Abstract:
In the present invention, a method is used for forming a poly-silicon film that uses sequential lateral solidification (SLS) with two laser irradiations using a mask for patterning the laser beam so as to increase the grain length. The method also achieves enhancing the throughput due to the use of a mask that is designed for the method.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a method for forming a poly-silicon film and, more particularly, to a method using sequential lateral solidification (SLS) with two laser irradiations using a mask for patterning the laser beam so as to increase the grain length. 
     2. Description of the Prior Art 
     In semiconductor manufacturing, amorphous silicon (a-Si) thin-film transistors (TFTs) are now widely used in the liquid crystal display (LCD) industry because a-Si films can be deposited on a glass substrate at low temperatures. However, the carrier mobility in an a-Si film is much lower than that in a poly-silicon (p-Si) film, so that conventional a-Si TFT-LCDs exhibit a relatively slow response time that limits their suitability for large area LCD devices. Accordingly, there have been lots of reports on converting low-temperature grown a-Si films into p-Si films using laser annealing. 
     Presently, p-Si films are used in advanced electronic devices such as solar cells, LCDs and organic light-emitting devices (OLEDs). The quality of a p-Si film depends on the size of the crystal grains that form the p-Si film. It is thus the greatest challenge to manufacture p-Si films having large grains with high throughput. 
       FIG. 1A  is a conventional system for forming a p-Si film using sequential lateral solidification (SLS). The system comprises: a laser generator  11  for generating a laser beam  12  and a mask  13  disposed in a traveling path of the laser beam  12 . The mask has a plurality of transparent regions  13   a  and a plurality of opaque regions  13   b . Each of the plurality of transparent regions  13   a  is a stripe region with a width W. The laser beam  12  passing through the transparent regions  13   a  irradiates an a-Si film  15  on the substrate  14  in back of the mask  13  so as to melt the a-Si film  15  in the stripe regions  15   a  with a width W. As the laser beam  12  is removed, the melted a-Si film  15  in the stripe regions  15   a  starts to solidify and re-crystallize to form laterally grown silicon grains. Primary grain boundaries  16  parallel to a long side of the stripe regions  15   a  are thus formed at the center of the stripe regions  15   a  and a p-Si film is formed to have crystal grains with a grain length equal to a half of the width W, as shown in  1 B. 
     In order to enhance the throughput, U.S. Pat. No. 6,908,835 discloses a method for forming a poly-silicon film using sequential lateral solidification with two laser irradiations. In U.S. Pat. No. 6,908,835, a mask is used to pattern the laser beam and thus control the grain length, as shown in  FIG. 2A  and  FIG. 2C . 
     In  FIG. 2A , the mask  20  comprises a plurality of first stripe-shaped transparent regions  21  and a plurality of second stripe-shaped transparent regions  22  so that an a-Si film (not shown) on a substrate (not shown) in back of the mask  20  undergoes two laser irradiations while moving relatively to the mask  20  along X-axis. In  FIG. 2B , it is given that each of the first and the second transparent regions  21  and  22  has a width W. The spacing between two adjacent first transparent regions  21  and between two adjacent second transparent regions  22  is S. An offset width OS appears between the first transparent regions and the second transparent regions, where OS≧½ W. Therefore, the distance λ between a first primary grain boundary (corresponding to a central line  211  in the first transparent regions  21 ) obtained after SLS using the first laser irradiation and a second primary grain boundary (corresponding to a central line  221  in the first transparent regions  22 ) obtained after SLS using the second laser irradiation is λ=(W+S)/2. 
     In practical cases, however, the system for forming a p-Si film in  FIG. 1A  can further comprise a projection lens apparatus (not shown) disposed on the traveling path of the laser beam  12  between the substrate  14  and the mask  13 . Given that the projection lens apparatus has an amplification factor of N, the grown p-Si film has crystal grains that have a grain length of λ/N. For example, if W=27.5 μm, S=7.5 μm and N=5, the grain length of the p-Si film is λ/N=[(W+S)/2]/5=3.5 μm, as shown in  FIG. 2C . 
     In order to obtain a larger grain length, U.S. Pat. No. 6,726,768 discloses a method for forming a poly-silicon film using sequential lateral solidification with multiple laser irradiations. In U.S. Pat. No. 6,726,768, a mask is used to pattern the laser beam and thus control the grain length, as shown in  FIG. 3 . In  FIG. 3 , the mask  30  comprises a plurality of first transparent regions  31 , a plurality of second transparent regions  32 , a plurality of third transparent regions  33 , a plurality of fourth transparent regions  34  and a plurality of fifth transparent regions  35 , so that an a-Si film (not shown) on a substrate (not shown) in back of the mask  30  undergoes multiple laser irradiations while moving relatively to the mask  30  along X-axis. Even though a larger grain length of crystal grains may be obtained using the method disclosed in U.S. Pat. No. 6,726,768, it takes longer time and results in low throughput. 
     Therefore, there exists a need in providing a method for forming a poly-silicon film, using sequential lateral solidification (SLS) with two laser irradiations using a mask for patterning the laser beam so as to increase the grain length. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to provide a method for forming a poly-silicon film, using sequential lateral solidification (SLS) with two laser irradiations using a mask for patterning the laser beam so as to increase the grain length. 
     It is another object of the present invention to provide a method for forming a poly-silicon film, using sequential lateral solidification (SLS) with two laser irradiations using a mask for patterning the laser beam so as to enhance the throughput. 
     In order to achieve the foregoing object, the present invention provides a method for forming an electrode, the method comprising steps of:
         providing a system for forming a poly-silicon film, comprising   a laser generator for generating a laser beam; and   a mask disposed in a traveling path of the laser beam, the mask including a plurality of first transparent regions with a spacing S and a plurality of second transparent regions with a spacing S, each transparent region having a width W and a length L, wherein the first transparent regions are adjacent to the second transparent regions and a central line of each first transparent region extends along the length L into one of the second transparent regions such that an offset width OS appears between the first transparent regions and the second transparent regions;   providing a substrate with an amorphous silicon film formed thereon behind the mask in the traveling path of the laser beam;   performing a first laser irradiation process on the amorphous silicon film using the laser beam irradiating through the mask so as to melt the amorphous silicon film in a plurality of first stripe-shaped regions corresponding to the first transparent regions on the mask;   removing the laser beam such that the melted amorphous silicon film in the first stripe-shaped regions turns into a poly-silicon film with a first grain length;   moving the substrate along the length L for a distance no longer than the length L such that the plurality of first stripe-shaped regions correspond to the plurality of second transparent regions on the mask;   performing a second laser irradiation process on the poly-silicon film using the laser beam irradiating through the mask so as to re-melt the poly-silicon film in a plurality of first stripe-shaped regions corresponding to the second transparent regions on the mask; and   removing the laser beam such that the re-melted poly-silicon film in the first stripe-shaped regions turns into a poly-silicon film with a second grain length.       

     The present invention further provides a mask for forming a poly-silicon film, comprising: 
     plurality of first transparent regions with a spacing S and a plurality of second transparent regions with a spacing S, each transparent region having a width W and a length L; 
     wherein the first transparent regions are adjacent to the second transparent regions and a central line of each first transparent region extends along the length L into one of the second transparent regions such that an offset width OS appears between the first transparent regions and the second transparent regions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, spirits and advantages of the preferred embodiments of the present invention will be readily understood by the accompanying drawings and detailed descriptions, wherein: 
         FIG. 1A  is a conventional system for forming a p-Si film using sequential lateral solidification (SLS); 
         FIG. 1B  is a top view of a p-Si film formed using the system in  FIG. 1A ; 
         FIG. 2A  is a top view of a mask disclosed in U.S. Pat. No. 6,908,835; 
         FIG. 2B  is an enlarged top view with detailed specification of the mask in  FIG. 2A ; 
         FIG. 2C  is a top view of a p-Si film formed using the method using sequential lateral solidification with two laser irradiations disclosed in U.S. Pat. No. 6,908,835; 
         FIG. 3  is a top view of a mask disclosed in U.S. Pat. No. 6,726,768; 
         FIG. 4A  is a top view of a mask according to the present invention; 
         FIG. 4B  is an enlarged top view with detailed specification of the mask according to the present invention; 
         FIG. 4C  is a top view of a p-Si film formed using the method using sequential lateral solidification with two laser irradiations according to the present invention; and 
         FIG. 5  is a flow chart showing a method for forming a p-Si film according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention providing a method for forming a poly-silicon film can be exemplified by the preferred embodiment as described hereinafter. 
     In the present invention, sequential lateral solidification (SLS) with two laser irradiations is employed using a mask for patterning the laser beam so as to increase the grain length.  FIG. 4A  and  FIG. 4B  show a top view of a mask and detailed specification thereof according to the present invention; and  FIG. 4C  is a top view of a p-Si film formed using the method using sequential lateral solidification with two laser irradiations according to the present invention. 
     In  FIG. 4A  and  FIG. 4B , the mask  40  comprises a plurality of first transparent regions  41  with a spacing S and a plurality of second transparent regions  42  with a spacing S. Each of the first and the second transparent regions  41  and  42  has a width W and a length L. The first transparent regions  41  are adjacent to the second transparent regions  42  and a central line  411  of each first transparent region extends along the length L into one of the second transparent regions  42  such that an offset width OS appears between the first transparent regions  41  and the second transparent regions  42 . 
     Therefore, the present invention provides a method for forming a p-Si film using the mask  40  described in  FIG. 4A  and  FIG. 4B  and the method comprises steps with reference to the steps described in  FIG. 5 , which is a flow chart showing the method according to the present invention. 
     First in Step  51 , a system for forming a p-Si film is provided to comprise a laser generator for generating a laser beam and a mask. The system is similar to the conventional one shown in  FIG. 1  and description thereof is not repeated. However, the mask employed in the present invention is shown in  FIG. 4A  and  FIG. 4B . 
     In Step  52 , a substrate with an a-Si film formed thereon is provided (as shown in  FIG. 4C ) in back of the mask in the traveling path of the laser beam. 
     In Step  53 , a first laser irradiation process is performed on the a-Si film using the laser beam irradiating through the mask  40  so as to melt the a-Si in a plurality of first stripe-shaped regions corresponding to the first transparent regions  41  on the mask  40 . 
     Then, the laser beam is removed so that the melted a-Si film in the first stripe-shaped regions starts to solidify and turn into a p-Si film with a first grain length, as described in Step  54 . First primary grain boundaries are formed on the plurality of first stripe-shaped regions (corresponding to the central line  411  in the first transparent region  41 ). Meanwhile, the first grain length is equal to a half of the width W. 
     In Step  55 , the substrate is moved along the length L (the a-axis orientation) for a distance no longer than the length L so that the plurality of first stripe-shaped regions correspond to the plurality of second transparent regions  42  on the mask  40 . 
     In Step  56 , a second laser irradiation process is performed on the p-Si film using the laser beam irradiating through the mask  40  so as to re-melt the p-Si in a plurality of first stripe-shaped regions corresponding to the second transparent regions  42  on the mask  40 . 
     Finally in Step  57 , the laser beam is removed so that the re-melted p-Si film in the first stripe-shaped regions starts to solidify and turn into a p-Si film with a second grain length. Second primary grain boundaries are formed on the plurality of first stripe-shaped regions (corresponding to the central line  421  in the second transparent region  42 ). Meanwhile, the second grain length λ=W+S. 
     In practical cases, however, the system for forming a p-Si film described in Step  51  can further comprise a projection lens apparatus (not shown) disposed on the traveling path of the laser beam between the substrate and the mask. Given that the projection lens apparatus has an amplification factor of N, the grown p-Si film has crystal grains that have a grain length of λ/N, as shown in  FIG. 4C . 
     Accordingly, the present invention is characterized in that the second laser irradiation re-melts the first primary grain boundaries on the p-Si film solidified after the first laser irradiation and further forms the second primary grain boundaries. Meanwhile, the final grain length is the sum of the width W and the spacing S (i.e., λ=W+S), i.e., the distance between two second central lines without the projection lens apparatus, while with the projection lens apparatus, the final grain length is λ/N. 
     More particularly, in the present invention, if it is given that the width for all the transparent regions  41  and  42  is W=5.5 μm, the spacing between two neighboring first transparent regions  41  is S=0.75 μm and the offset width between the first transparent region  41  and the second transparent region  42  is OS=1.75 μm, the distance between two second primary grain boundaries  421  of the p-Si film after sequential lateral solidification with two laser irradiations is λ=W+S=6.25 cm without using the projection lens apparatus. However, if a projection lens apparatus with an amplification factor of N=5 is used, the distance between two second primary grain boundaries  421  of the p-Si film after sequential lateral solidification with two laser irradiations is λ/N (W+S)/5=6.25 μm if W=27.5 μm, S=3.75 μm and OS=10 μm. 
     According to the above discussion, it is apparent that the present invention discloses a method for forming a poly-silicon film, using sequential lateral solidification (SLS) with two laser irradiations using a mask for patterning the laser beam so as to increase the grain length. Therefore, the present invention is novel, useful and non-obvious. 
     Although this invention has been disclosed and illustrated with reference to particular embodiments, the principles involved are susceptible for use in numerous other embodiments that will be apparent to persons skilled in the art. This invention is, therefore, to be limited only as indicated by the scope of the appended claims.