Abstract:
The present invention discloses a method for forming a polycrystalline semiconductor layer on a substrate at an atmospheric pressure, including: providing a chamber having an opening portion and a stage therein; forming an amorphous semiconductor layer on the substrate; positioning the amorphous semiconductor layer formed on the substrate on the stage of the chamber; and irradiating five to twelve laser beam shots to every position of a desired portion of the semiconductor layer over the stage through the opening portion of the chamber.

Description:
CROSS REFERENCE  
         [0001]    This application claims the benefit under 35 U.S.C. § 119, of Korean Patent Application No. 1999-11253, filed on Mar. 31, 1999, the entirety of which is hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method for manufacturing a polycrystalline silicon layer, and more particularly, to a system and a method for manufacturing a polycrystalline silicon layer for use in a thin film transistor (TFT) using a laser annealing technique.  
           [0004]    2. Description of Related Art  
           [0005]    In general, there are two methods for manufacturing an active layer for use in a polycrystalline thin film transistor (TFT). The first method involves depositing a polycrystalline material and patterning it to form a polycrystalline semiconductor layer as the active layer of the TFT. The other method involves depositing, patterning, and heat-treating an amorphous silicon layer to form a polycrystalline silicon layer as the active layer of the TFT using an excimer laser annealing technique or a furnace annealing technique.  
           [0006]    In case of the former method, a polycrystalline silicon layer is used as the active layer and an impurity ion gas of a nitrogen group or a boron group is doped into both ends of the polycrystalline silicon layer to define source and drain regions. At this point, the polycrystalline silicon layer undergoes a laser annealing using a rare gas halide laser such as Ar, ArF, KrF, or XeCl to activate the gas doped into the source and drain regions. Laser annealing is also performed to recrystallize amorphous portions of the polycrystalline silicon layer formed due to energy generated when the impurity ion gas is doped.  
           [0007]    In case of the latter method, first an amorphous silicon layer is deposited on a substrate using a sputtering technique. The layer then undergoes laser annealing to form the polycrystalline silicon layer.  
           [0008]    As described above, the laser annealing is mainly used to manufacture the active layer for use in a polycrystalline thin film transistor or to activate the doped impurity ion gas to define the source and drain regions of the polycrystalline silicon layer.  
           [0009]    [0009]FIG. 1 is a schematic view illustrating a configuration of a conventional laser annealing system.  
           [0010]    The conventional laser annealing system includes a vacuum chamber  1  having a laser beam passage window  13  made of a transparent material such as a rock-crystal, a vacuum device  3  having a vacuum pump  31  and a vacuum tube  33  to place the vacuum chamber  1  in a specific atmosphere, for example, a vacuum or nitriding atmosphere, a laser device  5  that irradiates a laser beam with a predetermined energy through the laser beam passage window  13 , and a stage  50  on which an amorphous silicon layer  23  on a substrate  21  as a workpiece is placed.  
           [0011]    A method of manufacturing the polycrystalline silicon layer using the conventional laser annealing system described above will be explained.  
           [0012]    First, the amorphous silicon layer  23  on the substrate  21  is located on the stage  50  in the vacuum chamber  1 , and then the vacuum device  3  is operated to place the amorphous silicon layer  23  in a vacuum or nitride atmosphere. A laser beam is irradiated from the laser device  5  through the laser beam passage window  13  to scan the amorphous silicon layer  23  on the substrate  21  on the stage  50 . At this point, the laser device  5  preferably produces about 200 to 300 laser beam pulses per second. As a result, the amorphous silicon layer  23  on the substrate  21  is converted into a polycrystalline silicon layer.  
           [0013]    Furthermore, the impurity-doped ion gas defining the source and drain regions of the polycrystalline silicon layer is activated through the laser annealing technique described above.  
           [0014]    In the vacuum method of the conventional art, as the number of shots increases, the layer quality increases. That is, the grain size becomes large, and the roughness of the surface can be improved as the number of shots is increased.  
           [0015]    The conventional laser annealing system and the method for polycrystallization and activation described above have the following disadvantages: 1) the process takes a long time because the inside of the vacuum chamber  1  must be maintained in a specific atmosphere, for example, a vacuum or a nitriding atmosphere; 2) high-cost because a high-cost material such as a rock-crystal must be used for the laser beam passage window  13  and the vacuum device  3  is required; and 3) possible damage to the laser beam passage window  13  may result if part of the polycrystalline silicon layer  23  comes undone from the substrate  21 . As shown in FIG. 2, a part  41  of the polycrystalline silicon layer  23  that comes undone may be deposited on the laser beam passage window  13  due to energy of the laser beam irradiated.  
         SUMMARY OF THE INVENTION  
         [0016]    To overcome the problems described above, a preferred embodiment of the present invention provides a laser annealing system and a method for laser annealing that can be performed in a normal atmosphere and that forms a polycrystalline silicon layer having layer-sized crystals as an active layer of a thin film transistor.  
           [0017]    In order to achieve the above object, the present invention provides a method for forming a polycrystalline semiconductor layer on a substrate at an atmospheric pressure, comprising: providing a chamber having an opening and a stage therein; forming an amorphous semiconductor layer on the substrate; positioning the amorphous semiconductor layer formed on the substrate on the stage of the chamber; and irradiating five to twelve laser beam shots to every position of a desired portion of the semiconductor layer over the stage through the opening of the chamber.  
           [0018]    The present invention further provides a method for activating impurity-doped ion gas in a polycrystalline semiconductor layer on a substrate at an atmospheric pressure, comprising: providing a chamber having an opening and a stage therein; providing the polycrystalline semiconductor layer including a region having impurity-doped ion gas therein, the polycrystalline semiconductor layer being positioned on the substrate; positioning the polycrystalline semiconductor layer having the impurity-doped ion gas therein together with the substrate on the stage of the chamber; and irradiating five to twelve laser beam shots to every position of the region having the impurity-doped ion gas of the polycrystalline semiconductor layer over the stage through the opening of the chamber.  
           [0019]    The laser beam includes one of a group consisting of Ar, ArF, KrF, and XeCl. The laser beams may be intermittently applied to the amorphous semiconductor layer, or the laser beams may be continuously applied to the amorphous semiconductor layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0021]    [0021]FIG. 1 is a schematic view illustrating a configuration of a conventional laser annealing system;  
         [0022]    [0022]FIG. 2 is a schematic view illustrating a problem that may occur when a laser annealing is performed in the conventional laser annealing system;  
         [0023]    [0023]FIG. 3 is a schematic view illustrating a configuration of a laser annealing system according to a preferred embodiment of the present invention;  
         [0024]    [0024]FIG. 4 is a graph illustrating transfer characteristics of polycrystalline silicon thin film transistors (Poly-Si TFTs) fabricated according to the conventional art and a preferred embodiment of the present invention;  
         [0025]    [0025]FIG. 5A is a photograph illustrating grain boundaries of the polycrystalline silicon layer fabricated in a vacuum with ten laser shots according to the conventional art;  
         [0026]    [0026]FIG. 5B is a photograph illustrating the roughness of the grain boundaries of a polycrystalline silicon layer fabricated in a vacuum with ten laser shots, according to the conventional art;  
         [0027]    [0027]FIG. 6A is a photograph illustrating the grain boundaries of a polycrystalline silicon layer fabricated in a normal atmosphere with ten laser shots, according to a preferred embodiment of the present invention; and  
         [0028]    [0028]FIG. 6B is a photograph illustrating the roughness of the grain boundaries of the polycrystalline silicon layer of FIG. 6A. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]    Reference will now be made in detail to a preferred embodiment of the present invention, an example of which is illustrated in the accompanying drawings.  
         [0030]    [0030]FIG. 3 is a schematic view illustrating a configuration of a laser annealing system according to a preferred embodiment of the present invention. As shown in FIG. 3, the laser annealing system includes a process chamber  105  having an opening  160  that allows a laser beam to pass through, a laser device  105  that irradiates a laser beam with a predetermined energy through the opening  160 , a stage  50  on which an amorphous silicon layer  123  deposited on a substrate  121  is placed as a workpiece, and a laser adjuster  115  for adjusting laser emission output. At this point, the laser beam includes one of a group consisting of Ar, ArF, KrF, and XeCl.  
         [0031]    The laser annealing system described above can be applied to both a polycrystallization process of the amorphous semiconductor layer and an activation process of the impurity-doped ion gas to define the source and drain regions. A preferred embodiment of the present invention will be discussed in the context of the polycrystallinzation process.  
         [0032]    First, either the amorphous silicon layer  123  on the substrate  121  or a polycrystalline silicon layer whose both ends are doped by impurity ion gas is placed on the stage  150  in the process chamber  101 . At this point, the process chamber  101  is maintained in a normal atmosphere. The laser device  105  can emit either of a continuous laser beam and an intermittent pulse laser beam, and can also move back and forth or right and left by a moving means such as servo motors (not shown) such that the amorphous silicon layer  123  is scanned uniformly. The laser device  105  preferably irradiates five to twelve laser beam pulses to a target of the amorphous silicon layer  123  under the control of the laser adjuster  115 .  
         [0033]    The energy of one laser beam pulse is between a complete melting energy and a surface melting energy. The complete melting energy, which is dependent on a thickness of the silicon layer, means the energy that can melt the amorphous silicon layer completely, thereby causing no seed of crystal or too many seeds of very tiny sized crystal. The surface melting energy means the energy that can melt only the surface of the silicon layer. One pulse of the laser beam can satisfy crystallization/activation. In this embodiment, the energy density per pulse is 290 to 400 mJ/cm2 for an amorphous silicon with a thickness of 600 Angstroms.  
         [0034]    When laser annealing is performed in a normal atmosphere for polycrystallization of the amorphous silicon layer, the amorphous silicon layer has an oxidation film on a surface thereof due to oxygen existing in the atmosphere when the amorphous silicon layer is heated during polycrystallization. Such an oxidation film deteriorates the roughness characteristic of grain boundaries, leading to bad electric characteristics of the polycrystalline silicon layer. In order to overcome this problem, the preferred embodiment of the present invention provides a method of adjusting the number of laser shots irradiated to a desired position of the amorphous silicon layer so that roughness of the grain boundaries on the surface is improved. Laser annealing is a technique in which the laser beam is irradiated to a specific position of the amorphous semiconductor layer for crystallization. A polycrystalline semiconductor layer having a good electric characteristics can be attained when the laser beam is irradiated several times. According to the experiments preformed by the present inventor, the preferred number of laser shots is five to twelve and the ideal number of laser shots irradiated to a specific position of the amorphous semiconductor layer has been found to be ten. In normal atmosphere, in the condition of below 5 shots, the uniformity of the grain size is not good, and transfer character of the TFT is not good. And as the number of shots increases, the small sized grains disappear and big-sized grain appear. With more than 12 laser shots, the grain boundary arises from the surface of the silicon layer. When the number of laser shots is more than fifteen, the amorphous semiconductor layer may be destroyed and ultimately come to be discarded.  
         [0035]    As shown in FIGS. 5B and 6B, roughness of the polycrystalline silicon layer fabricated according to the preferred embodiment of the present invention is much improved over the conventional art.  
         [0036]    There are several methods to adjust the number of laser shots to five to twelve. Two representative methods will be explained as follows.  
         [0037]    First, in case that the laser device  105  emits an intermittent pulse laser beam, the laser device  105  is adjusted to emit  200  to  300  laser pulses per second, and the laser beam generated from the laser device  105  has a duration of 25 ns to 50 ns per pulse. When the laser beam is emitted to the amorphous silicon layer  123  on the substrate  121 , the laser device  105  is adjusted by the laser adjuster  115  to irradiate 5-12 pulsed laser beams to a specific position of the amorphous silicon layer  123 . The entire surface of the amorphous silicon layer  123  is uniformly scanned in the same way. For instance, in case that the laser device  105  irradiates 200 pulses per second, if the laser device  105  scans the entire surface of the amorphous silicon layer  123  while moving at the speed of  5 / 200  second per a cross-sectional area of the laser beam, about  5  laser beam pulses are irradiated to a specific position of the amorphous silicon layer  123 . Furthermore, if the laser device  105  scans the entire surface of the amorphous silicon layer  123  while moving at the speed of 12/200 second per cross-sectioned area of the laser beam, about five to twelve laser beam pulses are irradiated to a specific position of the amorphous silicon layer  123 .  
         [0038]    Second, in the case that the laser device  105  emits a continuous laser beam, in order to have such an effect an the intermittent laser beam, it is necessary to adjust the scanning time according to the cross-sectional area of the laser beam. In an embodiment in which the laser adjuster  115  has a scanner with a scanning time of 25 ns to 50 ns per point of the amorphous silicon layer  123  corresponding to a size of the cross-sectional area of the laser beam, it is required that the laser beam irradiates the entire surface of the amorphous silicon layer  123  five to twelve times. Therefore, the scanning process is preformed five to twelve times.  
         [0039]    [0039]FIG. 4 is a graph illustrating transfer characteristics of Poly-Si TFTs, fabricated according to the conventional art and a preferred embodiment of the present invention, respectively. As shown in FIG. 4, the transfer characteristics of the Poly-Si TFT according to the preferred embodiment of the present invention are much better than that of the conventional art.  
         [0040]    Further, when laser annealing for polycrystallization is performed in a normal atmosphere, a SiO 2  film is formed on the surface of the amorphous silicon layer due to reaction of oxygen and silicon. The SiO 2  film prevents heat within the amorphous silicon layer from being rapidly ventilated, increasing crystal growing time leading to layer sized crystal. Therefore, as shown in FIGS. 5A and 6A, grains of the polycrystalline silicon layer according the inventive laser annealing technique is bigger in size than that according to the conventional art.  
         [0041]    Further, when the laser annealing for activation of the impurity-doped ion gas contained in the polycrystalline silicon layer is performed in the atmosphere, the source and drain regions formed are so rough that it is difficult for source and drain electrodes of the TFT to respectively contact with the source and drain regions. But, using the laser annealing technique described above, roughness of the regions of the polycrystalline silicon layer corresponding to the source and drain regions is improved. That is, if the two to twelve laser beam shots are applied to the regions of the polycrystalline silicon layer corresponding to the source and drain regions, roughness of the regions of the polycrystalline silicon layer corresponding to the source and drain regions is so much improved that it becomes easier for the source and drain electrodes of the TFT to respectively contact with the source and drain regions.  
         [0042]    As described hereinbefore, using the laser annealing system and the method of the same according to the preferred embodiment of the present invention, since roughness of the surface of the polycrystalline silicon layer is much improved over the conventional art, a TFT having good electric characteristics can be manufactured. Further, since the vacuum process is omitted, process time and cost are reduced.  
         [0043]    While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.