CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part of a prior application Ser. No. 10/248,372, filed Jan. 15, 2003, now U.S. Pat. No. 6,867,074 which claims the priority benefit of Taiwan application serial no. 91132242, filed Oct. 31, 2002. 


   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates to a method of fabricating a polysilicon layer. More particularly, the present invention relates to a method of fabricating a polysilicon layer through lateral crystallization using partially melted amorphous silicon inside a trench as nucleation seeds. 
   2. Description of Related Art 
   Low temperature polysilicon thin film transistor liquid crystal display (LTPS TFT-LCD) differs from a conventional amorphous silicon thin film transistor liquid crystal display (α-Si TFT-LCD) in that electron mobility as high as 200 cm 2 /V-sec can be reached. Hence, each thin film transistor device may occupy a smaller area so that a higher opening rate and hence a brighter display with smaller power consumption can be obtained. In addition, an increase in electron mobility also opens up the possibility of fabricating a portion of the driver circuit and the thin film transistor together on a glass substrate. Ultimately, reliability of the liquid crystal display panel is improved and cost of producing each display is reduced. Therefore, LTPS TFT-LCD has a fabrication cost considerably lower than α-Si TFT-LCD. Other advantages of the LTPS TFT-LCD has includes a slim package, a light body and a relatively high resolution. These advantages render the LTPS TFT-LCD especially suitable for implementing on portable and energy-short mobile terminal products. 
   The channel layer of the thin film transistor inside a LTPS TFT-LCD is formed in an excimer laser annealing (ELA) process. In general, quality of the channel layer depends largely on the average size of the polysilicon grains and their uniformity. However, the average size of the polysilicon grains and their uniformity are directly related to the energy provided to the excimer laser in the annealing process. 
     FIGS. 1A to 1C  are schematic cross-sectional views showing the steps for producing a conventional polysilicon layer. As shown in  FIG. 1A , a substrate  100  such as a glass substrate is provided. A buffer layer  102  is formed over the substrate  100 . In general, the buffer layer  102  is a composite layer that includes a silicon nitride layer or a silicon oxide layer. 
   As shown in  FIGS. 1B and 1C , an amorphous silicon layer  104  is formed over the buffer layer  102 . Thereafter, an excimer laser annealing (ELA) process is conducted. The amount of radiation energy on the amorphous silicon layer  104  provided by the excimer laser is so carefully controlled that the entire amorphous silicon layer  104  almost completely melts. Hence, only a few seed of crystallization remains on top of the buffer layer  102 . Thereafter, the melted silicon will start to crystallize from the seeds of crystallization to form a polysilicon layer  106  that contains lots of non-uniformly distributed grain boundaries. 
   In the aforementioned excimer laser annealing process, if the energy provided to the excimer laser exceeds the super lateral growth (SLG) point, density distribution of the seed of crystallization may drop to a very low value within a transient interval. The sudden loss of seed of crystallization may lead to the production of lots of small and highly non-uniform grains. Thus, energy to the excimer laser must be precisely controlled in order to fabricate a polysilicon layer with large and uniform grains therein. In other words, the process window is very small. 
     FIG. 2  is a perspective view showing a buffer layer with lots of openings capable of facilitating the fabrication of a polysilicon layer over the buffer layer. As shown in  FIG. 2 , a substrate  200  such as a glass substrate is provided. A buffer layer  202  is formed over the substrate  200 . In general, the buffer layer  202  is a composite layer that includes a silicon nitride layer and a silicon oxide layer. To increase the grain size and uniformity of the polysilicon layer and widen the process window of the fabrication process, a plurality of openings arranged into an array is formed on the buffer layer  202 . These openings  204  play a significant role during the excimer laser annealing process. During the annealing process, the amorphous silicon (not shown) outside the openings  204  melts completely and the silicon turns into a liquid state. However, some amorphous silicon (not shown) at the bottom of the openings  204  may remain solid and act as initiation sites for the lateral growth of crystal to form a polysilicon layer. In other words, crystallization starts out from the openings  204 . Consequently, the quantity and distribution of the seed of crystallization is precisely controlled. 
     FIG. 3  is a top view showing the grain boundaries of a polysilicon layer formed with an array of openings on the buffer layer as shown in  FIG. 2 . Since the amorphous silicon at the bottom of the openings  204  does not melt completely, crystallization of liquid silicon grows laterally from the bottom of each opening  204 . Due to the lateral growth of crystal from the bottom of the openings  204 , a grain boundary  300  is formed between neighboring openings  204 . In general, locations of the grain boundaries are directly related to the distance of separation between the openings. Because the openings  204  have an array arrangement, grain growth in the x and the y direction is influenced by the separation of neighboring openings. Thus, although the formation of an array of openings in the buffer layer is able to control grain size and uniformity, size of grains is still subjected to an intrinsic restriction. 
   SUMMARY OF THE INVENTION 
   Accordingly, one object of the present invention is to provide a method of fabricating a polysilicon layer with a uniform distribution of larger size crystals. 
   A second object of this invention is to provide a method of fabricating a polysilicon layer capable of increasing the processing window of an excimer laser annealing process. 
   A third object of this invention is to provide a method of fabricating a polysilicon layer such that the polysilicon layer has fewer grain boundaries. 
   To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of fabrication a polysilicon layer. The method includes: providing a substrate; forming a buffer layer having a plurality of first trenches over the substrate; forming an amorphous silicon layer over the buffer layer; and, conducting a laser annealing process so that the amorphous silicon layer melts and crystallizes into a polysilicon layer starting from the upper reach of the first trenches. 
   In this invention, the steps for forming the buffer layer with first trenches thereon over the substrate includes: forming a silicon nitride layer over the substrate; forming a plurality of second trenches within the silicon nitride layer; and, forming a conformal silicon oxide layer over the silicon nitride layer so that a plurality of first trenches are formed in the silicon oxide layer corresponding in position to the respective second trenches. 
   Alternative, the steps for forming the buffer layer with first trenches thereon over the substrate includes: forming a silicon nitride layer over the substrate; forming a silicon oxide layer over the silicon nitride layer; and, forming a plurality of first trenches in the silicon oxide layer. 
   In this invention, photolithographic/etching processes, for example, are used to form the first trenches and/or the second trenches. The laser annealing process includes an excimer laser annealing process. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
       FIGS. 1A to 1C  are schematic cross-sectional views showing the steps for producing a conventional polysilicon layer. 
       FIG. 2  is a perspective view showing a buffer layer with lots of openings capable of facilitating the fabrication of a polysilicon layer over the buffer layer. 
       FIG. 3  is a top view showing the grain boundaries of a polysilicon layer formed with an array of openings on the buffer layer as shown in  FIG. 2 . 
       FIG. 4  is a perspective view showing trenches on a buffer layer for fabricating a polysilicon layer according to one preferred embodiment of this invention. 
       FIG. 5  is a top view showing the crystal boundaries of a polysilicon layer formed over the buffer layer as shown in  FIG. 4 . 
       FIGS. 6A to 6D  are schematic cross-sectional view showing the progression of steps for fabricating a polysilicon layer according to one preferred embodiment of this invention. 
       FIGS. 7A to 7D  are schematic cross-sectional view showing the progression of steps for fabricating a polysilicon layer according to a second preferred embodiment of this invention. 
       FIGS. 8A to 8D  are schematic cross-sectional view showing the progression of steps for fabricating a polysilicon layer according to a third preferred embodiment of this invention. 
     FIGS.  8 B′ and  8 B″ are schematic cross-sectional view showing the alternative steps of  FIG. 8B  according to this invention. 
   


   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     FIG. 4  is a perspective view showing trenches on a buffer layer for fabricating a polysilicon layer according to one preferred embodiment of this invention. As shown in  FIG. 4 , a substrate  400  such as a glass panel is provided. Thereafter, a buffer layer  402  is formed over the substrate  400 . The buffer layer  402  can be, for example, a composite layer that includes a silicon nitride layer and a silicon oxide layer or at least a low-k material layer. To improve crystal size and distribution within a polysilicon layer and increase process window, this embodiment produces a plurality of parallel trenches  404  in the buffer layer  402 . The trenches  404  serve as seeds supplier of crystallization in an excimer laser annealing process. During excimer laser annealing, an amorphous silicon layer (not shown) on the buffer layer  402  outside the trench region melts completely while the amorphous silicon at the bottom of the trenches  404  melts only partially. Hence, silicon in the liquid state may start to solidify (lateral crystallization) into a polysilicon layer starting from the bottom of the trenches  404 . Since crystallization starts out from the trenches  404 , crystal growth can be manipulated through the distribution of seeds of crystallization. 
     FIG. 5  is a top view showing the crystal boundaries of a polysilicon layer  501  formed over the buffer layer as shown in  FIG. 4 . As shown in  FIG. 5 , since the amorphous silicon at the bottom of the trenches  404  melts partially, liquid silicon will crystallize outward from the bottom of the trenches  404 . The lateral crystallization of melted silicon crystallizes from the trenches  404  results in the formation of a grain boundary  500  between neighboring trenches  404 . Locations of the grain boundaries  500  depend largely on the distance of separation between neighboring trenches  404 . Because all the trenches  404  are parallel to each other and runs in a y direction, crystal growth is subjected to limitation in the x direction only. In other words, with the parallel trenches  404  serving as longitudinal seeds of crystallization, crystal size and uniformity of crystal distribution within a polysilicon layer are improved. In the following, the steps for fabricating a polysilicon layer are described in more detail. 
     FIGS. 6A to 6D  are schematic cross-sectional view showing the progression of steps for fabricating a polysilicon layer according to a first preferred embodiment of this invention. As shown in  FIG. 6A , a substrate  600  such as a glass panel is provided. Thereafter, a buffer layer  602  is formed over the substrate  600 . The buffer layer can be, for example, a composite layer that includes a silicon nitride layer  602   a  and a silicon oxide layer  602   b . The silicon nitride layer  602   a  and the silicon oxide layer  602   b  are formed, for example, in a plasma enhanced chemical vapor deposition (PECVD). 
   As shown in  FIG. 6B , a plurality of parallel trenches  604  is formed on the buffer layer  602 . The trenches  604  are formed on the upper silicon oxide layer  602   b , for example, by conducting photolithographic and etching processes. 
   As shown in  FIGS. 6C and 6D , an amorphous silicon layer  606  is formed over the buffer layer  602 . The amorphous silicon layer  606  is formed, for example, in a low-pressure chemical vapor deposition (LPCVD). Thereafter, a laser annealing process such as an excimer laser annealing process is conducted. In the laser annealing process, energy supplied to the excimer laser is carefully controlled such that the amorphous silicon  606  outside the trenches  604  region melts almost completely while the amorphous silicon  606  at the bottom of the trenches  604  melts only partially. Therefore, silicon in the liquid state crystallizes to form a polysilicon layer  608  starting from the bottom of the trenches  604 . Furthermore, the polysilicon layer  608  formed by the laser annealing process includes a plurality of crystal boundaries  610 . However, these crystal boundaries are located between each pair of neighboring trenches  604  only. 
     FIGS. 7A to 7D  are schematic cross-sectional view showing the progression of steps for fabricating a polysilicon layer according to a second preferred embodiment of this invention. As shown in  FIG. 7A , a substrate  700  such as a glass panel is provided. Thereafter, a silicon nitride layer  702   a  is formed over the substrate  700 . The silicon nitride layer  702   a  is formed, for example, in a plasma-enhanced chemical vapor deposition (PECVD). A plurality of parallel trenches  704   a  is formed in the silicon nitride layer  702   a . The parallel trenches  704   a  are formed, for example, by conducting photolithographic and etching processes. 
   As shown in  FIG. 7B , a conformal silicon oxide layer  702   b  is formed over the silicon nitride layer  702   a . The silicon nitride layer  702   a  and the silicon oxide layer  702   b  together constitute a buffer layer  702 . Since the silicon oxide layer  702   b  covers the silicon nitride layer  702   a , a plurality of trenches  704   b  are formed over the respective trenches  704   a . In addition, width of the trenches  704   b  is smaller than width of the trenches  704   a  due to step coverage. Hence, this embodiment is capable of fabricating trenches  704   b  whose width is smaller than the critical dimension (CD). 
   As shown in  FIGS. 7C and 7D , an amorphous silicon layer  706  is formed over the buffer layer  702 . The amorphous silicon layer  706  is formed, for example, in a low-pressure chemical vapor deposition (LPCVD). Thereafter, a laser annealing process such as an excimer laser annealing process is conducted. In the laser annealing process, energy supplied to the excimer laser is carefully controlled such that the amorphous silicon  706  outside the trenches  704   b  region melts almost completely while the amorphous silicon  706  at the bottom of the trenches  704   b  melts only partially. Therefore, silicon in the liquid state crystallizes to form a polysilicon layer  708  starting from the bottom of the trenches  704   b . Furthermore, the polysilicon layer  708  formed by the laser annealing process includes a plurality of crystal boundaries  710 . However, these crystal boundaries are located between each pair of neighboring trenches  704   b  only. 
     FIGS. 8A to 8D  are schematic cross-sectional view showing the progression of steps for fabricating a polysilicon layer according to a third preferred embodiment of this invention. As shown in  FIG. 8A , a substrate  800  such as a glass panel is provided. Thereafter, a buffer layer  802  is formed over the substrate  800 . The buffer layer can be, for example, a single layer of a low-k material or a composite layer of low-k materials. The low-k material can be an inorganic low-k material or an organic low-k material. If the buffer layer is made of an organic low-k material, a barrier layer may be further included and formed between the buffer layer and the subsequently formed silicon layer. 
   As shown in  FIG. 8B , a plurality of parallel trenches  804  is formed in the buffer layer  802 . The trenches  804  are formed, for example, by conducting photolithographic and etching processes. If the buffer layer  802  is a composite layer including a first low-k material layer  802   a  and a second low-k material layer  802   b , the trenches  804  can be formed in the upper second low-k material layer  802   b , as shown in FIG.  8 B′. Alternatively, first trenches  804   a  can be formed in the lower first low-k material layer  802   a , for example, by conducting photolithographic and etching processes, and the later formed second low-k material layer  802   b  covering the first low-k material layer  802   a  consequently includes a plurality of second trenches  804   b  corresponding to the first trenches  804   a , as shown in FIG.  8 B″. 
   As shown in  FIGS. 8C and 8D , an amorphous silicon layer  806  is formed over the buffer layer  802 . The amorphous silicon layer  806  is formed, for example, in a low-pressure chemical vapor deposition (LPCVD). Thereafter, a laser annealing process such as an excimer laser annealing process is conducted. In the laser annealing process, energy supplied to the excimer laser is carefully controlled such that the amorphous silicon  806  outside the trenches  804  region melts almost completely while the amorphous silicon  806  at the bottom of the trenches  804  melts only partially. Therefore, silicon in the liquid state crystallizes to form a polysilicon layer  808  starting from the bottom of the trenches  804 . Furthermore, the polysilicon layer  808  formed by the laser annealing process includes a plurality of crystal boundaries  810 . However, these crystal boundaries are located between each pair of neighboring trenches  804  only. 
   In conclusion, the method of fabricating a polysilicon layer according to this invention at least includes the following advantages: 
   1. The partially melted amorphous silicon material at the bottom of trenches provides an ideal side for the initialization of crystallization. Hence, the crystals within the polysilicon layer are more uniformly distributed and have a larger crystal size. 
   2. Since the trenches are produced in conventional photolithographic and etching processes, no particular equipment is required. 
   3. Because the partially melted amorphous silicon material inside the trenches provides seeds for lateral crystallization, processing window of the excimer laser annealing process is enlarged. 
   4. Since the trenches provide seeds for continuous crystallization, the ultimately formed polysilicon layer has fewer grain boundaries. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.
A method of fabrication a polysilicon layer is provided. A substrate is provided and then a buffer layer having a plurality of trenches thereon is formed over the substrate. Thereafter, an amorphous silicon layer is formed over the buffer layer. Finally, a laser annealing process is conducted so that the amorphous silicon layer melts and crystallizes into a polysilicon layer starting from the upper reach of the trenches. This invention can be applied to fabricate the polysilicon layer of a low temperature polysilicon thin film transistor liquid crystal display such that the crystals inside the polysilicon layer are uniformly distributed and have a larger average size.