JP4248743B2 - Purification method for molten silicon - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a silicon purification method for obtaining high-purity silicon suitable for, for example, a solar cell.
[0002]
[Prior art]
Photovoltaic power generation is expected worldwide as a new energy source to cope with an energy crisis expected in the near future and as a clean energy source that does not deteriorate the global environment. In Japan, due to national subsidies, solar cell production centering on residential solar power generation facilities has been growing at an annual rate of more than 10% in recent years. In order to reduce the cost of solar cells and promote their widespread use, various technological developments such as improvement of conversion efficiency from light energy to electric energy are being accomplished. However, as a technical problem that has been addressed since long ago, there is a problem of raw materials as semiconductors used in solar cells. Silicon as a raw material for semiconductor integrated circuits needs to be extremely high in purity, and most of the silicon is currently manufactured by the Siemens method. In the Siemens method, trichlorosilane gas is produced using 99% purity silicon as a raw material, and this is made into a high purity gas by distillation purification, and then extremely high purity silicon is produced by reduction deposition treatment using hydrogen gas. Since such high-purity silicon has excessive quality (excessive high purity) as a raw material for solar cells and is expensive, most of silicon for solar cells is a process for producing silicon for semiconductor integrated circuits. The end material generated in the (single crystal pulling step) has been used.
[0003]
On the other hand, research and development on manufacturing technology for low-cost solar cell grade purity silicon (SOG-Si) to replace the Siemens method has been continued from about 1980 to the present. Metallurgically, metal impurity elements such as Fe and Al in silicon have a very small equilibrium distribution coefficient in solid phase silicon with reference to liquid phase silicon. Therefore, it has been found that by slowly cooling and solidifying molten silicon in one direction, the metal impurity element is discharged into the melt and the impurities can be efficiently removed from the solid phase silicon. However, B and P, which are non-metallic light element impurities, have their equilibrium distribution coefficients in silicon of 0.8 and 0.35, respectively, which are difficult to remove by solid-liquid distribution. Among these impurities, P can be removed by evaporation by vacuum melting, but the removal of B is the biggest problem of SOG-Si manufacturing technology.
[0004]
The hint of the removal technique of B is given by the experimental result of Theuerer (Journal of Metals, October 1956, pp. 1316-1319), and the B removal effect can be obtained by subjecting the silicon to melting purification treatment in a humidified hydrogen gas atmosphere. It is said that.
[0005]
Based on the experimental results of Theuerer, a method of removing B by blowing an inert gas containing hydrogen and water vapor into a molten silicon is disclosed in US Pat. No. 4,097,584 and US Pat. No. 5,972. No. 107 and JP-A-4-193706.
[0006]
[Problems to be solved by the invention]
However, as one of the reasons that none of these methods has been industrialized, there are two facts that have become clear from our experiments. One is that the decrease rate of B is small in any of the methods, and the other is that the yield of silicon is lowered due to the splashing of silicon droplets by gas blowing into the molten silicon. The splash during the bubbling treatment of the molten silicon is larger than expected due to the physical properties of the silicon melt. The scattered silicon droplets adhere to a crucible wall such as silica or the ceiling of the processing tank, lowering the silicon yield after the purification process and making the processing tank difficult to maintain.
[0007]
Therefore, the present invention increases the B removal rate in silicon by the reaction between the processing gas containing water vapor and hydrogen and the molten silicon, and the reaction between the processing gas and the molten silicon proceeds without causing splash of the silicon melt. The purpose is to be able to.
[0008]
[Means for Solving the Problems]
In the purification process of silicon melt according to the present invention, while forcing stirred silicon melt in the treatment tank, seen write blow a process gas containing water vapor and hydrogen to the molten liquid surface, the silicon melt is forced by the impeller to rotate The process gas is agitated and blown through a gap between the rotating shaft of the impeller and the outer pipe covering the impeller . The process gas, but it may also be blown from above just below the liquid surface of the silicon melt.
[0009]
Sheet by inserting a baffle plate into the silicon melt, the silicon melt is forced stirring can cause turbulence, thereby making it possible to further enhance the reaction between the treatment gas and the silicon melt.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
According to Theuerer (Journal of Metals, October 1956, pp. 1316-1319), B in the fused silicon is an evaporating substance (this evaporating substance is not specified) due to the reaction between the humidified hydrogen gas and the fused silicon. It is removed and decreases. In order to increase the reaction efficiency between such gas and liquid, it is desirable to increase the area of the gas-liquid interface, and as in the above-mentioned plurality of patent documents, silicon purification in which a processing gas is bubbled into the molten silicon A method has been proposed. That is, B that reacts with the processing gas and becomes a component of the evaporating gas escapes from the liquid phase silicon as a gas phase component, so that the B concentration in the silicon near the gas-liquid interface decreases.
[0011]
However, when the movement of B atoms in the molten silicon is mainly due to diffusion, it takes time to move B atoms to the gas-liquid interface. Will not improve as expected. Therefore, stirring of the liquid phase silicon is desired to improve the removal rate. Here, conventionally, since the liquid flow is caused by bubbling by blowing gas into the liquid phase, it was considered that the bubbling method itself has a liquid stirring action. However, it has been found by the present inventors for the first time that the de-B rate is significantly improved by using artificial forced stirring in combination with bubbling.
[0012]
Also, during the bubbling process, the silicon droplets were scattered more than expected due to the physical properties of the molten silicon as the liquid phase, and the silicon droplets scattered on the crucible wall such as silica and the ceiling of the processing tank Adhesion occurs. Since it is difficult to peel off the silicon that has adhered and solidified in this way, the present inventors have considered a method of utilizing a gas-liquid reaction without causing splash. In addition, the method of spraying the processing gas directly on the surface of the molten silicon does not generate splash, and the method of blowing the processing gas directly above the surface of the molten silicon can substantially reduce the silicon yield after processing. It was found that the maintenance of the treatment tank would not be hindered. The combination of such a gas-liquid reaction method and forced stirring of the liquid phase makes it possible to increase the rate of decrease of B in the molten silicon.
[0013]
1, 2, and 3, to illustrate an example of one reference embodiment closely related to the present invention two embodiments according to the invention and, respectively, show a schematic cross-sectional view of the silicon purifier Yes. In the drawings of the present application, the same reference numerals represent the same or corresponding parts.
[0014]
1 and 2 includes a silicon melting furnace including an electromagnetic induction heating device 4, an impeller 7 attached to a rotary shaft 6 for forcibly stirring the molten silicon 12, and a processing gas. An outer tube 8 that is coaxial with the rotating shaft 6 so as to blow from above the silicon melt liquid surface or directly below the liquid surface, and an elongated baffle plate 9 that generates turbulence to enhance the forced stirring effect of the silicon melt 12. And an exhaust pipe 11 for discharging the treated waste gas from the melting furnace, and the atmosphere surrounding the molten silicon is kept airtight.
[0015]
In the melting furnace, a protective crucible 1 made of graphite is installed inside a refractory material 3 such as alumina, and a silica crucible 2 is installed further inside. The raw material silicon charged in the silica crucible 2 is melted by heat generated from the graphite crucible 1 heated by electromagnetic induction in an inert gas atmosphere to become a silicon melt 12. Thereafter, since the molten silicon 12 has electrical conductivity, it can be directly heated by electromagnetic induction and is maintained at a predetermined purification treatment temperature.
[0016]
The processing gas used for silicon purification reaches the processing gas outlet 13 through the gap between the shaft 6 rotating the impeller 7 that forcibly agitates the molten silicon 12 and the outer tube 8 covering the shaft 6, and the molten metal surface. It is blown out from above (FIG. 1) or from above (FIG. 2) to a position directly below the molten metal surface. The shaft 6 for rotating the impeller 7 and the cladding tube 8 for constituting the processing gas passage are integrated, and can be moved up and down at the same time. Such forcible stirrer and a processing gas blowing device that could be manufactured by refractory material such as graphite or silicon nitride.
[0017]
Melt forced stirring effect by diffraction Utatehanesha 7 increased by the generation of the molten metal turbulence due to the use of a baffle plate 9 may be made of a refractory material such as graphite, enhancing the rate of removal of B by the gas-liquid reaction of the silicon melt 12 Can be. Waste gas after the gas-liquid reaction is discharged from the gas discharge pipe 11, but it is also possible to keep the inside of the treatment tank in a reduced pressure atmosphere by vacuum suction from the discharge pipe 11.
[0018]
In FIG. 3 as a reference embodiment closely related to the present invention, a rotating magnetic field generating coil 16 for using the Lorentz force for forcibly stirring the molten silicon 12 is disposed, which is not shown for generating a rotating magnetic field. Connected to a three-phase AC transformer. In order to use this coil 16, a resistance heating element 15 is used as a method for heating and melting silicon. The heated and melted silicon melt 12 is forcibly agitated so as to rotate by Lorentz force. However, the forced agitation of the molten metal in the vertical direction is also applied by the turbulent action by the baffle plate 9, and a stronger forced agitation force is obtained. A processing gas is blown from the gas blowing pipe 14 directly onto the molten metal surface. The waste gas after the gas-liquid reaction is discharged from the gas discharge pipe 11. However, as described above, the inside of the treatment tank can be held in a reduced pressure atmosphere by vacuum suction from the discharge pipe 11.
[0019]
Example 1
In the silica crucible 2 shown in FIG. 1, 3 kg of high-purity silicon (SEG-Si) for semiconductor integrated circuits to which 7 ppm of B was added was charged. The inside of the purification apparatus was filled with an Ar + 3% H ₂ + 3% H ₂ O gas atmosphere, and the silicon was melted by the heat generation of the graphite protective crucible 1 by the electromagnetic induction coil 4. The impeller 7 for forced stirring and the baffle plate 9 were lowered from the ceiling portion and inserted into the silicon melt 12 held at 1480 ° C., and the treatment gas outlet 13 was set immediately above the melt surface. Thereafter, the forced stirring impeller 7 is rotated at a speed of 300 rpm while blowing Ar + 3% H ₂ + 3% H ₂ O gas as a processing gas at a flow rate of 2 (l / min) from the outlet 13 toward the molten metal surface. This treatment was continued for 2 hours. When the B concentration in the silicon after the treatment was analyzed, it was reduced to 3.5 ppm. It should be noted that no silicon droplets jumped during the purification process.
[0020]
(Example 2)
In the silica crucible 2 shown in FIG. 2, 3 kg of SEG-Si to which 7 ppm of B was added was charged. The inside of the purification apparatus was filled with an Ar + 3% H ₂ + 3% H ₂ O gas atmosphere, and the silicon was melted by the heat generation of the graphite protective crucible 1 by the electromagnetic induction coil 4. The impeller 7 and the baffle plate 9 for forced stirring are inserted into the silicon melt 12 held at 1480 ° C. while being lowered from the ceiling portion, and while the process gas is blown out from the blowout port 13, the blowout port 13 is opened from above. It was set immediately below (a slight bubbling occurred). The forced stirring impeller 7 is rotated at a speed of 300 rpm while Ar + 3% H ₂ + 3% H ₂ O gas as a processing gas is blown from the blowing port 13 at a flow rate of 2 (l / min). Continued for hours. When the B concentration in the silicon after the treatment was analyzed, it was reduced to 3.0 ppm. During this refining process, the molten silicon jumped very little.
[0021]
( Reference Example 1 )
In Reference Example 1 closely related to the present invention, 3 kg of SEG-Si to which 7 ppm of B was added was charged in the silica crucible 2 shown in FIG. The inside of the purifier was placed in an Ar + 3% H ₂ + 3% H ₂ O gas atmosphere, and silicon was melted by the graphite heater 15 and maintained at 1480 ° C. Then, while supplying a three-phase alternating current to the coil 16 to generate a 5000 Oersted rotating magnetic field, Ar + 3% H ₂ + 3% H ₂ O gas is blown onto the molten silicon surface at a flow rate of 2 (l / min). The purification process for 2 hours was continued. When the B concentration in the silicon after the treatment was analyzed, it was lowered to 4.0 ppm. It should be noted that no silicon droplets jumped during the purification process.
[0022]
(Example 3 )
The purification process was performed for 2 hours under the same conditions as in Example 1 except that the baffle plate 9 was not used. When the B concentration in the silicon after the treatment was analyzed, it was lowered to 4.2 ppm. During the purification process, no silicon droplet jumped.
[0023]
(Comparative Example 1)
In FIG. 4, the method of spraying process gas on the silicon melt surface without forced stirring as Comparative Example 1 is shown in a schematic cross-sectional view, and the heating furnace structure as shown in FIGS. 1 and 2 is omitted. Yes. In Comparative Example 1, the purification treatment was continued for 2 hours under the same conditions as in Reference Example 1 except that there was no forced stirring. When the B concentration in the silicon after the treatment was analyzed, it was reduced only to 6.3 ppm. During the purification process, no silicon droplet jumped.
[0024]
(Comparative Example 2)
In FIG. 5, as a comparative example 2, a method of blowing a processing gas into a silicon melt 12 without forced stirring and bubbling is shown in a schematic cross-sectional view, and the heating furnace structure as shown in FIGS. 1 and 2 is omitted. Is shown. In Comparative Example 2, the purification process was continued for 2 hours under the same conditions as in Reference Example 1 except that the processing gas was blown deeply into the silicon melt 12 without forced stirring and bubbling was performed. It was. When the B concentration in the silicon after the treatment was analyzed, it was only reduced to 5.3 ppm. During the refining process, silicon droplets jumped a lot, and the inner surface of the silica crucible 2 reached the ceiling of the refining device.
[0025]
【The invention's effect】
As described above, according to the method of the present invention, in the purification for removing the molten silicon by the treatment gas containing water vapor and hydrogen, the efficiency of the conventional method is improved without causing the silicon droplet to jump from the molten silicon. Purification becomes possible.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing an example of a purification apparatus that can be used in a method for purifying molten silicon according to the present invention.
FIG. 2 is a schematic cross-sectional view showing another example of a purification apparatus that can be used in the method for purifying molten silicon according to the present invention.
Figure 3 is a schematic sectional view showing an example of a can-purification system used in the method for purifying silicon melt you closely related to the present invention.
FIG. 4 is a schematic cross-sectional view for explaining a method for purifying molten silicon as a comparative example.
FIG. 5 is a schematic cross-sectional view for explaining a method for purifying molten silicon as another comparative example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Protective crucible made of graphite, 2 Silica crucible, 3 Alumina refractory material, 4 Electromagnetic induction coil, 5 Furnace shell, 6 Rotating shaft, 7 Impeller, 8 Outer tube covering the rotating shaft, 9 Baffle plate, 10 Ceiling board, 11 treated gas exhaust pipe, 12 silicon melt, 13 process gas outlet, 14 process gas inlet pipe, 15 resistance heating element, 16 rotating magnetic field generating coil.

Claims (3)

A method for purifying silicon melt by forcibly stirring the silicon melt in the treatment tank and blowing a treatment gas containing water vapor and hydrogen onto the melt surface ,
The molten silicon is forcibly stirred by a rotating impeller,
The process gas is blown through a gap between a rotating shaft of the impeller and an outer tube covering the impeller, and a method for purifying molten silicon. While forcibly stirring the silicon melt in the treatment tank, a method for purifying the silicon melt, in which a treatment gas containing water vapor and hydrogen is blown from above directly below the surface of the melt,
The molten silicon is forcibly stirred by a rotating impeller,
The process gas is blown through a gap between a rotating shaft of the impeller and an outer tube covering the impeller, and a method for purifying molten silicon. The method for purifying a molten silicon according to claim 1 or 2 , wherein a baffle plate is inserted into the molten silicon, and the forcedly stirred molten silicon generates a turbulent flow.

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