Crystal growth and annealing method and apparatus

Abstract

A method and apparatus for producing crystals that minimizes birefringence even at large crystal sizes, and is suitable for production of CaF 2 crystals. The method of the present invention comprises annealing a crystal by maintaining a minimal temperature gradient in the crystal while slowly reducing the bulk temperature of the crystal. An apparatus according to the present invention includes a thermal control system added to a crystal growth and annealing apparatus, wherein the thermal control system allows a temperature gradient during crystal growth but minimizes the temperature gradient during crystal annealing. An embodiment of the present invention comprises a secondary heater incorporated into a conventional crystal growth and annealing apparatus. The secondary heater supplies heat to minimize the temperature gradients in the crystal during the annealing process. The secondary heater can mount near the bottom of the crucible to effectively maintain appropriate temperature gradients.

We claim:  
     
         1 . A crystal annealing apparatus, comprising: 
 a) A support structure;    b) A crucible adapted to contain a crystal within the support structure;    c) A primary heating system mounted with the support structure proximal the top and sides of the crucible; and    d) A secondary heating system mounted with the support structure proximal the bottom of the crucible.    
     
     
         2 . The apparatus of  claim 1 , wherein the primary heating system and secondary heating system have heating capacities that maintain the crystal temperature uniform to within about 8° C. per inch.  
     
     
         3 . The apparatus of  claim 1 , wherein the primary heating system and secondary heating system have heating capacities that maintain the crystal temperature uniform to within about 4° C. per inch.  
     
     
         4 . The apparatus of  claim 1 , wherein the primary heating system comprises a primary heat shield mounted with the support structure and a primary heating element mounted between the primary heat shield and the crucible.  
     
     
         5 . The apparatus of  claim 1 , wherein the secondary heating system comprises a secondary heat shield mounted with the support structure and a secondary heating element mounted between the secondary heat shield and the crucible.  
     
     
         6 . The apparatus of  claim 1 , wherein the primary heating system comprises a primary heat shield mounted with the support structure and a primary heating element mounted between the primary heat shield and the crucible, and wherein the secondary heating system comprises a secondary heat shield mounted with the support structure and a secondary heating element mounted between the secondary heat shield and the crucible.  
     
     
         7 . The apparatus of  claim 1 , wherein the crucible is at least 8 inches in diameter, the primary heating system comprises electric resistive heaters mounted substantially surrounding the crucible, the secondary heating system comprises an electric resistive heater, the primary heat shield comprises graphite, and the secondary heat shield comprises graphite.  
     
     
         8 . The apparatus of  claim 1 , wherein the crystal is CaF 2 .  
     
     
         9 . The apparatus of  claim 1 , wherein the crystal is MgF 2 .  
     
     
         10 . The apparatus of  claim 1 , wherein the crystal is SiO 2 .  
     
     
         11 . A crystal growth and annealing apparatus, comprising: 
 a) A support structure;    b) A primary heating system mounted with the support structure;    c) A crucible support mounted with the support structure movable between first and second positions;    d) A crucible adapted to hold liquid crystal material and solid crystal material, mounted with the crucible support so that the crucible is proximal the primary heating system when the crucible support is in the first position and below the primary heating system when the crucible support is in the second position;    e) A secondary heating system mounted with the crucible support proximal the bottom of the crucible.    
     
     
         12 . The apparatus of  claim 11 , wherein the primary heating system and secondary heating system have heating capacities that maintain the temperature of a crystal within the crucible uniform to within about 8° C. per inch.  
     
     
         13 . The apparatus of  claim 11 , wherein the primary heating system and secondary heating system have heating capacities that maintain the temperature of a crystal within the crucible uniform to within about 4° C. per inch.  
     
     
         14 . The apparatus of  claim 11 , wherein the primary heating system comprises a primary heat shield mounted with the support structure and a primary heating element mounted between the primary heat shield and the crucible.  
     
     
         15 . The apparatus of  claim 11 , wherein the secondary heating system comprises a secondary heat shield mounted with the crucible support and a secondary heating element mounted between the secondary heat shield and the crucible.  
     
     
         16 . The apparatus of  claim 11 , wherein the primary heating system comprises a primary heat shield mounted with the support structure and a primary heating element mounted between the primary heat shield and the crucible, and wherein the secondary heating system comprises a secondary heat shield mounted with the crucible support and a secondary heating element mounted between the secondary heat shield and the crucible.  
     
     
         17 . The apparatus of  claim 11 , wherein the crucible is at least 8 inches in diameter, the primary heating system comprises electric resistive heaters mounted substantially surrounding the crucible, the secondary heating system comprises an electric resistive heater, the primary heat shield comprises graphite, and the secondary heat shield comprises graphite.  
     
     
         18 . The apparatus of  claim 11 , wherein the crystal is Ca F 2 .  
     
     
         19 . The apparatus of  claim 11 , wherein the crystal is MgF 2 .  
     
     
         20 . The apparatus of  claim 11 , wherein the crystal is SiO 2 .  
     
     
         21 . A method of annealing a crystal in an apparatus having a primary heating system proximal the top and sides of the crystal and a secondary heating system proximal the bottom of the crystal, comprising supplying heat using the primary heating system and the secondary heating system to the crystal to maintain the crystal's average temperature at a decreasing value over time without allowing a temperature gradient of more than about 8° C. per inch.  
     
     
         22 . The method of  claim 21 , wherein the temperature gradient is kept less than about 4° C. per inch.  
     
     
         23 . The method of  claim 11 , wherein the crystal material is CaF 2 .  
     
     
         24 . The method of  claim 11 , wherein the crystal material is MgF 2 .  
     
     
         25 . The method of  claim 11 , wherein the crystal material is SiO 2 .  
     
     
         26 . A method of producing a crystal in an apparatus having a primary heating system proximal the top and sides of the crystal and a secondary heating system proximal the bottom of the crystal, comprising: 
 a) Forming a liquid of crystal material in a crucible by heating the crystal material using heat from the primary heating system;    b) Lowering the crucible out of the primary heating system so that successive portions of said liquid crystal material cool to a temperature suitable for crystal formation;    c) Reducing the temperature of the primary heating system;    d) Raising the crucible into the primary heating system and supplying heat from the secondary heating system;    e) Reducing the heat output of the primary and secondary heating systems so that the average temperature of the crystal is reduced over time without allowing a temperature gradient in the crystal more than about 8° C. per inch.    
     
     
         27 . The method of  claim 26 , wherein the temperature gradient is kept less than about 4° C. per inch.  
     
     
         28 . The method of  claim 26 , wherein the crystal material is CaF 2 .  
     
     
         29 . The method of  claim 26 , wherein the crystal material is MgF 2 .  
     
     
         30 . The method of  claim 26 , wherein the crystal material is SiO 2 .