Patent Publication Number: US-9891501-B2

Title: Polycrystalline silicon thin film transistor device and method of fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This non-provisional application claims priority to and benefit of, under 35 U.S.C. § 119(a), Patent Application No. 104131229 filed in Taiwan R.O.C. on Sep. 22, 2015, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a polycrystalline silicon thin film transistor device and a method of fabricating the same, and more particularly to a polycrystalline silicon thin film transistor device having high electron mobility and a method of fabricating the same. 
     Related Art 
     A low temperature polycrystalline silicon (LTPS) thin film transistor device has a characteristic of relatively high electron mobility, and therefore theoretically has better electrical performance compared with an amorphous silicon thin film transistor device. However, because a process of an LTPS thin film transistor device is relatively complex, and the LTPS thin film transistor device needs more photolithography and etching processes (PEP) than an amorphous silicon thin film transistor device does, not only a fabrication cost is relatively high, but also a yield is also reduced. 
     SUMMARY 
     An objective of the present invention is to provide a polycrystalline silicon thin film transistor device and a method of fabricating the same, so as to simplify process steps, reduce a fabrication cost, and improve electron mobility and device characteristics. 
     An embodiment of the present invention provides a method of fabricating a polycrystalline silicon thin film transistor device, including the following steps. A substrate is provided, and a buffer layer having dopants is formed on the substrate. An amorphous silicon layer is formed on the buffer layer having the dopants. A thermal process is performed to convert the amorphous silicon layer into a polycrystalline silicon layer by means of polycrystalization, and to simultaneously out-diffuse a portion of the dopants in the buffer layer into the polycrystalline silicon layer for adjusting a threshold voltage. The polycrystalline silicon layer is patterned to form an active layer. A gate insulating layer is formed on the active layer. A gate electrode is formed on the gate insulating layer. A source doped region and a drain doped region are formed in the active layer. 
     Another embodiment of the present invention provides a polycrystalline silicon thin film transistor device, disposed on a substrate. The polycrystalline silicon thin film transistor device includes a buffer layer, a polycrystalline silicon layer, a gate insulating layer, and a gate electrode. The buffer layer is disposed on the substrate, and the buffer layer has dopants. The polycrystalline silicon layer is disposed on the buffer layer, where the polycrystalline silicon layer includes a channel, a source doped region, and a drain doped region, and the source doped region and the drain doped region are respectively located on two sides of the channel. The gate insulating layer is disposed on the polycrystalline silicon layer. The gate electrode is disposed on the gate insulating layer and corresponds to the channel of the polycrystalline silicon layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart of a method of fabricating a polycrystalline silicon thin film transistor device according to the present invention; 
         FIG. 2  to  FIG. 8  are schematic diagrams of a method of fabricating a polycrystalline silicon thin film transistor device according to a first embodiment of the present invention; 
         FIG. 9  is a schematic diagram of fabricating a display panel according to an embodiment of the present invention; 
         FIG. 10  and  FIG. 11  are schematic diagrams of a method of fabricating a polycrystalline silicon thin film transistor device according to a second embodiment of the present invention; 
         FIG. 12  is a drain current-gate voltage relationship diagram and an electron mobility-gate voltage relationship diagram of a polycrystalline silicon thin film transistor device according to a comparative embodiment; 
         FIG. 13  is a drain current-gate voltage relationship diagram and an electron mobility-gate voltage relationship diagram of a polycrystalline silicon thin film transistor device in a first sample according to the present invention; and 
         FIG. 14  is a drain current-gate voltage relationship diagram and an electron mobility-gate voltage relationship diagram of a polycrystalline silicon thin film transistor device in a second sample according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     To enable a person skilled in the art of the present invention to further understand the present invention, the constituent content and the efficacy to be achieved of the present invention are described below in detail by using preferred embodiments of the present invention with reference to the accompanying drawings. 
     Refer to  FIG. 1 .  FIG. 1  is a flowchart of a method of fabricating a polycrystalline silicon thin film transistor device according to the present invention. As shown in  FIG. 1 , the method of fabricating a polycrystalline silicon thin film transistor device of the present invention includes the following steps. 
     Step  10 : Provide a substrate. 
     Step  12 : Form a buffer layer having dopants on the substrate. 
     Step  14 : Form an amorphous silicon layer on the buffer layer having the dopants. 
     Step  16 : Perform a thermal process, to convert the amorphous silicon layer into a polycrystalline silicon layer by means of polycrystalization, and simultaneously to out-diffuse a portion of the dopants in the buffer layer into the polycrystalline silicon layer for adjusting a threshold voltage. 
     Step  18 : Pattern the polycrystalline silicon layer to form an active layer. 
     Step  20 : Form a gate insulating layer on the active layer. 
     Step  22 : Form a gate electrode on the gate insulating layer. 
     Step  24 : Form a source doped region and a drain doped region in the active layer. 
     Continue to refer to  FIG. 2  to  FIG. 8 , and refer to  FIG. 1  together.  FIG. 2  to  FIG. 8  are schematic diagrams of a method of fabricating a polycrystalline silicon thin film transistor device according to a first embodiment of the present invention. As shown in  FIG. 2 , a substrate  30  is provided first. The substrate  30  may include a transparent substrate such as a glass substrate, a plastic substrate, a quartz substrate, a sapphire substrate or another suitable substrate, and a rigid substrate or a flexible substrate may be selected for the substrate  30 . Next, a buffer layer  32  having dopants  34  is formed on the substrate  30 . In this embodiment, the buffer layer  32  is a single-layer-structure buffer layer, and the material of the buffer layer  32  may include an inorganic insulating material such as silicon oxide, but is not limited thereto. The material of the buffer layer  32  may also include silicon nitride, silicon oxynitride or another suitable inorganic or organic insulating material. In another embodiment, the buffer layer  32  may be a multi-layered-stack-structure buffer layer. The buffer layer  32  may be formed by using a deposition process such as a chemical vapor deposition (CVD) process, a physical vapor deposition (PVD) or another suitable deposition process. In addition, the method in this embodiment may include introducing a gas that contains the dopants  34  during a deposition process for forming the buffer layer  32 , so as to form the dopants  34  in the buffer layer  32 . That is, the buffer layer  32  and the dopants  34  may be formed together in a same reaction chamber. The dopants  34  in the buffer layer  32  may include P-type dopants such as boron ions or N-type dopants such as phosphor ions. In this case, the gas that is introduced in the reaction chamber may be gas that contains boron ions or phosphor ions, but is not limited thereto. In addition, in a variant embodiment, the buffer layer  32  and the dopants  34  may be fabricated separately. That is, a deposition process may be performed first to deposit the buffer layer  32  on the substrate  30 , and an ion implantation process is then performed to form the dopants  34  in the buffer layer  32 . In this embodiment, a doping concentration of the dopants  34  in the buffer layer  32  may be, for example, between 8×10 14  atom/cm 3  to 4×10 15  atom/cm 3 , but is not limited thereto. The doping concentration of the dopants  34  may be changed according to a threshold voltage of a polycrystalline silicon thin film transistor device and a different thermal process. In addition, the dopants  34  may be evenly distributed within the buffer layer  32 , or a large portion of the dopants  34  are distributed within an area near the surface of the buffer layer  32 , or the dopants  34  are distributed on a gradient. 
     As shown in  FIG. 3 , an amorphous silicon layer  36  is then formed on the buffer layer  32  having the dopants  34 . The amorphous silicon layer  36  may be formed by using a deposition process such as a CVD process, a plasma enhanced chemical vapor deposition (PECVD) process or another suitable deposition process. 
     As shown in  FIG. 4 , a thermal process is performed subsequently, to convert the amorphous silicon layer  36  into a polycrystalline silicon layer  38  by means of polycrystalization, and simultaneously to out-diffuse a portion of the dopants  34  in the buffer layer  32  to the polycrystalline silicon layer  38  for adjusting the threshold voltage. The method in this embodiment may be a method of fabricating an LTPS thin film transistor device, where thermal process may include excimer laser annealing (ELA) process, in which a laser beam  37  is used to perform polycrystalization sequentially on the amorphous silicon layer  36  at different locations in a scan manner, so as to rearrange amorphous silicon into polycrystalline silicon. In a variant embodiment, thermal process may also include a solid phase crystallization (SPC) process or another suitable thermal process. In the ELA process, the buffer layer  32  is also radiated by the laser beam  37  and is heated, so that a portion of the dopants  34  is diffused into the polycrystalline silicon layer  38  to form doping of the polycrystalline silicon layer  38 , thereby achieving an effect of adjusting a threshold voltage. When the dopants  34  in the buffer layer  32  is diffused to the polycrystalline silicon layer  38 , the buffer layer  32  may have a rough surface, and the buffer layer  32  may be formed into a porous buffer layer. In this case, after thermal process, the doping concentration of the dopants  34  in the buffer layer  32  is less than the doping concentration of the dopants  34  in the buffer layer  32  before thermal process. In this case, after radiation by the laser beam  37 , thermal energy accumulates in the rough surface and pores of the buffer layer  32  to increase a grain boundary size of the polycrystalline silicon in the polycrystalline silicon layer  38  (that is, a grain size of the polycrystalline silicon increases) to optimize seed growth. That is, thermal process in this embodiment may simultaneously perform polycrystalization on the amorphous silicon layer  36 , and together perform doping on the polycrystalline silicon layer  38  for adjusting a threshold voltage. In addition, compared with a manner of directly performing doping on the polycrystalline silicon layer  38  by using an ion implantation process and a manner of performing polycrystalization on the amorphous silicon layer  36  by using a thermal process, in the approach of performing doping on the polycrystalline silicon layer  38  by using the dopants  34  in the buffer layer  32  in this embodiment, only one thermal process is needed to achieve an effect of adjusting a threshold voltage and performing polycrystalization, and the pores in the buffer layer  32  may further exert an effect of accumulating thermal energy to increase a grain boundary size of polycrystalline silicon in the polycrystalline silicon layer  38  to optimize seed growth, thereby helping improve electron mobility. 
     In addition, to further improve a crystallization capability of an ELA process, in this embodiment, the thickness and a refractive index of the buffer layer  32  and a wavelength of and excimer laser may meet the relationship in the following Formula (1).
 
2nd=mλ  (1)
 
     where n is the refractive index of the buffer layer  32 , d is the thickness of the buffer layer  32 , λ is the wavelength of the excimer laser, and m is positive integer. 
     Under the condition that the following Formula (1) is met, the excimer laser may produce resonance in the buffer layer  32  to amplify energy, thereby improving a crystallization status of polycrystalline silicon and increase the grain boundary size, so that electron mobility and device characteristics are further improved. 
     Before thermal process is performed, in the method in this embodiment, a dehydrogenation process may be selectively performed first on the amorphous silicon layer  36 , and during the dehydrogenation process, a portion of the dopants  34  in the buffer layer  32  may be simultaneously diffused into the amorphous silicon layer  36 . 
     As shown in  FIG. 5 , next, the polycrystalline silicon layer  38  is patterned, to remove a portion of the polycrystalline silicon layer  38  to form an active layer  381 . The patterning process may be a PEP, but is not limited thereto. 
     As shown in  FIG. 6 , a channel  38 C, a source doped region  38 S, and a drain doped region  38 D are formed in the active layer  381 , where the source doped region  38 S and the drain doped region  38 D are respectively located on two sides of the channel  38 C. In addition, a soft doped region  38 L may be selectively formed between the channel  38 C and the source doped region  38 S and between the channel  38 C and the drain doped region  38 D between respectively, where the doping concentrations in the source doped region  38 S and the drain doped region  38 D are greater than the doping concentration in the soft doped region  38 L, and the doping concentration in the channel  38 C is less than that in the source doped region  38 S, the drain doped region  38 D, and the soft doped region  38 L. 
     In this embodiment, the source doped region  38 S, the drain doped region  38 D, and the soft doped region  38 L may be formed by using an ion implantation process in combination with a mask (not shown) such as a resist pattern. For example, the resist pattern may be first formed on the surface of the active layer  381 , where the resist pattern covers the channel  38 C and a predetermined region to form the soft doped region  38 L and exposes predetermined regions to form the source doped region  38 S and the drain doped region  38 D. Next, the resist pattern is used as a mask to perform the ion implantation process on the exposed active layer  381  to form the source doped region  38 S and the drain doped region  38 D. Subsequently, a portion of the resist pattern is removed by using, for example, an ashing process, to further expose the predetermined region to form the soft doped region  38 L, and the reduced resist pattern is then used as a mask to perform the ion implantation process again to form the soft doped region  38 L. Eventually, the remaining resist pattern is removed. The manner of fabricating the source doped region  38 S, the drain doped region  38 D, and the soft doped region  38 L of the present invention is not limited to the foregoing embodiment. In a variant embodiment, the source doped region  38 S and the drain doped region  38 D may be formed by using an ion implantation process in combination with a mask, and the soft doped region  38 L may be formed by using another ion implantation process in combination with another mask. Alternatively, the source doped region  38 S, the drain doped region  38 D, and the soft doped region  38 L may be fabricated by using a subsequently formed gate electrode as a mask. Subsequently, the method in this embodiment may include performing a hydrogenation process on the polycrystalline silicon layer  38 , to repair defects of the polycrystalline silicon layer  38 . In addition, the method in this embodiment may also include performing an activation process such as a thermal annealing process or a laser annealing process on the source doped region  38 S and the drain doped region  38 D, to reduce resistance values of the source doped region  38 S and the drain doped region  38 D. 
     As shown in  FIG. 7 , next, a gate insulating layer  40  is formed on the active layer  381 . The material of the gate insulating layer  40  may be an inorganic material such as silicon oxide, silicon nitride or silicon oxynitride, but is not limited thereto. Subsequently, a gate electrode  42 G is formed on the gate insulating layer  40 , where the gate electrode  42 G and the channel  38 C may overlap in a perpendicular projection direction. In this embodiment, the material of the gate electrode  42 G may include a metal or an alloy, for example, metal such as gold, silver, copper, aluminum, titanium, and molybdenum or an alloy thereof, or another suitable conductive material. 
     As shown in  FIG. 8 , next, an interlevel dielectric layer  44  is formed on the gate electrode  42 G, and the interlevel dielectric layer  44  and the gate insulating layer  40  are patterned to expose the source doped region  38 S and the drain doped region  38 D. The patterning process may be a PEP, but is not limited thereto. In this embodiment, the interlevel dielectric layer  44  may be a multi-layered-stack-structure interlevel dielectric layer, and may include, for example: a first interlevel dielectric layer  441  is located on the gate electrode  42 G, and a second interlevel dielectric layer  442  is stacked on the first interlevel dielectric layer  441 , where the first interlevel dielectric layer  441  may be an inorganic dielectric layer such as a silicon oxide layer, a silicon nitride layer or a silicon oxynitride layer, but is not limited thereto; and the second interlevel dielectric layer  442  may be an organic dielectric layer, but is not limited thereto. In a variant embodiment, the interlevel dielectric layer  44  may also be single-layer structure layer. Subsequently, a source electrode  46 S and a drain electrode  46 D are formed on the interlevel dielectric layer  44 , where the source electrode  46 S and the source doped region  38 S are electrically connected, and the drain electrode  46 D and the drain doped region  38 D are electrically connected. The material of the source electrode  46 S and the drain electrode  46 D may include a metal or an alloy, for example, a metal such as gold, silver, copper, aluminum, titanium, and molybdenum or an alloy thereof, or another suitable conductive material. At this point, the polycrystalline silicon thin film transistor device  1  in this embodiment may be fabricated. The interlevel dielectric layer  44  in this embodiment may be a multi-layered-stack-structure design, and may add an effect of resonance in a laser annealing process, so that a grain boundary size can be increased to optimize seed growth, thereby improving electron mobility and device characteristics. 
     The polycrystalline silicon thin film transistor device  1  in this embodiment may be applied to a display panel such as a liquid crystal display panel, an electroluminescent display panel or a display panel of any other type, a touch panel or any electronic apparatus or photoelectric apparatus, and is used as a switch device or a driving device. Refer to  FIG. 9  together with  FIG. 8 .  FIG. 9  is a schematic diagram of fabricating a display panel according to an embodiment of the present invention. As shown in  FIG. 9 , next, a passivation layer  48  and a pixel electrode PE are sequentially formed on the polycrystalline silicon thin film transistor device  1 . The passivation layer  48  may partially expose the drain electrode  46 D, and the pixel electrode PE and the drain electrode  46 D are electrically connected. Next, another substrate  50  is provided, and a common electrode CE is formed on the substrate  50 . Next, the substrate  30  and the substrate  50  are combined, and a display medium layer  52  is formed between the substrate  30  and the substrate  50 , so as to form a display panel  100  in this embodiment. The display panel  100  in this embodiment is, for example, a liquid crystal display panel, and therefore, the display medium layer  52  may include a liquid crystal layer, but is not limited thereto. In a variant embodiment, the display panel may also be electroluminescent display panel such as an organic light-emitting diode display panel, and the display medium layer  52  may be an electroluminescent layer or another suitable non-self-luminous display medium layer or self-emission optical display medium layer. The display panel  100  in this embodiment is, for example, vertical-field-driven liquid crystal display panel, for example, a twisted nematic (TN) liquid crystal display panel or a vertical alignment (VA) liquid crystal display panel, but is not limited thereto. For example, the display panel of the present invention may also be a horizontal-field-driven liquid crystal display panel such as an in-plane switching (IPS) liquid crystal display panel, a fringe field switching (FFS) liquid crystal display panel or a display panel of another type. In a variant embodiment, the common electrode CE may also be formed on the substrate  30  and is located on a same plane as the pixel electrode PE, for an application as the IPS liquid crystal display panel. In another variant embodiment, the common electrode CE may also be formed on the substrate  30  but is located on a different plane from the pixel electrode PE, for an application as the FFS liquid crystal display panel. 
     As can be seen from the above, in the method of fabricating a polycrystalline silicon thin film transistor device of the present invention, an amorphous silicon layer can be converted into a polycrystalline silicon layer by using a single thermal process, and simultaneously dopants in a buffer layer are diffused into the polycrystalline silicon layer for adjusting a threshold voltage, so that process steps can be simplified and a fabrication cost can be reduced. In addition, when the dopants in the buffer layer are diffused into the polycrystalline silicon layer, the buffer layer is formed into a porous buffer layer, so that in thermal process, thermal energy can accumulate in pores to increase a grain boundary size of polycrystalline silicon in the polycrystalline silicon layer to optimize seed growth, thereby improving electron mobility and device characteristics. 
     The polycrystalline silicon thin film transistor device and the method of fabricating the same of the present invention are not limited to the foregoing embodiments. The polycrystalline silicon thin film transistor device and the method of fabricating the same in other preferred embodiments of the present invention are sequentially introduced below. To facilitate comparison of differences between the embodiments and to simplify description, in the following embodiments, the same symbols are used to represent the same devices, and the differences between the embodiments are mainly described, and repetitive parts are no longer elaborated. 
     Continue to refer to  FIG. 10  and  FIG. 11 , and refer to  FIG. 1  together.  FIG. 10  and  FIG. 11  are schematic diagrams of a method of fabricating a polycrystalline silicon thin film transistor device according to a second embodiment of the present invention. As shown in  FIG. 10 , in this embodiment, the buffer layer  32  is a multi-layered-stack-structure buffer layer, and includes at least one bottom buffer layer  32 B located on the substrate  30  and a top buffer layer  32 T located on the bottom buffer layer  32 B. For example, the bottom buffer layer  32 B of the buffer layer  32  in this embodiment may include a first bottom buffer layer  32 B 1 , a second bottom buffer layer  32 B 2 , and a third bottom buffer layer  32 B 3 , which are sequentially formed on the substrate  30 , and the top buffer layer  32 T may be formed on the third bottom buffer layer  32 B 3 . In this embodiment, the first bottom buffer layer  32 B 1 , the second bottom buffer layer  32 B 2 , the third bottom buffer layer  32 B 3 , and the top buffer layer  32 T may be sequentially stacked film layers of different materials. For example, the material of the first bottom buffer layer  32 B 1  and the third bottom buffer layer  32 B 3  may be silicon nitride, and the material of the second bottom buffer layer  32 B 2  and the top buffer layer  32 T may be silicon oxide, thereby increasing and the adhesiveness of the substrate  30  and reduce stress. In addition, the thickness of the first bottom buffer layer  32 B 1 , the second bottom buffer layer  32 B 2 , the third bottom buffer layer  32 B 3 , and the top buffer layer  32 T may be adjusted according to the adhesiveness, stress, and another consideration. In an implementation aspect, the thickness of the second bottom buffer layer  32 B 2  and the top buffer layer  32 T may be greater than that of the first bottom buffer layer  32 B 1  and the third bottom buffer layer  32 B 3 . For example, the thickness of the second bottom buffer layer  32 B 2  or the top buffer layer  32 T may be a multiple of the thickness of the first bottom buffer layer  32 B 1  or the third bottom buffer layer  32 B 3 ; however, the present invention is not limited thereto. 
     In this embodiment, the dopants  34  may be formed on only the top buffer layer  32 T of the multi-layered-stack-structure buffer layer, where a manner of forming the dopants  34  in the top buffer layer  32 T may be as described in the foregoing embodiments. For example, a gas that contains the dopants  34  is simultaneously introduced in a deposition process of forming the top buffer layer  32 T, so as to form the dopants  34  in the top buffer layer  32 T, whereas the dopants  34  are not formed in the bottom buffer layer  32 B. Alternatively, the top buffer layer  32 T is formed, and an ion implantation process is then performed to form the dopants  34  in the top buffer layer  32 T, whereas the dopants  34  are not formed in the bottom buffer layer  32 B. In a variant embodiment, in addition to being formed in the top buffer layer  32 T, the dopants  34  may also be formed in the bottom buffer layer  32 B. 
     As shown in  FIG. 11 , the process in  FIG. 3  to  FIG. 8  in the first embodiment is then performed, so that a polycrystalline silicon thin film transistor device  2  in this embodiment can be fabricated. 
     The polycrystalline silicon thin film transistor device  2  in this embodiment may be applied to a display panel such as a liquid crystal display panel, an electroluminescent display panel or a display panel of any other type, a touch panel or any electronic apparatus or photoelectric apparatus, and is used as a switch device or a driving device, as shown in the foregoing embodiment, which is no longer elaborated herein. 
     Refer to  FIG. 12  to  FIG. 14 .  FIG. 12  is a drain current-gate voltage relationship diagram and an electron mobility-gate voltage relationship diagram of a polycrystalline silicon thin film transistor device according to a comparative embodiment,  FIG. 13  is a drain current-gate voltage relationship diagram and an electron mobility-gate voltage relationship diagram of the polycrystalline silicon thin film transistor device in a first sample of the present invention, and  FIG. 14  is a drain current-gate voltage relationship diagram and an electron mobility-gate voltage relationship diagram of the polycrystalline silicon thin film transistor device in a second sample of the present invention. In the method in the comparative embodiment ( FIG. 12 ), in the polycrystalline silicon thin film transistor device, a threshold voltage is directly adjusted for a polycrystalline silicon layer by using an ion implantation process, there is no dopant in a buffer layer, and tests are performed respectively in conditions in which a drain voltage Vd is 0.1 V, 5.1 V, and 10.1 V. In the method of the present invention ( FIG. 13  and  FIG. 14 ), the ion implantation process is first performed on the buffer layer, and a thermal process is then performed to perform polycrystalization and simultaneously to diffuse dopants into the polycrystalline silicon layer of polycrystalline silicon thin film transistor device, and tests are performed respectively in conditions in which a drain voltage Vd is 0.1 V, 5.1 V, and 10.1 V. In  FIG. 13 , the energy in the ion implantation process in the first sample is 60 key, the dopants are boron ions, and the doping concentration is 5*10 13  atoms/cm 3 . In  FIG. 14 , the energy in the ion implantation process in the second sample is 60 kev, the dopants are phosphor ions, and the doping concentration is 2*10 14  atoms/cm 3 . As shown in  FIG. 12 , the threshold voltage (Vth) of the polycrystalline silicon thin film transistor device in the comparative embodiment is approximately −2.71 V. By comparison, as shown in  FIG. 13 , the threshold voltage (Vth) of the polycrystalline silicon thin film transistor device in the first sample is approximately −1.17 V. As shown in  FIG. 14 , the threshold voltage (Vth) of the polycrystalline silicon thin film transistor device in the second sample is approximately −2.02 V. As can be seen from the above experimental data, the approach in which the dopants are first formed in the buffer layer and a thermal process is then performed to perform polycrystalization and to simultaneously diffuse the dopants into the polycrystalline silicon layer of the polycrystalline silicon thin film transistor device actually can effectively adjust the threshold voltage into a predetermined range. 
     In addition, the experiment conducted for a grain boundary size of the polycrystalline silicon layer in the present invention also shows that the approach in which the dopants are first formed in the buffer layer and a thermal process is then performed to perform polycrystalization and to simultaneously diffuse the dopants into the polycrystalline silicon layer of the polycrystalline silicon thin film transistor device can effectively increase the grain boundary size (that is, increase the grain size). Refer to Table 1, Table 1 shows grain boundary sizes in a comparative embodiment and an embodiment of the present invention, where in the comparative embodiment, a grain boundary size is obtained through actual measurement in a condition in which there is no dopant in the buffer layer, whereas in this embodiment, a grain boundary size is obtained through actual measurement in a condition in which argon (Ar) ions are doped in the buffer layer. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Comparative Embodiment 
                 This Embodiment 
               
               
                   
               
             
            
               
                 Grain boundary size 
                 0.3098 micrometer 
                 0.3432 micrometer 
               
               
                   
               
            
           
         
       
     
     As can be seen from the result in Table 1, in a condition in which there is no dopant in the buffer layer, the grain boundary size in the comparative embodiment is approximately 0.3098 micrometer, whereas in the condition in which dopants are formed in the buffer layer, the grain boundary size in this embodiment is approximately 0.3432 micrometer; therefore, the grain boundary size is increased by approximately 10.81%. 
     In conclusion, in the method of fabricating a polycrystalline silicon thin film transistor device of the present invention, an amorphous silicon layer can be converted into a polycrystalline silicon layer by using a single thermal process, and dopants in a buffer layer are diffused into the polycrystalline silicon layer for adjusting a threshold voltage to a predetermined range, so that process steps can be simplified and a fabrication cost can be reduced. In addition, when the dopants in the buffer layer are diffused into the polycrystalline silicon layer, the buffer layer is formed into a porous buffer layer, so that in thermal process, thermal energy can accumulate in pores to increase a grain boundary size of polycrystalline silicon in the polycrystalline silicon layer to optimize seed growth, thereby improving electron mobility and device characteristics. 
     The foregoing provides only preferred embodiments of the present invention, and all equivalent variations and modifications made in accordance with the scope of the present invention shall fall within the coverage of the present invention.