JPH07267624A - Purification of silicon and apparatus therefor - Google Patents

Abstract

PURPOSE:To enable high-efficiency production of silicon of high purity by permitting the track of the electric current generated between electrodes provided with the bottom of the vessel for the molten silicon and the plasma torch jetting a plasma jet gas to scan the surface of the molten silicon. CONSTITUTION:In the silicon purifier provided with the vessel 30 for holding the molten silicon and the plasma torch 20 which is connected to the plasma generator 28 and jets the plasma gas containing less than 20 vol. % of steam toward the surface of the silicon melt, a plurality of electrodes 26a-d are set to the bottom of the vessel 30 and the power source is provided so that the voltage which has phase difference and varies periodically may be loaded between the plasma torch 20 and the bottom electrodes 26a-d and the track of the electric current generated between the plasma torch 20 and the electrodes 26a-d is made it possible to scan the surface of the silicon melt. Thus, the contacting area between the high-temperature plasma and the silicon surface is largely expanded to increase the removal rate of boron in the silicon whereby the purification time can be shortened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for producing high-purity silicon that can be used as a raw material for solar cells.

[0002]

2. Description of the Related Art As a raw material for solar cells, generally, high-purity silicon having a specific resistance value of 0.1 Ωcm or more and p-type conductivity is used, and the content of impurities contained in silicon is
It is necessary to remove up to ppmw order or less. Conventionally,
Various studies have been made on the removal of these impurities, but boron, which is a doping element, is the most difficult element to remove.

In Japanese Patent Laid-Open No. 63-218506, silicon is melted using thermal plasma excited by high frequency,
A method of removing boron in silicon is shown. According to this method, a part of solid silicon is irradiated with plasma generated by high-frequency excitation of a mixed gas of hydrogen and oxygen to cause zone melting, and boron is removed at the melted part. However, in this method, a part of solid silicon is melted and purified only by heat from plasma, resulting in poor heat utilization efficiency and low productivity. However, there is a drawback that the loss of silicon due to evaporation increases.

On the other hand, the inventors of the present invention have proposed a method (JP-A-4-228414) for injecting a plasma gas jet stream using an inert gas containing water vapor as the plasma gas onto the surface of the molten silicon. In order to further reduce the manufacturing cost, a plasma torch for injecting the plasma jet stream is moved horizontally above the surface of the molten silicon (Japanese Patent Laid-Open No. 4-338108).
Proposed.

However, in order to manufacture a large amount of high-purity silicon used for solar cells, it is desired to develop a method and apparatus for removing boron continuously, which requires a shorter time for removing boron than this technique. Was there.

[0006]

SUMMARY OF THE INVENTION In order to solve the above-mentioned demands for the prior art, the present invention efficiently removes boron by efficiently treating a silicon raw material having a high boron content to a concentration usable as silicon for solar cells. It aims at providing the method and apparatus which do. Furthermore, it is an object of the present invention to provide a silicon refining method and apparatus capable of supplying silicon in a molten state to the next step.

[0007]

The present invention is a method for purifying silicon by injecting a plasma jet gas onto the molten metal surface of molten silicon contained in a holding container, wherein a plurality of electrodes are provided at the bottom of the holding container. A silicon purification method characterized by applying a voltage having a phase difference and changing periodically between the plasma torch and each electrode, and scanning the trajectory of the current generated between the plasma torch and the electrode on the surface of the molten silicon. is there. In this case, the solid silicon can be supplied to the next step by continuously supplying the solid silicon to the holding container and melting the solid silicon while allowing the overflow portion to flow out.

As an apparatus of the present invention capable of suitably carrying out the method of the present invention, a molten silicon holding container and a plasma torch for injecting a plasma jet gas containing water vapor toward a molten metal surface of the molten silicon are provided. In a silicon refining device, a plurality of electrodes are provided at the bottom of the container, a power source for applying a voltage having a phase difference and changing periodically between the plasma torch and each electrode, and a current generated between the plasma torch and the electrodes. The present invention provides a silicon refining apparatus characterized in that the orbit of (1) can be scanned on the surface of the molten silicon.
In this apparatus, the molten silicon holding container has an overflow outlet and is provided with an apparatus for continuously supplying solid silicon, so that the molten silicon can be supplied to the next step.

[0009]

FIG. 1 is a schematic cross-sectional view of the main part of the apparatus of the embodiment of the present invention.
FIG. 2 shows the electrode arrangement installed on the bottom of the silicon holding container. Electrodes (transfer plasma anodes) 26a, 26b, 26c, 2 are provided on the bottom of the silicon holding container 30 via an insulator 27.
6d is provided and is connected to the plasma power source 28 together with the plasma cathode (electrode) 23 provided on the plasma torch 20. FIG. 3 shows the time change of the electrode potential in the device shown in FIG. Electrodes 26a, 26b, 26c, 2
By changing the phase of the electrode potential applied to 6d, the time (t ₁ , t
₂ , t ₃ , t ₄ ), the trajectories of the current generated between the plasma torch and the electrodes can be scanned over the surface of the silicon melt by periodically changing it sequentially. Therefore, the contact area between the high temperature plasma gas and the silicon surface is greatly expanded, so that the removal rate of boron in silicon can be increased and the purification time can be shortened. The number and arrangement of the transfer plasma anodes (electrodes), the period for changing the electrode potential, etc. are appropriately selected.

If the concentration of water vapor contained in the plasma jet is 20% by volume or more with respect to the plasma gas, a SiO ₂ film is suddenly formed on the surface of the molten silicon and the evaporation of boron in the silicon is significantly hindered. The addition concentration of water vapor is preferably 20% by volume or less. As a material for the molten silicon holding container 30 (crucible, hearth), graphite, quartz, water-cooled copper, or a graphite container lined with silica can be used. Particularly in the case of a container lined with quartz or silica, carbon dissolved in silicon can be removed together with removal of boron.

FIG. 4 is a schematic vertical sectional view of an essential part of an apparatus according to another embodiment of the present invention. In FIG. 4, 31 is a silicon holding container having an outlet, and 15 is a receiving container for the purified silicon 14. An electrode (transfer plasma anode) 26a is provided on the bottom of the silicon holding container 31 having an outlet through an insulator 27.
26b, 26c, and 26d are provided, and are connected to a plasma power source (not shown) together with the plasma cathode (electrode) 23 of the plasma torch 20. The time change of the electrode potential is the same as that shown in FIG.

Granular solid silicon 11 as a raw material is continuously supplied and melted into a molten silicon 12 by a plasma flame 13, and subsequently, a steam-added plasma jet 21 is sprayed for a predetermined time to remove boron, that is, purify. While flowing, it overflows from the outlet and becomes purified silicon 14. That is, the purified silicon 14 containing no boron can be continuously taken out in a molten state from the raw material silicon.

The purified silicon 14 is melted and held by installing a resistance heating device or an induction heating device around the receiving container 15 for the purified silicon 14, or an electron beam heating device together with a differential pressure exhaust device. In this state, it is possible to cool from the bottom of the container, unidirectionally solidify the silicon, solidify and purify, or manufacture a cast ingot for solar cells, and it is possible to reduce costs by continuous manufacturing processes. . At this time, it is more effective to add an ingot drawing device.

[0014]

【Example】

Examples 1 to 4 and Comparative Examples 1 to 4 FIG. 1 is a schematic vertical cross-sectional view of a main part of a silicon refining apparatus used for carrying out the present invention. A plasma torch 20 connected to a 30 kW transfer type argon plasma generator 28 (plasma power supply) was provided so that the height was 50 mm above the surface of the molten metal when the silicon melted down. As the silicon holding container 30, a quartz crucible, a graphite crucible, a water-cooled copper crucible, and a graphite crucible each having an inner diameter of 150 mm and silica lined therein were used. Four transfer plasma anodes (electrodes) 26a, 26b, 26c and 26d are installed on the bottom of the silicon holding container 30 via an insulator. Each plasma anode (electrode) is 26a, 26b, 26c, 26.
The voltage applied between d and the plasma torch 20 is shown in FIG.
As shown in (1), it has a period of 50 cycles / sec with a phase difference. 2 kg of solid metallic silicon (purity 99.5%, boron concentration about 21 ppmw) was charged into the holding container 30, and plasma arc melting was performed in an argon gas atmosphere. When the entire amount of silicon had melted down, water vapor was supplied into the plasma from the addition nozzle to start the removal of boron.
The plasma working gas is argon gas 19 liters /
min, steam is 1 liter / min (addition ratio 5.0
(% By volume) and the power input to the plasma is 20
kW.

After injecting the steam-added plasma jet gas into the molten silicon 12, the molten silicon 12 was sampled with a quartz tube at regular intervals, and the change in boron concentration was determined by IPC emission spectroscopy. As a comparative example, one plasma anode,
Others were the same as in Example 1.

Table 1 shows the relationship between the injection time of the steam-added plasma jet gas and the boron concentration. The reaction rate coefficient K in the table is a value obtained from d [B] / dt = -K [B] because boron is removed by the first-order reaction of the concentration. As can be seen from Table 1, in any of the examples, boron is removed more rapidly than the comparative example. That is, the present invention can reduce boron to a target concentration in a shorter time. Although not shown in the table, the evaporation loss of silicon generated at the same time can be small because the time is short.

[0017]

[Table 1]

Examples 5, 6 and Comparative Example 5 FIG. 4 is a schematic vertical cross-sectional view of a main part of a silicon refining apparatus used for carrying out the present invention. A plasma torch 20 connected to a 30 kW transfer type argon plasma generator,
The height was 50 mm above the surface of the molten metal when the silicon melted down. As the silicon holding container 31, a quartz container having an inner dimension of 250 mm in length and 70 mm in width and having a flow outlet for overflow at one end was used.
Transfer plasma anodes (electrodes) 26a, 26b, 26c, 2
6d is placed in the center of the bottom of the holding container 31 via an insulator 27.
The book is installed. 2 kg of solid metallic silicon was charged into the holding container 31 under an argon atmosphere, and when the total amount of silicon was melted down by the plasma arc, 5% by volume of water vapor was supplied into the plasma from the addition nozzle so that the boron concentration was 0. Silicon holding container 3 at the time of reaching 3 ppmw
The solid metal silicon 11 in the form of particles was continuously supplied from one end of No. 1 and the silicon for the overflow was received in the container 15. That is, the receiving container 15 has a boron concentration of 0.
Purified silicon 14 purified to 3 ppmw is accumulated.

As in the first to fourth embodiments, the plasma torch 20 and the plasma anodes 26a, 26b, 26c and 2 are formed.
An electric potential that periodically changes at 50 and 100 cycles / sec was applied to 6d to scan the trajectory of the current generated between the plasma torch and the electrode. Further, the quartz crucible (silicon holding container 3) having an inner diameter of 150 mm used in Comparative Example 1 was used.
2 kg of solid metallic silicon was charged in 0), and when the silicon melted down, argon plasma was blown onto the silicon from a position at a height of 50 mm above the molten metal surface to melt it, and when the total amount of silicon melted down. , 5% of water vapor is supplied into the plasma from the addition nozzle, and the boron concentration becomes 0.
When the concentration reached 3 ppmw, the addition of water vapor was stopped and the plasma output was gradually reduced to solidify the silicon in the holding container.

In each of Examples 5 and 6 and Comparative Example 5, the plasma working gas was 19 liters of argon gas /
Min, plasma added gas is water vapor 1 liter / min (5.0
(Volume%), and the input power to the plasma was 20 kW. At this time, the processing time from the start of steam supply until the boron concentration in the initially charged silicon of 2 kg reached 0.3 ppmw was 7 hours.

As a result, in the embodiment, the purified silicon 14 is stored in the receiving container 15 of the purified silicon 14 with the lapse of time after 7 hours have passed. The purification amount of was used as the purification rate. In the comparative example, one cycle from cooling the purified silicon in the holding container to solidifying and then taking it out and charging the next raw material silicon requires a total of 13 hours,
Moreover, since the amount adhered to the holding container must be excluded, the number obtained by dividing the actually obtained purified silicon by 13 hours can be considered as the purification rate. This is shown in FIG. The purification rate values thus obtained are shown in Table 2. Note that the boron concentration in silicon was determined by ICP emission spectroscopy, which was performed by suctioning silicon with a quartz tube during the experiment.

Comparing Examples 5 and 6 with Comparative Example 5, in the case of Example, purified silicon was continuously obtained as the raw material silicon was charged, and the purified silicon was retained after the completion of purification as in Comparative Example. Since the container replacement work and the melting / solidification time are unnecessary, and the current trajectory of the transfer type plasma is scanned on the surface of the silicon melt, the evaporation area of boron oxide is expanded and the substantial boron removal rate is increased. The amount of silicon purified per unit time increased, and the purification rate in the example was 3.5 times higher than that in the comparative example.

In the above embodiment, the case where argon is used as the plasma working gas has been described, but the present invention is not limited to this, and one in which helium, hydrogen, nitrogen is used as the working gas or a mixed gas thereof is used. May be used.

[0024]

[Table 2]

[0025]

According to the present invention, a large number of plasma anodes (electrodes) are provided at the bottom of a silicon holding container, and a voltage having a phase difference is applied to these plasma anodes, whereby the current trajectory can be scanned on the surface of the molten silicon. It became possible to remove boron efficiently. Also, while continuously supplying silicon to a container having an overflow outlet at one end, the surface of the molten silicon is irradiated with steam-added inert gas plasma from above the holding container, and the bottom of the molten silicon holding container is irradiated. Since multiple electrodes are provided on the surface of the molten metal and the trajectory of the current generated between the plasma torch and the electrodes is scanned on the surface of the molten silicon, the boron in silicon can be purified at a speed three times or more that of the conventional batch type purification method. Now you can do it, and your productivity has improved.

[Brief description of drawings]

FIG. 1 is a schematic vertical sectional view of an essential part of an apparatus used in an example of the present invention.

FIG. 2 is a plan view of an essential part of an apparatus used in an embodiment of the present invention.

FIG. 3 is a diagram showing an example of a change with time of an electrode potential of the device.

FIG. 4 is a longitudinal sectional view of an essential part of an apparatus in which four electrodes used in an embodiment of the present invention are provided on the bottom of a container.

FIG. 5 is a diagram showing a refining status of silicon over time.

[Explanation of symbols]

 11 Solid Silicon 12 Molten Silicon 13 Plasma Flame 14 Purified Silicon 15 Purified Silicon Receiving Container 20 Plasma Torch 21 Water Vapor Addition Plasma Jet 23 Plasma Torch Cathode (Electrode) 25 Water Vapor Addition Tube 26 Transfer Plasma Anode (Electrode) 27 Insulator 28 Plasma Generator (Power source) 30 Silicon holding container 31 Silicon holding container having an outlet

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hisae Terajima, 1 Kawasaki-cho, Chuo-ku, Chiba City, Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor, Yasuhiko Sakaguchi, 1 Kawasaki-machi, Chuo-ku, Chiba City Kawasaki Steel Co., Ltd. Research headquarters

Claims (4)

[Claims] 1. A method for purifying silicon by injecting a plasma jet gas onto a molten metal surface of molten silicon held in a holding container, wherein a plurality of electrodes are provided at the bottom of the holding container, and a periodic phase difference is provided. A method for purifying silicon, characterized in that a voltage that changes to a voltage is applied between the plasma torch and each electrode, and the trajectory of the current generated between the plasma torch and the electrode is scanned on the surface of the molten silicon. 2. The method for purifying silicon according to claim 1, wherein solid silicon is continuously supplied to the holding container, and the solid silicon is melted while allowing an overflow portion to flow out. 3. A silicon refining apparatus comprising: a molten silicon holding container; and a plasma torch for injecting a plasma jet gas containing water vapor toward a molten silicon level, wherein the holding container bottom has a plurality of electrodes. A power supply for applying a periodically changing voltage having a phase difference between the plasma torch and each of the electrodes is provided, and the trajectory of the current generated between the plasma torch and the electrodes can be scanned on the molten silicon surface. And silicon refining equipment. 4. The apparatus for purifying silicon according to claim 3, wherein the holding container has an outlet for overflow and has a device for continuously supplying silicon.