Abstract:
A method of forming a polysilicon film having smooth surface using a lateral growth and a step-and-repeat laser process. Amorphous silicon formed in a first irradiation region of a substrate is crystallized to form a first polysilicon region by a first laser shot. Then, the substrate is moved a predetermined distance, and irradiated by a second laser shot. The polysilicon region is then recrystallized and locally planarized by subsequent laser shots. After multiple repetitions of the irradiation procedure, the amorphous silicon film formed on a substrate is completely transformed into a polysilicon film. The polysilicon film includes lateral growth crystal grains and nano-trenches formed in parallel on the surface of the polysilicon film. A longitudinal direction of the nano-trenches is substantially perpendicular to a lateral growth direction of the crystal grains.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of U.S. application Ser. No. 11/539,650, filed Oct. 9, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates in general to a polysilicon film and a method of forming the same, and more particularly to a polysilicon film having a smooth surface and to a method of forming the same. 
     2. Description of the Related Art 
     The techniques for manufacturing thin film transistor (TFT) on a glass substrate include the amorphous silicon (a-Si) process and the low-temp polysilicon (LTPS) process. The major differences between the a-Si process and the LTPS process are their complexity and the electrical characteristics of the manufactured devices. The LTPS TFT possesses higher mobility, but the process for fabricating the LTPS TFT is more complicated than that for fabricating the a-Si TFT. 
     Several methods, including the excimer laser annealing (ELA) method, the continuous grain silicon (CGS) method, the continuous wave (CW) laser method, the sequential lateral solidification (SLS) method and the metal induced lateral crystallization (MILC) method, have been developed to convert the amorphous silicon layer into a polysilicon film. A laser beam, such as one produced by an excimer laser, a continuous wave (CW) laser, or abeam pulse laser, can be used for irradiating the amorphous silicon layer. Recently, the beam pulse laser is commonly used, and the method of lateral crystallization by causing a temperature gradient is also popular. 
     Taking the sequential lateral solidification (SLS) method for example, the use of optical phase shift masks that have different transparency can cause a lateral temperature gradient in an amorphous silicon layer so as to induce lateral grain growth. 
     No matter what polysilicon film is used for fabricating the metal-oxide-semiconductor (MOS) device or TFT device, the surface roughness of the polysilicon film has a significant effect on the electrical reliability and uniformity of the device. Using the conventional laser annealing methods to convert the amorphous silicon into the polysilicon grains usually produces protrusions (or tips) on the surface of the polysilicon film, and therefore roughens the surface. The root mean square (RMS) roughness of the polysilicon film formed by the conventional laser annealing methods is in a range of approximately 7-9 nm.  FIG. 1  shows a SEM (scanning electron microscope) image of a polysilicon sample annealed by the ELA method. The SEM image indicated that the height of the protrusions on the polysilicon surface is in the range of about 1000 Å to 1200 Å.  FIG. 2  is perspective view, in the form of an AFM (atomic force microscope) image, of a polysilicon sample also annealed by the ELA method. The image of  FIG. 2  clearly shows that a lot of protrusions project from the polysilicon&#39;s surface. 
     Therefore, a method for forming a polysilicon film having a smooth surface to improve the electrical reliability and uniformity of devices is desirable. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a method of forming a polysilicon film having a smooth surface, by using a lateral growth and a step-and-repeat laser process. According to this method, a first part of the amorphous silicon on a substrate is completely melted by a first laser shot, and then crystallizes to become polysilicon when the laser shot has ceased. Several poly-Si protrusions are also developed. The substrate is then moved in a predetermined step distance, and a second part of the amorphous silicon (which partially overlaps the first part) and the poly-Si protrusions are completely melted by a second laser shot, and become crystallized to form polysilicon again when the second laser shot has ceased. The procedure is repeated until the amorphous silicon film formed on the substrate is completely transformed into a polysilicon film. 
     The invention achieves this object of the invention by providing a method for forming a polysilicon layer, comprising steps of: 
     (a) forming an amorphous silicon layer on a substrate; 
     (b) irradiating a first irradiation area of the substrate by a first laser shot with sufficient energy to melt the amorphous silicon in the first irradiation area, whereupon the melted amorphous silicon crystallizes to form polysilicon; 
     (c) moving the substrate a distance d, and irradiating a second irradiation area of the substrate by a second laser shot with sufficient energy to melt amorphous silicon in the second irradiation area and a portion of the polysilicon in the first irradiation area overlapping the second irradiation area, whereupon the melted amorphous silicon and melted polysilicon crystallize and re-crystallize, respectively; and 
     (d) repeating step (c) until the amorphous silicon layer on the substrate is transformed completely into a polysilicon film. 
     According to the method of the invention, a polysilicon layer formed on the substrate has lateral growth crystal grains and nano-trenches formed in parallel on a surface of the polysilicon layer. Also, a longitudinal direction of the nano-trenches is substantially perpendicular to the direction of growth of the lateral growth crystal grains. 
     Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
         FIG. 1  shows a SEM image of a polysilicon sample annealed by the ELA method. 
         FIG. 2  is perspective view, in the form of an AFM image, of a polysilicon sample also annealed by the ELA method. 
         FIG. 3A˜FIG .  3 D illustrate a method of forming a polysilicon film according to a preferred embodiment of the invention. 
         FIG. 4  is a top view of the polysilicon film formed by the method illustrated in  FIG. 3A˜FIG .  3 D. 
         FIG. 5A  and  FIG. 5B  are plan views illustrating a laser beam applied in the present invention and an excimer laser beam applied according to conventional laser annealing technology, respectively. 
         FIG. 6  illustrates a laser energy profile corresponding to the substrate according to the embodiment of the invention. 
         FIG. 7A  and  FIG. 7B  respectively are cross-sectional view and perspective view of a polysilicon layer formed by the preferred embodiment of the method of the invention. 
         FIG. 8  is perspective view, in the form of an AFM image, of a polysilicon sample annealed according the preferred embodiment of the method of the invention. 
         FIGS. 9A and 9B  are a graph and another AFM image, illustrating a polysilicon sample annealed by the preferred embodiment of the method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A polysilicon film, i.e. a polysilicon layer, formed according to the invention as described below has a smooth surface. The polysilicon film of the invention includes a plurality of lateral growth grains and parallel nano-trenches formed on the surface of the polysilicon film. The roughness of the polysilicon film formed by the method of the invention is less than that formed by the conventional method. 
     Additionally, an embodiment disclosed herein merely illustrates the invention, and the scope of the invention is not limited thereto. The drawings used for illustrating the embodiments of the invention show only the major characteristic parts in order to avoid obscuring the invention. Accordingly, the specification and the drawings are to be regarded in an illustrative sense rather than in a restrictive sense. 
       FIG. 3A˜FIG .  3 D illustrate a method of forming a polysilicon film according to a preferred embodiment of the invention.  FIG. 4  is a top view of the polysilicon film formed by the method illustrated in  FIG. 3A˜FIG .  3 D. First, a substrate  101  is provided, and an amorphous silicon layer  103  is formed on the substrate  101 . Then, a first irradiation area  11  of the substrate  101  is irradiated by a first laser shot with sufficient energy to completely melt the amorphous silicon in the first irradiation area  11 , as shown in  FIG. 3A . The duration of the first laser shot should be long enough to completely melt portion of the amorphous silicon layer  103  in the first irradiation area  11 . When the first laser shot ceases the melted amorphous silicon is crystallized from the edges to the center of the first irradiation area  11 , so as to form a first polysilicon layer  105 . As shown in  FIG. 3B , “A” denotes a lateral growth length (defined as the distance crystals can grow laterally from the edges of the irradiation area), and the first polysilicon layer  105  within a first polysilicon region  21  develops the first poly-Si protrusions or tips  106 . 
     The substrate  101  is then moved along the direction of the arrow in  FIG. 4  in steps of distance d (to be described below), and after each step a melting and crystallization procedure similar to that described is repeated. Thus, following a first step of distance d from the position shown in  FIG. 3A  and  FIG. 3B , a second irradiation area  12  of the substrate  101  is irradiated by a second laser shot with sufficient energy to melt or remelt the silicon, wherein the first irradiation area  11  and the second irradiation area  12  are partially overlapping, as shown in  FIG. 3C . Preferably, the step of distance d is shorter than the lateral growth length A, in order to remelt the protrusions (or tips)  106 . After the amorphous silicon layer  103  in the second irradiation area  12  and a portion of the first polysilicon layer  105  in the first irradiation area  11  are completely melted and the second laser shot has ceased, the melted amorphous silicon is crystallized from the edges to the center of the second irradiation area  12  so as to form a second polysilicon layer  107 . As shown in  FIG. 3D , the second polysilicon layer  107  within a second polysilicon region  22  develops the second poly-Si protrusions  108 . Accordingly, a laterally grown polysilicon region  30  is developed, and the nano-trench  205  (i.e nanometers in depth) is formed at the boundary between the laterally grown polysilicon region  30  and second polysilicon region  22 . 
     The procedures described above are repeated, until the amorphous silicon layer  103  is completely crystallized into a polysilicon layer with a smooth surface. For example, the substrate  101  is further moved by a step of distance d, and a third irradiation area of the substrate  101  is irradiated by a third laser shot with sufficient energy to remelt the amorphous silicon and polysilicon in the third irradiation area, and also to remelt the second poly-Si protrusions  108 . When the third laser shot has ceased, the completely melted silicon grows laterally. According to the method of the invention, the poly-Si protrusions will move across the substrate  101  corresponding to the irradiation produced by the laser shots. Also, each laser shot after the first laser shot produces a laterally grown polysilicon region (e.g. region  30 ) and a nano-trench (e.g. nano-trench  205 ) at the surface of the polysilicon layer, successively spaced apart in the lateral direction. 
     It is noted that the step of distance d is in a range of about 0.5 μm˜5 μm, and preferably about 2 μm ˜5 μm. Also, the laser shot energy should be high enough sufficiently to melt the amorphous silicon layer  103  on the substrate  101 . According to the invention, laser energy of fluence 900 J/cm 2  is applicable, and laser energy of fluence in a range of 1000 J/cm 2  to 1400 J/cm 2  is preferably used in practical applications. Conventional excimer laser irradiation with fluence of 350 to 450 is J/cm 2  is not enough completely to melt the amorphous silicon layer  103 . 
       FIG. 5A  and  FIG. 5B  are plan views respectively illustrating a laser beam applied according to the invention and an excimer laser beam applied in the conventional laser annealing technology. The width of the excimer laser beam EL S  ( FIG. 5B ) is about 0.4 mm, which it is too broad to be used in the invention. According to the invention, the width of laser beam L S  ( FIG. 5A ) is no more than about 100 μm, and preferably is less than 50 μm. The length of the excimer laser beam EL I  ( FIG. 5B ) and the length of laser beam L I  (FIG.  5 A) are approximately equal to the length of the substrate  101 . 
     Additionally, the step of distance d corresponds to the profile of the laser energy for effective completion of the lateral growth.  FIG. 6  illustrates a laser energy profile corresponding to the substrate according to the embodiment of the invention. D LE , a lateral distance along the substrate  101  from where the intensity of the laser beam&#39;s energy impacting upon it is 10% of its maximum to where the intensity first reaches 90% of its maximum. Preferably, D LE  is larger than the step of distance d ( FIG. 3C  and  FIG. 4 ). For example, the lateral distance D LE  will be larger than 2 μm if the distance d is 2 μm, and the lateral distance L E  will be larger than 5 μm if the step of distance d is 5 μm. 
       FIG. 7A  and  FIG. 7B , respectively, are a cross-sectional view and a perspective view of a polysilicon layer formed by the preferred embodiment of the method of the invention. The polysilicon layer  203  formed by the method of the invention has lateral growth crystal grains in the regions  30 , and nano-trenches  205  respectively developed in parallel on the surface  204  of the polysilicon layer  203  at the boundaries of the successively adjacent laterally grown polysilicon regions  30 . As can be seen in  FIG. 7B , in the resulting polysilicon layer, from a plane  300  defined by the average height of its upper surface the surface falls sharply to define the trenches, and from the lateral sides of the trenches the surface gradually rises slightly above the defined plane  300 . According to experimental results, the surface  204  of the polysilicon layer  203  has root-mean-square (RMS) roughness no greater than 5 nm, and in the vicinity of each nano-trench  205  the surface of the polysilicon layer will be at a height in a range of about −10 nm˜+10 nm relative to the level of the plane  300 . In other words, the nano-trench  205  is no greater than about 20 nm in depth. According to the aforementioned description, distance D (in  FIG. 7A ) between two adjacent nano-trenches  205  corresponds to the step of distance d, which is in a range of about 0.5 μm-5 μm. Since the distance d is related to the lateral distance D LE  between 10% of laser energy intensity and 90% of laser energy intensity (as shown in  FIG. 6 ), the lateral distance D LE  preferably will be larger than the distance D. 
     Further, the longitudinal direction T L  of the nano-trench  205  is substantially perpendicular to the lateral growth direction P L  of crystal grains, as shown in  FIG. 7B . Also, the longitudinal direction T L  of the nano-trench  205  and the grain boundary of the polysilicon layer  203  form an angle in a range of about 60 degrees to 90 degrees. 
     According to the invention, a laser beam utilized in the method according to the present embodiment has a shorter width (of less than 50 μm, preferably) than that of a laser beam used in the conventional ELA method. When a portion of the amorphous silicon on the substrate is irradiated by the first laser shot, the crystal grains grow laterally from the edges to the center so as to form the poly-Si protrusions. The substrate is then shifted by a step of distance d, and the amorphous silicon is irradiated by the second laser shot to completely melt the poly-Si protrusions. Repeating the step and laser shot procedure, the amorphous silicon can be laterally crystallized to a polysilicon layer with a smooth surface, and this is supported by experimental results.  FIG. 8  is perspective view, in the form of an AFM (atomic force microscope) image, of a polysilicon sample annealed by the method according to the present embodiment.  FIG. 8  clearly shows the surface of the polysilicon sample to be smooth (without any protrusions), and only several nano-trenches are formed in parallel on the surface.  FIGS. 9A and 9B  illustrate the contour of a polysilicon sample annealed by the method according to the present embodiment, wherein  FIG. 9A  shows the height of the polysilicon surface along the lateral direction and  FIG. 9B  is another AFM image of the polysilicon layer in plan view. As shown in  FIGS. 9A and 9B , the uppermost and lowermost points of the polysilicon surface are +5 nm and −5 nm, respectively relative to the plane  300 . The polysilicon sample annealed by the method according to the present embodiment has a surface smoother than that annealed by the conventional ELA method (see  FIG. 2 , with protrusions of 1000 Å to 1200 Å). 
     Additionally, TFT devices having polysilicon layers formed by the method according to the present embodiment were fabricated, and experiments were performed to determine their electrical properties. The experimental results listed in Table 1 indicate that the mobility increases from 108 to 301 cm 2 /V-S. Thus, generally, the uniformity and reliability of electrical characteristics of TFT devices having polysilicon layers formed by the method of the invention are improved over those formed by a conventional method. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Reciprocal 
                   
               
               
                   
                   
                 Threshold 
                 of IV Curve 
               
               
                   
                 Mobility 
                 Voltage 
                 Slope (SS) 
                 I off   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 TFT devices having 
                 301 cm 2 /V-S 
                 3.14 V 
                 0.19 
                 5 × 10 −13   
                 pA 
               
               
                 polysilicon layers formed by 
               
               
                 the method according the 
               
               
                 invention 
               
               
                 TFT devices having 
                 108 cm 2 /V-S 
                  2.0 V 
                 0.22 
                 3.76 × 10 −13   
                 pA 
               
               
                 polysilicon layers formed by 
               
               
                 conventional excimer laser 
               
               
                 annealing (ELA) 
               
               
                   
               
             
          
         
       
     
     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, the invention covers various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.