Crystal pulling system and methods for producing monocrystalline ingots with reduced edge band defects

Abstract

A crystal pulling system for growing a monocrystalline ingot from a melt of semiconductor or solar-grade material includes a crucible for containing the melt of material, a pulling mechanism configured to pull the ingot from the melt along a pull axis, and a multi-stage heat exchanger defining a central passage for receiving the ingot as the ingot is pulled by the pulling mechanism. The heat exchanger defines a plurality of cooling zones arranged vertically along the pull axis of the crystal pulling system. The plurality of cooling zones includes two enhanced-rate cooling zones and a reduced-rate cooling zone disposed vertically between the two enhanced-rate cooling zones.

What is claimed is: 
     
         1 . A crystal pulling system for growing a monocrystalline ingot from a melt of semiconductor or solar-grade material, the system comprising:
 a crucible for containing the melt of semiconductor or solar-grade material;   a pulling mechanism configured to pull the ingot from the melt along a pull axis; and   a multi-stage heat exchanger defining a central passage for receiving the ingot as the ingot is pulled by the pulling mechanism, wherein the heat exchanger defines a plurality of cooling zones arranged vertically along the pull axis of the crystal pulling system, wherein the plurality of cooling zones includes two enhanced-rate cooling zones and a reduced-rate cooling zone disposed vertically between the two enhanced-rate cooling zones.   
     
     
         2 . The crystal pulling system of  claim 1 , wherein the pulling mechanism includes a seed crystal connected to a pull cable configured to raise and lower the seed crystal and to rotate the seed crystal about the pull axis, the system further comprising a controller configured to control a melt/crystal interface profile by controlling a rate of rotation of the seed crystal. 
     
     
         3 . The crystal pulling system of  claim 1  further comprising a heat shield disposed radially outward from the heat exchanger, the heat shield defining an elongate passage for receiving the ingot as the ingot is pulled from the melt, wherein the heat exchanger is positioned within the passage defined by the heat shield. 
     
     
         4 . The crystal pulling system of  claim 3 , wherein the heat exchanger includes a cylindrical fluid-cooled housing having an upper portion and a lower portion spaced vertically from the upper portion by an annular gap, and wherein the heat shield is positioned and oriented to reflect heat towards the ingot through the gap. 
     
     
         5 . The crystal pulling system of  claim 3 , wherein the heat shield includes insulation. 
     
     
         6 . The crystal pulling system of  claim 3 , wherein the heat shield includes a radiation shield. 
     
     
         7 . The crystal pulling system of  claim 3 , further comprising a gas flow barrier extending under the heat exchanger and between the heat exchanger and the melt such that an annular opening is defined between the gas flow barrier and the heat exchanger. 
     
     
         8 . The crystal pulling system of  claim 7 , wherein the gas flow barrier is connected to a lower end of the heat shield and extends radially inward from the heat shield and under the heat exchanger. 
     
     
         9 . The crystal pulling system of  claim 7 , wherein the gas flow barrier occludes the heat exchanger from a direct vertical line-of-sight with the melt. 
     
     
         10 . The crystal pulling system of  claim 1 , wherein the heat exchanger includes a cylindrical fluid-cooled housing having an upper portion and a lower portion spaced vertically from the upper portion by an annular gap, and wherein the lower portion of the housing defines a first enhanced-rate cooling zone, the upper portion of the housing defines a second enhanced-rate cooling zone, and a reduced-rate cooling zone is defined between the lower portion of the housing and the upper portion of the housing. 
     
     
         11 . The crystal pulling system of  claim 10 , wherein the first enhanced-rate cooling zone has a first cooling rate, the reduced-rate cooling zone has a second cooling rate less than the first cooling rate, and the second enhanced-rate cooling zone has a third cooling rate greater than the second cooling rate. 
     
     
         12 . The crystal pulling system of  claim 10 , wherein the heat exchanger includes a quartz baffle disposed proximate the gap defined by the housing, the quartz baffle configured to inhibit gas flow through the gap.