Patent Publication Number: US-2007111113-A1

Title: Photo mask for mitigating stitching effect and exposing method using the same

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to photo masks and exposure methods using photo masks, and particularly to a photo mask for mitigating the so-called stitching effect and an exposure method using such photo mask.  
      2. General Background  
      The process of manufacturing thin film transistors (TFTs) for an array substrate of a liquid crystal display generally includes several steps of treatment of a glass substrate. Such steps include thin film coating, photolithographic processes, forming of TFTs, etc. The photolithographic processes mainly include film coating, cleaning, photo-resist coating, exposing, developing, etching, stripping, inspecting, subsequent film coating, subsequent cleaning, etc. In the exposing process, a photo mask (also known simply as a “mask”) is used for selectively irradiating a coating of photo-resist on the glass substrate with light such as UV light. The photo-resist coating is then developed. The quality of the exposure process has a direct impact on the quality of the TFTs produced, which in turn affects the display quality of a screen of the liquid crystal display. Therefore, the mask plays an important role in the exposure process.  
      Referring to  FIG. 6 , this is a schematic plan view of a layout of a conventional mask. The mask  20  has three blocks, which are designated as block I, block II, and block III. Each of so-called stitching regions between respective adjacent blocks I, II, and III is wavy. In an exposure procedure, the mask  20  is utilized to expose a glass substrate  10  from a left side of the glass substrate  10  to a right side of the glass substrate  10 . Typically, the glass substrate  10  has a photo-resist coating (not shown) thereon. During the exposing process, light is irradiated through block I of the mask  20  to form an exposed area  1  on the glass substrate  10 . Then light is irradiated through block II of the mask  20  to form exposed areas  2 ,  3 , and  4  on the glass substrate  10 . Finally, light is irradiated through block III of the mask  20  to form an exposed area  5  on the glass substrate  10 . Due to the stitching regions among the blocks I, II, III of the mask  20  being wavy, boundaries between the respective adjacent exposed areas  1 ,  2 ,  3 ,  4 ,  5  on the glass substrate  10  have corresponding stitching regions, which are also wavy. For example, a stitching region  8  is formed between the two adjacent exposed areas  4  and  5 .  
       FIG. 7  is a schematic, enlarged view of the stitching region  8 , showing how it is finally formed over the course of two exposure steps in the above-described exposure procedure. The stitching region  8  includes three regions  81 ,  82 ,  83 . The reference number  8 A indicates the stitching region  8  when the stitching region  8  undergoes an earlier of said exposure steps. In this earlier exposure step, the regions  81  and  82  are exposed through the block II of the mask  20 . The reference number  8 B indicates the stitching region  8  when the stitching region  8  undergoes a later of said exposure steps. In this later exposure step, the regions  82  and  83  are exposed through the block III of the mask  20 . The stitching region  8  finally obtained is in effect a combination of the stitching region  8  represented by reference number  8 A and the stitching region  8  represented by reference number  8 B. As seen, the region  82  finally obtained has undergone double exposure through block II and then block III of the mask  20 . This can result in serious variation occurring in subsequent photolithographic processes performed, such as in developing, etching, stripping, inspecting, etc. For example, subsequent processes performed on photo-resist coating may be non-uniform. The upshot may be significant gray level variation on the screen of the liquid crystal display.  
      For example, a viewer may see a whole line that corresponds to the stitching region  8 . Referring to  FIG. 8 , this is a schematic, enlarged view of portions of the regions  82 ,  83  where they adjoin each other, showing the stitching effect. When the intensity of exposure through the block II of the mask  20  is greater than the intensity of exposure through the block III, as seen, a corresponding shadow distribution in region  82  is more concentrated than a corresponding shadow distribution in region  83 . Therefore, the result of the exposing process at the stitching region  8  is different from that of other portions of the exposed areas  4 ,  5 . Thus the optical characteristics of the glass substrate  10  once finally formed are different as between the stitching region  8  and the exposed areas  4 ,  5 . Accordingly, a straight bright line (or dark line) may exist on the glass substrate  10 , and diminish the display quality of images shown on the screen of the liquid crystal display.  
     SUMMARY  
      A exemplary mask for improving stitching effect includes several blocks where the stitching areas between two adjacent blocks is nonlinear.  
      An exemplary exposing method for using a mask to miligate stitching effect is also provided, the mask having at least a first block and a second block wherein the first block adjoins the second block. The method has following steps: forming a first exposed area on a target substrate by exposing the substrate with light through the first block of the mask; and forming a second exposed area on the substrate by exposing the substrate through the second block of the mask. A stitching area where the first block and the second block adjoin each other is nonlinear.  
      A detailed description of embodiments of the present invention is given below with reference to the accompanying drawings. In the drawings, all the views are schematic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a plan view of a mask according to a first embodiment of the present invention, together with a substrate used with the mask in an exposing process according to a fourth embodiment of the present invention, the substrate having a stitching region formed thereon.  
       FIG. 2  is an enlarged view of part of the stitching region shown in  FIG. 1 , the part shown corresponding to a plurality of pixels of a liquid crystal display that employs the substrate once completed.  
       FIG. 3  is an enlarged view of an alternative form of appearance of the pixel the part shown corresponding to a plurality of pixels of a liquid crystal display that employs the substrate once completed.  
       FIG. 4  is a plan view of a mask layout according to the second embodiment of the present invention.  
       FIG. 5  is a plan view of a mask layout according to the third embodiment of the present invention.  
       FIG. 6  is a schematic plan view of a layout of a conventional mask.  
       FIG. 7  is a schematic, enlarged view of the stitching region  8 , showing how it is finally formed over the course of two exposure steps in the above-described exposure procedure.  
       FIG. 8  is a schematic, enlarged view of portions of the regions  82 ,  83  where they adjoin each other, showing the stitching effect. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      Referring to  FIG. 1 , this is a plan view of a mask  200  according to a first embodiment of the present invention, together with a substrate used with the mask in an exposing process according to a fourth embodiment of the present invention, the substrate having a stitching region formed thereon. The mask  200  includes three blocks including block I, block II, and block III. The boundary shape of each of the adjacent two blocks includes alternated linear shape portion and curve shape portion. The exposing steps include forming a thin film transistor (TFT) electrode pattern on the glass substrate  100  by projecting light beams to the substrate  100  through the mask  200  then exposing the substrate  100  from left side to right side so as to form an exposed area  11  through block I. Subsequently, three exposed areas  12 ,  13 ,  14  are formed by illuminate the block II of the mask  200  sequentially. Finally, an exposed area  15  is formed by exposing through block III of the mask  200 .  
      It should be noted that the boundary structure of each two exposed areas is not shaped as linear and is shaped as nonlinear structure instead. The preferred embodiment of the nonlinear structure is an alternative linear portion, and curve portion. The preferred embodiment to form the nonlinear structure of the exposed area does not increase difficulty of design and process flow.  
      Referring to  FIG. 2 , this is an enlarged view of part of the stitching region shown in  FIG. 1 , the part shown corresponding to a plurality of pixels of a liquid crystal display that employs the substrate once completed. As shown in the drawing, the bright pixels and the dark pixels are arranged irregularly due to dispersion of dark areas so as to mitigate the so-called beeline effect. Therefore, viewers cannot easily see a bright line or dark line in the viewing zone of display.  
      Referring to  FIG. 3 , this is an enlarged view of an alternative form of appearance of the pixel the part shown corresponding to a plurality of pixels of a liquid crystal display that employs the substrate once completed. The appearance of pixels may figure as S shape. This kind of structure can increase the difficulty for viewers to notice the beeline effect on the stitching area  18  exposed through the mask  200  so as to raise the viewing quality. Alternatively, the appearance of pixels can be a bending shape or another kind of shape such as crooked shape.  
      Referring to  FIG. 4 , this is a mask layout according to the second embodiment of the present invention. The mask  300  includes three blocks of block I, block II, and block III. The stitching areas between the aforesaid three blocks may figure as a curve shape.  
      Referring to  FIG. 5 , this is a mask layout according to the third embodiment of the present invention. The mask  400  may include three blocks illustrating block I, block II, and block III. The stitching areas between the aforesaid three blocks may figure as a bending shape.  
      There may be other alternative embodiments, such as the stitching structure between each of two blocks can be an alternate combination of a curve segment and a bending segment or an alternate combination of a linear segment and a bending segment. It is noted that three blocks in the aforementioned mask  200 ,  300 , and  400  are to be taken as examples only. The number of blocks and the corresponding dimensions can be adjustable according to the size of the glass substrate.  
      As would be understood by a person skilled in the art, the foregoing preferred and exemplary embodiments illustrate the present invention rather than limit the present invention. The embodiments are intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.