Abstract:
A directional solidification furnace comprises a crucible assembly including a crucible for containing a melt having walls and a base with an opening therein, a crucible support for supporting the crucible, and a lid covering the crucible. A plate is received in the opening in the base. The plate has a higher thermal conductivity than that of the base. The base can include a composite having an additive such that the composite base has a higher thermal conductivity than a comparable without the additive.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/299,133 filed Jan. 28, 2010, the entire disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    This invention generally relates to directional solidification furnaces and, more specifically, to a directional solidification furnace for improving the solidification rate. 
         [0003]    Directional solidification furnaces, such as those shown in  FIG. 1  and referred to generally at  100 , are often used in the production of multi-crystalline silicon ingots. The directional solidification furnace  100  of  FIG. 1  comprises a crucible  102  supported by a crucible support  103  having graphite support walls that add structural rigidity to the crucible. The crucible  102  includes walls  104  (crucible support walls) and a base  106 . The crucible  102  is typically constructed of quartz, or another suitable material that can withstand high temperatures while remaining essentially inert. 
         [0004]    Together with a lid  112 , the crucible  102  and crucible support  103  form an inner assembly  105 . This inner assembly  105  may also include a heat exchanger  107  disposed beneath the base  106 . Heaters  108  are positioned around the walls  104  and within a containment vessel  110 . The heaters  108  may suitably be radiant heaters configured to apply the heat necessary to melt charge material within the crucible. The charge material of this embodiment is silicon, though other materials are contemplated. 
         [0005]    Side insulation  109  is disposed around the crucible and may be partially opened, such as by vertical movement. Once the silicon charge has melted, a cooling medium may be introduced to the heat exchanger  107  and/or the insulation  109  may be raised to aid in the directional solidification of the silicon. The heat output of the heaters  108  may be adjusted so that less heat is applied to the melt  111 . The position of the heaters  108  may also be adjusted relative to the crucible by moving them away from the crucible  102 , especially away from the crucible base  106 . 
         [0006]    After the crucible  102  has been charged with silicon, the area surrounding the crucible is sealed from the outside ambient environment. To aid in the separation of the crucible  102  from the outside environment, the crucible is placed within the containment vessel  110  that forms part of the furnace. The pressure within the containment vessel  110  is then reduced. The content of the atmosphere within the containment vessel  110  can also be monitored and controlled. 
         [0007]    The crucible  102  and the charge are then heated to a temperature sufficient to melt the silicon. After the charge has completely melted it is cooled at a controlled rate to achieve a directional solidification structure. The controlled rate of cooling is established by any combination of reducing the amount and location of heat applied by the heaters  108 , the movement of or the opening of a heat vent in insulation  109  surrounding the crucible  102 , or the circulation of a cooling medium through the heat exchanger  107  (e.g., a cooling plate). Any of these methods transfer heat away from the surface of the crucible  102 . If the rate of cooling of the base  106  of the crucible  102  is greater than that of the walls  104  of the crucible, then a relatively flat, horizontal solidification isotherm with predominately axial thermal gradients is generated. The ingot thereby solidifies in the region closest to the cooler side of the crucible  102  and proceeds in a direction away from that side of the crucible. The last portion of the melt  111  to solidify is generally at the top of the ingot. 
         [0008]    A significant concern in the production of multi-crystalline silicon ingots in directional solidification furnaces is the amount of time required to generate an ingot from raw silicon. The rate at which the ingot solidifies directly affects the amount of time required to form the ingot from the raw materials. 
         [0009]    This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
       BRIEF SUMMARY 
       [0010]    A first aspect of the disclosure is a directional solidification furnace comprising a crucible assembly. The crucible assembly includes a crucible for containing a melt, the crucible including walls and a base, the base having an opening therein. A crucible support supports the crucible and a lid covers the crucible. A plate is received in the opening in the base. The plate has a higher thermal conductivity than that of the base. 
         [0011]    Another aspect of the disclosure is a directional solidification furnace comprising a crucible assembly. The crucible assembly includes a crucible for containing a melt, the crucible including walls and a composite base. A crucible support supports the crucible and a lid covers the crucible. The composite base includes an additive such that the composite base has a higher thermal conductivity than a comparable base without the additive. 
         [0012]    Yet another aspect of the disclosure is a method of producing an ingot in a directional solidification furnace. The method comprises melting a silicon charge in a crucible of the furnace to form a liquid melt. The crucible includes a base having a first portion and a second portion. The first portion has a higher thermal conductivity than the second portion. Heat is then transferred from the melt through the base of the crucible. Heat is transferred through the first portion of the base at an increased rate compared to the second portion. The transfer of heat from the melt results in the solidification of the melt and production of the ingot. 
         [0013]    Various refinements exist of the features noted in relation to the above-mentioned aspects. Further features may also be incorporated in the above-mentioned aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above-described aspects, alone or in any combination. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a partially schematic cross-section of a directional solidification furnace in accordance with one embodiment; 
           [0015]      FIG. 2  is a top plan view of a first embodiment of a crucible for use in the directional solidification furnace of  FIG. 1 ; 
           [0016]      FIG. 3  is a cross-section of the crucible of  FIG. 2  taken along line  3 - 3  in  FIG. 2 ; 
           [0017]      FIG. 4  is an enlarged portion of  FIG. 3 ; 
           [0018]      FIG. 5  is a top plan view of a second embodiment of a crucible for use in the directional solidification furnace of  FIG. 1 ; 
           [0019]      FIG. 6  is a cross-section of the crucible of  FIG. 5  taken along line  6 - 6  in  FIG. 5 ; 
           [0020]      FIG. 7  is a top plan view of a third embodiment of a crucible for use in the directional solidification furnace of  FIG. 1 ; 
           [0021]      FIG. 8  is a cross-section of the crucible of  FIG. 7  taken along line  8 - 8  in  FIG. 7 ; 
           [0022]      FIG. 9  is a top plan view of a fourth embodiment of a crucible for use in the directional solidification furnace of  FIG. 1 ; 
           [0023]      FIG. 10  is a cross-section of the crucible of  FIG. 9  taken along line  10 - 10  of  FIG. 9 ; 
           [0024]      FIG. 11  is an enlarged portion of  FIG. 10 ; and 
           [0025]      FIG. 12  is a flow diagram depicting a method of producing an ingot in a directional solidification furnace using any of the crucibles shown in  FIGS. 2-11 . 
       
    
    
       [0026]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0027]      FIGS. 2 ,  3 , and  4  show respective top plan, cross-sectional, and enlarged views of a first embodiment of a crucible  200  for use in any directional solidification furnace, such as the furnace  100  shown in  FIG. 1 . The crucible  200  has a base  206  (generally, a “second portion”) and four walls  204  extending upward from the base. The base  206  and walls  204  may be integrally formed together or joined together such that the melt  111  ( FIG. 1 ) is contained therein. The base  206  has an upper surface  208  and a lower surface  210  and an opening  220  extending between the upper and lower surfaces. 
         [0028]    The opening  220  is defined by a void formed in the crucible  200  that is bounded by four sides  222 . In other embodiments, the opening  220  may be a shape other than rectangular, such as circular, oval, or any other suitable shape and the plate  250  placed therein is shaped accordingly. The opening  220  may be formed in the crucible  200  by machining or otherwise removing a section of base  206 . In other embodiments, the opening  220  may be formed during manufacture of the crucible  200 . 
         [0029]    The opening  220  has a length L and a width W adjacent the upper surface  208  of the base  206  and a length L′ and a width W′ adjacent the lower surface  210 . Each of the sides  222  slopes inward away from the walls  204  of the crucible  200  such that the length L is greater than the length L′ and the width W is greater than the width W′. In some embodiments, the length L, length L′, width W, and width W′ may be between 50 mm and 630 mm. As more clearly seen in  FIG. 4 , the sides  222  are angled at approximately 45 degrees with respect to the lower surface  210  of the base  206 . However, the sides  222  may be oriented at a different angle without departing from the scope of the embodiments. For example, in some embodiments, the sides  222  may be oriented at approximately 35 degrees with respect to the lower surface  210  of the base  206 . 
         [0030]    A plate  250  (generally, a “first portion”) having four sides  252  is sized for positioning within the opening  220 . While the embodiments herein disclose placing the plate  250  in the base  206  of the crucible  200 , additional plates may be placed within any or all of the walls  204 . Moreover, in some embodiments the plate  250  may not be used, and instead plates similar to the plate  250  may be placed within any or all of the walls  204 . 
         [0031]    The plate  250  is formed from a material having a higher thermal conductivity than the base  206  of the crucible  200 . In one embodiment, the plate  250  may be formed from fused quartz. In some embodiments, the plate  250  has a thermal conductivity (k) that is approximately 
         [0000]    
       
         
           
             3 
              
             
               
                 W 
                 
                   m 
                    
                   
                       
                   
                    
                   °K 
                 
               
               . 
             
           
         
       
     
         [0000]    compared to a typical thermal conductivity of the base  206  of 
         [0000]    
       
         
           
             1 
              
             
                 
             
              
             
               
                 W 
                 
                   m 
                    
                   
                       
                   
                    
                   °K 
                 
               
               . 
             
           
         
       
     
         [0000]    In other embodiments, the plate  250  is formed from any material having a thermal conductivity greater than the base  206  of the crucible  200  that has a higher melting point than the melt  111 . For example, the plate  250  may be formed from MgO, AlN, SiC, graphite, or a composite of MgO and SiC. According to some embodiments, the plate  250  may only be used one time, after which it is removed and repaired or discarded. In other embodiments, the plate  250  is used multiple times before removal. 
         [0032]    The plate  250  has a thickness T 1  that is substantially the same as a thickness T 2  of the base  206  of the crucible  200  adjacent the plate  250 . In some embodiments, T 1  may be equal to between 5 mm and 25 mm and T 2  may be equal to between 5 mm and 25 mm. In other embodiments, the thickness T 1  of the plate  250  may be greater or less than the thickness T 2  of the base  206 . As seen in  FIG. 4 , the plate  250  has four sides  252  with a sloped profile that corresponds to the sides  222  of the opening  220  such that the plate is in registry with the sides of the opening. The sloped sides  222  of the opening form a first angle that is complementary a second angle formed by the sloped profile of the four sides  252  of the plate  250 . The geometry of the sides  222  of the opening  220  and the sides  252  of the plate  250  thus result in the weight of the melt  111  pressing the sides of the plate against the sides of the opening. A bonding agent may be used to further secure the plate  250  within the sides  222  of the opening  220  such that the melt  111  is not able to pass between opening and the plate. 
         [0033]    In some embodiments, the bonding agent is a slip cast silica compound  256 . The size of the joint  254  and amount of slip cast silica compound  256  contained therein shown in  FIG. 4  is greatly exaggerated for illustration. Prior to use of the crucible  200 , a joint  254  between the plate  250  and the lower surface  210  of the base  206  may be sealed with tape or other material. Slip cast silica  256  in a fluid state is the poured into the joint  254  adjacent the upper surface  208  of the base  206 . The solvent in the fluid slip cast silica  256  then evaporates, and the silica remains as a joint filler. The crucible  200  may then be fired to cure the slip cast silica  256  in the joint  254 . 
         [0034]      FIGS. 5 and 6  show respective top plan and cross-sectional views of a second embodiment of a crucible  300  for use in any directional solidification furnace, such as the furnace  100  shown in  FIG. 1 . The crucible  300  has a base  306  (generally, a “second portion”) and four walls  304  extending upward from the base. The base  306  and walls  304  may be integrally formed together or joined together such that the melt  111  ( FIG. 1 ) is contained therein. The base  306  has an upper surface  308  and a lower surface  310 . A recess  320  extends upward from the lower surface  310  towards the upper surface  308 , but does not pass through the upper surface  308  of the base  306 . 
         [0035]    A plate  350  (generally, a “first portion”) having four sides  352  is sized for positioning within the recess  320 . In other embodiments, the recess  320  and plate  250  may not be rectangular, and instead be circular, oblong, or any other suitable shape. While the embodiments herein disclose placing the plate  350  in the recess  320  in the base  306  of the crucible  300 , additional plates may be placed within any or all of the walls  304 . Moreover, the plate  350  may not be used, and instead plates may be placed within any or all of the walls  304 . 
         [0036]    A portion  360  of the base  306  separates the plate  350  from the melt disposed in the crucible. The portion  360  prevents the melt from damaging, wearing, or corroding the plate  350 . According to some embodiments the portion  360  may have a thickness T PORTION  between 1 mm and 20 mm while the plate  350  may have a thickness T PLATE  between 1 mm and 20 mm and the base  306  may have a thickness T BASE  between 1 mm and 20 mm. Moreover, while the thicknesses of the plate T PLATE  and portion T PORTION  are shown as being uniform, the thicknesses may vary. As the plate  350  is shielded from the melt  111  by the portion  360  of the base  306 , the plate may thus be used multiple times before replacement. 
         [0037]    The plate  350  is attached to the base  306  by any bonding mechanism, such as adhesive or fasteners. In some embodiments, slip cast silica is used to fasten the plate  360  to the recess  320 . In other embodiments, a friction fit between the recess  320  and the plate  350  retains the plate in the recess. 
         [0038]    The plate  350  is formed from a material having a higher thermal conductivity than the base  306  of the crucible  300 . In one embodiment, the plate  350  may be formed from fused quartz. In these embodiments the plate  250  and the portion  360  of the base  306  have a combined effective thermal conductivity (k eff ) that is up to approximately 
         [0000]    
       
         
           
             10 
              
             
                 
             
              
             
               W 
               
                 m 
                  
                 
                     
                 
                  
                 °K 
               
             
           
         
       
     
         [0000]    compared to a typical thermal conductivity of the base  306  of 
         [0000]    
       
         
           
             1 
              
             
                 
             
              
             
               
                 W 
                 
                   m 
                    
                   
                       
                   
                    
                   °K 
                 
               
               . 
             
           
         
       
     
         [0039]    In other embodiments, the plate  350  is formed from any material having a thermal conductivity greater than the base  306  of the crucible  300 . For example, the plate  350  may be formed from MgO, AlN, SiC, graphite, or composites of MgO and SiC, SiO 2  and AlN, SiO 2  and MgO, and SiO 2  and TiO 2 . Moreover, as the plate  350  is shielded from the melt  111  by the portion  360 , materials that would otherwise not be suitable for contact with the melt may be used in its construction (e.g., TiO 2 ). 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 T BASE   
                 T PORTION   
                 T PLATE   
                 k PORTION   
                 k PLATE   
                 k eff   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 20 
                 5 
                 15 
                 1 
                 3 
                 2 
               
               
                 20 
                 10 
                 10 
                 1 
                 3 
                 1.5 
               
               
                 20 
                 15 
                 5 
                 1 
                 3 
                 1.2 
               
               
                 20 
                 5 
                 15 
                 1 
                 50 
                 3.77 
               
               
                 20 
                 10 
                 10 
                 1 
                 50 
                 1.96 
               
               
                 20 
                 15 
                 5 
                 1 
                 50 
                 1.32 
               
               
                 20 
                 2 
                 18 
                 1 
                 50 
                 8.47 
               
               
                   
               
             
          
         
       
     
         [0040]    As shown in Table 1 above, the effective thermal conductivity (k eff ) of the base  306  is dependent on the thermal conductivities of the portion  360  (k PORTION ) and the plate  350  (k PLATE ) and their thicknesses, T PORTION  and T PLATE . The combined effective thermal conductivity (k eff ) of the plate  350  and the portion  360  is dependent on the composition of the plate and the thicknesses of the plate and the portion. The combined thermal conductivity (K eff ) of the base  206  is expressed as: 
         [0000]    
       
         
           
             
               k 
               eff 
             
             ≡ 
             
               
                 
                   T 
                   BASE 
                 
                 
                   ( 
                   
                     
                       
                         T 
                         PLATE 
                       
                       
                         k 
                         PLATE 
                       
                     
                     + 
                     
                       
                         T 
                         PORTION 
                       
                       
                         k 
                         PORTION 
                       
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
         [0041]      FIGS. 7 and 8  show respective top plan and cross-sectional views of a third embodiment of a crucible  400  for use in any directional solidification furnace, such as the furnace  100  shown in  FIG. 1 . The crucible  400  has a base  406  (generally, a “second portion”) and four walls  404  extending upward from the base. The base  406  and walls  404  may be integrally formed together or joined together such that the melt  111  ( FIG. 1 ) is contained therein. The base  406  has an upper surface  408  and a lower surface  410 . 
         [0042]    The base  406  has a portion  450  (generally, a “first portion”) of increased thermal conductivity. While the embodiments herein disclose placing the portion  450  in the base  406  of the crucible  400 , additional portions may be placed within any or all of the walls  404 . Moreover, the portion  450  may not be used, and instead portions of increased thermal conductivity are placed within any or all of the walls  404 . 
         [0043]    The portion  450  includes one or more additive materials  452  that are intermixed with the material from which the base  406  is formed to form a composite. The number of additive materials  452  shown in  FIG. 7  is greatly reduced for the sake of clarity and the relative size of the additive materials is likewise greatly increased for clarity. The additive materials  452  have a greater thermal conductivity than the material from which the base  406  is formed. The thermal conductivity of the portion  450  of the base  406  having the increased thermal conductivity is thus generally in the range of three to ten times greater than that of the remainder of the base and the walls  404  of the crucible  400 . The additive materials  452  in the portion  450  may be selected from any material that is capable of being intermixed with the material from which the base  406  is formed during construction of the base. For example, the additive materials  452  may be any one of or a combination of MgO, SiC, AlN, or TiO 2 . 
         [0044]      FIGS. 9 ,  10 , and  11  show respective top plan, cross-sectional, and enlarged views of a fourth embodiment of a crucible  500  for use in any directional solidification furnace, such as the furnace  100  shown in  FIG. 1 . The crucible  500  has a base  506  (generally, a “second portion”) and four walls  504  extending upward from the base. The base  506  and walls  504  may be integrally formed together or joined together such that the melt  111  ( FIG. 1 ) is contained therein. The base  506  has an upper surface  508  and a lower surface  510 . An opening  520  extends between the upper surface  508  and the lower surface  510 . 
         [0045]    The opening  520  is defined by a void formed in the crucible  500  that is bounded by four sides  522 . The opening  520  may be formed in the crucible  500  by machining or otherwise removing a section of base  506 . In other embodiments, the opening  520  may be formed during manufacture of the crucible  500  such that a section of the base is not removed to form the opening. 
         [0046]    A portion  530  of the base  506  extends inward from the sides  522  of the opening  520  and away from the walls  504 . The portion  530  has a thickness T 3 , measured from the bottom surface  510 , which is less than a thickness T 4  of the base  506 . The portion  530  thus extends out from the base  506  and forms a ledge structure. 
         [0047]    A plate  550  (generally, a “first portion”) is sized for placement within the opening  520 . In other embodiments, the opening  520  and plate  550  may not be rectangular, and instead be circular, oblong, or any other suitable shape. While the embodiments herein disclose placing the plate  550  in the base  506  of the crucible  500 , additional plates may be placed within any or all of the walls  504 . Moreover, plate  550  may not be used, and instead plates may be placed within any or all of the walls  504 . 
         [0048]    The plate  550  is formed from a material having a higher thermal conductivity than the base  506  of the crucible  500 . In one embodiment, the plate  550  may be formed from fused quartz. In some embodiments, the plate  550  has a thermal conductivity (k) that is approximately 
         [0000]    
       
         
           
             3 
              
             
               W 
               
                 m 
                 * 
                 °K 
               
             
           
         
       
     
         [0000]    compared to a typical thermal conductivity of the base  506  of 
         [0000]    
       
         
           
             1 
              
             
               
                 W 
                 
                   m 
                   * 
                   °K 
                 
               
               . 
             
           
         
       
     
         [0000]    In other embodiments, the plate  550  is formed from any material having a thermal conductivity greater than the base  506  of the crucible  500  that has a higher melting point than the melt  111 . For example, the plate  550  may be formed from MgO, AlN, SiC, graphite, or a composite of MgO and SiC. 
         [0049]    The plate  550  has a lip portion  552  extending outward from the remainder of the plate along its circumference. The lip portion has a width W 1  approximately equal to a width W 2  of the portion  530  extending from the sides  522  of the base  506 . During use, the lip portion  552  is positioned directly above the portion  530  of the base  506 . The lip portion  552  and portion  530  of the base  506  thus together form a lap joint. The lap joint of the plate  550  and base  506  thus result in the weight of the melt  111  pressing the plate into contact with the base. A bonding agent may be used to further secure the plate  550  within the opening  520  such that the melt  111  is not able to pass between the opening and the plate. In some embodiments, the bonding agent is a slip cast silica compound  556 . Prior to use of the crucible  500 , a joint  554  between the plate  550  and the lower surface  510  of the base  506  may be sealed with tape or other material. Slip cast silica  556  in a fluid state is the poured into the joint  554  adjacent the upper surface  508  of the base  506 . The solvent in the fluid slip cast silica  556  then evaporates, and the silica remains as a joint filler. The crucible  500  may then be fired to cure the slip cast silica  556  in the joint  554 . 
         [0050]    The increased thermal conductivity of the bases of the crucibles described above in  FIGS. 2 through 11  results in an increased thermal flux (i.e., flow of thermal energy) through the respective bases. The increased heat flux through the base of the crucible results in an increase in the solidification rate of the melt contained within the crucible. In some embodiments, the solidification rate may increase by two to three times that shown in conventional crucibles. 
         [0051]    The increase in the solidification rate of the melt thus reduces the amount of time required for the melt to cool and form the solidified ingot. The reduction in the amount of time required to form the melt increases the rate (i.e., throughput) at which ingots can be produced in directional solidification furnaces using crucibles like those described above. 
         [0052]      FIG. 12  depicts an exemplary method  600  for producing an ingot in a directional solidification furnace using any of the crucibles shown in  FIGS. 2-11 . As described above, the furnace includes a crucible having a base with a first portion (e.g., the plate  250 , the plate  350 , the portion  450 , or the plate  550 ) and a second portion (e.g., the base  206 , the base  306 , the base  406 , or the base  506 ). The first portion has a higher thermal conductivity than the second portion. In the exemplary embodiment, the first portion has a thermal conductivity that is at least double that of the second portion. 
         [0053]    The crucible is first loaded with a silicon charge. The method  600  then begins in block  610  with the melting of the silicon charge to form a liquid melt. In block  620 , heat is transferred from the melt through at least the base of the crucible. Heat is transferred through the first portion at an increased rate compared to the rate at which it is transferred through the second portion. The transfer of heat from the melt solidifies the melt into an ingot. 
         [0054]    While the invention has been described in terms of various specific embodiments, it will be recognized that the invention can be practiced with modification within the spirit and scope of the claims. 
         [0055]    When introducing elements of the present disclosure or the embodiments thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of terms indicating a particular orientation (e.g., “top”, “bottom”, “side”, etc.) is for convenience of description and does not require any particular orientation of the item described. 
         [0056]    As various changes could be made in the above constructions and methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.