Abstract:
A method for manufacturing a liquid crystal display device is disclosed, comprising the following steps: (a) providing a substrate having a transparent electrode layer and a first metal layer, wherein the transparent electrode layer is sandwiched in between the first metal layer and the substrate; (b) defining a patterned area having a thin film diode area and a pixel area by a first mask; (c) forming a first insulating layer and a second metal layer on the patterned area having the thin film diode area and the pixel area in sequence, wherein the first insulating layer is sandwiched in between the second metal layer and the first metal layer; and (d) defining the thin film diode area and the pixel area by a second mask, and removing the second metal layer, the first insulating layer, and the first metal layer on the pixel area to expose the transparent electrode layer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for manufacturing a liquid crystal display device, more particularly, to a method for manufacturing a liquid crystal display device having a thin film diode as a switch element. 
   2. Description of Related Art 
   In general, a thin film transistor (TFT) is used as a pixel switch element for an active LCD device, and a TFT exhibits the high on/off current ratio (&gt;10 6 ) to display images with improved quality. However, the process for preparing a TFT usually requires five to six masking steps. The process for preparing a TFT is complex and the yield is reduced. Thereby, those skilled in the field focus on reducing the steps and enhancing the yield. 
   The idea that a thin film diode (TFD) is used as a pixel switch element was brought up in 1983. The process for preparing a TFD requires three masking steps, and thereby the manufacturing process of a TFD is simpler than that of a TFT. A TFD comprises a selective line extending to a pixel area, a pixel electrode overlapped by the selective line, and a semi-insulating layer sandwiched between the selective line and the pixel electrode. The area of the three-layered structure is the area of a TFD. The driving action is performed by applying a voltage to the selective line to turn on the TFD, and thereby the pixel signal can be written to the pixel area. In addition, a black matrix of the color filter covering the TFD area can protect the semi-insulating layer from light that would affect electrical properties of the TFD. 
   U.S. Pat. No. 5,204,764 discloses that a TFD of a Metal-Insulator-Metal (MIM) structure is used in combination with a dual selective line to drive an LCD device so as to improve the display quality, as shown in  FIG. 1 . However, the area of the TFD for the TFD display device disclosed by U.S. Pat. No. 5,204,764 has to be increased to make the driven current large enough, and thereby the treatment reduces the aperture ratio of the pixel area. In addition, since the material of the upper metal layer is different from that of the lower metal layer, the current-voltage (I-V) characteristic of the thin film diode is asymmetrical, resulting in a poor display quality. 
   U.S. Pat. No. 6,243,062 discloses a structure of two back-to-back thin film diodes to improve the I-V symmetry (as shown in  FIG. 2 ). Although the method disclosed by U.S. Pat. No. 6,243,062 can efficiently improve the I-V symmetry, the manufacturing process comprises one more step and the aperture ratio is reduced. 
   According to one of the conventional methods, a metal layer, a transparent electrode layer, and a semi-insulating layer are coated in sequence, and a selective line is defined by a first masking step; subsequently, a photosensitive polymer insulating film is deposited, and then the TFD area is defined by a second masking step, and a transparent electrode layer is deposited; and then a pixel area and a thin film diode are accomplished by a third masking step, as the structure shown in  FIG. 3 . The conventional method can resolve the issue of asymmetrical I-V characteristic and enhances the breakdown voltage. However, the process of the conventional method still comprises three masking steps, and thereby the issue of complex processing is not resolved. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to provide a method for manufacturing a liquid crystal display (LCD) device to resolve the issues of the insufficient aperture ratio and the asymmetrical current-voltage characteristic, and the LCD device having a TFD as a switch element can be accomplished by using only two masking steps. 
   In the present invention, the TFD area overlaps the selective line, and is not in the display area. Thereby, the selective line functions as the element communicating the signal and the component of the TFD. In addition, owing to there being no switch elements in the display area, the aperture ratio can be improved. Furthermore, the material of the first metal layer is the same as that of the second metal layer of the TFD of the present invention, so the TFD exhibits the symmetrical current-voltage characteristic. The area of the TFD of the present invention can be greatened without affecting the aperture ratio to raise the driven current, and accelerate the pixel charging. 
   The present invention provides a process for manufacturing an LCD device, comprising: (a) providing a substrate having a transparent electrode layer and a first metal layer thereon, wherein the transparent electrode layer is sandwiched in between the substrate and the first metal layer; (b) defining a patterned area having a TFD structure area and a pixel area by use of a first mask; (c) forming a first insulating layer and a second metal layer on the substrate in sequence, wherein the first insulating layer is sandwiched between the second metal layer and the first layer; and (d) defining the TFD structure area and the pixel area, and removing the first metal layer, the first insulating layer, and the second metal layer on the pixel area by use of a second mask to expose the transparent electrode layer. According to the aforementioned process of the present invention, the step (a) can further comprise a step for forming a second insulating layer on the first metal layer to thicken the insulating layer. 
   The present invention further provides another process for manufacturing an LCD device, comprising: (a) providing a substrate having a transparent electrode layer, a first metal layer, and a first insulating layer thereon, wherein the transparent electrode layer is sandwiched between the substrate and the first metal layer, and the first metal layer is sandwiched between the first insulating layer and transparent electrode layer; (b) defining a patterned area having a TFD structure area and a pixel area by use of a first mask; (c) forming a second insulating layer and exposing the patterned area having the TFD structure area and the pixel area; (d) forming a second metal layer over the second insulating layer and the patterned area having the TFD structure area and the pixel area; and (e) defining the TFD structure area and the pixel area, and removing the second metal layer, the first insulating layer, and the first metal layer by use of a second mask on the pixel area to expose the transparent electrode layer. 
   In addition, the material of the transparent electrode on the substrate can be any transparent conductive material. Preferably, the material of the transparent electrode layer is ITO or IZO. The material of the first metal layer and the second metal layer can be any suitable metal material. Preferably, the material of the first metal layer and the second metal layer is metal with low resistance, such as tantalum. The material of the insulating layer of the present invention can be any insulating material. Preferably, the material of the insulating layer is SiNx. 
   In the step (b) of the present invention, in addition to defining the patterned area having the TFD structure area and the pixel area, a patterned signal line can be defined simultaneously. According to the method of the present invention, the TFD structure area can overlap the part patterned signal line of the LCD device. 
   The process for defining the patterned area having the TFD structure area and the pixel area of the step (c) is performed by backside exposure. The material of the insulating layer of the step (c) can be any suitable insulating material. Preferably, the material of the insulating layer is SiNx, SiO 2 , positive photoresist, or negative photoresist. 
   According to the LCD device accomplished by the process of the present invention, each different TFD structure area overlaps each different signal line on the same pixel. 
   Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a chart of a conventional TFD structure of a metal-insulator-metal structure; 
       FIG. 2  is a chart of a conventional TFD structure of a back-to-back structure; 
       FIG. 3  is a chart of a conventional TFD structure; 
       FIGS. 4A to 4D  are charts of a manufacturing process of a preferred embodiment (Embodiment 1) of the present invention; 
       FIGS. 5A to 5D  are charts of a manufacturing process of a preferred embodiment (Embodiment 2) of the present invention; 
       FIGS. 6A to 6F  are charts of a manufacturing process of a preferred embodiment (Embodiment 3) of the present invention; 
       FIGS. 7A to 7F  are charts of a manufacturing process of a preferred embodiment (Embodiment 4) of the present invention, wherein the original insulating layer is replaced by a negative photoresist planarization layer; and 
       FIGS. 8A to 8F  are charts of a manufacturing process of a preferred embodiment (Embodiment 5) of the present invention, wherein the original insulating layer is replaced by an organic positive photoresist layer. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Generally, the relationship between the voltage and the current of a thin film diode can be shown as the following equation (1). 
   
     
       
         
           
             
               
                 I 
                 = 
                 
                   α 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   V 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   A 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ⅇ 
                     
                       
                         V 
                       
                       ⁢ 
                       
                         
                           β 
                           ⁡ 
                           
                             ( 
                             
                               1 
                               d 
                             
                             ) 
                           
                         
                         
                           1 
                           2 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   I: current value through a thin film diode 
   V: applied voltage value to both terminals of a thin film diode 
   A: area of a thin film diode 
   d thickness of an insulator layer 
   α, β: constant; 
   For example, when the area of a thin film diode is increased from A 0  to A 1 , it is known by the equation (1) that the current is thereby increased to A 1 /A 0 -fold. 
   The equation about liquid crystal charge of an active LCD device comprising a thin film diode can be shown as the following equation (2). 
   
     
       
         
           
             
               
                 V 
                 = 
                 
                   
                     V 
                     0 
                   
                   ⁡ 
                   
                     ( 
                     
                       1 
                       - 
                       
                         ⅇ 
                         
                           - 
                           
                             τ 
                             RC 
                           
                         
                       
                     
                     ) 
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   V 0 : final voltage value during liquid crystal charge 
   τ: charge time, related to the resolution of a panel 
   R: line resistance value (including TFD resistance value) 
   C: liquid crystal capacitance 
   R is about equal to the addition of line resistance value (R line ) and TFD resistance value for two TFDs in parallel connection (R diode =V diode /I diode ), as the equation (3). 
   
     
       
         
           
             
               
                 R 
                 = 
                 
                   
                     
                       R 
                       line 
                     
                     + 
                     
                       
                         R 
                         diode 
                       
                       2 
                     
                   
                   = 
                   
                     
                       R 
                       line 
                     
                     + 
                     
                       
                         V 
                         diode 
                       
                       
                         2 
                         ⁢ 
                         
                           I 
                           diode 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   Thereby, if it is assumed that liquid crystals are charged to the voltage as large as the 90% final voltage at least, as shown in the equation (4), 
   
     
       
         
           
             
               
                 
                   0.9 
                   ⁢ 
                   
                     V 
                     0 
                   
                 
                 ≤ 
                 
                   
                     V 
                     0 
                   
                   ⁡ 
                   
                     ( 
                     
                       1 
                       - 
                       
                         ⅇ 
                         
                           - 
                           
                             τ 
                             RC 
                           
                         
                       
                     
                     ) 
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   I diode  value can be obtained by substituting the equation (3) into the equation (4), as the equation (5). 
   
     
       
         
           
             
               
                 
                   I 
                   diode 
                 
                 ≥ 
                 
                   
                     1 
                     2 
                   
                   ⁢ 
                   
                     
                       
                         C 
                         LC 
                       
                       ⁢ 
                       
                         V 
                         diode 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ln 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       0.1 
                     
                     
                       τ 
                       - 
                       
                         
                           C 
                           LC 
                         
                         ⁢ 
                         
                           R 
                           line 
                         
                         ⁢ 
                         ln 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         0.1 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
         
       
     
   
   According to the equation (5), it is known that when the panel resolution is increased (τ is reduced) or line resistance (R line ) is larger, the larger current through the TFD is required so as to charge liquid crystals completely during the turn-on period. The present invention can enhance the area of the TFD and the driven-current by making the TFD overlap on the selective line directly, without the effect on aperture ratio of the pixel. 
   Embodiment 1 
     FIGS. 4A-4D  are charts for the manufacturing process of a preferred embodiment of the present invention. In each figure, the lower figure is a top view, and the upper figure is a cross-section view of the line A-B area. As shown in  FIG. 4A , a transparent electrode layer  10  (such as ITO), and a first metal layer  20  with low resistance (such as Ti) are formed on a substrate  00  in sequence. The transparent electrode layer  10  functions as a pixel electrode, and thereby the material of the transparent electrode layer  10  can be ITO, IZO, or other transparent conductive materials. The first metal layer  20  functions as the lower metal layer of a thin film diode to communicate the signal to the pixel electrode. Preferably, the material of the first metal layer  20  is Ti, Mo, or other metals. 
   Subsequently, a photoresist layer is formed (not shown in the figures), and a patterned area  100  having both a thin film diode area and a pixel area is defined by a first mask, as the structure shown in  FIG. 4B . 
   After the patterned area  100  having both the thin film diode area and the pixel area is defined, a first insulating layer  30  (such as SiNx), and a second metal layer  40  with low resistance (such as Ti) are formed over the surfaces of the substrate and the first metal layer  20 , as shown in  FIG. 4C . Since the condition of the insulating layer  30  would affect the current-voltage characteristic of the thin film diode, the condition of the process has to be regulated to provide the best quality. The second metal layer  40  is both the upper metal layer of the thin film diode, and the pathway for communicating the IC output signal as well. The material of the second metal layer  40  can be Ti, Ti/Al, Mo, or other metals to reduce the resistance, and the RC delay is thereby reduced. 
   After a photoresist layer is applied on the second metal layer  10  (not shown in  FIG. 4D ), the TFD structure area  101  and the pixel area  102  are defined by a second mask. The first metal layer  20 , the first insulating layer  30 , the second metal layer  40  on the transparent electrode  10  in the pixel area  102 , the first insulating layer  30 , and the second metal layer  40  surrounding the pixel area  102  are removed by etching to insulate each pixel area, and thereby the preparation of the array is accomplished, as the structure shown in  FIG. 4D . The leakage area of the array can be blocked by a black array of the color filter. 
   Embodiment 2 
   In Embodiment 1, the first insulating layer  30  on the TFD structure area  101  is as thick as the first insulating layer  30  in the side area  103 . In order to control the electrical property more easily, the first insulating layer  30  on the TFD structure area  101  can be thickened without affecting the thickness of the first insulating layer  30  in the side area  103 , please refers to  FIGS. 5A to 5D . 
   As shown in  FIG. 5A , after a transparent electrode layer  10  and a first metal layer  20  are formed on a substrate  00 , a second insulating layer  31  is further formed on the first metal layer  20 . A patterned area  100  having a TFD structure area and a pixel area is defined by a first mask, as shown in  FIG. 5B . Subsequently, a first insulating layer  30 , and a second metal layer  40  are formed over the substrate  00 , as shown in  FIG. 5C . Finally, the TFD structure area  101  and the pixel area  102  are defined by a second mask and etching to accomplish the process for preparing the array, as shown in  FIG. 5D . 
   Embodiment 3 
   In Embodiment 1, the first insulating layer  30  on the TFD structure area  101  is as thick as the first insulating layer  30  in the side area  103 . In order to inhibit the insufficient breakdown voltage of the element resulting from the insufficient thickness of the side area  103 , the present embodiment provides a process for preparing a TFD structure, as shown in  FIGS. 6A to 6F . 
   As shown in  FIG. 6A , a transparent electrode layer  10 , a first metal layer  20  and a first insulating layer  30  are formed on a substrate in sequence first. As shown in  FIG. 6B , a patterned area  100  having both a TFD structure area and a pixel area is defined by a first mask. Then, a second insulating layer  50  (such as SiNx or SiO 2  with a preferred insulating property) formed over the substrate  00 , as shown in  FIG. 6C . Preferably, the second insulating layer  50  is thicker than the combination of the transparent electrode layer  10 , the first metal layer  20  and the first insulating layer  30 . 
   A negative photoresist layer is deposited over the second insulating layer  50  (not shown in  FIG. 6D ), and then the first metal layer  20  functions as a mask to enable removal of the second insulating layer  50  over the patterned area  100  and expose the patterned area  100  having the TFD structure area and the pixel area by backside exposure, as shown in  FIG. 6D . Subsequently, a second metal layer  40  is formed over the substrate  00 , as shown in  FIG. 6E , and then the TFD structure area  101  and the pixel area  102  are defined by a second mask and etching to accomplish the TFD with the thicker side area, as shown in  FIG. 6F . 
   Embodiment 4 
   The process of the embodiment is similar to that of Embodiment 3. However, the second insulating layer  50  is replaced by a negative photoresist planarization layer  51 , as shown in  FIG. 7C . Similarly, the photoresist over the area  100  is removed by backside exposure to expose the area  100  having the TFD structure area and the pixel area, as shown in  FIG. 7D . Then, a second metal layer  40  is formed over the substrate  00 , as shown in  FIG. 7E . Finally, the TFD structure area  101  and the pixel area  102  are defined by a second mask and etching to accomplish the process for preparing the array, as shown in  FIG. 7F . 
   Embodiment 5 
   The process of the embodiment is similar to that of Embodiment 3. However, the second insulating layer  50  is replaced by an organic positive photoresist layer  52 , as shown in  FIG. 8C . Then, the part positive photoresist  52  is removed to reduce the thickness of the positive photoresist layer  52  and make the positive photoresist layer  52  as high as the first insulating layer  30 . In order to avoid the organic positive photoresist  52  from remaining on the first insulating layer  30 , the duration for removing the photoresist can be extended to make the height of the positive photoresist layer  52  slightly lower than the first insulating layer  30 , as shown in  FIG. 8D . The first insulating layer  30  of the present embodiment is thicker than that of the aforementioned embodiments to inhibit excessively etching the first insulating layer  30  and thereby affecting the electrical property of the TFD during removing the photoresist. 
   Subsequently, the organic positive photoresist layer  52  is solidified, and then a second metal layer  40  is formed, as shown in  FIG. 8E . As shown in  FIG. 8F , the TFD structure area  101  and the pixel area  102  are defined by a second mask and etching to accomplish the TFD with the thicker side area. 
   According to the aforementioned process, the TFD structure can be accomplished by using only two masking steps. In addition, the aperture ratio and the area of the TFD can be enhanced to raise the driven current and accelerate the pixel charging by overlapping the TFD and the selective line. 
   Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.