Method for manufacturing a silicon ingot having uniform phosphorus concentration

Abstract

A method for manufacturing a silicon ingot having uniform phosphorus concentration. The method includes at least the steps of: (i) providing a quasi-uniform molten silicon bath containing at least phosphorus; and (ii) proceeding to the directional solidification of the silicon, wherein a speed (VI) for solidifying the silicon and a rate (JLV) of evaporation of the phosphorus at the liquid/vapor interface of the bath are controlled such that, at each moment of the directional solidification, the following equation is verified: VI=k/(2k) (E), wherein k is the phosphorus transfer coefficient, and k is the distribution coefficient of the phosphorus in the silicon. Also relates to a silicon ingot having uniform phosphorus concentration across a height of at least 20 cm.

Claims

1. A process for manufacturing a silicon ingot having a uniform phosphorus concentration, comprising at least the steps consisting in: (i) providing an almost uniform bath of molten silicon comprising at least phosphorus; and (ii) carrying out the directional solidification of the silicon, with a speed of solidification (V.sub.I) of the silicon and a rate of evaporation (J.sub.LV) of the phosphorus at the liquid-vapor interface of the bath which are controlled such that, at each moment of the directional solidification, the following equation is verified:
V.sub.I=k/(2k)(E) k representing the phosphorus transfer coefficient, and k representing the partition coefficient of the phosphorus in the silicon.

2. The process as claimed in claim 1, wherein the silicon bath undergoes, prior to its use in step (i), a heat treatment conducive to the partial evaporation of the phosphorus.

3. The process as claimed in claim 1, wherein the phosphorus is present in the molten bath in step (i) in a concentration of less than or equal to 50 ppm.

4. The process as claimed in claim 1, wherein the phosphorus is present in the molten bath in step (i) in a concentration of less than or equal to 25 ppm.

5. The process as claimed in claim 1, wherein the phosphorus is present in the molten bath in step (i) in a concentration of less than or equal to 5 ppm.

6. The process as claimed in claim 1, wherein the phosphorus is present in the molten bath in step (i) in a concentration of between 0.3 and 3 ppm.

7. The process as claimed in claim 1, wherein the molten bath is stirred, prior to its use in step (ii) and/or during the solidification in step (ii), using a stirring system, so as to ensure almost uniform dispersion of the phosphorus in the bath of molten silicon.

8. The process as claimed in claim 1, wherein the silicon solidification step (ii) is carried out by means of a gradient freeze method.

9. The process as claimed in claim 1 wherein the phosphorus evaporation rate (J.sub.LV) in step (ii) is controlled via an adjustment of the phosphorus transfer coefficient (k).

10. The process as claimed in claim 9, wherein the phosphorus evaporation rate (J.sub.LV) in step (ii) is controlled by adjusting the temperature of the liquid-vapor interface of the molten bath and/or the pressure in the chamber of the directional solidification furnace used.

11. The process as claimed in claim 1, wherein the silicon solidification speed (V.sub.I) in step (ii) is controlled via an adjustment of the heat flow in the directional solidification furnace used.

12. The process as claimed in claim 11, wherein the silicon solidification speed (V.sub.I) in step (ii) is controlled by controlled extraction of the heat.

13. The process as claimed in claim 1, wherein the average silicon solidification speed (V.sub.I) in step (ii) is between 1 and 20 m/s.

14. The process as claimed in claim 1, wherein the average silicon solidification speed (V.sub.I) in step (ii) is between 5 and 10 m/s.

15. The process as claimed in claim 1, wherein the phosphorus transfer coefficient k in step (ii) is between 1.510.sup.6 and 310.sup.5.

16. The process as claimed in claim 1, wherein the phosphorus transfer coefficient k in step (ii) is between 810.sup.6 and 1.510.sup.5 m/s.

17. The process as claimed in claim 1, wherein the phosphorus evaporation rate (J.sub.LV) is kept constant during the directional solidification in step (ii), the silicon solidification speed (V.sub.I) being adjusted so as to satisfy the equation (E).

18. A silicon ingot having a uniform phosphorus concentration across a height of at least 20 cm.

19. The ingot as claimed in claim 18, said phosphorus concentration being less than or equal to 20 ppm.

20. The ingot as claimed in claim 19, said phosphorus concentration being less than or equal to 5 ppm.

21. The ingot as claimed in claim 18, having a uniform phosphorus concentration across its entire height.

22. The ingot as claimed in claim 21, said ingot having a height of between 10 cm and 1 m.

Description

FIGURES

(1) FIG. 1: Curves, established according to Scheil's law, of variation of the concentration of species incorporated into the solid as a function of the solidified fraction for various values of the partition coefficient k;

(2) FIG. 2: Diagrammatic representation of the controlled parameters, silicon solidification speed V.sub.L and phosphorus evaporation rate J.sub.LV, at a given moment of the directional solidification of the silicon according to the process of the invention.

(3) The invention will now be described by means of the following example, given by way of nonlimiting illustration of the invention.

Example

(4) A silicon feedstock of approximately 60 kg, in the form of lumps with centimetric dimensions, is introduced into a graphite crucible coated with a layer of Si carbide, having interior dimensions of 393939 cm, onto which a release coating of silicon nitride had previously been deposited. The phosphorus content of the feedstock is 15 ppm weight, measured by the Glow Discharge Mass Spectroscopy technique.

(5) The whole is then introduced into the solidification device, a gradient freeze technology vertical furnace, with heating elements (graphite resistors) located in the top part and on the sides of the crucible.

(6) Reduction of the Phosphorus Concentration

(7) The silicon is first subjected to a temperature increase in order to ensure complete melting thereof, and a gradient of 10 K/cm is applied across the height of the bath, the top of which is kept at a temperature of 1650 C. and the bottom of which at a temperature close to 1410 C. for approximately three hours at a pressure of 510.sup.3 mbar in order to promote evaporation of the phosphorus and to bring the phosphorus concentration of the bath back to a value of 2 ppm weight.

(8) Directional Solidification of the Silicon

(9) The temperature in the hot part (temperature of the liquid-vapor interface) is kept equal to 1650 C. throughout the solidification; the phosphorus transfer coefficient k is thus kept constant during the directional solidification.

(10) The natural convection in the molten bath is sufficient for it to be possible for said bath to be considered an almost uniform bath.

(11) The silicon solidification speed is adjusted so as to satisfy the equation V.sub.I=k/(2k) (E), with k the partition coefficient of the phosphorus in the silicon, of approximately 0.35.

(12) The growth of the silicon is initiated by opening insulating shutters. The gradual opening of the insulating shutters makes it possible to control the heat to be evacuated throughout the process.

(13) Complete solidification of the liquid bath 22 cm in height is carried out in approximately 10 hours, the average solidification speed is therefore 2.2 cm/h (6 m/s). In the growth regime, the power consumed by the furnace is approximately 38 kW. The heating power is then reduced and the ingot is brought back to ambient temperature and demolded from the crucible.

(14) Result

(15) An analysis of the variations in resistivity within the silicon ingot, by 4-point measurement, makes it possible to verify that the phosphorus is uniformly distributed in the ingot.

REFERENCES

(16) [1] W. G. Pfann, Zone melting (2.sup.nd edition), Wiley, New York 1966; [2] S. S Zheng et al., Mass Transfer of Phosphorus in Silicon Melts Under Vacuum Induction Refining, Metall. Mat. Trans. B, 41B (2010) 1268-1273.