Abstract:
An active matrix substrate for a liquid crystal display and method of forming the same. To form the active matrix substrate five masks are needed. The first mask forms gate lines on the transparent substrate. The second mask patterns a stacked layer of a metal layer/an n-doped layer/a semiconductor layer formed on a gate insulating layer to form data lines. After forming a low k dielectric layer, the third mask forms openings therein. The forth mask patterns pixel electrodes and conducting lines with source pattern on the low k dielectric layer and further patterns the metal layer and the n-doped layer. After depositing a passivating layer the fifth mask defines the passivating layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to an active matrix substrate for a liquid crystal display (LCD) and method of forming the same. In particular, the present invention relates to a liquid crystal display having a high pixel aperture ratio (pixel opening size) and method of forming the same. 
     2. Description of the Related Art 
     Generally, LCDs have an upper and a lower substrate with electrodes thereon. These substrates are sealed with adhesive materials, and a liquid crystal material is sealed between these two substrates. Before the liquid crystal is injected between the two substrates, spacers are sprayed between the substrates in order to hold a constant distance therebetween. Conventionally, many TFTs are formed above the lower substrate as switching devices. Each TFT has a gate electrode connected to a scanning line, a drain electrode connected with a signal line, and a source electrode connected to a pixel electrode. The lower substrate is also called an active matrix substrate. The upper substrate includes a color filter and a common electrode. 
     The higher the pixel aperture ratio of a LCD, the higher the display transmission. Thus, by increasing the pixel aperture ratio of a LCD, transmission may be increased using the same backlight power, or alternatively, the backlight power consumption may be reduced while maintaining the same display transmission. 
     In order to enhance the pixel aperture ratio, a thicker insulating layer is formed over source and drain electrodes before forming pixel electrodes, thus, the pixel electrodes are formed over the insulating layer so as to overlap portions of the address lines, as disclosed in U.S. Pat. Nos. 5,955,744, 5,780,871, 5,641,974. Thus, the capacitance between pixel electrodes and underlying conducting material can be reduced, and an effective display area, i.e. the area of the pixel electrode, can be enlarged. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of forming an active matrix substrate for a liquid crystal display with a high pixel aperture ratio and method of forming the same. 
     According to one aspect of the invention, an active matrix substrate for a liquid crystal display is provided. A plurality of gate lines are disposed on a transparent substrate, parallel to a first direction and with protruding portions covering active device regions. A gate insulating layer is disposed on the gate lines and the transparent substrate. A low k dielectric layer is disposed on the gate insulating layer, and has first openings corresponding to the active device regions. A plurality of data lines are disposed between the low k dielectric layer and the gate insulating layer, perpendicular to the first direction and with protruding portions covering the active device regions. The data lines and the gate lines define regions including the active device regions and the pixel regions. A first n-doped layer is disposed under the data lines, contacts the data lines and has the same pattern as a gathering of the data lines, wherein the first n-doped layer at one side of the first openings providing sources. Conducting lines with source pattern are disposed at the first openings on the sources, with the protruding portions of the data lines contact source electrodes. Second n-doped layer is disposed under the data lines, and contacts the drain electrodes belonging to the protruding portions of the data lines, wherein the second n-doped layer at the other side of the first openings provides drains. A channel exists between each source and its corresponding drain. A semiconductor layer is disposed under the first n-doped layer and the second n-doped layer on the gate insulating layer and has the same pattern as a gathering of the first n-doped layer, the second n-doped layer and the channels. Pixel electrodes are disposed on the low k dielectric layer at the pixel regions and contact the drains. A passivating layer is disposed on the first openings. 
     According to another aspect of the invention, an active matrix substrate for a liquid crystal display can be formed by the following steps. Gate lines are formed on the transparent substrate, parallel to a first direction and with protruding portions covering a plurality of active device regions. A gate insulating layer is formed on the gate lines and the transparent substrate. A semiconductor layer is formed on the gate insulating layer. An n-doped layer is formed on the semiconductor layer. A metal layer is formed on the n-doped layer. The semiconductor layer, the n-doped layer and the metal layer are patterned and the metal layer is transferred to data lines perpendicular to the first direction. A low k dielectric layer is formed on the transparent substrate having the data lines thereon. First openings are formed in the low k dielectric layer corresponding to the active device regions. A transparent conducting layer is formed on the low k dielectric layer. The transparent conducting layer is patterned to pixel electrodes and conducting lines with source pattern. The data lines and the n-doped layer corresponding to the first openings are etched, so the data lines and the n-doped layer have the same local pattern as the pixel electrodes and the conducting lines in the first openings. A passivating layer is formed on the first openings. 
     To form the active matrix substrate five masks are needed. The first mask forms gate lines on the transparent substrate. The second mask patterns a stacked layer of a metal layer/an n-doped layer/a semiconductor layer formed on a gate insulating layer to form data lines. After forming a low k dielectric layer, the third mask forms openings therein. The forth mask patterns pixel electrodes and conducting lines with source pattern on the low k dielectric layer and further patterns the metal layer and the n-doped layer. After depositing a passivating layer the fifth mask defines the passivating layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention. 
     FIGS. 1A-1D are a series of layouts of the active matrix substrate showing the manufacturing steps in fabricating an active matrix substrate for a liquid crystal display in accordance with the first embodiment of the present invention. 
     FIGS. 2A-2E are cross sections showing the manufacturing steps in fabricating an active matrix substrate for a liquid crystal display in accordance with the present invention, wherein FIGS. 2A-2D are cross sections taken along line II—II of FIGS. 1A-1D respectively. 
     FIGS. 3 and 4 are layouts of the active matrix substrate showing the manufacturing steps in fabricating an active matrix substrate for a liquid crystal display in accordance with another embodiment of the present invention. 
     FIGS. 5A and 5B are cross sections taken along A—A and B—B lines in FIG.  3 . 
     FIG. 6 is a cross section taken along C—C line in FIG.  4 . 
     FIGS. 7A-7E are cross sections showing the manufacturing steps in fabricating an active matrix substrate for a liquid crystal display in accordance with the present invention, wherein FIGS. 7A-7D are cross sections taken along line II—II of FIGS. 1A-1D respectively. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An active matrix substrate having a high pixel aperture ratio is shown in FIG.  1 D and FIG. 2E. A detailed description of the active matrix substrate and method for forming the same is given in the following embodiments. 
     First Embodiment 
     FIGS. 2A-2E are cross sections showing the manufacturing steps in fabricating an active matrix substrate for a liquid crystal display in accordance with the first embodiment of the present invention. FIGS. 1A-1D are a series of layouts of the active matrix substrate of FIGS. 2A-2D respectively. 
     Referring to FIGS. 1A and 2A, a transparent substrate  11  is provided. A metal layer, such as Al or Al alloy, is formed on the transparent substrate  11 . After defining the metal layer using lithography and etching, gate lines  13  are formed. The gate lines  13  have protruding portions covering active device regions A, and the protruding portions of the gate lines  13  function as gates for thin film transistors (TFTs). Each active device region A is located at a corner of each pixel region. 
     Referring to FIGS. 1B and 2B, a gate insulating layer  15  is formed on the gate lines  13  and the transparent substrate  11 . A semiconductor layer, an n-doped layer and a metal layer are sequentially formed on the gate insulating layer  15 . The insulating layer can be silicon nitride, the semiconductor layer can be amorphous silicon, and the metal layer can be Cr or Cr alloy. The metal layer/the n-doped layer/the semiconductor layer are etched by one photolithography and etching process to form data lines  21 , a patterned n-doped layer  19  and a patterned semiconductor layer  17 . 
     Referring to FIGS. 1C and 2C, a low k dielectric layer  23  is formed on the data lines  21  and the gate insulating layer  15 . The low k dielectric layer  23  has a dielectric constant (k) less than 5 and with high transparency. Because the low k dielectric layer  23  can reduce the capacitance between the data and gate lines and the pixel electrodes to be formed, the pixel electrodes can be formed over the low k dielectric layer  23  to overlap portions of the data and gate lines to increase the pixel aperture ratio. The low k dielectric layer  23  can be photosensitive material or non-photosensitive material, such as benzocyclobutene (BCB). The thickness of the low k dielectric layer  23  is about 1-5 μm. 
     Openings  24  are then formed in the low k dielectric layer  23  and corresponding to the active device regions A. 
     Referring to FIGS. 1D and 2D, a transparent conducting layer, such as indium tin oxide (ITO), is formed on the low k dielectric layer  23 . The transparent conducting layer, the data lines  21  and the n-doped layer  19  are etched, therefore, openings  24  are formed in the transparent conducting layer, the date lines  21  and the n-doped layer  19 , sources S and drains D are defined in the n-doped layer  19 , and the transparent conducting layer is transferred to pixel electrodes  25   a  and conducting lines  25   b  with source patterns. A channel exists in the semiconductor layer  17  between each source S and its corresponding drain D. Each source S contacts the corresponding source electrode  21 S connecting to the corresponding conducting line  25   b . Each drain D contacts the corresponding drain electrode  21 D connecting to the corresponding pixel electrode  25   a . The source electrodes  21 S and the drain electrodes  21 D belong to the protruding portions of the data lines  21 . 
     Referring to FIG. 2E, a passivating layer  27 , such as a silicon nitride layer, is formed on the conducting lines  25   b , the channels and the pixel electrodes  25   a  at active device regions A to protect the channels between the sources S and the drains D. 
     After forming the passivating layer  27  the active matrix substrate is obtained. The following processes of fabricating upper substrate and filling liquid crystal therebetween follow. 
     Second Embodiment 
     FIGS. 7A-7E are cross sections showing the manufacturing steps in fabricating an active matrix substrate for a liquid crystal display in accordance with the second embodiment of the present invention. FIGS. 1A-1D are a series of layouts of the active matrix substrate of FIGS. 7A-7D respectively. 
     Referring to FIGS. 1A and 7A, a transparent substrate  11  is provided. A metal layer, such as Al or Al alloy, is formed on the transparent substrate  11 . After defining the metal layer using lithography and etching, gate lines  13  are formed. The gate lines  13  have protruding portions covering active device regions A, and the protruding portions of the gate lines  13  function as gates for thin film transistors (TFTs). Each active device region A is located at a corner of each pixel region. 
     Referring to FIGS. 1B and 7B, a gate insulating layer, a semiconductor layer, an n-doped layer and a metal layer are sequentially formed on the gate lines  13  and the transparent substrate  11 . The insulating layer can be silicon nitride, the semiconductor layer can be amorphous silicon, and the metal layer can be Cr or Cr alloy. The metal layer/the n-doped layer/the semiconductor layer/the gate insulating layer are etched by one photolithography and etching process to form data lines  21 , a patterned n-doped layer  19 , a patterned semiconductor layer  17  and a patterned gate insulating layer  15   a.    
     Referring to FIGS. 1C and 7C, a low k dielectric layer  23  is formed on the data lines  21  and the transparent substrate  11 . The low k dielectric layer  23  has a dielectric constant (k) less than 5 and high transparency. Because the low k dielectric layer  23  can reduce the capacitance between the data and gate lines and the pixel electrodes to be formed, the pixel electrodes can be formed over the low k dielectric layer  23  to overlap portions of the data and gate lines to increase the pixel aperture ratio. The low k dielectric layer  23  can be photosensitive material or non-photosensitive material, such as benzocyclobutene (BCB). The thickness of the low k dielectric layer  23  is about 1-5 μm. 
     Openings  24  are then formed in the low k dielectric layer  23  and corresponding to the active device regions A. 
     Referring to FIGS. 1D and 7D, a transparent conducting layer, such as indium tin oxide (ITO), is formed on the low k dielectric layer  23 . The transparent conducting layer, the data lines  21  and the n-doped layer  19  are etched, therefore, openings  24  are formed in the transparent conducting layer, the date lines  21  and the n-doped layer  19 , sources S and drains D are defined in the n-doped layer  19 , and the transparent conducting layer is transferred to pixel electrodes  25   a  and conducting lines  25   b  with source patterns. A channel exists in the semiconductor layer  17  between each source S and its corresponding drain D. Each source S contacts the corresponding source electrode  21 S connecting to the corresponding conducting line  25   b . Each drain D contacts the corresponding drain electrode  21 D connecting to the corresponding pixel electrode  25   a.    
     Referring to FIG. 7E, a passivating layer  27 , such as a silicon nitride layer, is formed on the conducting lines  25   b , the channels and the pixel electrodes  25   a  at active device regions A to protect the channels between the sources S and the drains D. 
     After forming the passivating layer  27  the active matrix substrate is obtained. The following processes of fabricating upper substrate and filling liquid crystal therebetween follow. 
     Third Embodiment 
     Concerning the fabrication of the electrostatic discharge (ESD) protection circuit, a detailed description is given below accompanying FIGS. 3-4,  5 A,  5 B and  6 . 
     Referring to FIGS. 3,  5 A and  5 B, while the openings  24  are formed in the low k dielectric layer  23  in the active device regions A, as shown in FIGS. 1C and 2C, openings  44  are formed in the low k dielectric layer  23  and the gate insulating layer  15  on the bonding pads  13   a  located at the terminal of the gate lines  13 . The gate insulating layer  15  are then transferred into the patterned gate insulating layer  15   c , as shown in FIGS. 5A and 5B. 
     Referring to FIGS. 4 and 6, while the transparent conducting layer formed on the low k dielectric layer  23  is patterned into the pixel electrodes  25   a  and the conducting lines  25   b  with source patterns, as shown in FIGS. 1D and 2D, conducting lines  25   c  are formed to connect the bonding pads  13   a  and the sources S. After etching the transparent conducting layer, the etching process continues with the data lines  21  and the n-doped layer  19 , therefore, openings  24  and  44  are formed in the transparent conducting layer, the date lines  21  and the n-doped layer  19  to define the sources S and the drains D. Each conducting line  25   c  is formed in the opening  44  to connect the bonding pad  13   a  and the corresponding source S, which constitutes a diode for ESD protection circuit. 
     The following processes of fabricating the active matrix substrate as mentioned in first embodiment follow. 
     The foregoing description of the preferred embodiments of this invention has been presented for purposes of illustration and description. Obvious modifications or variations are possible in light of the above teaching. The embodiments were chosen and described to provide the best illustration of the principles of this invention and its practical application to thereby enable those skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.