Abstract:
A mask used in a sequential lateral solidification process to fabricate a multi-boundary polysilicon. The mask comprises a first portion, a second portion and a third portion. The first and the third portions are translucent to light, and the second portion is opaque. These three portions have the same shape but different sizes. The first portion surrounds the second portion, and the third portion is parallel to both the first and the second portions.

Description:
This application claims the benefit of Taiwan Application Serial No. 094118439, filed Jun. 3, 2005, the subject matter of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a mask for polycrystalline silicon fabrication processes, especially to a sequential lateral solidification (SLS) mask which could be used for crystallizing an amorphous silicon film onto a multi-boundary polycrystalline silicon. 
     (2) Description of the Prior Art 
     Thin film transistors (TFTs) are widely applied to liquid crystal display (LCD) devices. In the art, two materials, the amorphous silicon (a-Si) and the polycrystalline silicon (poly-Si), are known to be two of major materials for forming the TFTs. 
     The polycrystalline silicon has lots of characters and is superior to the amorphous silicon while being used in the LCD. For instance, the polycrystalline silicon has bigger grains so that the speed of the response time of the TFT using the polycrystalline silicon is faster than that using the amorphous silicon. Furthermore, a poly-Si TFT LCD can use a backlight module to lower its power consumption. 
     Low Temperature Polycrystalline silicon (LTPS) process is a common process to crystallize the amorphous silicon. The LTPS usually uses Excimer laser as its heat source. While the laser beam irradiates onto an amorphous silicon film, the amorphous silicon film absorbs energy of the laser beam and is transformed into a polycrystalline silicon film. 
     One method of crystallizing the amorphous silicon into polycrystalline silicon is a sequential lateral solidification (SLS), which using a technique of Excimer laser annealing. SLS crystallization uses the fact that silicon grains tend to grow laterally from the boundary of the liquid silicon and the solid silicon. With the SLS, the amorphous silicon is crystallized by using a laser beam having a sufficient magnitude and a relative motion to melt the amorphous silicon, such that the melted silicon can grow laterally silicon grains in recrystallization. 
     Please refer to  FIG. 1A , which is a schematic cross-section view showing how a poly-Si film is fabricated by the SLS. As shown, an amorphous silicon film  11  formed by a chemical vapor deposition or sputtering is located on a substrate  10 , and a mask  2  is disposed above the amorphous silicon film  11 . The mask  2  comprises a plurality of light transmitting slits  21  and light absorptive slits  22 . Please refer to  FIG. 1B , a laser beam irradiates the mask  2  along the direction shown by arrows in  FIG. 1A  with a back-and-forth scanning pattern of stroking horizontally but feeding vertically (as a figure view of  FIG. 1B ). After the amorphous silicon film  11  is well irradiated, a lateral-growth poly-Si is then formed. 
     Please refer to  FIG. 1C , which shows the pattern formed by the above process via the mask  2 . The grain boundary  111  in  FIG. 1C  is perpendicular to the growing direction of the grains. According to various tests, channels of the TFTs could have better electrical performance while they are arranged parallel to the growing direction of the grains. For the above reason, almost all channels of the TFTs in the art have been designed parallel to each other, and such arrangements restrict the design variety in circuit patterning. 
     Therefore, how to remove the limitation caused by the grain-growing direction is necessary and important. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an SLS mask which can be used for crystallizing an amorphous silicon film into a multi-boundary polycrystalline silicon. 
     Another object of the present invention is to provide an SLS mask which can be used for producing poly-Si pattern with at least two grain growing directions. 
     According, a mask used in a SLS process to fabricate a multi-boundary polysilicon has a first portion, a second portion, and a third portion. The first and the third portions are pervious to light, and the second portion is opaque to shield light. The peripheral configurations of above three portions are the same but the sizes are different. The first portion surrounds the second portion, and the third portion is parallel to both the first and the second portions. 
     Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood form the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention, and in which: 
         FIG. 1A  is a schematic cross-section view showing how a poly-Si film is fabricated by an SLS; 
         FIG. 1B  is a top view of the mask  2  of  FIG. 1A ; 
         FIG. 1C  shows the pattern of the poly-Si formed via the mask  2  of  FIG. 1B ; 
         FIG. 2  is a schematic top-view of one embodiment of the mask of the invention; 
         FIG. 3  shows a preferred flowchart of using the mask of the present invention in an SLS; 
         FIG. 4A˜FIG .  4 C show the pattern of the poly-Si film produced from processes in  FIG. 3 . 
         FIG. 5A˜FIG .  5 B show another pattern of the poly-Si film produced from the processes in  FIG. 3 ; 
         FIG. 6  shows another embodiment of the mask of the present invention; 
         FIG. 7A˜FIG .  7 B show the pattern of the poly-Si film produced from the mask in  FIG. 6 ; 
         FIG. 8  shows a further embodiment of the mask of the present invention; 
         FIG. 9A˜FIG .  9 B show the pattern of the poly-Si film produced from the mask in  FIG. 8 ; and 
         FIG. 10  shows one more embodiment of the mask of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Please refer to  FIG. 2 , which is a schematic top-view of one embodiment of the mask of the invention. As shown, the mask  3  comprises a first portion  31 , a second portion  32 , and a third portion  33 , where the first portion  31  and the third portion  33  are pervious to light, and the second portion  32  is light absorptive or configured to shield light. 
     As shown, the second portion  32  is surrounded by the first portion  31 . The peripheral configurations of the first portion  31 , the second portion  32 , and the third portion  33  are all rectangular. The third portion  33  is disposed parallel to both the first portion  31  and the second portion  32  by spaced by a predetermined direction. 
     The mask of the present invention is an SLS mask which can be used for crystallizing an amorphous silicon film to a multi-boundary polycrystalline silicon. Please refer to  FIG. 3 , which shows the flowchart of using the mask in the SLS. The processes of utilizing the mask include: 
     STEP  301 (S 301 ): performing a first crystallization process by irradiating the amorphous silicon film by a laser beam applied through the first portion  31  or the third portion  33  of the mask  3 . 
     STEP  302  (S 302 ): moving a relative position of the mask  3  and the irradiation direction of the laser beam, in which moving the mask relative to the irradiation direction of the laser beam can be accomplished via moving the mask and/or moving the irradiation direction of the laser beam. 
     STEP  303  (S 303 ): performing a second crystallization process by irradiating the laser beam applied through the mask  3 , in which the irradiation direction is moved from the first portion  31  to the third portion  33  (in the case that, in S 303 , the first portion  31  is irradiated first) or from the third portion  33  to the first portion  31  (in the case that, in S 303 , the third portion  33  is irradiated first). 
     Please refer to  FIG. 4A˜FIG .  4 B, which show the pattern of poly-Si film produced from the above processes. After the first crystallization process, the melted silicon begins lateral growing at the interface between the liquid and the solid silicon (where the temperature is lower than other melted portions) to form the grain boundaries as shown in  FIG. 4A . In the second crystallization process, for the light transmitted portion and the light absorptive/or shielding portion of the mask are reversed, hence the pattern of  FIG. 4B  is formed. 
     In the present invention, the irradiation direction of the laser beam applied through the mask could influence the poly-Si pattern.  FIG. 4A  and  FIG. 4B  show the poly-Si pattern after the laser beam irradiates the mask in direction from the first portion  31  to the third portion  33 . If the laser beam irradiates the mask in direction from the third portion  33  to the first portion  31 , the poly-Si pattern would be the one shown in  FIG. 5A  and  FIG. 5B . The object of the present invention is to provide a mask and get a poly-Si pattern with at least two direction grain boundaries. Hence, no matter what direction of the laser beam is, the object of the present invention can be accomplished by having the laser beam to irradiate different portions of the mask in a predetermined sequence. 
     In the present invention, the arrangement between the light transmitted portion and the light absorptive/or shielding portion follows a conception of graph complementary. Furthermore, the sizes of different portions in the mask are different. For instance, the third portion  33  has an area between the area of the first portion  31  and that of the second portion  32 . This is because silicon grains tend to grow laterally from the interface between the liquid and the solid silicon (i.e. the boundary of the second portion  32 ). To resolve the problem or fact that a higher probability of bad grains is correlated to a quicker grain growth, the overlapping of the laser beam irradiation in accordance with the present invention is right provided to help achieving a perfect grain structure. 
     By providing the mask of the present invention, a poly-Si film with at least two growth directions of the grains can be obtained. With the poly-film of the present invention, a dual-gate thin film transistor (TFT) can present a uniform and better electrical performance. Please refer to  FIG. 4C , which is a schematic view of a TFT channel  3   a  and a gate line  3   b  in the poly-Si patterns that are fabricated via the mask  3  described above. 
     Herein, it should be aware and thus understood that, in the present invention, the graph peripheral configuration on the mask is not only limited to that shown in  FIG. 2 . In practice, a graph design which is frame shaped and has at least two major directions is suitably applied to the mask of the present invention. Please refer to  FIG. 6  and  FIG. 8 , which show another two embodiments of the present invention. The mask  4  shown in  FIG. 6  has a first portion  41 , a second  42 , and a third portion  43 , in which these three portions  41 ,  42  and  43  are all circular shape, contrarily to the rectangular shape portions in the mask  3  of  FIG. 2 . By applying the mask  4  to the SLS, the poly-Si pattern shown in  FIG. 7A  and  FIG. 7B  can be obtained. On the other hand, in  FIG. 8 , the first portion  51 , the second portion  52 , and the third portion  53  of the mask  5  are all triangular shape, and thereby the poly-Si pattern as shown in  FIG. 9A  and  FIG. 9B  can be obtained from utilizing the mask  5  of  FIG. 8 . 
     The number of portions pervious to light and the number configured to shieldlight in the present invention are not limited. As long as the design fits in the complementary frame shaped structure, a plurality of different light transmitted and absorptive/or shielding portions can also be designed into the mask of the present invention. In particular, please refer to  FIG. 10 , which shows one more embodiment of the present invention. The mask  6  shown in  FIG. 10  comprises a first portion  61 , a second  62 , a third portion  63 , a fourth portion  64 , a fifth portion  65 , a sixth portion  66 , a seventh portion  67 , and an eighth portion  68 . As shown, the first portion  61 , the third portion  63 , the fourth portion  64 , the seventh portion  67 , and the eighth portion  68  are defined as portions pervious to light, while the second portion  62 , the fifth portion  65 , and the sixth portion  66  are defined as portions configured to shield light. As noted, the peripheral configurations of all the portions  61 - 68  are all rectangular form. 
     Namely, the design of different sized portions in the mask  6  is resembled to the other embodiments mentioned above. For instance, the sixth portion  66  has an area between the area of the second portion  62  and that of the fourth portion  64 , and the eighth portion  68  has an area between the area of the fourth portion  64  and that of the fifth portion  65 . 
     Comparing to prior arts, the present invention further has advantages as follows. 
     1. Having the design concept in complementary graphing of the mask in accordance with the present invention can help to form a desired poly-Si pattern with at least two grain-growing directions. 
     2. A perfect grain boundary can be obtained by controlling the sizes of the different portions on the mask. 
     While the present invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be without departing from the spirit and scope of the present invention.