Abstract:
The present invention relates to a fabrication method for polycrystalline silicon thin that is capable of providing uniform crystallization of polycrystalline silicon thin film by laser using a mask having a mixed structure of laser transmission regions and laser non-transmission regions, wherein the laser transmission regions exist asymmetrically on the basis of a laser scanning directional axis, and the laser transmission regions exist symmetrically on the basis of a certain central axis, and the laser transmission regions are shifted to a certain distance on the basis of another axis parallel to the certain central axis, so that the laser transmission regions and non laser transmission regions are alternately positioned.

Description:
This application claims the benefit of Korean Patent Application No. 2003-71592, filed on Oct. 14, 2003, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates to a method for fabricating a polycrystalline silicon thin film for an apparatus and an apparatus that uses polycrystalline silicon thin film fabricated by the method, more particularly, to a method for fabricating a polycrystalline silicon thin film capable of controlling the shape of grains of polycrystalline silicon thin film and an apparatus using the polycrystalline silicon thin film. 
     2. Discussion of Related Art 
     Generally, the sequential lateral solidification (SLS) crystallizing method is used to laterally grow grain silicon by irradiating a laser beam on an amorphous silicon layer two or more times. Polycrystalline silicon grains thus fabricated are formed in a columnar shape, and grain boundaries are formed between adjacent grains due to the grains&#39; limited size. 
     Polycrystalline or single crystal large silicon grains may be formed on a substrate using SLS crystallization technology, and characteristics similar to characteristics of a thin film transistor (TFT) fabricated of single crystal silicon may be obtained. 
       FIG. 1A ,  FIG. 1B  and  FIG. 1C  show an ordinary SLS crystallizing method. 
     In the SLS crystallizing method as illustrated in  FIG. 1A , a laser beam is irradiated onto an amorphous silicon thin film layer through a mask having a laser beam transmission region and a laser beam non-transmission region, thereby melting the amorphous silicon in the laser beam transmission region. 
     Crystallization preferentially occurs at an interface between amorphous silicon and molten silicon if cooling is started after finishing the laser beam irradiation, wherein a temperature gradient is formed in which temperature is gradually decreased in a direction from the interface between amorphous silicon and molten silicon to a molten silicon layer. 
     Therefore, referring to  FIG. 1B , a polycrystalline silicon thin film layer, with laterally grown grains formed in a columnar shape, is formed since heat flux flows in a direction from the interface of the mask to a central part of the molten silicon layer. The polycrystalline silicon grains grow laterally until the molten silicon layer is completely solidified. 
     As illustrated in  FIG. 1C , amorphous silicon and crystalline silicon are melted by moving stage, thereby moving mask, by irradiating a laser beam onto partially exposed portions of the amorphous silicon thin film layer and already crystallized polycrystalline silicon layer. Silicon atoms are adhered to already formed polycrystalline silicon grains that are covered by the mask so that the length of the grains is increased as the melted amorphous silicon and crystalline silicon cool after being melted. 
       FIG. 2A ,  FIG. 2B  and  FIG. 2C  are plan figures that show a method for crystallizing grain silicon using a mask structure of an ordinary fabrication method of polycrystalline silicon thin film, and  FIG. 3A ,  FIG. 3B  and  FIG. 3C  are plan figures of polycrystalline silicon thin films produced in respective stages. 
     In  FIG. 2A , amorphous silicon is melted by irradiating a laser beam onto the amorphous silicon using an ordinary mask with a laser beam transmission region and a laser beam non-transmission region. Polycrystalline silicon is formed as the melted amorphous silicon solidifies. 
     The mask is shifted as far as a certain distance illustrated in  FIG. 2B , and a laser beam is irradiated onto a portion of the previously formed polycrystalline silicon and amorphous silicon as illustrated in  FIG. 2C . By continuously scanning the polycrystalline silicon and irradiating a laser beam onto the polycrystalline silicon in this manner, at a part where mask patterns of the amorphous silicon and transmission region overlap with each other, polycrystalline silicon is melted and crystallized as it solidifies. 
     Polycrystalline silicon crystallinity varies per parts onto which a laser pulse is irradiated due to laser shot energy density deviation, or energy density in the laser beam can be uneven in amorphous silicon onto which a laser beam is once irradiated as illustrated in  FIG. 3A . 
     Particularly, laser scan line causes striped defects on upper and lower boundaries between different laser shots as illustrated in  FIG. 3B  and  FIG. 3C . 
     These striped defects cause luminance non-uniformity on a display device, particularly an organic electroluminescent device. 
     PCT international patent No. WO 97/45827 and U.S. Pat. No. 6,322,625, disclose technologies for converting amorphous silicon on a substrate into polycrystalline silicon, or for crystallizing only a selected region on the substrate, by the sequential lateral solidification (SLS) method. 
     Additionally, obtaining TFT characteristics second only to single crystal silicon is disclosed in U.S. Pat. No. 6,177,391, since the barrier effect of grain boundaries for a carrier direction is minimized when an active channel direction is parallel to a direction of grains grown by SLS crystallizing method. But the patent also discloses that large numbers of the grain boundaries act as a trap of charge carriers, and TFT characteristics greatly deteriorate when the active channel direction is perpendicular to the grain growing direction. 
     There is a case, however, where an active matrix display device is fabricated with driving circuit TFTs generally perpendicular to pixel cell region TFTs, wherein uniformity of the display device is improved when an active channel region direction is inclined 30 to 60 degrees to a crystal growing direction. 
     However, with this method, since the grains are formed by the SLS crystallizing method, the problem of non-uniform grains due to non-uniformity of laser energy density still exists. 
     Also with this method, crystallization can not be carried out all over a substrate, therefore an uncrystallized region will always exist, although a method is described in Korean Patent Laid-open Publication No. 2002-93194 in which laser beam patterns are formed in a triangle shape (“           ”), and crystallization is proceeded by moving the triangle shaped (“         ”) laser beam patterns widthwise.
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a fabrication method for polycrystalline silicon thin film and an apparatus using the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     This invention provides a method for improving uniformity of polycrystalline silicon thin film fabricated by the SLS crystallizing method. 
     This present invention also provides an apparatus using polycrystalline silicon thin film fabricated by the method above. 
     Additional features of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. 
     The present invention discloses a method of fabricating a polycrystalline silicon thin film in which amorphous silicon is crystallized by laser, comprising forming a mask having a laser transmission region and a laser non-transmission region, and shifting the laser transmission region a distance on the basis of an axis parallel to a predetermined central axis. The laser transmission region exists asymmetrically on the basis of a laser scanning directional axis, and the laser transmission region exists symmetrically on the basis of a predetermined central axis. The laser transmission region and the laser non-transmission region are alternately positioned. The predetermined central axis is x and/or y. 
     This present invention also discloses a display device using a polycrystalline silicon thin film fabricated by the process of forming a mask having a laser transmission region and a laser non-transmission region, and shifting the laser transmission region a distance on the basis of an axis parallel to a predetermined central axis. The predetermined central axis is x and/or y. The laser transmission region exists asymmetrically on the basis of a laser scanning directional axis, and the laser transmission region exists symmetrically on the basis of a predetermined central axis. The predetermined central axis is x and/or y. The laser transmission region and the laser non-transmission region are alternately positioned. 
     This present invention also discloses a mask used for fabricating polycrystalline silicon thin film, comprising a laser transmission region and a laser non-transmission region. The laser transmission region exists asymmetrically on the basis of a laser scanning directional axis and the laser transmission region exists symmetrically on the basis of a predetermined central axis. The predetermined central axis is x and/or y. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
         FIG. 1A ,  FIG. 1B  and  FIG. 1C  show an ordinary sequential lateral solidification (SLS) crystallization method. 
         FIG. 2A ,  FIG. 2B  and  FIG. 2C  are plan figures that show a method for crystallizing grain silicon using a mask structure of an ordinary fabrication method of polycrystalline silicon thin film. 
         FIG. 3A ,  FIG. 3B  and  FIG. 3C  are plan figures that show polycrystalline silicon thin film produced in each stage of  FIG. 2A ,  FIG. 2B  and  FIG. 2C . 
         FIG. 4A  is a plan figure that shows a case when a laser is once irradiated onto line shaped transmission patterns. 
         FIG. 4B  is a plan figure that shows a case when a laser is irradiated onto the line shaped transmission patterns twice by moving the mask patterns to a certain distance. 
         FIG. 5A  is a plan figure that shows a case when a laser is once irradiated onto two line shaped transmission patterns of a different length. 
         FIG. 5B  is a plan figure that shows a case when a laser is irradiated onto the differing length line shaped transmission patterns twice by moving the mask patterns to a certain distance. 
         FIG. 6A  is a plan figure that shows a case when a laser is once irradiated onto two line shaped transmission patterns where the patterns are of differing length but shorter than the length of the transmission pattern of the first embodiment. 
         FIG. 6B  is a plan figure that shows a case when a laser is irradiated onto the differing length line shaped transmission patterns twice by moving the mask patterns to a certain distance. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to an embodiment of the present invention, example of which is illustrated in the accompanying drawings. 
     For reference, like reference characters designate corresponding parts throughout several views. 
       FIG. 4A  and  FIG. 4B  are plan figures showing mask patterns and scanning according to a first exemplary embodiment of the present invention.  FIG. 4A  shows a case when a laser is once irradiated onto line shaped transmission patterns.  FIG. 4B  shows a case when a laser is irradiated onto the line shaped transmission patterns twice by moving the mask patterns to a certain distance 
     Referring to  FIG. 4A , transmission pattern groups have transmission regions formed in upper and lower mask patterns. The transmission regions are asymmetrical to each other based on an x axis, which is a directional axis parallel to a scan direction, and the transmission regions are symmetrical to each other based on axes y, y′, which are perpendicular to the x axis. 
     The mask patterns are shifted to a certain distance based on another axis that is parallel to the certain central axis so that transmission regions and non-transmission regions are reversed when the mask is moved after once irradiating the laser on the mask patterns formed as described above. 
     That is, a part of the transmission region before moving the mask and a part of the transmission region after moving the mask overlap as illustrated in  FIG. 4B , wherein a part of the transmission region is shifted to be formed on a non-transmission region while a part of the non-transmission region is shifted to be formed on the transmission region. 
     Therefore, the same grains are formed on the region b as on the region a since the region b is crystallized a second time using the same energy as a laser energy irradiated a region a by crystallizing the melted grains after melting grains of the region b using laser after moving mask although grains are vertically differently formed in case that laser is irradiated on mask patterns once according to energy deviation of laser irradiated as illustrated in  FIG. 3B . Eventually, the same effect as that transmission region is moved as much as a distance of d−I of  FIG. 4A  is generated, where the distance d−I is obtained by subtracting I, the shortest distance in width of a non-transmission region between transmission regions, from a distance d, where d is greater than I. 
     The mask moves along the scan directional axis as much as ¼ of width of the mask based on a y″ axis. 
     Therefore, a polycrystalline silicon thin film is formed in such a way that grains may be formed evenly. 
     Although  FIG. 4A  illustrates line shaped transmission regions, transmission regions are not limited to the line shaped pattern. 
       FIG. 5A  and  FIG. 5B  are plan figures showing mask patterns and scanning according to a second exemplary embodiment of the present invention.  FIG. 5A  shows a case when a laser is once irradiated onto three line shaped transmission patterns of a different length, and  FIG. 5B  shows a case when a laser is irradiated onto the differing length line shaped transmission patterns twice by moving the mask patterns to a certain distance. 
     Referring to  FIG. 5A  and  FIG. 5B , mask patterns of the second exemplary embodiment of the present invention have a structure with first line shaped transmission regions, second line shaped transmission regions, and third line shaped transmission regions where the second line shaped transmission regions have a shorter length than the first line shaped transmission regions, and the third line shaped transmission regions have a shorter length than the second line shaped transmission regions. 
     The mask patterns also exist in such a way that transmission regions are asymmetrical to each other based on an x axis, which is an axis parallel to a scan line direction, and the transmission regions are symmetrical to each other based on axes y, y′, which are perpendicular to the x axis. 
     The mask patterns are shifted to a certain distance based on another axis that is parallel to the certain central axis so that transmission regions and non-transmission regions are reversed when the mask is moved after once irradiating the laser on the mask patterns formed as described above. 
     That is, a part of the transmission region before moving the mask and a part of the transmission region after moving the mask overlap as illustrated in  FIG. 5B , wherein a part of the transmission region is shifted to be formed on a non-transmission region while a part of the non-transmission region is shifted to be formed on the transmission region. 
     Therefore, the same grains are formed on the region b as on the region a since the region b is crystallized a second time using the same energy as a laser energy irradiated a region a by crystallizing the melted grains after melting grains of the region b using laser after moving mask although grains are vertically differently formed in case that laser is irradiated on mask patterns once according to energy deviation of laser irradiated as illustrated in  FIG. 3B . Eventually, the same effect as that transmission region is moved as much as a distance of d−I of  FIG. 5A  is generated. 
       FIG. 6A  and  FIG. 6B  are plan figures showing mask patterns and scanning according to a third exemplary embodiment of the present invention.  FIG. 6A  shows a case when a laser is once irradiated onto two line shaped transmission patterns where the patterns are of differing length but shorter than the length of the transmission pattern of the first embodiment.  FIG. 6B  shows a case when a laser is irradiated onto the differing length line shaped transmission patterns twice by moving the mask patterns to a certain distance. 
     The mask patterns also exist in such a way that transmission regions are asymmetrical to each other based on an x axis, which is an axis parallel to a scan line direction, and the transmission regions are symmetrical to each other based on axes y, y′, which are perpendicular to the x axis. 
     The transmission patterns are shifted to a certain distance based on another axis that is parallel to the certain central axis so that transmission regions and non-transmission regions are reversed when the mask is moved after once irradiating the laser on the mask patterns formed as described above. 
     That is, a part of the transmission region before moving the mask and a part of the transmission region after moving the mask overlap as illustrated in  FIG. 6B , wherein a part of the transmission region is shifted to be formed on a non-transmission region while a part of the non-transmission region is shifted to be formed on the transmission region. 
     Therefore, the same grains are formed on the region b as on the region a since the region b is crystallized a second time using the same energy as a laser energy irradiated a region a by crystallizing the melted grains after melting grains of the region b using laser after moving mask although grains are vertically differently formed in case that laser is irradiated on mask patterns once according to energy deviation of laser irradiated as illustrated in  FIG. 3B . 
     As described in the above, non-uniformed crystallization of polycrystalline silicon according due to laser energy deviation during crystallization may be prevented by designing mask patterns as described in exemplary embodiments of the present invention. 
     A display device used in the present invention is preferably a flat panel display device such as liquid crystal display device or organic electroluminescent device. 
     As described in the above, the present invention may be capable of solving luminance non-uniformity of a display device by forming mask patterns so that non-uniformity of crystallization of polycrystalline silicon according to laser deviation is overcome. 
     It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.