Abstract:
Solid or semi-solid feedstock is melted in an open bottom electric induction cold crucible furnace. Directionally solidified multi-crystalline solid purified material continuously exits the bottom of the furnace and may optionally pass through a thermal conditioning chamber before being gravity fed into a transport mold where an ingot of the purified multi-crystalline solid material is transported to a remote holding area after the transport mold is filled with the multi-crystalline material and cut from the continuous supply of material. Cool down of the ingot is accomplished remote from the open bottom of the cold crucible furnace.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/222,994, filed Jul. 3, 2009, hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to directional solidification of a multi-crystalline ingot of a purified material from an open bottom cold crucible induction furnace where the cool-down of the hot solid ingot is accomplished remote from the furnace. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electromagnetic casting of a multi-crystalline material such as silicon can be accomplished in an open bottom electric induction cold crucible furnace. Feedstock is continuously supplied to the crucible&#39;s interior volume and melted in the cold crucible while a solidified hot mass of the multi-crystalline material exits from the open bottom of the crucible. The solidified hot mass of the multi-crystalline material is typically cooled down as an integral step in the casting process after it exits from the electric induction cold crucible without immediate removal from the casting process line. 
         [0004]    One object of the present invention is to produce a purified multi-crystalline solid ingot from feedstock material where the feedstock is melted and purified in an open bottom cold crucible induction furnace and a hot multi-crystalline solid mass exits from the bottom of the furnace for deposit in a mold that is removed from the casting process line for remote passive cool-down and directional solidification of the hot multi-crystalline solid mass in the mold without further application of external heating or cooling of the mold. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    In one aspect the present invention is apparatus for, and method of, purifying a crystalline material by directional solidification. Solid or semi-solid feedstock is melted in an open bottom cold crucible electric induction furnace. Hot purified multi-crystalline solidified material continuously exits from the open bottom of the furnace, and may optionally pass through a thermal conditioning chamber, before being deposited in a transport mold and moved to a remote holding area for passive cool-down and directional solidification of the hot purified multi-crystalline solidified material. 
         [0006]    The above and other aspects of the invention are set forth in this specification and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    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: 
           [0008]      FIG. 1  illustrates on example of an apparatus used in the present invention. 
           [0009]      FIG. 2  illustrates another example of an apparatus used in the present invention. 
           [0010]      FIG. 3  illustrates another example of an apparatus used in the present invention. 
           [0011]      FIG. 4  graphically illustrates the change in thermal coefficient of expansion of solid silicon relative to the temperature of the silicon. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    There is shown in  FIG. 1  one example of the apparatus and process of the present invention. Feedstock  90  is supplied in a suitable form from a suitable source to electric induction cold crucible  12 , which is surrounded by one or more induction coils  14  that are connected to one or more suitable alternating current sources  16 . The open bottom electric induction cold crucible  12  is similar in construction and operation to a closed bottom electric induction cold crucible furnace, for example, as described in U.S. Patent Application Publication No. 2005/0175064 A1 except that there is no bottom structure to inhibit bottom exit of a continuous hot solidified mass of a purified multi-crystalline material from the bottom of the cold crucible. 
         [0013]    Initially feedstock  90  may be in a semi-solid state (particularly if the feedstock material is non-electrically conductive in the solid state, and electrically conductive in the liquid state, such as silicon feedstock) to establish a generally steady state solidification front  90 ′ of molten feedstock  90   a  (horizontally line shaded region) over purified multi-crystalline hot solid mass  90   b  (stipple-shaded region) exiting from the bottom of the cold crucible furnace. The multi-crystalline hot solid mass will gravity free-fall into mold  81 . The mold can have a moveable inner bottom  81   a  with powered driver  92  controlling the drop speed of the moveable inner bottom, and therefore controlling the rate of vertical formation of the multi-crystalline solid mass in the mold. The bottom opening of the induction cold crucible  12  may be of any shape, for example, to produce a cylindrical or rectangular solid mass. 
         [0014]    In-process fill mold  81 ; pre-filled mold  81 ″ and post-filled mold  81 ′ can each be formed from a suitable liner material  81   b  as consistent with the type of material mass  90   b  contained within the mold. Thermal insulation  81   c  may surround at least the exterior wall of the mold. In some examples of the invention, bottom  81   a  may be formed from a susceptor. The susceptor may optionally be suitably heated, for example either resistively or inductively, prior to filing of the mold. 
         [0015]    When the multi-crystalline solid mass has achieved a desired vertical height in mold  81 , cut-off saw  18  is activated to slice through a horizontal plane of the multi-crystalline solid mass. Downward movement of the multi-crystalline solid mass can be controlled at a sufficiently slow rate relative to the cut through speed of the cut-off saw so that a substantially horizontal cut of the multi-crystalline solid mass is achieved with the cut-off saw. 
         [0016]    Mold  81 , when filled with a hot multi-crystalline solid mass, is moved away from the furnace, as represented by filled mold  81 ′ in  FIG. 1  and covered with thermal insulating lid  81   d  so that the multi-crystalline solid mass in mold  81  cools down and directionally solidifies passively without application of controlled zone external heating or cooling. A multi-crystalline metal or metalloid, such as silicon, will initially solidify (amorphous-to-crystalline phase transition) at a high temperature (nominally 1,380° C. for silicon), which is the hot multi-crystalline solid mass deposited in the mold prior to cool-down and directional solidification. Most important in any material directional solidification process is to avoid an effect know as “shrink defects” caused by volume contraction of the material as it transitions inwardly from the liquid state to the hot solid state by controlling the volume rate of transition from liquid to hot solid state, and then controlling the cool-down (directional solidification) rate of the hot solid mass of material in the mold. For example the graph in  FIG. 4  illustrates the expansion and contraction characteristics of solid silicon over a temperature range. Thermal monitoring systems can be provided to monitor the passive rate of cool-down of the hot material mass in the mold, so that, for example, the temperature of the hot solidified mass of purified multi-crystalline material exiting the open bottom of the crucible can be adjusted for an optimum passive cool-down rate. For example thermocouples or other temperature sensors  83  can be buried along the height of the mold&#39;s wall to monitor the rate of cool-down in the mold. The entire process may be automated to minimize manpower required for operation of the process as long as a constant material feedstock is available. 
         [0017]    The molds can be supplied to the furnace (such as pre-filled mold  81 ″) and removed from the furnace on a suitable conveyance apparatus. The conveyance apparatus can move filled molds to a remote cool-down storage zone that automatically moves molds containing material that have completed the cool-down process to a process finishing zone, for example, for further trimming of the ingot for shipping and recycling of ingot trimmings of sufficient purity as feedstock  90 . The entire process may be automated to minimize manpower requirements for the operation. 
         [0018]    In some examples of the invention, the filled mold may be inserted into a thermally controlled chamber, or tunnel, at the remote area that further regulates the rate of heat dissipation from the multi-crystalline mass in the mold. The chamber or tunnel can provide passive thermal control, for example, by lining the chamber or tunnel with thermal insulating material. 
         [0019]    Generally the mold filing and remote cool-down process is not an extremely fast process. 
         [0020]    For example for a mold with a cross section of 32 centimeters square, and an interior height of 1 meter, process time to fill the mold with a multi-crystalline solid silicon mass can be on the order of six hours. Completion of a remote passively controlled cool-down of the same mass of silicon in a mold can be on the order of thirty-six hours. 
         [0021]    In some examples of the present invention, thermal conditioning of the multi-crystalline solid mass exiting from the cold crucible furnace may be required. This may be achieved, for example, as shown in  FIG. 2  wherein graduated vertical heat zone control apparatus  20  is provided. In this example the zone heat source is an induced electromagnetic field selectively applied to vertical zones of susceptor  21  as the multi-crystalline solid mass drops through apparatus  20 . The electromagnetic field can be established via current flow through induction coils  22   a ,  22   b  and  22   c  (separated from the susceptor by thermal insulator  23 ) with current supplied from alternating current source  24  and controlled by switching devices  26   a ,  26   b  and  26   c . Generally, applied induced heat to the susceptor is reduced in the direction from top to bottom of apparatus  20  to establish a temperature gradient along the height of the susceptor wall. Susceptor  21  performs a secondary function of retaining the outer wall shape of the solidified mass of multi-crystalline material as it passes through apparatus  20 . 
         [0022]    In a directional solidification process, impurities generally migrate to the top of the liquid material phase of the melt. One method of removing these impurities in the present invention is illustrated in  FIG. 3 . In this example at least the open top of induction cold crucible  12  is contained within sealed pressure chamber  30  so that a positive (above atmospheric) pressure can be applied to the surface of melt  90   a  below which surfaces the impurities tend to migrate. For example valve  32  may be used to supply an inert gas to the pressure chamber, which forces impurities-enriched melt  90   a  up tube  34  and out of the pressure chamber. In the particularly arrangement shown in  FIG. 3  tube  34  is arranged as a siphon tube. Consequently once the pressurized gas primes siphon tube  34  the interior of chamber  30  can be vented to atmospheric pressure via connecting open valve  32  to atmosphere and the siphon drain of impurities-enriched melt  90   a  will stop when the height of melt  90   a  in the cold crucible reaches y 1 , which is the vertical position of the opening of siphon tube  34  in the uppermost portion of the melt containing the impurities. 
         [0023]    Tube  34  may be formed at least in part along its length from a high temperature electrically conductive material. If the tube is formed from an electrically conductive material, the tube may be electrically heated by connecting the external end of the tube to a first terminal of electric power source  36  that has a second terminal connected to system ground. If the cold crucible induction furnace is system grounded, for example by system ground connection  12   a  to the wall of the crucible, the electrical conducting path through the furnace skull (solid material formed during cold crucible operation) and melt in the crucible that is in contact with the end of the tube in the melt will establish a Joules heating current flow from power source  36  (via the system grounded return) through the electrically conductive tube that will keep the impurities-enriched melt flowing through the tube without solidification (freezing). One must make certain that the system ground is safely installed and operating correctly in any particular installation as established by applicable standards. Impurities-enriched melt drawn from the crucible may be sent to an impurities separator so that sufficiently pure feedstock can be separated from the impurities and fed back into the feedstock supply to the cold crucible. 
         [0024]    While the present invention is particularly applicable to a cool-down process for directionally solidified silicon, the invention can also be applied to other metalloids or metals, and alloys thereof, capable of purification by directional solidification that require a controlled cool-down process. 
         [0025]    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.