Patent Publication Number: US-2013247620-A1

Title: Silicon refining device and silicon refining method

Description:
TECHNICAL FIELD 
     The present invention generally relates to a silicon refining device and a silicon refining method. 
     BACKGROUND ART 
     In silicon used for a solar cell, a large number of impurity elements need to be reduced to the order of ppm. Thus, metal silicon as a starting material is refined separately in processes for removing boron, phosphorous, and other metal elements. Elements such as iron, aluminum, calcium or the like in the impurity elements are removed and refined by directional solidification process due to their low segregation coefficients. 
     In order to conduct the directional solidification, solidification at a constant speed upward from a bottom part of silicon molten metal is needed. Thus, the molten metal was heated from above, while a bottom surface of a crucible was cooled. 
     If this method is used, since the crucible bottom part is cooled, solidification progresses in a stable manner at a relatively fast solidification speed from a lower part of the silicon molten metal. However, if heat removal at the crucible bottom part is too strong, an undissolved part (scull) is generated in a contact portion between the crucible bottom surface and the molten silicon during silicon dissolving; the part remains at the impurity concentration of the material silicon; and as a result, it was found that refining is insufficient. 
     PRIOR-ART REFERENCES 
     Patent Document 
     
         
         [Patent Document 1] Japanese Patent Laid-Open No. 11-199216 
       
    
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     The present invention was made in order to solve the above-described disadvantages of prior-art technologies and has an object to provide a technology of removing impurities from a silicon material at a low cost. 
     Means for Solving the Problems 
     In order to solve the above problems, the present invention is a silicon refining device comprising: a vacuum chamber; cooling means arranged in the vacuum chamber; a dissolution vessel arranged in the vacuum chamber, spaced from the cooling means; and heating means for melting silicon in the dissolution vessel. 
     The present invention is a silicon refining device further comprising: a supporting member for holding the dissolution vessel on the cooling means, wherein an opening portion is provided in the supporting member so that an outer bottom surface of the dissolution vessel faces the surface of the cooling means. 
     The present invention is a silicon refining device wherein an area ratio of a section in parallel with the inner bottom surface of the opening portion to an inner bottom surface of the dissolution vessel is at least 50%. 
     The present invention is a silicon refining device wherein a first insulating material is provided between the supporting member and the dissolution vessel. 
     The present invention is a silicon refining device wherein a second insulating material is provided on the opening portion. 
     The present invention is a silicon refining device wherein the first insulating material and the second insulating material are composed of carbon felt, respectively. 
     The present invention is a silicon refining method comprising the steps of: arranging a base material composed of metal silicon in a dissolution vessel; heating the base material arranged in the dissolution vessel in a vacuum ambience and fully melting the same; cooling a bottom surface of the dissolution vessel and solidification the silicon from a portion where an inner bottom surface of the dissolution vessel is in contact with molten silicon; crystallizing upward; and removing unsolidified silicon located above the solidified silicon from the dissolution vessel, wherein, when the bottom surface of the dissolution vessel is to be cooled, cooling an outer bottom surface of the dissolution vessel which is spaced from and facing the cooling means. 
     The present invention is a silicon refining method wherein, when the bottom surface of the dissolution vessel is to be cooled, a second insulating material in contact with both the outer bottom surface of the dissolution vessel and the surface of the cooling means is arranged between the outer bottom surface of the dissolution vessel and the cooling means. 
     Effect of the Invention 
     Since heat removal efficiency of the dissolution vessel bottom surface is suppressed and scull is not generated in a portion in contact with the inner bottom surface of the dissolution vessel, in this invention, refining without refining unevenness can be achieved. 
     By providing the cooling means facing the outer bottom surface of the dissolution vessel, heat is efficiently removed from the bottom surface, and temperature gradient of the solid-liquid interface can be increased. 
     By insulating portions other than the bottom surface in the dissolution vessel, electron beam outputs can be reduced to at most 50% as compared with water-cooling copper crucibles. 
     Since a lowering mechanism for the cooling means is unnecessary, a device structure can be simplified. 
     By removing a portion where impurities cohered in a liquid state, cutting of an ingot is no longer necessary, and the silicon refining cost can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an internal configuration diagram of a silicon refining device of the present invention. 
         FIG. 2  is a diagram for explaining a second example for the structural arrangement of a dissolution vessel and cooling means. 
         FIG. 3  is a diagram for explaining the structural arrangement of a second insulating material. 
         FIG. 4  is a diagram for explaining the structural arrangement of a third insulating material. 
         FIG. 5  is a diagram for explaining an area ratio of a sectional area in parallel with an inner bottom surface of an opening portion to an area of the inner bottom surface of the dissolution vessel. 
         FIG. 6  is a diagram for explaining the second example of the shape of a facing space. 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Reference numeral  10  in  FIG. 1  denotes a silicon refining device of the present invention. 
     This silicon refining device  10  has a vacuum chamber  11 . Cooling means  21  is arranged inside the vacuum chamber  11 ; and a dissolution vessel  31  is arranged above the cooling means  21  and spaced from the cooling means  21 . 
     Here, the dissolution vessel  31  is formed by a carbon material (graphite, for example). 
     An outer bottom surface of the dissolution vessel  31  and the upward-oriented surface of the cooling means  21  are flat and made parallel with each other. In the outer surface of the dissolution vessel  31 , nothing is arranged between a region having a predetermined size including the center of the outer bottom surface and the surface of the cooling means  21 , and a region including at least the center of the outer bottom surface is made to face the surface of the cooling means  21 . 
     The cooling means  21  is made of copper or stainless. In the cooling means  21 , a circulation path  23  for a cooling medium is arranged, and by operating a cooling device  25  provided outside the vacuum chamber  11  and by having cooled cooling medium flow into the cooling means  21 , the bottom surface of the dissolution vessel  31  facing the cooling means  21  is cooled. 
     To the vacuum chamber  11 , an evacuating device  13  is connected and the inside is evacuated and maintained in a vacuum ambience. 
     In the vacuum chamber  11 , heating means  12  for melting silicon in the dissolution vessel  31  is provided. The heating means  12  is an electron gun, but it is not limited to an electron gun as long as the silicon in the dissolution vessel  31  can be melted, and it may be an induction heating means. 
     A silicon raw material, which is a base material constituted by chunk-shaped or small-flake shaped metallurgical silicon, is arranged inside the dissolution vessel  31 ; an electron beam is irradiated to the silicon raw material so as to melt all the silicon raw material in the dissolution vessel  31 ; and the inside of the dissolution vessel  31  is filled with the molten silicon. The silicon is in contact only with the carbon material of the dissolution vessel  31 . 
     The electron beam is irradiated to the silicon while evacuating the inside of the vacuum chamber  11 ; and impurities with a vapor pressure higher than that of the silicon contained in the silicon raw material is evaporated and discharged to the outside of the vacuum chamber  11  by the evacuation process. More particularly, phosphorous contained in the silicon raw material is evaporated and removed, and molten silicon is refined to high purity. 
     If the cooling medium is made to flow inside the cooling means  21  and power of the electron beam (irradiation density) is gradually weakened without changing an irradiation width (surface) in a state where the bottom surface of the dissolution vessel  31  is cooled, silicon starts to solidify at a portion in contact with the surface of the inner bottom surface of the dissolution vessel  31 , and solidified silicon grows from the lower part to the upper part. Unsolidified silicon which is molten silicon is located above the solidified silicon. 
     Elements (such as, Fe and Al) having low-segregation coefficients are discharged from a solid phase to a liquid phase when silicon is solidified; and thus, a concentration difference of impurities is generated between the solid phase and the liquid phase (the impurity concentration in the liquid phase is higher than the impurity concentration in the solid phase). 
     Therefore, if the molten silicon in the dissolution vessel  31  is cooled on a vertically lower side of the dissolution vessel  31  and the solidification of the molten silicon is made to progress from the vertically lower side to the upper side, the impurity concentration in the molten silicon located above the solidified portion gradually rises. 
     In the dissolution vessel  31 , a tilting device  39  is provided. 
     In this embodiment, when the unsolidified silicon decreases to 20% of the total, while the irradiation of the electron beam is stopped, the dissolution vessel  31  is tilted by the tilting device  39 , and a molten portion flows out of the dissolution vessel  31  and is received and recovered by a recovering vessel  14  arranged in the vacuum chamber  11 . 
     Alternatively, it may be so configured that the entire molten silicon is solidified from the lower part to the upper part once and then, the electron beam is irradiated again to the solidified silicon, a portion corresponding to 20% of the whole from the upper part is melted again, the irradiation of the electron beam is stopped, and the dissolution vessel  31  is tilted by the tilting device  39  so that the re-molten portion flows out towards the outside, and the molten silicon that flows out is received and recovered by the recovering vessel  14 . 
     In the above-described embodiment, the unsolidified silicon is recovered when the unsolidified silicon decreases to 20% of the total, but the ratio of the unsolidified silicon when being recovered is not limited to the 20% of the total but may be determined in advance depending on the purity of the raw material. 
     Subsequently, the heating means (electron gun)  12  is re-operated so as to irradiate the electron beam to the solidified silicon, and the solidified silicon is melted. 
     After the solidified silicon in the dissolution vessel  31  is fully melted, while continuing to cool the bottom surface of the dissolution vessel  31  by the cooling means  21 , gradually weakening the power of the electron beam without changing the irradiation width (surface), solidifying the silicon from the portion in contact with the inner bottom surface of the dissolution vessel  31 , and making to grow the solidified silicon from the portion in contact with the inner bottom surface of the dissolution vessel  31  to the upper part. 
     When the unsolidified silicon located at the upper part decreases to 20% of the silicon in the dissolution vessel  31  in this embodiment, the irradiation of the electron beam is stopped and the dissolution vessel  31  is tilted by the tilting device  39 , the unsolidified silicon located above the solidified silicon is made to flow out towards the outside of the dissolution vessel  31 , and the molten silicon that flows out is received and recovered by the recovery vessel  14 . 
     Alternatively, after the entire molten silicon is solidified from the lower part to the upper part once, the electron beam is irradiated to the solidified silicon so as to re-dissolve the portion corresponding to 20% of the total from the upper part, the irradiation of the electron beam is stopped, the dissolution vessel  31  is tilted by the tilting device  39  so as to have the re-molten portion flow to the outside, and the molten silicon that flows out is received and recovered by the recovery vessel  14 . 
     In the above-described embodiment, the unsolidified silicon is recovered when the unsolidified silicon decreases to 20% of the total, but the ratio of the unsolidified silicon when being recovered is not limited to 20% of the total but may be determined in advance depending on purity of the raw material. 
     As described above, by repeating the processes of melting the base material made of metallurgical silicon, growing the solidified silicon from the portion in contact with the inner bottom surface of the dissolution vessel  31 , and removing the unsolidified silicon located above the solidified silicon in which the impurities are concentrated, the impurities in the solidified silicon can be reduced since the impurities are contained in the removed molten silicon. 
     The silicon refining device  10  of the present invention has a supporting member  33  for holding the dissolution vessel  31  on the cooling means  21 , and an opening portion  29  is provided on the supporting member  33  so that the outer bottom surface of the dissolution vessel  31  and the upward-oriented surface of the cooling means  21  are facing each other. 
     As long as the outer bottom surface of the dissolution vessel  31  and the surface of the cooling means  21  are facing each other, the opening portion  29  is not limited to a region where an outline of the outer periphery is closed by the supporting member  33  (see,  FIG. 5 ) but may be in a region where the outline of the outer periphery is open (see,  FIG. 6 ). 
     In other words, by referring to a region located inside the outer periphery of the outer bottom surface of the dissolution vessel  31  in a space between the dissolution vessel  31  and the cooling means  21  as a facing space, the opening portion  29  is constituted by a portion in the facing space other than the supporting member  33 . 
     In this embodiment, the dissolution vessel  31  is arranged on the cooling means  21  when the first insulating material  32  is brought into contact with the outer peripheral portion or the bottom surface portion of the outer surface (that is, the outer side surfaces or outer bottom surface) and the portion with which the first insulating material  32  is brought into contact is supported by the holding tool (supporting member)  33 , and the dissolution vessel  31  and the cooling means  21  are in a non-contact state. The first insulating material  32  herein is carbon felt. 
     If the holding tool (supporting member)  33  and the dissolution vessel  31  are in contact with each other, since the contact surface can be easily cooled, solidification from the inner peripheral surface (inner side surface) of the dissolution vessel  31  can easily occur; and thus, favorable solidification from the bottom surface toward the upper surface of the dissolution vessel  31  is obstructed. Accordingly, in this embodiment, the cooling is suppressed by placing the first insulating material  32  between the holding tool (supporting member)  33  and the dissolution vessel  31 , and thus, favorable solidification growth is enabled. 
     Moreover, since the dissolution vessel  31  and the cooling means  21  are not in contact with each other, the inner bottom surface of the dissolution vessel  31  is also heated to a silicon melting point (1414° C.) or higher. Thus, occurrence of scull can be suppressed, and a contact surface between the molten silicon and the dissolution vessel  31  can also be solidified and refined. 
     As illustrated in  FIG. 2 , a space between the dissolution vessel  31  and the cooling means  21  may be used as a part of the internal space of the vacuum chamber  11  so that the whole portion of the bottom surface of outer surface (that is, the outer bottom surface) of the dissolution vessel  31  faces the surface of the cooling means  21  or as illustrated in  FIG. 1 , the outer peripheral portion of the outer bottom surface is in contact with the first insulating material  32  and the inner portion of the portion in contact is facing the cooling means  21 . 
     In reference to  FIG. 5 , an area ratio R=B/A, where (A) is an area of the inner bottom surface of the dissolution vessel  31 , and (B) is a sectional area of the opening portion  29  in parallel with the inner bottom surface of the dissolution vessel  31  that is, an area of a portion in the outer bottom surface of the dissolution vessel  31  facing the surface of the cooling means  21  is preferably at least 50% and at most 200%. 
     In other words, a case where the area ratio R is less than 50%, the solid/liquid interface does not become horizontal and solidification cannot progress favorably in one direction toward the center in the upper surface of the dissolution vessel  31 . Moreover, in a case where the area ratio R is larger than 200%, the heat removal efficiency becomes large, and the scull (those solidified without being refined remaining at the concentration of the impurities in the raw material) occurs on the bottom surface of the dissolution vessel  31 . 
     In the above embodiment, the opening portion  29  is provided with a concentric configuration at the inner bottom surface of the dissolution vessel  31 , but it is not limited in the present invention, as long as the area ratio R is at least 50% and at most 200%, and, for example, as illustrated in  FIG. 6 , the dissolution vessel  31  may be held by at least two columnar holding tools (supporting members)  33  in a non-contact relationship with the cooling means  21 . 
     As illustrated in  FIG. 3 , between the surface on the bottom surface of the outer surface of the dissolution vessel  31  (that is, the outer bottom surface) and the surface of the cooling means  21 , a second insulating material  35  in contact with both the outer bottom surface and the surface of the cooling means  21  may be arranged. The second insulating material  35  is carbon felt. 
     In a case where the second insulating material  35  is arranged between the outer bottom surface of the dissolution vessel  31  and the surface of the cooling means  21 , the second insulating material  35  is in contact with the surface of the cooling means  21  and the outer bottom surface of the dissolution vessel  31 , cooling is performed mainly by thermal conductivity of the second insulating material  35 . 
     The heat removal efficiency of the bottom surface of the dissolution vessel  31  becomes smaller than in the case where nothing is arranged in the facing space, and the occurrence of scull on a portion in contact with the inner bottom surface of the dissolution vessel  31  can be prevented. 
     The outer periphery (outer side surface) of the dissolution vessel  31  may be surrounded by the first insulating material  32  sandwiched by the dissolution vessel  31  and the holding tool (supporting member)  33  as illustrated in  FIGS. 1 to 3  or may be surrounded by a third insulating material  36  different from the first insulating material  32  as illustrated in  FIG. 4 . Here, the third insulating material  36  is not sandwiched between the dissolution vessel  31  and the holding tool (supporting member)  33 . 
     Among outer surfaces of the dissolution vessel  31 , in the case where nothing is arranged between the outer bottom surface and the surface of the cooling means  21 , the bottom surface of the vessel is cooled since, in the dissolution vessel  31 , the radiation emitted mainly from the bottom surface is larger than the radiation emitted from the side surfaces. 
     By enhancing the cooling of the bottom surface of the vessel while suppressing the cooling of side surfaces of the vessel, the silicon is solidified in one direction from the lower part to the upper part. 
     In the above-described embodiment, the cooling of the bottom surface of the dissolution vessel  31  by the cooling means  21  is started before the heating of the silicon by the electron beam is started, but the cooling by the cooling means  21  may be started during irradiation of the electron beam. 
     Example 
     Example 1 
     In reference to  FIG. 1 , the dissolution vessel  31  (depth: 60 mm, inner diameter: 300 mm) made of graphite was arranged above the cooling means  21  which surface is made of oxidized copper, and spaced from the cooling means  21 . Here, emissivity of the surface of the cooling means  21  is at least 0.1. 
     Aluminum (Al) and iron (Fe) were added to 7.5 kg of high purified silicon (Si) so as to have a weight ratio of 250 ppm, respectively; the fabricated silicon raw material was loaded in the dissolution vessel  31 ; the electron beam was irradiated at irradiation density of 1000 kW/m 2 ; and the silicon raw material was fully dissolved. 
     The power of the electron beam was gradually weakened without changing an irradiation width (surface) so that the solidification speed becomes 1 mm/min; and when the molten silicon located above reached 20% of the whole, the dissolution vessel  31  was tilted and the molten silicon was removed. 
     After the molten silicon was removed, the silicon remaining in the dissolution vessel  31  was cut out into an arbitrary size, sliced in a layered state having a 4-mm thickness in a height direction, and each was analyzed by using ICP-MS. The analysis result is shown in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Analysis Result of Example 1 
               
            
           
           
               
               
            
               
                   
                 Distance* 1  (in mm) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 2 
                 6 
                 10 
                 14 
                 18 
                 22 
                 26 
                 30 
                 34 
                 38 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Al concentration 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
               
               
                 (in ppm) 
               
               
                 Fe concentration 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
               
               
                 (in ppm) 
               
               
                   
               
               
                 * 1 Distance from inner bottom surface in dissolution vessel 
               
            
           
         
       
     
     It is found that Al and Fe which are impurities could be fully removed. 
     Example 2 
     In reference to  FIG. 3 , the second insulating material  35  (carbon felt) having thermal conductivity of 0.3 W/m·K was sandwiched by the cooling means  21  and the dissolution vessel  31  made of graphite (depth: 60 mm, inner diameter: 300 mm) and arranged. 
     The silicon raw material fabricated by adding Al and Fe to 7.5 kg of high purified Si so as to have a weight ratio of 250 ppm, respectively, was loaded in the dissolution vessel  31 ; the electron beam was irradiated at irradiation density of 1000 kW/m 2 ; and the silicon raw material was fully dissolved. 
     The power of the electron beam was gradually weakened without changing an irradiation width (surface) so that the solidification speed becomes 1 mm/min; and when the molten silicon reached 20% of the whole, the dissolution vessel  31  was tilted and the molten silicon was removed. 
     After the molten silicon was removed, the silicon remaining in the dissolution vessel  31  was cut out into an arbitrary size, sliced in a layered state having a 4-mm thickness in a height direction, and each was analyzed by using ICP-MS. The analysis result is shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Analysis Result of Example 2 
               
            
           
           
               
               
            
               
                   
                 Distance* 2  (in mm) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 2 
                 6 
                 10 
                 14 
                 18 
                 22 
                 26 
                 30 
                 34 
                 38 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Al concentration 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 0.15 
                 0.23 
                 5.1 
                 250 
                 498 
                 596 
               
               
                 (in ppm) 
               
               
                 Fe concentration 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 0.21 
                 7.9 
                 98 
                 523 
                 498 
               
               
                 (in ppm) 
               
               
                   
               
               
                 * 2 Distance from inner bottom surface in dissolution vessel 
               
            
           
         
       
     
     By providing the second insulating material  35  between the cooling means  21  and the dissolution vessel  31 , it is found that the heat removal efficiency from the bottom surface of the dissolution vessel  31  becomes small and the occurrence of scull on a portion in contact with the inner bottom surface can be suppressed. 
     However, in the case where the heat removal efficiency becomes small, the temperature gradient of the solid-liquid interface becomes small. Thus, in this embodiment, it is found that constitutional undercooling occurred and the impurity concentration rapidly rose during the refining. 
     Comparative Example 1 
     The silicon raw material fabricated by adding Al and Fe to 7.5 kg of high purified Si so as to have a weight ratio of 250 ppm, respectively, was loaded in a water-cooling copper crucible (depth: 60 mm; inner diameter: 300 mm), the electron beam was irradiated at irradiation density of 2000 kW/m 2 ; and the silicon raw material was fully dissolved. 
     Here, the reason why the irradiation density of the electron beam is twice as large as those in the examples 1 and 2 is that heat removal efficiency of the water-cooling copper crucible was large and solid silicon could not be fully dissolved at a portion in contact with the inner bottom surface of the water-cooling copper crucible with the same irradiation density as those of examples 1 and 2. 
     The power of the electron beam was gradually weakened without changing an irradiation width (surface) so that the solidification speed becomes 1 ram/min; and when the molten silicon reached 20% of the whole, the water-cooling copper crucible was tilted and the molten silicon was removed. 
     After the molten silicon was removed, the silicon remaining in the water-cooling copper crucible was cut out into an arbitrary size, sliced in a layered state having a 4-mm thickness in a height direction, and each was analyzed by using ICP-MS. The analysis result is shown in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Analysis Result of Comparative Example 1 
               
            
           
           
               
               
            
               
                   
                 Distance* 3  (in mm) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 2 
                 6 
                 10 
                 14 
                 18 
                 22 
                 26 
                 30 
                 34 
                 38 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Al concentration 
                 10.2 
                 0.32 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
               
               
                 (in ppm) 
               
               
                 Fe concentration 
                 10 
                 0.23 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
               
               
                 (in ppm) 
               
               
                   
               
               
                 * 3 Distance from inner bottom surface in dissolution vessel 
               
            
           
         
       
     
     Since the heat removal efficiency of the water-cooling copper crucible is too large, it is found that scull occurred on a portion in contact with the inner bottom surface of the water-cooling copper crucible, and the refining efficiency was low in that portion. 
     Comparative Example 2 
     The dissolution vessel made of graphite (depth: 60 mm, inner diameter: 300 mm) was arranged on the cooling means and in contact with the cooling means. 
     The silicon raw material fabricated by adding Al and Fe to 7.5 kg of high purified Si so as to have a weight ratio of 250 ppm, respectively, was loaded in the dissolution vessel; the electron beam was irradiated at irradiation density of 1000 kW/m 2 ; and the silicon raw material was fully dissolved. 
     The power of the electron beam was gradually weakened without changing an irradiation width (surface) so that the solidification speed becomes 1 mm/min; and when the molten silicon reached 20% of the whole, the dissolution vessel was tilted and the molten silicon was removed. 
     After the molten silicon was removed, the silicon remaining in the dissolution vessel was cutout into an arbitrary size, sliced in a layered state having a 4-mm thickness in a height direction, and each was analyzed by using ICP-MS. The analysis result is shown in Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Analysis Result of Comparative Example 2 
               
            
           
           
               
               
            
               
                   
                 Distance* 4  (in mm) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 2 
                 6 
                 10 
                 14 
                 18 
                 22 
                 26 
                 30 
                 34 
                 38 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Al concentration 
                 3.2 
                 0.18 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
               
               
                 (in ppm) 
               
               
                 Fe concentration 
                 0.54 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
               
               
                 (in ppm) 
               
               
                   
               
               
                 * 4 Distance from inner bottom surface in dissolution vessel 
               
            
           
         
       
     
     In the case where the dissolution vessel is brought into direct contact with the cooling means, since the heat removal efficiency is too large, similar to the water-cooling copper crucible in the comparative example 1, it is found that scull occurred on the contact portion with the inner bottom surface of the dissolution vessel and the refining efficiency was low in that portion. 
     Comparative Example 3 
     In reference to  FIG. 1 , the dissolution vessel  31  (depth: 60 mm, inner diameter: 300 mm) made of graphite was arranged above the cooling means  21  which surface is made of mirror-polished copper, and separated from the cooling means  21 . By means of the mirror-polishing, the emissivity of the cooling means  21  is less than 0.1. 
     The silicon raw material fabricated by adding Al and Fe to 7.5 kg of high purified Si so as to have a weight ratio of 250 ppm, respectively, was loaded in the dissolution vessel  31 ; the electron beam was irradiated at irradiation density of 1000 kW/m 2 ; and the silicon raw material was fully dissolved. 
     The power of the electron beam was gradually weakened without changing an irradiation width (surface) so that the solidification speed becomes 1 mm/min; and when the molten silicon reached 20% of the whole, the dissolution vessel  31  was tilted and the molten silicon was removed. 
     After the molten silicon was removed, the silicon remaining in the dissolution vessel  31  was cut out into an arbitrary size, sliced in a layered state having a 4-mm thickness in a height direction, and each was analyzed by using ICP-MS. The analysis result is shown in Table 5. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Analysis Result of Comparative Example 3 
               
            
           
           
               
               
            
               
                   
                 Distance* 5  (in mm) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 2 
                 6 
                 10 
                 14 
                 18 
                 22 
                 26 
                 30 
                 34 
                 38 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Al concentration 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 0.15 
                 0.23 
                 4.5 
                 25 
               
               
                 (in ppm) 
               
               
                 Fe concentration 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 0.19 
                 3.6 
                 56 
               
               
                 (in ppm) 
               
               
                   
               
               
                 * 5 Distance from inner bottom surface in dissolution vessel 
               
            
           
         
       
     
     Since the emissivity of the cooling means  21  is less than 0.1, radiation heat from the outer bottom surface of the dissolution vessel  31  is reflected by the cooling means  21  and the heat removal efficiency becomes low. Thus, it is found that the same phenomenon as that in example 2 occurred, and the impurity concentration rapidly rose during the refining. 
     Example 3 
     In reference to  FIG. 1 , the dissolution vessel  31  (depth: 60 mm, inner diameter: 300 mm) made of graphite was arranged above the cooling means  21  which surface is made of oxidized copper, and spaced from the cooling means  21 . 
     An area ratio R of a sectional area of the opening portion  29  in parallel with the inner bottom surface of the dissolution vessel  31  to an area of the inner bottom surface of the dissolution vessel  31  was set to a value of at least 40% and at most 200%. 
     The silicon raw material fabricated by adding Al and Fe to 7.5 kg of high purified Si so as to have a weight ratio of 250 ppm, respectively, was loaded in the dissolution vessel  31 ; the electron beam was irradiated at irradiation density of 1000 kW/m 2 ; and the silicon raw material was fully dissolved. 
     The power of the electron beam was gradually weakened without changing an irradiation width (surface) so that the solidification speed becomes 1 mm/min; and when the molten silicon reached 20% of the whole, the dissolution vessel  31  was tilted and the molten silicon was removed. 
     After the molten silicon was removed, the silicon remaining in the dissolution vessel  31  was molten again and when the silicon was fully dissolved, 5 cc of it was taken out by a sampler and analyzed by using ICP-MS. 
     The area ratio R is changed within a range of at least 40% and at most 200%; and the above-described analysis test was repeated. 
     The analysis result is shown in Table 6. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Analysis Result of Example 3 
               
            
           
           
               
               
            
               
                   
                 Ratio* 6  (in %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 200 
                 180 
                 160 
                 140 
                 120 
                 100 
                 80 
                 60 
                 50 
                 40 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Al concentration 
                 0.12 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 0.35 
                 0.98 
               
               
                 (in ppm) 
               
               
                 Fe concentration 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 0.45 
               
               
                 (in ppm) 
               
               
                   
               
               
                 * 6 Ratio R to inner surface of dissolution vessel 
               
            
           
         
       
     
     It is found that, in the case where the area ratio R is less than 50%, solidification could not favorably progress in one direction toward the center in the upper surface of the dissolution vessel  31 , while in the case where the area ratio R is larger than 200%, the heat removal efficiency became large and scull occurred on the bottom surface. That is, it is found that the area ratio R is preferably at least 50% and at most 200%. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
           10 . silicon refining device 
           11 . vacuum chamber 
           12 . heating means 
           21 . cooling means 
           25 . cooling device 
           29 . opening portion 
           31 . dissolution vessel 
           32 . first insulating material 
           33 . supporting member (holding tool) 
           35 . second insulating material 
           36 . third insulating material 
           39 . tilting device