Process for producing low defect density silicon

Abstract

The present invention relates to single crystal silicon, in ingot or wafer form, which contains an axially symmetric region which is free of agglomerated intrinsic point defects, and a process for the preparation thereof.

What is claimed is:  
     
         1 . A process for growing a single crystal silicon ingot in which the ingot comprises a central axis, a seed-cone, an end-cone and a constant diameter portion between the seed-cone and the end-cone having a circumferential edge and a radius extending from the central axis to the circumferential edge, the ingot being grown from a silicon melt and then cooled from the solidification temperature in accordance with the Czochralski method, the process comprising 
 controlling (i) a growth velocity, v, (ii) an average axial temperature gradient, G 0 , during the growth of the constant diameter portion of the crystal over the temperature range from solidification to a temperature of no less than about 1325° C., and (iii) the cooling rate of the crystal from the solidification temperature to about 1,050° C. to cause the formation of an axially symmetrical segment which is substantially free of agglomerated intrinsic point defects wherein the axially symmetric region extends inwardly from the circumferential edge of the ingot, has a width as measured from the circumferential edge radially toward the central axis of the ingot which is at least about three-tenths the length of the radius of the ingot, and has a length as measured along the central axis of at least about two-tenths the length of the constant diameter portion of the ingot.    
     
     
         2 . The process of  claim 1  wherein the crystal has a nominal diameter of about 150 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 10 hours.  
     
     
         3 . The process of  claim 1  wherein the crystal has a nominal diameter of about 150 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 15 hours.  
     
     
         4 . The process of  claim 1  wherein the crystal has a nominal diameter of about 200 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 10 hours.  
     
     
         5 . The process of  claim 1  wherein the crystal has a nominal diameter of about 200 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 20 hours.  
     
     
         6 . The process of  claim 1  wherein the crystal has a nominal diameter of greater than 200 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 40 hours.  
     
     
         7 . The process of  claim 1  wherein the crystal has a nominal diameter of greater than 200 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 60 hours.  
     
     
         8 . The process of  claim 1  wherein the length of the axially symmetric region is at least about three-tenths the length of the constant diameter portion of the ingot.  
     
     
         9 . The process of  claim 8  wherein the crystal has a nominal diameter of about 150 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 10 hours.  
     
     
         10 . The process of  claim 8  wherein the crystal has a nominal diameter of about 150 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 15 hours.  
     
     
         11 . The process of  claim 8  wherein the crystal has a nominal diameter of about 200 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 10 hours.  
     
     
         12 . The process of  claim 8  wherein the crystal has a nominal diameter of about 200 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 20 hours.  
     
     
         13 . The process of  claim 8  wherein the crystal has a nominal diameter of greater than 200 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 40 hours.  
     
     
         14 . The process of  claim 8  wherein the crystal has a nominal diameter of greater than 200 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 60 hours.  
     
     
         15 . The process of  claim 1  wherein the growth velocity, v, and the instantaneous axial temperature gradient, G 0 , are controlled such that the ratio, v/G 0 , ranges in value from about 0.6 to about 1.5 times the critical value of v/G 0 .  
     
     
         16 . The process of  claim 1  wherein the growth velocity, v, and the instantaneous axial temperature gradient, G 0 , are controlled such that the ratio v/G 0  ranges in value from about 0.75 to about 1 times the critical value of v/G 0 .  
     
     
         17 . The process of  claim 1  wherein the average axial temperature gradient, G 0 , is controlled over the temperature range from solidification to a temperature of no less than about 1350° C.  
     
     
         18 . The process of  claim 17  the crystal has a nominal diameter of about 150 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 15 hours.  
     
     
         19 . The process of  claim 17  the crystal has a nominal diameter of about 200 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 15 hours.  
     
     
         20 . The process of  claim 17  the crystal has a nominal diameter of greater than 200 mm and is cooled from the solidification temperature to a temperature of at least about 1,050° C. over a period of at least about 40 hours.