INDUCTION MELTING OF SILICON USING TANTALUM AS A HEAT-DELIVERY SEED
Methods and configurations for melting silicon by induction using tantalum as a heat-delivery seed are presented. The tantalum supports induction-generated electrical eddy currents and resultantly heats to relatively high temperatures. The heat of the tantalum then conducts into surrounding silicon, which melts. Because the tantalum does not melt at the relatively low melting temperature of silicon, the tantalum will not contaminate the molten silicon, which may thus have a high purity. The melted silicon may then be used in processes that form thin films on a substrate or ingots.
1 . A method for melting silicon, the method comprising:
combining into a mixture, solid silicon and solid tantalum;
subjecting, in a vessel, the mixture to a time-varying magnetic field to heat the solid tantalum; and
allowing the heated solid tantalum to conduct heat to the solid silicon to melt the solid silicon into liquid silicon.
2 . The method of claim 1 , wherein the solid tantalum is cylindrical.
3 . The method of claim 1 , wherein the mixture is substantially uniform such that the solid tantalum and the solid silicon are homogeneously mixed.
4 . The method of claim 1 , wherein the mixture is non-uniform such that a concentration of the solid tantalum varies vertically in the vessel.
5 . The method of claim 1 , further comprising adjusting a spatial distribution of the time-varying magnetic field based, at least in part, on a rate of sinking of the solid tantalum in the liquid silicon.
6 . The method of claim 1 , further comprising adjusting a magnitude of the time-varying magnetic field based, at least in part, on a rate of sinking of the solid tantalum in the liquid silicon.
7 . The method of claim 1 , wherein the time-varying magnetic field is produced by a vertically-moveable solenoid, the method further comprising:
controlling the vertical motion of the solenoid based, at least in part, on a rate of sinking of the solid tantalum in the liquid silicon.
8 . The method of claim 1 , wherein the time-varying magnetic field is produced by a vertically-moveable solenoid, the method further comprising:
controlling the vertical motion of the solenoid based, at least in part, on a rate of melting of the solid silicon.
9 . The method of claim 1 , wherein the time-varying magnetic field is produced by a solenoid that comprises two or more individually operable portions, the method further comprising:
selectively operating the two or more individually operable portions of the solenoid based, at least in part, on a rate of sinking of the solid tantalum in the liquid silicon.
10 . The method of claim 1 , wherein the time-varying magnetic field is produced by a solenoid that comprises two or more individually operable portions, the method further comprising:
selectively operating the two or more individually operable portions of the solenoid based, at least in part, on a rate of melting of the solid silicon.
11 . The method of claim 1 , further comprising, after the solid tantalum has sunk toward the bottom of the vessel:
condensing silicon vapor from the liquid silicon onto a surface that is above the vessel.
12 . The method of claim 1 , further comprising, after the solid tantalum has sunk toward the bottom of the vessel:
placing a seed crystal into the liquid silicon; and
growing a silicon crystal from the seed crystal.
13 . A system for melting silicon, the system comprising:
a refractory crucible;
a mixture of solid silicon and solid tantalum in the refractory crucible; and
an induction coil to produce a time-varying magnetic field in the mixture to heat the solid tantalum, wherein the heated solid tantalum conducts heat to the solid silicon to melt the solid silicon into liquid silicon.
14 . The system of claim 13 , wherein the solid tantalum is cylindrical.
15 . The system of claim 13 , wherein the mixture is substantially uniform such that the solid tantalum and the solid silicon are homogeneously mixed.
16 . The system of claim 13 , wherein the mixture is non-uniform such that a concentration of the solid tantalum varies vertically in the vessel.
17 . The system of claim 13 , further comprising a controller to adjust a distribution of the time-varying magnetic field based, at least in part, on a rate of sinking of the solid tantalum in the liquid silicon.
18 . The system of claim 13 , further comprising a controller to adjust the time-varying magnetic field based, at least in part, on a rate of sinking of the solid tantalum in the liquid silicon.
19 . The system of claim 13 , wherein the induction coil is vertically moveable, the system further comprising:
a controller to vertically move the solenoid based, at least in part, on a rate of sinking of the solid tantalum in the liquid silicon.
20 . The system of claim 13 , wherein the induction coil comprises two or more individually operable portions, the system further comprising:
a controller to selectively operate the two or more individually operable portions of the solenoid based, at least in part, on a rate of melting of the solid silicon.