Reusable crucible for silicon ingot growth

Abstract

A silicon nitride crucible is coated with a crucible release coating for use in directional solidification of multicrystalline silicon ingots. The crucible preferably includes reaction bonded silicon nitride crucible. After removing the silicon ingot, the release coating is easily removed and the crucible can be repeatedly recoated and reused.

What is claimed is:  
     
         1 . A reusable crucible for use in directional solidification of multicrystalline silicon ingots wherein the crucible comprises reaction bonded silicon nitride.  
     
     
         2 . The reusable crucible of  claim 1 , wherein the crucible is coated with a release coating selected from the group consisting of Si 3 N 4 , SiO 2 , a mixture of SiO 2  and Si 3 N 4 , SiC, graphite wool/cloth and silica wool/cloth.  
     
     
         3 . The reusable crucible of  claim 2 , wherein the release coating is Si 3 N 4 .  
     
     
         4 . The reusable crucible of  claim 1 , wherein the crucible walls have a thickness between 10 mm and 25 mm.  
     
     
         5 . The reusable crucible of  claim 1 , wherein the silicon nitride crucible has a coefficient of expansion that is similar to that of silicon.  
     
     
         6 . A method for production of multicrystalline silicon ingots in a furnace wherein the method comprises: 
 a. coating a silicon nitride crucible with a release coating;    b. baking the coated crucible to remove impurities;    c. directionally solidifying a multicrystalline silicon ingot in the reusable crucible;    d. removing the silicon ingot and the release coating;    e. recoating the crucible; and    f. using the recoated crucible for producing at least one additional silicon ingot.    
     
     
         7 . The method of  claim 6 , wherein the release coating is selected from the group consisting of Si 3 N 4 , SiO 2 , a mixture of SiO 2  and Si 3 N 4 , SiC, graphite wool/cloth and silica wool/cloth.  
     
     
         8 . The method of  claim 7 , wherein the release coating is Si 3 N 4 .  
     
     
         9 . The method of  claim 6 , wherein the furnace is a graphite resistance heated furnace.  
     
     
         10 . The method of  claim 6  wherein the furnace is an induction heated furnace.  
     
     
         11 . The method of  claim 6 , wherein the furnace is backfilled at between 1200 and 1450° C. and with a pressure of at least 300 torr.  
     
     
         12 . The method of  claim 9  wherein the furnace is backfilled at between 1200 and 1450° C. and with a pressure of at least 300 torr.  
     
     
         13 . The method of  claim 6 , wherein the crucible is made of RBSN.  
     
     
         14 . The method of  claim 6 , wherein the furnace temperature and pressure are sufficient to prevent silicon in molten form from wicking up inner walls of the crucible.  
     
     
         15 . The method of  claim 6 , wherein the crucible is made of isopressed silicon nitride.  
     
     
         16 . The method of  claim 11 , wherein the pressure is at least 500 torr.  
     
     
         17 . The method of  claim 9  wherein the pressure is at least 300 torr.