining a pixel region on the first substrate in which the lines are arranged perpendicularly and/or horizontally in a matrix. Common lines are formed parallel to the gate bus lines. TFTs are formed at respective cross points of the data bus lines and the gate bus lines in the pixel region. At least one data electrode is formed in the pixel region, and at least one transparent common electrode is formed parallel to the data bus lines. A passivation layer is formed on the whole surface of the first substrate. A first alignment layer with a fixed alignment direction is deposited on the passivation layer. A black matrix is formed over the second substrate to prevent light leakage around the TFTs, the gate bus lines, and the data bus lines. A color filter layer is formed on the black matrix and the second substrate. A second alignment layer is deposited on the color filter layer. A liquid crystal layer is between the first and second substrates. The transparent common electrode is connected to the common line, and the data electrode is connected to source/drain electrode of the TFT. The common line may be formed with the common electrode in a single process using the same material or another process using a different material. When the common and data electrodes are formed on the gate insulator, the common line which may be formed with the data bus line in a single process using the same material may also be formed on the gate insulator. Further, the gate pad and the data pad are prevented from oxidizing by the metal layer including ITO (indium tin oxide) is formed thereon by forming the transparent common electrode on the passivation layer. 
     FIGS. 2 a  and  2   b  are drawings showing an in-plane switching mode LCD according to one embodiment of the present invention. As shown in FIG. 2 a , gate and data bus lines  101 ,  102  defining a pixel region are arranged perpendicularly and/or horizontally in a matrix on the first substrate  110 . Common line  103  is formed parallel to the gate bus line  101 . TFTs are respectively formed at cross points of the data bus line  102  and the gate bus line  101  in the pixel region. As shown in FIG. 2 b , a TFT includes a gate electrode  105 , a gate insulator  112  deposited on the gate electrode  105 , a semiconductor layer  115  formed on the gate insulator  112 , a channel layer  116  formed on the semiconductor layer  115 , and source/drain electrode  106  formed on the channel layer  116 . The gate insulator  112  is deposited on the whole surface of the substrate  110 . 
     The gate electrode  105  and the gate bus line  101  are preferably formed by sputtering and photoetching a method such as Al, Mo, or Al alloy in a single process at a surface of the substrate. At this time, it is possible to form an anodic oxidation layer by anodizing the gate bus line  101  and the gate electrode  105  to improve the insulating characteristic. The gate insulator  112  including inorganic material such as SiNx or SiOx is formed by PCVD (plasma chemical vapor deposition) method. 
     The semiconductor layer  115  is formed by depositing and etching an amorphous silicon by PCVD method, and the channel layer  116  is formed by depositing n + a-Si. The source/drain electrode  106  is formed at the same time with the data electrode  108  by depositing and etching a metal such as Al, Cr, Ti, and Al alloy by a sputtering method. At this time, it is possible to form the semiconductor layer  115 , the channel layer  116 , and the source/drain electrode  106  by different processes. Also, it is possible to form the semiconductor layer  115  and the channel layer  116  by etching the a-Si layer and the n + a-Si layer on the gate insulator  112 . Furthermore, an etch stopper may be formed on the semiconductor layer  115  to prevent the channel region from being etched. 
     The common line  103  and the common electrode  109  are formed on the passivation layer  120  including an inorganic material such as SiNx or SiOx, or organic material such as BCB (benzocyclobutene) or acryl resin by depositing and etching a transparent metal such as ITO (indium tin oxide) by a sputtering method. At this time, it is possible to form the common line  103  and the common electrode  109  in a same process. Also, these may be formed by different processes using different materials. For instance, the common line  103  may be formed with the gate bus line  101  made of an opaque metal such as Al, Mo, Ta, or Al alloy and the common electrode  109  may be made of a transparent metal such as ITO. Further, the first alignment layer  123   a  is formed on the passivation layer  120 . 
     FIG. 3 a  is a drawings showing a gate pad region, and FIG. 3 b  is a drawing showing a data pad region according to one embodiment of the present invention. A gate pad  135  is preferably formed at the same time with the gate bus line  102  and the gate electrode  105  on the first substrate  110  at the same time, and a data pad  136  is preferably formed at the same time with the data bus line  101  and the source/drain electrode  106  on the gate insulator  112 . The gate pad  135  and the data pad  136  are exposed to outside air, thereby forming oxidation layers on the pads  135  and  136  which cause inferior connections to an outer driving circuit. Therefore, if a metal layer  140  including ITO is formed on the pads  135 ,  136  with the common electrode  109 , it is possible to prevent the pads  135 ,  136  from oxidizing. 
     The first alignment layer  123   a  on the passivation layer includes polyimide, polyamide, or photosensitive material such as PVCN (polyvinylcinnamate) or PSCN (polysiloxanecinnamate). An alignment direction of the layer  123   a  is determined by rubbing in case of polyimide or polyamide, and by light irradiation using UV (ultraviolet) light in case of a photosensitive material in which the alignment direction is controlled by the polarization direction of an irradiated light. 
     On the second substrate  111 , a black matrix  128  for preventing light leakage at around the gate bus line  101 , the data bus line  102 , and TFT is formed by depositing a metal such as Cr or CrOx as a shielding layer. Color filter elements R, G, and B are formed in the color filter layer  129  on each of pixel regions. An overcoat layer may be formed on the color filter layer in order to improve its flatness or planarity. 
     After depositing the second alignment layer  123   b  made of polyimide or photosensitive material on the overcoat layer, a fixed alignment direction is determined by rubbing or light irradiating thereon, as discussed above. The liquid crystal layer  130  is formed between the first substrate  110  and second substrate  111  by injecting a liquid crystal in a vacuum. 
     In an LCD having the above structure, when a voltage is applied from the outer driving circuit (not illustrated) to the liquid crystal panel through the TFT, a plane electric field at the surface of the substrate is created between the common electrode  109  and the data electrode  108 . By this plane electric field, the liquid crystal molecules aligned according to the alignment directions of the first and second alignment layers  123   a ,  123   b  are switched parallel to the substrate, thereby controlling the amount of transmitted light through the liquid crystal layer. However, when a voltage is not applied, the alignment directions of the first and second alignment layers  123   a ,  123   b  are vertical to each other, thereby the liquid crystal molecules are in a twist mode in which the liquid crystal is a nematic liquid crystal. If a thickness of the liquid crystal is smaller than a gap between the electrodes  108 ,  109 , the liquid crystal molecules are aligned or in parallel in the liquid crystal layer  130  because the plane electric field is uniformly aligned on the whole liquid crystal layer  130 . Although not illustrated, if polarization directions of two polarizers attached to the first and second substrates  110 ,  111  are parallel, light is transmitted in the liquid crystal only when the voltage is applied. This is called the normally black mode. Further, in the present embodiment, only the liquid crystal molecules are aligned by the first and second alignment layers  123   a ,  123   b , and a desirable normally black mode is also obtained by adding a chiral dopant to the liquid crystal after depositing an alignment layer on the first substrate  110  or the second substrate  111 . 
     As discussed above, a difference between the present embodiment and the prior art is that the common electrode  109  including a transparent metal is formed on the passivation layer  120 , thereby improving the aperture ratio. 
     FIG. 4 is a drawing showing another embodiment in accordance with the present invention. In the present embodiment, the common electrode  209  including an ITO is formed with the data electrode  208  on the gate insulator  212 . At this time, the common line  203  may be formed with the common electrode  209  by a single process using ITO, or with the data electrode  208  by a different process using an opaque metal such as Al, Cr, Ti, or Al alloy. 
     A plane electric field is generated at the surface of the substrate where the electrodes  208 ,  209  are formed in parallel in the same plane, thereby improving the viewing angle characteristic. Further, as shown in the drawing, if the passivation layer  220  in the pixel region is etched, a strong electric field is created between the electrodes  208 ,  209  by the electric field which is directly applied to the liquid crystal layer  230  without passing through the passivation layer  220  because the passivation layer  220  only covers the TFT region. This strong electric field causes the liquid crystal molecules in the liquid crystal layer  230  to switch faster, thereby making it possible to obtain an improved moving image. 
     In accordance with the present invention, since the common electrode includes a transparent metal, the opening or aperture ratio is improved. Also, the gate and/or date pads are prevented from oxidizing because a metal layer is formed by a same process in the pad region when the common electrode is formed on the passivation layer. Furthermore, since a strong plane electric field is applied to the liquid crystal layer where the passivation in the pixel region is etched when the common and data electrodes are formed on the same plane, it is possible to obtain an improved viewing angle characteristic and to prevent a break down of the moving image by making the liquid crystal molecules to switch faster. Accordingly, the in-plane switching mode liquid crystal display device of the present invention obtains a high ratio of aperture by using a transparent metal as the common electrode. 
     It will be apparent to those skilled in the art that various modifications and variation can be made in the in-plane switching mode liquid crystal display device of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.