Abstract:
A liquid crystal display, and a fabricating method thereof, having TFTs with specially shaped channels produced by protrusions on source and drain electrodes.

Description:
CROSS REFERENCE  
         [0001]    This application claims the benefit of Korean Patent Application No. P00-87051, filed Dec. 30, 2000, under 35 U.S.C. §119, the entirety of which is hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that is adaptable for use in large, high picture quality displays. The present invention also is directed to a method of fabricating such a liquid crystal display device.  
           [0004]    2. Description of the Related Art  
           [0005]    Generally, an active matrix liquid crystal display (LCD) uses thin film transistors (TFT&#39;s) as switching devices to produce a natural-looking moving picture. Since such LCDs can be made smaller than existing Cathode Ray Tube (CRT) displays, they have become widely used as monitors for devices such as personal computers, notebook computers, office equipment, cellular phones, and pagers.  
           [0006]    Referring now to FIG. 1, which is a plan view showing a partial structure of a conventional liquid crystal display device, that LCD includes a TFT that is arranged extended from the intersection of a data line  4  and a gate line  2  to a portion of the data line  4 . A pixel electrode  22  is connected to the drain electrode  10  of the TFT.  
           [0007]    The TFT includes a gate electrode  6  that is connected to the gate line  2 , a source electrode  8  that is connected to the data line  4 , and the drain electrode  10 . The source electrode  8  projects over the gate line  2  and is a part of the data line  4 . The data line  4  is applied with a gap to the semiconductor layer  14  and  16 . The drain electrode  10  and the pixel electrode  22  are connected together via a contact hole  20 . The TFT also includes semiconductor layers  14  and  16  that define a channel  24  between the source electrode  8  and the drain electrode  10  when a gate voltage is applied to the gate electrode  6 . Referring now to FIG. 2, the channel  24  typically has a length L of about 5 to 6 μm and a width W of about 25 μm. Referring now back to FIG. 1, the channel is arranged along the data line  4  so as to increase the ON current. In operation, a TFT responds to a gate signal on the gate line  2  by selectively applying a data signal on the data line  4  to the pixel electrode  22 .  
           [0008]    In practice, an LCD includes a plurality of TFTs and pixel electrodes  22 . Each pixel electrode  22  is positioned in a cell area defined between data lines  4  and gate lines  2 . The pixel electrodes are comprised of a transparent conductive material. Each pixel electrode  22  can produce a potential difference with a common transparent electrode (not shown) provided on an upper substrate (not shown). By way of a potential difference, a liquid crystal positioned between the lower substrate  1  (not shown) and the upper substrate (not shown) is rotated due to dielectric anisotropy in the liquid crystal. Thus, a molecular arrangement of the liquid crystal varies according to the voltage that is applied to each pixel electrode. As a result, the light transmission through the liquid crystal display varies pixel by pixel such that video information is displayed.  
           [0009]    [0009]FIG. 3A to FIG. 3E are plan views and sectional views, taken along line a-a′ of FIG. 1, which are useful for explaining a method of fabricating an LCD device as shown in FIG. 1. Referring first to FIG. 3A, the gate line  2  and the gate electrode  6  are provided on a substrate  1 . The gate electrode  6  is formed by depositing a metal, such as aluminum (Al) or copper (Cu), such as by sputtering, and then etching to form the gate line and gate electrode.  
           [0010]    Referring now to FIG. 3B, an active layer  14  and an ohmic contact layer  16  are then fabricated on a gate insulating film  12  formed on the gate line  2  and the gate electrode  6 . The gate insulating film  12  is typically formed by depositing an insulating material, such as silicon nitride (SiN x ) or silicon oxide (SiO x ), by plasma enhanced chemical vapor deposition (PECVD). The active layer  14  is then fabricated from undoped amorphous silicon, while the ohmic contact layer  16  is formed from amorphous silicon that is heavily doped with an n-type or p-type impurity. The active layer  14  and the ohmic contact layer  16  are fabricated by deposition and subsequent patterning using chemical etching.  
           [0011]    Referring now to FIG. 3C, the data line  4 , the source electrode  8  and the drain electrode  10  are then provided on the gate insulating film  12 . The data line  4  and the source and drain electrodes  8  and  10  are typically made from chrome (Cr) or molybdenum (Mo). The source electrode  8  projects over the gate line  2  and is a part of the data line  4 . The data line  4  and the source and drain electrodes  8  and  10  are formed by first depositing a metal layer (i.e., Cr or Mo) using chemical vapor deposition or sputtering and then patterning that metal layer. After the data line  4 , source electrode  8 , and drain electrode  10  are formed, the ohmic contact layer  16  over the gate electrode  6  is patterned to expose the active layer  14 . The area over the gate electrode  6  between the source and drain electrodes  8  and  10  in the active layer forms a channel  24 .  
           [0012]    Referring to FIG. 3D, a protective layer  18  and a contact hole  20  are then added to the structure. The protective layer  18  and the contact hole  20  are formed by depositing an insulating material over the exposed surface. The protective layer is then patterned to form the contact hole  20  that exposes the drain electrode. The protective layer  18  is beneficially made from an inorganic insulating material such as silicon nitride (SiN x ) or silicon oxide (SiO x ), an acrylic organic compound, or an organic insulating material having a small dielectric constant such as Teflon, BCB (benzocyclobutene), Cytop or PFCB (perfluorocyclobutane).  
           [0013]    Referring now to FIG. 3E, a pixel electrode  22  is then formed on the protective layer  18 . The pixel electrode  22  is formed by depositing a transparent conductive material, such as indium-tin-oxide (ITO), indium-zinc-oxide (IZO) or indium-tin-zinc-oxide (ITZO), on the protective layer  18  and into the contact hole  20 . The transparent conductive layer is then patterned. The pixel electrode  22  is electrically connected, via the contact hole  20 , to the drain electrode  10 .  
           [0014]    In such LCD devices, the gate electrode  6  extends along the data line  4 , as shown in FIG. 1, so as to enlarge the channel  24 . When a typical photo resist pattern is used to fabricate an extended gate electrode  6 , the uniformity of the resulting gate electrode is not particularly good as the photo resist deteriorates during fabrication. The result can be a defective gate electrode and consequent reduced manufacturing throughput. Additionally, an LCD device having an extended gate electrode has a problem in that, since the TFT area extends along the data line to correspond to the lengthy gate electrode, the display area has a reduced aperture ratio.  
           [0015]    In addition, as the overlapping area A of the gate electrode  6  and the data line  4  increases (see FIG. 1), the parasitic capacitance Cgs, which is proportional to the overlapping area A, increases. A large parasitic capacitance Cgs can cause flicker and a residual image, both of which reduce picture quality. Also, a large parasitic capacitance reduces the speed of response such that acceptable large-dimension LCDs are not possible.  
         SUMMARY OF THE INVENTION  
         [0016]    Accordingly, it is an object of the present invention to provide a liquid crystal display device, and a fabricating method thereof, that are adaptive for high quality, large dimension liquid crystal display devices.  
           [0017]    In order to achieve that and other objects of the invention, a liquid crystal display device according to one aspect of the present invention includes a gate electrode on a substrate; a gate insulating film on the substrate and over the gate electrode; a semiconductor layer on the gate insulating layer and over the gate electrode; source and drain electrodes formed on the semiconductor layer, wherein each of those electrodes includes a protrusion that extends toward the other electrode and over the semiconductor layer; a protective layer on the gate insulating film and over the source and drain electrodes; and a pixel electrode on the protective layer.  
           [0018]    The semiconductor layer has a          -shaped channel formed along the source and drain electrodes. The semiconductor layer includes an active layer formed on the gate insulating film, and an ohmic contact layer formed on the active layer and provided with a          -shaped opening corresponding to the shape of the channel.  
           [0019]    The channel beneficially has a width of more than 50 μm.  
           [0020]    A method of fabricating a liquid crystal display device according to the principles of the present invention includes the steps of forming a gate electrode on a substrate; forming a gate insulating film on the substrate and over the gate electrode; forming a semiconductor layer on the gate insulating film and adjacent the gate electrode; forming source and drain electrodes on the semiconductor layer such that at least one protrusion projects from each electrode toward the other electrode, forming a protective layer on the gate insulating film and over the source and drain electrodes; and forming a pixel electrode on the protective layer. Beneficially, the semiconductor layer forms a TFT having a channel that follows the area between the source and drain electrodes. Furthermore, a          shaped channel is formed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The principles of the present invention will be apparent from the following detailed descriptions of an embodiment of the present invention, and with reference to the accompanying drawings, in which:  
         [0022]    [0022]FIG. 1 is a plan view showing a partial structure of a conventional liquid crystal display device;  
         [0023]    [0023]FIG. 2 is a plan view of the channel of FIG. 1;  
         [0024]    [0024]FIG. 3A to FIG. 3E are plan views and sectional views, along line a-a, of the liquid crystal display device shown in FIG. 1;  
         [0025]    [0025]FIG. 4 is a plan view showing a structure of a liquid crystal display device according to an embodiment of the present invention;  
         [0026]    [0026]FIG. 5 is a plan view of the channel of FIG. 4; and  
         [0027]    [0027]FIG. 6A to FIG. 6E are plan views and sectional views, along line b-b′, of the liquid crystal display device shown in FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    Refer now to FIG. 4 for a plan view of a partial structure of an LCD according to the principles of the present invention. On a lower substrate (element  31 , shown in FIGS.  6 A- 6 E) is a TFT arranged near an intersection of a data line  34  and a gate line  32 . Also shown is a pixel electrode  52 , which connects to a drain electrode  40  of the TFT.  
         [0029]    The TFT includes a gate electrode  36  that is connected to the gate line  32 , a source electrode  38  that is connected to the data line  34 , and the drain electrode  40 , which connects via a contact hole  50  to the pixel electrode  52 . The TFT also includes semiconductor layers  44  and  46  that define a conductive channel  54  between the source electrode  38  and the drain electrode  40  when a gate voltage is applied to the gate electrode  36 . As shown in FIG. 4, the source electrode  38  and the drain electrode  40  are each formed with at least one protrusion such that the channel  54  has the shape shown in FIG. 5.  
         [0030]    Referring now to FIG. 5, the channel  54  is fabricated with a width W of about 50 to 100 μm and with a length L of about 4 to 6 μm. The length L of the channel  54  is beneficially equal to or less than that of the channel in the prior art (see FIG. 2) while its width W is beneficially greater (by more than 25 μm) than that of the prior art channel. In other words, the width W of the channel  54  is increased. Furthermore, by forming a channel as shown in FIG. 5, the overlapping area B between the source electrode  38  and the gate metal layers ( 32  and  36 ) can be reduced.  
         [0031]    The TFT responds to gate signals on the gate line  32  so as to selectively apply a data signal on the data line  34  to the pixel electrode  52 . Of course, in practice an LCD includes a plurality of TFTs and pixel electrodes. The pixel electrodes  52  are individually positioned in cells defined by data lines  34  and gate lines  32 . The pixel electrodes are made from a transparent conductive material, each pixel electrode  52  can produce a potential difference with a common transparent electrode (not shown) on an upper substrate (not shown) when a data signal is applied to that electrode through a contact hole  50 . A liquid crystal positioned between the lower substrate and the upper substrate is rotated due to its dielectric anisotropy by this potential difference. Thus, the liquid crystal selectively allows light from a light source to pass through the pixel electrode  52  to the upper substrate.  
         [0032]    [0032]FIG. 6A to FIG. 6E are plan views and sectional views taken along line b-b′ of FIG. 4. Those view are useful for explaining a method of fabricating an LCD device according to FIG. 4.  
         [0033]    Referring first to FIG. 6A, the gate line  32  and the gate electrode  36  are formed on the substrate  31 . Those elements are formed by depositing aluminum (Al) or copper (Cu), such as by sputtering, and then patterning the deposited layer to form the gate line  32  and the gate electrode  36 .  
         [0034]    Referring now to FIG. 6B, an active layer  44  and an ohmic contact layer  46  are then formed on a gate insulating film  42 . The gate insulating film  42  is produced first, beneficially by depositing an insulating material, such as silicon nitride (SiN x ) or silicon oxide (SiO x ), on the substrate  31  and over the gate electrode  36 . Typically, deposition is by plasma enhanced chemical vapor deposition (PECVD). The active layer  44  is then formed by depositing undoped amorphous silicon, and then the ohmic contact layer  46  is formed from depositing amorphous silicon that is heavily doped with an n-type or p-type impurity. The active layer  44  and the ohmic contact layer  46  are then completed by chemical etching.  
         [0035]    Referring now to FIG. 6C, the data line  34 , the source electrode  38 , and the drain electrode  40  are then provided on the gate insulating film  42 . The data line  34  and the source and drain electrodes  38  and  40  are beneficially comprised of chrome (Cr) or molybdenum (Mo). The data line  4  and the source and drain electrodes  38  and  40  are formed by first depositing a metal layer (i.e., Cr or Mo) using chemical vapor deposition or sputtering. The deposited metal layer is then photolithographically patterned to produce the data line  34 , the source electrode  38 , and the drain electrode  40 . As shown, the source and drain electrodes each have at least one protrusion that extends toward the other electrode. After the source and drain electrodes  38  and  40  were formed, the ohmic contact layer  46  over the gate electrode  36  is patterned to expose the active layer  44 . The area over the gate electrode  36  between the source and drain electrodes  38  and  40 , including the protrusions, forms a channel  54  having the shape shown in FIG. 5.  
         [0036]    Referring now to FIG. 6D, a protective layer  48  and a contact hole  50  are then provided on the gate insulating layer  42 . The protective layer  48  is formed by depositing an insulating material on the gate insulating layer  42  in such a manner as to cover the exposed structure. The hole  50  is then formed by patterning the protective layer so as to expose the drain electrode  40 . The protective layer  48  is beneficially comprised of an inorganic insulating material, such as silicon nitride (SiN x ) or silicon oxide (SiO x ), an acrylic organic compound or an organic insulating material having a small dielectric constant such as Teflon, BCB (benzocyclobutene), Cytop or PFCB (perfluorocyclobutane).  
         [0037]    Referring now to FIG. 6E, a pixel electrode  52  is then formed on the protective layer  48 . The pixel electrode  52  is formed by depositing a transparent conductive material, such as indium-tin-oxide (ITO), indium-zinc-oxide (IZO) or indium-zinc-oxide (ITZO), on the protective layer  48  and into the contact hole  50 . That transparent conductive material is then patterned to produce the pixel electrode  52 . Significantly, the pixel electrode  52  is electrically connected, via the contact hole  50 , to the drain electrode  40 .  
         [0038]    As described above, according to the principles of the present invention, protrusions from source and drain electrodes form a TFT channel into a          pattern. Accordingly, the width of the gate electrode is enlarged over that of a comparable prior art TFT. When a photo resist pattern for such a gate electrode is provided, the uniformity of the photo resist pattern can be improved, which reduces defects in the gate electrode, and thus improves manufacturing throughput. Also, a display according to the principles of the present invention can have an increased aperture ratio.  
         [0039]    Furthermore, as the overlap of a gate electrode and a data line is decreased, a parasitic capacitance Cgs is decreased. Accordingly, flicker and residual images can be reduced, resulting in an improved picture quality. Furthermore, the switching times of the TFTs can be reduced, enabling higher quality, large-dimension LCDs.  
         [0040]    While the foregoing has generally described the principles of the present invention in relation to a standard LCD, those principles are also applicable to other types of LCDs, such as ferroelectric liquid crystal (FLC) displays.  
         [0041]    Although the principles of the present invention has been illustrated using a particular embodiment shown in the drawings, it should be understood to the ordinary skilled person in the art will recognize that those principles are not limited to the illustrated embodiment, but rather that various changes, modifications, and adaptations are possible without departing from the spirit of the invention. Accordingly, the scope of the invention shall be determined only by the appended claims and their equivalents.