METHODS FOR PRODUCING SINGLE CRYSTAL SILICON INGOTS WITH REDUCED SEED END OXYGEN
Methods for producing single crystal silicon ingots with a reduced oxygen content toward the seed end of the ingot are disclosed. The methods may involve controlling growth conditions during crown formation and, in some embodiments, controlling the rate of crucible rotation during crown rotation to increase the time the crucible is rotated at or below a threshold value during crown growth.
1 . A method for producing a single crystal silicon ingot from a silicon melt held within a crucible, the ingot having a constant diameter portion, a neck portion, a crown portion disposed between the neck portion and the constant diameter portion and tapering radially outward toward the constant diameter portion, and a terminal end, the method comprising:
contacting the melt with a seed crystal to initiate crystal growth;
pulling the seed crystal away from the melt to form the neck portion of the ingot;
forming a crown portion of the ingot, the crucible rotating while forming the crown at a crucible rotation rate;
forming the constant diameter portion of the ingot after the crown portion reaches a target diameter, the constant diameter portion having a seed region that extends from the crown portion and toward the terminal end of the ingot, the crucible rotation rate during formation of the crown portion being controlled to reduce the oxygen content in the seed region of the constant diameter portion of the ingot.
2 . The method as set forth in claim 1 wherein the crucible rotation rate is controlled to be at or less than a threshold value during formation of at least a portion the crown portion.
3 . The method as set forth in claim 1 further comprising rotating the crucible during formation of the neck portion, wherein controlling the crucible rotation rate during formation of the crown portion comprises maintaining the rate of rotation during formation of at least about the last 10% of the length of the crown to about 20% or less of the rate of rotation during formation of the neck.
4 . The method as set forth in claim 1 further comprising rotating the crucible during formation of the neck portion, wherein controlling the crucible rotation rate during formation of the crown portion comprises maintaining the rate of rotation during formation of at least about the last 20% of the length of the crown to about 20% or less of the rate of rotation during formation of the neck.
5 . The method as set forth in claim 1 wherein controlling the crucible rotation rate during formation of the crown portion comprises maintaining the rate of rotation during formation of at least about the last 10% or about the last 20% of the length of the crown to less than about 3.0 RPM.
6 . The method as set forth in claim 1 further comprising rotating the crucible during formation of the neck portion, wherein controlling the crucible rotation rate during formation of the crown portion comprises reducing the rate of rotation by at least about 40% during formation of about the first 60% of the crown length relative to the rate of rotation of the crucible during formation of the neck.
7 . The method as set forth in claim 1 further comprising rotating the crucible during formation of the neck portion, wherein controlling the crucible rotation rate during formation of the crown portion comprises reducing the rate of rotation by at least about 40% during formation of about the first 50% of the crown length relative to the rate of rotation of the crucible during formation of the neck.
8 . The method as set forth in claim 1 further comprising rotating the crucible during formation of the neck portion, wherein controlling the crucible rotation rate during formation of the crown portion comprises reducing the rate of rotation by at least about 50% during formation of about the first 60% of the crown length relative to the rate of rotation of the crucible during formation of the neck.
9 . The method as set forth in claim 1 further comprising rotating the crucible during formation of the neck portion, wherein controlling the crucible rotation rate during formation of the crown portion comprises reducing the rate of rotation by at least about 80% during formation of about the first 80% of the crown length relative to the rate of rotation of the crucible during formation of the neck.
10 . The method as set forth in claim 1 wherein the melt is heated by a heating system, the method further comprising controlling the power supplied to the heating system during formation of the crown portion to compensate for reduced crown diameter caused by a lowered crucible rotation during crown growth.
11 . The method as set forth in claim 10 wherein controlling the power supplied to the heating system comprises reducing the power during at least a portion of formation of the crown portion.
12 . The method as set forth in claim 11 wherein the heater power is reduced by at least about 1% in about the first 25% of crown diameter growth.
13 . The method as set forth in claim 11 wherein the heater power is reduced by at least about 5% during growth of about the first 50% of the crown diameter growth.
14 . The method as set forth in claim 1 wherein the constant diameter portion of the ingot has a length and the seed region of the constant diameter portion has a length, the length of the seed region being less than about 10% of the length of the constant diameter portion.
15 . The method as set forth in claim 14 wherein at least about 25% of the seed region of the constant diameter portion of the ingot has an oxygen concentration, the oxygen concentration being less than about 4.4 ppma.
16 . An ingot produced by the method of claim 1 .
17 . A single crystal silicon ingot grown by a Czochralski method and comprising:
a constant diameter portion;
a neck portion;
a crown portion disposed between the neck portion and the constant diameter portion and tapering radially outward toward the constant diameter portion;
a terminal end, the constant diameter portion having a seed region that extends from the crown portion and toward the terminal end of the ingot and having a length of about 150 mm or less, a portion of the seed region having an oxygen concentration of about 4.4 ppma or less.
18 . The single crystal silicon ingot as set forth in claim 17 wherein at least about 15% of the length of the seed region has an oxygen content of about 4.4 ppma or less.
19 . The single crystal silicon ingot as set forth in claim 17 wherein at least about 25% of the length of the seed region has an oxygen content of about 4.4 ppma or less.
20 . The single crystal silicon ingot as set forth in claim 17 wherein from about 5% to about 50% of the length of the seed region has an oxygen concentration of about 4.4 ppma or less.
21 . The single crystal silicon ingot as set forth in claim 17 wherein the constant diameter portion of the ingot has a diameter of at least about 150 mm, about 200 mm, about 300 mm or about 450 mm.
22 . A population of wafers sliced from the seed region of the constant diameter portion of the ingot of claim 17 , wherein at least about 5% of the wafers have an oxygen concentration of about 4.4 ppma or less.
23 . A population of wafers as set forth in claims 22 wherein from about 5% to about 50% of the wafers have an oxygen concentration of about 4.4 ppma or less.