Abstract:
An apparatus and process are provided for directional solidification of silicon by electric induction susceptor heating in a controlled environment. A susceptor vessel is positioned between upper and lower susceptor induction heating systems and a surrounding induction coil system in the controlled environment. Alternating current selectively applied to induction coils associated with the upper and lower susceptor heating systems, and the induction coils making up the surrounding induction coil system, result in melting of the silicon charge in the vessel and subsequent directional solidification of the molten silicon. A fluid medium can be directed from below the vessel towards the bottom, and then up the exterior sides of the vessel to enhance the directional solidification process.

Description:
CROSS REFERENCE TO RELATED TO APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/093,347, filed Aug. 31, 2008, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to directional solidification of silicon by electric induction heating in a controlled environment. 
     BACKGROUND OF THE INVENTION 
     Pure silicon does not exist in nature. Various methods can be used to remove the impurities from silicon ore. Some methods involve heating and melting the silicon so that impurities are driven out of the molten silicon by outgassing or sedimentation. Outgassing is enhanced in a controlled vacuum environment. Methods of heating and melting silicon ore must take into consideration the fact that silicon is essentially non-electrically conductive in the solid state, and electrically conductive in the molten state. 
     It is one object of the present invention to provide a method of purifying silicon by heating and melting a charge of impure solid silicon in a controlled environment by inductively heating multiple susceptor elements surrounding a crucible containing solid silicon. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is in one aspect, apparatus for, and method of, heating and melting a charge of impure silicon and directionally solidifying the molten silicon in a controlled environment such as a vacuum chamber, or a chamber having an inert gas environment. Silicon in suitable solid form is placed in a susceptor vessel and load into the chamber. The susceptor vessel is positioned between upper and lower susceptor heating systems and an exterior surrounding induction coil system in the controlled environment. Alternating current selectively applied to induction coils associated with the upper and lower susceptor heating systems, and the induction coils making up the surrounding induction coil system, result in melting of the silicon charge in the vessel and subsequent directional solidification of the molten silicon. A fluid medium can be directed from below the vessel towards the bottom, and up the exterior of the wall of the vessel to enhance the directional solidification process. 
     The above and other aspects of the invention are set forth in this specification and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing brief summary, as well as the following detailed description of the invention, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings exemplary forms of the invention that are presently preferred; however, the invention is not limited to the specific arrangements and instrumentalities disclosed in the following appended drawings: 
         FIG. 1  is a cross sectional view of one example of the present invention with a susceptor vessel containing silicon charge loaded into a controlled environment chamber. 
         FIG. 2  is a cross sectional view of one example of the present invention with the susceptor vessel positioned inside the controlled environment chamber for melting of the silicon charge and directional solidification. 
         FIG. 3  is a cross sectional view of one example of the present invention with directional solidification of the silicon in the susceptor vessel partially completed. 
         FIG. 4  is one example of an induction coil used in the upper or lower susceptor heating system of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     There is shown in the figures one non-limiting example of the apparatus and process of present invention. Controlled environment chamber  90  contains upper susceptor heating system  30 , vessel surrounding induction coil system  40 , and lower susceptor heating system  50 . Susceptor vessel  20  can be filled with impure silicon charge  92  and loaded into chamber  90  with a suitable mechanical transport system. 
     Susceptor vessel  20  may comprise susceptor  20   a , formed, for example, from a graphite composition, which can be physically isolated from silicon  92  by a protective liner material  20   b , such as quartz, to prevent contamination of the molten silicon in the vessel with the graphite composition. Alternatively the graphite composition may be treated to inhibit reaction with the silicon charge. The silicon charge in the susceptor vessel, which is initially in the solid non-electrically conductive state, is heated and melted to separate impurities from the silicon, and directionally solidified as purified silicon, as described below. The solid silicon may be in any form, for example, silicon pellets or crushed silicon compositions. Thermal insulation  20   c  can be provided around the exterior of susceptor  20   a  to direct heat induced in the susceptor into the mass of silicon  92  in vessel  20 , and to protect induction coil system  40  from heat generated in the susceptor. In alternative examples of the invention the thermal insulation may be, at least in part provided around the interior of surrounding induction coil system  40  rather than integral with susceptor vessel  20 . In some examples of the invention, a susceptor may not be used in the bottom region of the vessel, or the bottom susceptor region may be formed from segmented one or more susceptors. 
     Upper susceptor induction heating system  30  comprises upper susceptor  30   a  with upper induction coil  30   b  positioned adjacent to susceptor  30   a . Coil  30   b  may be configured as a “pancake” type coil, as shown for example in  FIG. 4 , suitably arranged over susceptor  30   a  to achieve a desired heat distribution profile in the mass of susceptor  30   a . High temperature thermal insulation  30   c  can be provided to direct heat to silicon  92  in vessel  20 , as further described below, and to protect induction coil  30   b  from heat generated in the susceptor. A suitable high temperature thermal insulation is CALCARB® insulation material available from Calcarb Ltd (Bellshill, Scotland, UK). 
     Lower susceptor induction heating system  50  comprises susceptor  50   a  with lower induction coil  50   b  positioned adjacent to susceptor  50   a . Coil  50   b  may be configured as a “pancake” type coil suitably arranged under susceptor  50   a  to achieve a desired heat distribution profile in the mass of susceptor  50   a . High temperature thermal insulation  50   c  can be provided to direct heat to the bottom of the mass of silicon  92  in vessel  20 , as further described below, and to protect induction coil  50   b  from heat generated in the susceptor. A suitable cooling medium, for example air or other gaseous composition, can be supplied to cooling conduit  50   d , as further described below, to flow through passages in lower susceptor  50   a  and up along the exterior sides of vessel as illustrated by representative air flow arrows in  FIG. 3  where the silicon mass is shown in half molten state  92 ′ and half solid (purified) state (shown in solid black). 
     Upper induction coil  30   b , surrounding induction coil system  40  and lower induction coil  50   b  are suitably connected to one or more alternating current (ac) power supplies, such as power supply  60  in the figures. Although the upper and lower induction coils are represented as a single coil, any number of coils, and arrangements thereof, may be used in other examples of the invention. Although the surrounding induction coil system  40  is represented as three helical coils surrounding the exterior wall of the susceptor vessel, any number of coils, and arrangements thereof, may be used to in other examples of the invention. Suitable switching devices S 1  through S 3  are used to control power (current flow) through each of the surrounding induction coils; suitable switching devices S U  and S L  are used to control current flow through the upper and lower induction coils, respectively. The one or more power supplies may output fixed or variable frequencies and/or power levels as required during a particular induction heating process. Electromagnetic shunts  42  may be used around the exterior of the surrounding induction coil system to direct the magnetic flux fields towards the susceptor vessel. 
     In  FIG. 1  susceptor vessel  20 , which is loaded into chamber  90  can be seated upon a suitable support platform (not shown in the figures) formed from a material composition having a high value of thermal conductivity or otherwise configured (for example, with a plurality of holes or passages) to facilitate heat transfer from the inductively heated lower susceptor  50   a  to the bottom of vessel  20  when positioned as shown in  FIG. 2 . 
     A suitable vertical translation drive system (not shown in the figures) can be used to raise susceptor vessel  20  and lower susceptor induction heating system  50  to the position shown in  FIG. 2  where silicon is shown in molten state  92 ′ so that the vessel is located within surrounding induction coil system  40  and chamber  90  is closed to the external environment. Alternatively in other examples of the invention, the upper susceptor induction heating system  30  and surrounding induction coil system  40  can be lowered so that the vessel is located within surrounding induction coil system  40  and chamber  90  is closed to the external environment. 
     With susceptor vessel  20  positioned as shown in  FIG. 2  heating and melting of silicon  92 ′ in the vessel can be accomplished by closing switches S U , S L , S 1 , S 2  and S 3  to inductively heat the upper and lower susceptors, and the susceptor vessel, so that heat from the susceptors transfers by radiation to silicon  92  in the vessel. Current flows through the upper and lower induction coils, and the coils making up the surrounding induction coil system generate magnetic flux fields that couple with the associated susceptors to inductively heat them. Depending upon the frequency of the current and configuration of the susceptors, some flux may penetrate the susceptors and inductively couple with the silicon in the vessel once it becomes electrically conductive. In some examples of the invention power supply  60  may have a variable frequency output that is adjusted to increase magnetic coupling within the susceptor vessel as the silicon transfers to the molten state. 
     After the mass of silicon in the vessel is melted, the switches are selectively opened to facilitate directional solidification of the molten silicon in the vessel from the bottom upwards. For example, switching device S L  may be opened first, followed sequentially by switching devices S 3 , S 2 , S 1  and S U  as the directional solidification process progresses. Further a suitable cooling medium, for example air or other gaseous medium, can be supplied to cooling conduit  50   d , as further described above, to flow through passages in the lower susceptor and up along the exterior sides of the vessel as illustrated by representative air flow arrows in  FIG. 3 , which illustrates partial directional solidification (shown in solid black) of purified silicon in the susceptor vessel. 
     In alternative examples of the invention the susceptor vessel may be gradually withdrawn from within the surrounding induction coil system to enhance the directional solidification process while the cooling medium is supplied from below the vessel. 
     The present invention has been described in terms of preferred examples and embodiments. Equivalents, alternatives and modifications, aside from those expressly stated, are possible and within the scope of the invention.