Methods for producing a product ingot having low oxygen content
Methods for producing a product ingot from a silicon melt held within a crucible are disclosed. The methods involve evaluating one or more ingot puller apparatus to determine if the apparatus is capable of producing low oxygen content silicon product ingots. A sample rod is pulled from the silicon melt and the oxygen content of the sample rod is measured.
1. A method for producing a product ingot having an oxygen concentration of less than 5 ppma, the method comprising:
adding solid silicon to a crucible of a first ingot puller apparatus;
heating the solid silicon in the crucible of the first ingot puller apparatus to cause a first silicon melt to form in the crucible;
pulling a first sample rod from the first silicon melt, the first sample rod having a first sample rod diameter;
measuring an oxygen content of the first sample rod;
adding solid silicon to a crucible of a second ingot puller apparatus;
heating the solid silicon in the crucible of the second ingot puller apparatus to cause a second silicon melt to form in the crucible;
pulling a second sample rod from the second silicon melt, the second sample rod having a second sample rod diameter;
measuring an oxygen content of the second sample rod;
comparing the oxygen content of the first sample rod to the oxygen content of the second sample rod; and
growing the product ingot in the ingot puller apparatus in which the sample rod having a lower oxygen content was grown, the product ingot having a diameter, the first sample rod diameter and second sample rod diameter each being less than the diameter of the product ingot.
2. The method as set forth in claim 1 further comprising:
adding solid silicon to a crucible of a third ingot puller apparatus;
heating the solid silicon in the crucible of the third ingot puller apparatus to cause a third silicon melt to form in the crucible;
pulling a third sample rod from the third silicon melt, the third sample rod having a sample rod diameter;
measuring an oxygen content of the third sample rod;
comparing the oxygen content of the first sample rod to the oxygen content of the third sample rod; and
growing the product ingot in the ingot puller apparatus in which the sample rod having the lowest oxygen content of the first, second and third sample rods was grown.
3. The method as set forth in claim 1 wherein the product ingot is a first product ingot, the method comprising growing a second product ingot in the ingot puller apparatus in which the sample rod having a higher oxygen content was grown.
4. The method as set forth in claim 1 wherein:
a first slab is formed from the first sample rod;
measuring the oxygen content of the first sample rod comprises measuring the oxygen content of the first slab;
a second slab is formed from the second sample rod; and
measuring the oxygen content of the second sample rod comprises measuring the oxygen content of the second slab.
5. The method as set forth in claim 1 the oxygen contents of the first and second sample rods are measured by Fourier-transform infrared spectroscopy.
6. The method as set forth in claim 3 wherein the second product ingot has an oxygen content of greater than 5 ppma.
7. The method as set forth in claim 4 wherein:
the first slab comprises at least a portion of a central axis of the first sample rod, the first slab having a thickness of between about 5 mm and about 0.1 mm; and
the second slab comprises at least a portion of a central axis of the second sample rod, the second slab having a thickness of between about 5 mm and about 0.1 mm.
8. The method as set forth in claim 4 comprising etching the first slab prior to measuring the oxygen content of the first sample rod.
9. The method as set forth in claim 8 comprising etching the second slab prior to measuring the oxygen content of the second sample rod.
10. The method as set forth in claim 1 comprising forming one or more coins from the first sample rod, wherein measuring an oxygen content of the first sample rod comprises measuring the oxygen content of one or more of the coins.
11. The method as set forth in claim 10 comprising etching the one or more coins prior to measuring the oxygen content of the sample rod.
12. The method as set forth in claim 10 comprising forming one or more coins from the second sample rod, wherein measuring an oxygen content of the second sample rod comprises measuring the oxygen content of one or more of the coins.
13. The method as set forth in claim 1 wherein the diameter of the first sample rod is less than 50 mm and the diameter of the second sample rod is less than 50 mm.
14. The method as set forth in claim 1 wherein the diameter of the first sample rod is from about 5 mm to about 25 mm and the diameter of the second sample rod is from about 5 mm to about 25 mm.