Abstract:
A method of manufacturing a semiconductor film capable of inhibiting the quality of a semiconductor film from destabilization is obtained. This method of manufacturing a semiconductor film includes steps of introducing source gas for a semiconductor, controlling the pressure of an atmosphere formed by the source gas to a prescribed level, heating a catalytic wire to at least a prescribed temperature after controlling the pressure of the atmosphere to the prescribed level and forming a semiconductor film by decomposing the source gas with the heated catalytic wire.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing a semiconductor film and a method of manufacturing a photovoltaic element, and more particularly, it relates to a method of manufacturing a semiconductor film and a method of manufacturing a photovoltaic element each comprising a step of forming a semiconductor film by decomposing source gas with a catalytic wire. 
     2. Description of the Background Art 
     A method of manufacturing a semiconductor film comprising a step of forming a semiconductor film by decomposing source gas with a catalytic wire is known in general, as disclosed in Japanese Patent No. 3453214, for example. 
     According to the aforementioned Japanese Patent No. 3453214, a gas mixture of gas (source gas) of a silicon compound such as silane (SiH 4 ) and gas of another material such as hydrogen (H 2 ) is introduced into a catalytic body (catalytic wire) supplied with power to be heated to at least the thermal decomposition temperature of the source gas, thereby decomposing the silicon compound and forming a silicon film (semiconductor film) on the surface of a substrate. 
     However, the aforementioned Japanese Patent No. 3453214 discloses neither the timing for starting supplying power to (starting heating) the catalytic body (catalytic wire) nor the timing for introducing the source gas in formation of the silicon film (semiconductor film). In general, heating of the catalytic body is started and the source gas is introduced at the same time. In this case, a constant time is required for stabilizing the pressure of an atmosphere formed by the source gas after the introduction of the source gas, and hence the semiconductor film is formed in a state where the pressure of the atmosphere formed by the source gas is not yet stabilized in the initial state. In this case, the quality of the semiconductor film formed in the state where the pressure of the atmosphere formed by the source gas is not yet stabilized is disadvantageously destabilized. 
     SUMMARY OF THE INVENTION 
     The present invention has been proposed in order to solve the aforementioned problem, and an object of the present invention is to provide a method of manufacturing a semiconductor film and a method of manufacturing a photovoltaic element each capable of inhibiting the quality of a semiconductor film from destabilization. 
     A method of manufacturing a semiconductor film according to a first aspect of the present invention comprises steps of introducing source gas for a semiconductor, controlling the pressure of an atmosphere formed by the source gas to a prescribed level, heating a catalytic wire to at least a prescribed temperature after controlling the pressure of the atmosphere to the prescribed level and forming a semiconductor film by decomposing the source gas with the heated catalytic wire. 
     A method of manufacturing a photovoltaic element according to a second aspect of the present invention comprises steps of introducing source gas for a semiconductor, controlling the pressure of an atmosphere formed by the source gas to a prescribed level, heating a catalytic wire to at least a prescribed temperature after controlling the pressure of the atmosphere to the prescribed level and forming a semiconductor film functioning as a photoelectric conversion layer by decomposing the source gas with the heated catalytic wire. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a catalytic wire CVD apparatus employed in the present invention; 
         FIG. 2  is a sectional view of a thin-film photovoltaic element manufactured according to the present invention; and 
         FIG. 3  is a sectional view showing a heterojunction photovoltaic element manufactured according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are now described with reference to the drawings. 
     First Embodiment 
     First, the structure of a catalytic wire CVD apparatus employed for manufacturing a semiconductor film according to a first embodiment of the present invention is described with reference to  FIG. 1 . 
     As shown in  FIG. 1 , the catalytic wire CVD apparatus comprises a reaction chamber  1 , a gas supply portion  2  for supplying source gas and pressure control gas into the reaction chamber  1 , a catalytic wire  4  connected to a DC power source  3 , an exhaust valve  5 , a set portion  6  for setting an underlayer  20  for forming a semiconductor film  10  and a heater  7  for heating the underlayer  20  set on the set portion  6 . 
     The catalytic wire  4  is made of tungsten (W). This catalytic wire  4  is heated by excitation with the DC power source  3 . The reaction chamber  1  can be evacuated with a vacuum pump (not shown), and the exhaust valve  5  opens/closes an exhaust passage. 
     A method of manufacturing a semiconductor film according to the first embodiment of the present invention is described with reference to  FIG. 1 . According to the first embodiment, a hydrogenated amorphous silicon film is formed on the underlayer  20  as the semiconductor film  10 . Table 1 shows exemplary conditions for manufacturing the amorphous silicon film. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Material for Catalytic Wire 
                 Tungsten 
               
               
                   
                 Diameter of Catalytic Wire 
                 0.5 mm 
               
               
                   
                 Temperature of Catalytic Wire 
                 1700° C. 
               
               
                   
                 Temperature of Underlayer 
                 200° C. 
               
               
                   
                 Pressure 
                 3 Pa 
               
               
                   
                 Flow Rate of SiH 4   
                 500 sccm 
               
               
                   
                 Flow Rate of H 2   
                 1000 sccm 
               
               
                   
                   
               
             
          
         
       
     
     As shown in Table 1, the catalytic wire  4  of tungsten having a diameter of about 0.5 mm is employed for forming the amorphous silicon film. The underlayer  20  is set on the set portion  6  of the catalytic wire CVD apparatus provided with this catalytic wire  4 . The underlayer  20  is formed by an amorphous silicon film, a transparent conductive oxide film or a single-crystalline silicon substrate, for example. Formation of the amorphous silicon film is started in this state. Table 2 shows manufacturing process conditions for the amorphous silicon film according to the first embodiment. 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Introduction 
                   
                 Heating 
                   
                   
                   
               
               
                   
                   
                 of 
                   
                 of 
               
               
                   
                   
                 Source 
                 Pressure 
                 Catalytic 
                 Film 
               
               
                   
                 Step 
                 Gas 
                 Control 
                 Wire 
                 Formation 
                 Evacuation 
                 End 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Embodiment 
                 Catalytic 
                   
                   
                 ON 
                 ON 
                 ON 
                 OFF 
               
               
                   
                 Wire 
               
               
                   
                 Source 
                 ON 
                 ON 
                 ON 
                 ON 
                 OFF 
               
               
                   
                 Gas 
               
               
                   
               
             
          
         
       
     
     In order to form the amorphous silicon film by the method of manufacturing a semiconductor film according to the first embodiment, the underlayer  20  is heated to about 200° C. with the heater  7  as shown in Table 1, and the source gas of SiH 4  for forming the amorphous silicon film (semiconductor film  10 ) is introduced into the reaction chamber  1  from the gas supply portion  2 , as shown in Table 2. The source gas of SiH 4  is diluted with non-source gas of H 2 . As shown in Table 1, SiH 4  and H 2  are introduced at flow rates of about 500 sccm and about 1000 sccm respectively. Thereafter the pressure of the source gas is controlled according to the present invention. In other words, the pressure of the source gas is controlled to about 3 Pa (with partial pressures of about 1 Pa and about 2 Pa of SiH 4  and H 2  respectively), as shown in Table 1. Thereafter the catalytic wire  4  is heated to about 1700° C. by excitation with the DC power source  3 , as shown in Table 2. 
     The catalytic wire  4  heated to about 1700° C. and the source gas of SiH 4  present in the reaction chamber  1  come into contact with each other. Thus, the catalytic wire  4  heated to about 1700° C. decomposes SiH 4 , and the decomposed species is deposited on the underlayer  20 , for forming the hydrogenated amorphous silicon film (semiconductor film  10 ) on the underlayer  20 . 
     After the formation of the hydrogenated amorphous silicon film, the exhaust valve  5  is opened for evacuating the reaction chamber  1  with the vacuum pump (not shown), as shown in Table 2. After the source gas (SiH 4 ) is substantially exhausted from the reaction chamber  1 , the DC power source  3  stops exciting the catalytic wire  4 . Thus, the temperature of the catalytic wire  4  is reduced in the state where the source gas is substantially exhausted from the reaction chamber  1 . The amorphous silicon film (semiconductor film  10 ) according to the first embodiment is formed in this manner. 
     According to the first embodiment, as hereinabove described, formation of the semiconductor film  10  can be started in the state where the pressure of the atmosphere is stable by controlling the pressure of the atmosphere to about 3 Pa, thereafter heating the catalytic wire  4  to about 1700° C. and decomposing the source gas of SiH 4  with the heated catalytic wire  4  thereby forming the semiconductor film  10 , whereby formation of the semiconductor film  10  in a state where the pressure of the atmosphere is instable can be suppressed. Thus, the quality of the semiconductor film  10  can be inhibited from destabilization. 
     The source gas of SiH 4  is diluted with the non-source gas of H 2 , whereby the partial pressure of the source gas can be reduced to about 1 Pa when controlling the pressure of the atmosphere to about 3 Pa (total pressure of the source gas and the non-source gas). Thus, the pressure in the reaction chamber  1  can be controlled to about 3 Pa with a smaller quantity of the source gas as compared with a case of not diluting the source gas with the non-source gas. If heating of the catalytic wire  4  is started in a state introducing the source gas of SiH 4  into the reaction chamber  1 , a constant time is required for heating the catalytic wire  4  to about 1700° C., and hence the catalytic wire  4  at a temperature of less than about 1700° C. and the source gas of SiH 4  come into contact with each other immediately after heating of the catalytic wire  4  is started. At this time, the source gas of SiH 4  easily remains on the insufficiently heated catalytic wire  4  (at the temperature of less than about 1700° C.), and hence a compound (tungsten silicide) of the catalytic wire  4  of tungsten (W) and the source gas of SiH 4  may be formed on the surface of the catalytic wire  4 . According to the first embodiment, however, the pressure in the reaction chamber  1  is controlled to about 3 Pa with a relatively small quantity of the source gas of SiH 4 , whereby the surface of the catalytic wire  4  can be prevented from formation of a silicide due to the small quantity of SiH 4 . Thus, the resistivity of the catalytic wire  4  can be inhibited from variation resulting from formation of this compound, whereby difficulty in temperature control of the catalytic wire  4  can be suppressed. 
     According to the first embodiment, as hereinabove described, the source gas is exhausted from the reaction chamber  1  after the formation of the semiconductor film  10  and heating of the catalytic wire  4  heated to about 1700° C. is stopped after the source gas (SiH 4 ) is substantially exhausted from the reaction chamber  1 , whereby the catalytic wire  4  and SiH 4  can be prevented from coming into contact with each other in a state where the temperature of the catalytic wire  4  is lower than about 1700° C. At the end of the manufacturing process for the amorphous silicon film (semiconductor film  10 ), therefore, formation of a compound (tungsten silicide) of the catalytic wire  4  made of tungsten (W) and the source gas of SiH 4  can be suppressed. Therefore, difficulty in temperature control of the catalytic wire  4  can be suppressed similarly to the above. 
     Second Embodiment 
     According to a second embodiment of the present invention, a thin-film photovoltaic element  100  is manufactured by the method of manufacturing a semiconductor film according to the aforementioned first embodiment. First, the structure of the thin-film photovoltaic element  100  manufactured by the method of manufacturing a semiconductor film according to the present invention is described with reference to  FIG. 2 . 
     As shown in  FIG. 2 , the photovoltaic element  100  comprises a substrate  101 , a surface electrode layer  102 , a photoelectric conversion layer  103  and a rear electrode layer  104 . 
     The substrate  101  has an insulating surface, and is made of glass having transparency. The surface electrode layer  102  is formed on the upper surface of the substrate  101 . This surface electrode layer  102  is formed by a TCO (transparent conductive oxide) film of tin oxide (SnO 2 ) or the like having conductivity and transparency. 
     The photoelectric conversion layer  103  made of a p-i-n-type amorphous silicon-based semiconductor is formed on the upper surface of the surface electrode layer  102 . This photoelectric conversion layer  103  of the p-i-n-type amorphous silicon-based semiconductor is constituted of a p-type hydrogenated amorphous silicon carbide (a-SiC:H) layer  103   a  (hereinafter referred to as a p layer  103   a ), an i-type hydrogenated amorphous silicon (a-Si:H) layer  103   b  (hereinafter referred to as an i layer  103   b ) and an n-type hydrogenated amorphous silicon (a-Si:H) layer  103   c  (hereinafter referred to as an n layer  103   c ). 
     The rear electrode layer  104  is formed on the upper surface of the photoelectric conversion layer  103 . The rear electrode layer  104  is formed by holding the front and back surfaces of a silver (Ag) layer with a pair of ZnO layers. 
     A manufacturing process for the photovoltaic element  100  shown in  FIG. 2  is now described. In the manufacturing process for the photovoltaic element  100 , the surface electrode layer  102  of tin oxide is first formed on the upper surface of the substrate  101  having the insulating surface by thermal CVD (chemical vapor deposition). 
     Then, the p layer (p-type hydrogenated amorphous silicon carbide layer)  103   a , the i layer (i-type hydrogenated amorphous silicon layer)  103   b  and the n layer (n-type hydrogenated amorphous silicon layer)  103   c  are successively formed on the upper surface of the surface electrode layer  102  by catalytic wire CVD, thereby forming the photoelectric conversion layer  103  of the amorphous silicon-based semiconductor. At this time, source gas diluted with hydrogen (H 2 ) is introduced, the pressure of the atmosphere formed by the source gas is controlled and thereafter a catalytic wire  4  (see  FIG. 1 ) is heated similarly to the aforementioned first embodiment, for forming the p layer (p-type hydrogenated amorphous silicon carbide layer)  103   a  on the surface electrode layer  102  consisting of the transparent conductive oxide film, forming the i layer  103   b  on the p layer  103   a  and forming the n layer  103   c  on the i layer  103   b  respectively. 
     Thereafter the rear electrode layer  104  consisting of the metallic material layers (the ZnO layer (upper layer), the Ag layer (intermediate layer) and the ZnO layer (lower layer)) mainly composed of silver is formed on the upper surface of the photoelectric conversion layer  103  (n layer  103   c ) by sputtering. The thin-film photovoltaic element  100  is manufactured in this manner. 
     According to the second embodiment, as hereinabove described, the thin-film photovoltaic element  100  is manufactured by forming the photoelectric conversion layer  103  by the method of manufacturing a semiconductor film according to the aforementioned first embodiment, whereby the quality of the photoelectric conversion layer  103  can be inhibited from destabilization. Thus, the thin-film photovoltaic element  100  can be manufactured with stable performance. 
     Third Embodiment 
     According to a third embodiment of the present invention, a heterojunction photovoltaic element  200  is manufactured by the method of manufacturing a semiconductor film according to the aforementioned first embodiment. First, the structure of the heterojunction photovoltaic element  200  manufactured by the method of manufacturing a semiconductor film according to the present invention is described with reference to  FIG. 3 . 
     In the photovoltaic element  200  according to the third embodiment, an amorphous silicon (a-Si) layer  202  functioning as a photoelectric conversion layer and a surface electrode layer  203  are successively formed on the upper surface of an n-type single-crystalline silicon (c-Si) substrate  201 , as shown in  FIG. 3 . The surface electrode layer  203  is formed by a transparent conductive oxide film of ITO (indium tin oxide). The amorphous silicon layer  202  is constituted of a substantially intrinsic i-type amorphous silicon layer  202   a  formed on the upper surface of the n-type single-crystalline silicon substrate  201  and a p-type amorphous silicon layer  202   b  doped with boron (B) formed on the i-type amorphous silicon layer  202   a . The i-type amorphous silicon layer  202   a  has a small thickness, in order not to substantially contribute to power generation as an optical active layer. 
     An amorphous silicon layer  204  functioning as a photoelectric conversion layer and a rear electrode layer  205  are formed on the back surface of the n-type single-crystalline silicon substrate  201  successively from the side closer to the back surface of the n-type single-crystalline silicon substrate  201 . The rear electrode layer  205  is formed by a transparent conductive oxide film of ITO. The amorphous silicon layer  204  is constituted of a substantially intrinsic i-type amorphous silicon layer  204   a  formed on the back surface of the n-type single-crystalline silicon substrate  201  and an n-type amorphous silicon layer  204   b  doped with phosphorus (P) formed on the back surface of the i-type amorphous silicon layer  204   a . The i-type amorphous silicon layer  204   a  has a small thickness, in order not to substantially contribute to power generation. The i-type amorphous silicon layer  204   a , the n-type amorphous silicon layer  204   b  and the rear electrode layer  205  constitute the so-called BSF (back surface field) structure. 
     A manufacturing process for the photovoltaic element  200  is now described with reference to  FIG. 3 . 
     First, the n-type single-crystalline silicon substrate  201  is cleaned and set in a vacuum chamber (not shown) and thereafter heated under a temperature condition of not more than 200° C., for removing moisture adhering to the surface of the n-type single-crystalline silicon substrate  201  to the utmost. Thus, oxygen contained in the moisture adhering to the surface of the n-type single-crystalline silicon substrate  201  is inhibited from binding to silicon and forming defects. 
     Then, hydrogen (H 2 ) gas is introduced while keeping the substrate temperature at 170° C., for hydrogenating the upper surface of the n-type single-crystalline silicon substrate  201 . Thus, the upper surface of the n-type single-crystalline silicon substrate  201  is cleaned, and hydrogen atoms are adsorbed around the upper surface of the n-type single-crystalline silicon substrate  201 . The adsorbed hydrogen atoms inactivate (terminate) defects on the upper surface of the n-type single-crystalline silicon substrate  201 . 
     Thereafter the respective layers are formed on the front and back surfaces of the n-type single-crystalline silicon substrate  201 . 
     More specifically, the i-type amorphous silicon layer  202   a  is formed on the upper surface of the n-type single-crystalline silicon substrate  201  by catalytic wire CVD. At this time, the i-type amorphous silicon layer  202   a  is formed by introducing source gas diluted with hydrogen (H 2 ), controlling the pressure of the atmosphere formed by the source gas and thereafter heating a catalytic wire  4  (see  FIG. 1 ), similarly to the aforementioned first embodiment. 
     Then, the p-type amorphous silicon layer  202   b  doped with boron (B) is formed on the i-type amorphous silicon layer  202   a  by catalytic wire CVD. At this time, the p-type amorphous silicon layer  202   b  is formed by introducing the source gas diluted with hydrogen (H 2 ), controlling the pressure of the atmosphere formed by the source gas and thereafter heating the catalytic wire  4  (see  FIG. 1 ), similarly to the aforementioned first embodiment. 
     Then, the surface electrode layer  203  of ITO (indium tin oxide) is formed on the upper surface of the p-type amorphous silicon layer  202   b  by sputtering. 
     Then, the i-type amorphous silicon layer  204   a  is formed on the back surface of the n-type single-crystalline silicon substrate  201  by catalytic wire CVD. At this time, the i-type amorphous silicon layer  204   a  is formed by introducing the source gas diluted with hydrogen (H 2 ), controlling the pressure of the atmosphere formed by the source gas and thereafter heating the catalytic wire  4  (see  FIG. 1 ), similarly to the aforementioned first embodiment. 
     Then, the n-type amorphous silicon layer  204   b  doped with phosphorus (P) is formed on the back surface of the i-type amorphous silicon layer  204   a  by catalytic wire CVD. At this time, the n-type amorphous silicon layer  204   b  is formed by introducing source gas diluted with hydrogen (H 2 ), controlling the pressure of the atmosphere formed by the source gas and thereafter heating the catalytic wire  4  (see  FIG. 1 ), similarly to the aforementioned first embodiment. 
     Finally, the rear electrode layer  205  of ITO is formed on the back surface of the n-type amorphous silicon layer  204   b  by sputtering. The heterojunction photovoltaic element  200  shown in  FIG. 3  is formed in this manner. 
     According to the third embodiment, as hereinabove described, the heterojunction photovoltaic element  200  is manufactured by the method of manufacturing a semiconductor film according to the aforementioned first embodiment, whereby the qualities of the amorphous silicon layers  202  and  204  can be inhibited from destabilization when the heterojunction photovoltaic element  200  is manufactured by catalytic wire CVD. Thus, the heterojunction photovoltaic element  200  can be manufactured with stable performance, similarly to the aforementioned second embodiment. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims. 
     For example, while silane (SiH 4 ) gas is employed as the source gas in the aforementioned first embodiment, the present invention is not restricted to this but another silane-based gas such as disilane (Si 2 H 6 ) or trisilane (Si 3 H 8 ) may alternatively be employed, or silicon fluoride-based gas such as SiF 2  or SiH 2 F 2  may be employed. 
     While the catalytic wire  4  is made of tungsten (W) in the aforementioned first embodiment, the present invention is not restricted to this but a catalytic wire made of another high-melting point material such as tantalum (Ta) may alternatively be employed. When the catalytic wire made of tantalum is employed, the surface of the catalytic wire can be more inhibited from formation of a silicide as compared with the case of employing the catalytic wire  4  made of tungsten. 
     While the amorphous silicon film is formed on the underlayer  20  as the semiconductor film  10  under the film forming conditions shown in Table 1 in the aforementioned first embodiment, the present invention is not restricted to this but a semiconductor film of microcrystalline silicon or polycrystalline silicon may alternatively be formed on the underlayer  20  as the semiconductor film  10  by changing the film forming conditions. 
     While hydrogen (H 2 ) gas is employed as the non-source gas diluting the source gas in the aforementioned first embodiment, the present invention is not restricted to this but rare gas such as argon (Ar) gas, fluorine (F 2 ) gas, chlorine (Cl 2 ) gas, nitrogen (N 2 ) gas, carbon dioxide (CO 2 ) gas or methane (CH 4 ) gas may alternatively be employed as the non-source gas. 
     While the pressure of the atmosphere is controlled to about 3 Pa in total with the partial pressures of about 1 Pa and about 2 Pa of the source gas and the non-source gas in the aforementioned first embodiment, the present invention is not restricted to this but the pressure of the atmosphere may not be controlled to about 3 Pa. The partial pressure of the source gas is preferably set to not more than about 1 Pa. 
     While the reaction chamber  1  is evacuated with the vacuum pump in the aforementioned first embodiment, the present invention is not restricted to this but the reaction chamber  1  may alternatively be evacuated with supply of gas (H 2  gas or Ar gas) containing no film forming species such as SiH 4 . Thus, the speed for exhausting SiH 4  from the reaction chamber  1  can be increased. After film formation, the source gas (SiH 4 ) may simply be exhausted, while the remaining gas (H 2  gas or the like) may remain in the reaction chamber  1 . When H 2  gas remains in the reaction chamber  1 , a compound (silicide) formed on the surface of the catalytic wire  4  can be removed by etching. 
     While the thin-film photovoltaic element  100  and the heterojunction photovoltaic element  200  are manufactured in the aforementioned second and third embodiments respectively, the present invention is not restricted to these but is generally applicable to a photoelectric element having a semiconductor film manufactured by catalytic wire CVD. Further, the present invention is not restricted to the photovoltaic element but is generally applicable to a semiconductor element having a semiconductor film manufactured by catalytic wire CVD.