JPH0251493A - Silicone casting device - Google Patents

Abstract

PURPOSE:To take out ingot without the pollution of casting atmosphere or without the need of decompression by providing the seal member which seals a chamber without the contact with the ingot at an ingot taking-out opening of the chamber provided with an inert gas supply means. CONSTITUTION:Seed crystal of polycrystal silicone in a induction coil 7 is molten by a heat generating body 12 in such a state that the inside of the chamber 1 is substituted with an inert gas, such as Ar, to form molten silicone 13a. After that, granular silicone material 10 is added on the surface of the molten silicone 13a from a duct 11 to melt the granular silicone material 10, while the silicone ingot 13b containing seed ingot is taken out from a nonbottomed crucible 6 and from a gradual cooling unit 8. When the lower end of the silicone ingot 13b is passed through a noncontact seal member, such as a labyrinth packing, a gate valve 16 is opened in such a condition that the pressure in the chamber 1 is slightly higher than the atmospheric pressure. When the silicone ingot 13b taken out to the outside of the chamber 1 is reached to a specified length, it is cut by a diamond cutter 18.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a silicon casting apparatus using electromagnetic casting for producing a unidirectionally solidified ingot of polycrystalline silicon used for solar cells and the like.

[Conventional technology]

Conventionally, in the field of metal casting, a casting method called electromagnetic casting has been known, and the induct slag melting method (
Note) There is a history of its being put into practical use.

(Note) P, G, CLITES and R, A, BE
ALL:Proc, theFifth Intern
ational Conf, on Electr.

slag and 5special Melting
Technology (1975) P477 As shown in Fig. 2, inside the induction coil 7,
In this method, an electrically conductive bottomless crucible 6 divided into a plurality of parts in the circumferential direction is installed, and the material 20 is melted therein and sequentially pulled downward to solidify it.

Note that the electromagnetic casting in this specification also includes a method in which the bottomless crucible 6 is not used, and the induction coil 7 is formed into a cylindrical shape so that the induction coil 7 also serves as the bottomless crucible 6.

According to the method shown in FIG. 2, since the bottomless crucible 6 is divided into a plurality of parts in the circumferential direction, the current flowing through the induction coil 7 generates a current in each crucible division piece 60, which causes the material 2 in the crucible 6 to A current is generated to heat and melt the material 20, and a repulsive force is generated between the current flowing through the crucible division piece 60 and the current flowing through the material 20, causing the material 20 to be in a non-contact state with respect to the crucible 6. It is believed that it can be maintained.

Since there is no material contact between this type of electromagnetic casting and the bottomless crucible, when it is used to produce a unidirectionally solidified ingot of polycrystalline silicon, it completely eliminates impurity contamination from the outside of the silicon ingot. It is hoped that this can be prevented. If there is no contamination from the crucible, the material of the crucible can be lower graded, no mold is required, and equipment costs are significantly reduced.By combining with a large-capacity power supply, large, high-quality silicon ingots can be produced. can be manufactured continuously at low cost. Furthermore, from a crystallographic point of view, crystallization from the side walls of the crucible can be suppressed, making it very preferable.

Based on this idea, the United States has filed a patent application in Japan for a continuous silicon casting method using electromagnetic induction in Japanese Patent Laid-Open No. 61-52962. In addition, the present inventors are also continuing research on the production of unidirectionally solidified polycrystalline silicon ingots by electromagnetic casting, and have already achieved some results (
Japanese Patent Application No. 62-211551, No. 62-211552, No. 63-93684, No. 63-93685, etc.).

[Problem to be solved by the invention]

However, in the production of unidirectionally solidified polycrystalline silicon ingots using conventional electromagnetic casting, although in theory ingot products can be produced continuously, in reality it is a batch process. Development is progressing only on a large scale. This is due to the difficulty in removing the ingot from the chamber during casting.

That is, in the production of a unidirectionally solidified ingot of polycrystalline silicon, it is essential to reduce the pressure inside the chamber to about 10 to 20 Torr in order to ensure the quality of the ingot product. When attempting to extract an ingot product from the reduced pressure chamber, the seal structure can only secure the product to the extent that the ingot product can be extracted, preventing external air from entering the chamber through the seal. I can't.

It is well known that in the production of unidirectionally solidified ingots of polycrystalline silicon, the inflow of air into the chamber causes decisive damage to the quality of the ingot product.

Therefore, it is conceivable to cut the ingot inside the chamber and take it out of the chamber through a take-out section having a two-chamber structure. Needless to say, in that case as well, it is a prerequisite that the casting atmosphere within the chamber is not contaminated.

Cutting methods that do not contaminate the casting atmosphere include those using a laser or an electron beam. However, these cutting methods are inefficient, and unless the chamber is extremely extended in the direction of ingot withdrawal, it is impossible to finish cutting the ingot without following the ingot withdrawal. Although this method does not cause contamination because it is heated and melted, it does cause quality deterioration due to thermal distortion and the like. Taking these things into consideration, we reach the conclusion that in industrial scale casting, we must rely on mechanical cutting means such as a diamond cutter.

However, mechanical cutting means use lubricants such as water. Water, like air, causes decisive damage to the quality of ingots. Therefore, when employing mechanical cutting means, it is a prerequisite that the cutting is performed outside the chamber. However, as described above, it is impossible at the current technological level to pull the ingot out of the chamber during casting.

SUMMARY OF THE INVENTION An object of the present invention is to provide a silicon casting apparatus capable of extracting a unidirectionally solidified ingot of polycrystalline silicon cast in a chamber to the outside of the chamber without any problem.

[Means for Solving the Problem] In the process of conducting research and development toward the practical application of a method for producing a unidirectionally solidified ingot of polycrystalline silicon by electromagnetic casting, the present inventors discovered that the casting atmosphere was inert. If there is, it has been found that an ingot with no quality problems can be obtained even if casting is performed at atmospheric pressure without the need for depressurization. There are three reasons for this:

As is well known, the factors that reduce the quality of unidirectionally solidified polycrystalline silicon ingots are SiO□, Ot, and other P.
and other impurities. Conventionally, these are brought in from the crucible and the polycrystalline silicon material, and in the casting method, they are also brought in from the mold.

In the production of unidirectionally solidified polycrystalline silicon ingots by electromagnetic casting, no crucible is used, and the silicon melt and ingots do not come into contact with the mold, so all impurities contained in the ingot product are made of polycrystalline silicon. It will be brought in from the material. As the polycrystalline silicon material, polycrystalline silicon rods manufactured by the CVD method are usually used.

Great efforts have been made to eliminate impurities from this material during the manufacturing process, but elements such as hydrogen and chlorine have not reached the level of impurities that can be used as ingot products. is the current situation. Therefore, there is a need to eliminate impurities during the casting process.

In the case of electromagnetic casting, O! from the crucible or mold is used for SlO□. Since there is no supply of SiO□, SiO□ will not be generated to a problem level even without reducing the pressure. This is the first reason why casting can be carried out at atmospheric pressure.

Regarding 0□, as long as the casting atmosphere is maintained as an inert gas atmosphere, the 0. It was found that the content was reduced.

This is considered to be because the oxygen partial pressure in the atmosphere is more dominant than the atmospheric pressure in removing 0□.

This is the second basis.

Impurities such as P are hardly removed at atmospheric pressure. However, P and the like are not removed even when the casting method is performed at a pressure of about 10 Torr. In the case of a unidirectionally solidified ingot of polycrystalline silicon, even if P and the like in the polycrystalline silicon material are carried into the ingot product as they are, it is acceptable in terms of product quality. This is the third basis.

Based on the above three fils, in the case of manufacturing polycrystalline silicon unidirectionally solidified ingots by electromagnetic casting, as long as the casting atmosphere is maintained in an inert gas atmosphere, an atmosphere of atmospheric pressure or higher is required. Even under these conditions, a higher quality ingot product can be obtained than by conventional casting.

The silicon casting apparatus of the present invention was developed based on such knowledge, and includes a chamber equipped with an electromagnetic casting means inside and a drawer opening for the ingot cast by the electromagnetic casting means, and and a sealing member that is attached to the outlet and seals the ingot drawn out from the outlet in a substantially non-contact manner.

[Function] When casting is performed by the electromagnetic casting means in the chamber while the inside of the chamber is replaced with inert gas from the inert gas supply means, the cast ingot flows out of the chamber from the drawer opening at the bottom of the chamber. derived.

At this time, the substantially non-contact sealing member provided at the draw-out port does not inhibit drawing out of the ingot. Additionally, if the pressure inside the chamber is adjusted to be slightly higher than atmospheric pressure by supplying inert gas from the inert gas supply means, the inert gas will flow out of the chamber from between the ingot and the non-contact sealing member. However, outside air does not flow into the chamber.

If outside air is prevented from flowing into the chamber, casting is performed in the chamber in an inert gas atmosphere at atmospheric pressure or higher. As described above, even if a unidirectionally solidified ingot of polycrystalline silicon is produced by electromagnetic casting in an inert gas atmosphere at atmospheric pressure or higher, sufficient ingot quality is ensured.

If the manufactured ingot can be taken out of the chamber during casting, there is no need to cut the ingot inside the chamber (
Therefore, it is only necessary to surround the electromagnetic casting means with a chamber, and the chamber can be significantly downsized. Furthermore, if cutting can be performed outside the chamber, there is no need to consider contamination within the chamber, and highly efficient mechanical cutting means such as a diamond cutter can be employed.

If the ingot extracted from the chamber is cut below the chamber, there is no need to secure a large space below the chamber. In addition, mechanical cutting means such as a diamond cutter can cut the ingot at high speed, so even if the cutting is performed in synchronization with the lowering of the ingot, the movement stroke of the cutting means is short, and the quality of the material due to thermal distortion etc. No deterioration occurs in the ingot.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a longitudinal sectional view of a casting apparatus showing an embodiment of the present invention.

The chamber 1 is a water-cooled container with a double wall structure to be protected from internal heat generation, and has a raw material charging chamber 3 partitioned off by a shutoff device 2 at the top, and a chamber 3 at the bottom for extracting the ingot. It has a drawer opening 4. An inert gas supply means (not shown) is connected to an inert gas inlet 5 provided on the upper side wall of the chamber l.

At approximately the center of the chamber 1, a bottomless crucible 6, an induction coil 7, and an annealing device 8 are provided as tuff casting means. The bottomless crucible 6 is a copper water-cooled cylindrical body that is divided into multiple parts in the circumferential direction except for the upper part, and the inner peripheral surface of the crucible is inserted into the divided parts to prevent difficulty in pulling down the ingot. Therefore, it has a tapered surface that expands downward and toward the outer periphery. The induction coil 7 is disposed around the outer periphery of the bottomless crucible 6 and is connected to a power source via a coaxial cable (not shown). The slow cooling device 8 is connected directly below the bottomless crucible, heats the ingot pulled down from the bottomless crucible 6, and provides a predetermined temperature gradient in the axial direction thereof.

A raw material hopper 9 is located below the raw material charging chamber 3 in the chamber 1.
is provided, and the granular or lumpy polycrystalline silicon material 10 charged into the omper 9 is supplied to the molten silicon 13a in the bottomless crucible 6 through a rotating charging duct 11. A resistive heating element 12 made of graphite or the like is provided directly above the bottomless crucible 6 so as to be movable up and down, and is inserted into the bottomless crucible 6 in a lowered state.

A support and extraction device 14 is provided below the slow cooling device 8 to support and draw out the silicon ingot 13b downward. The supporting and pulling means 14 may be of a clamp roll type, but the silicon ingot 13b
In consideration of the influence on
It is preferable that a plurality of clamp bodies, which release the clamp body 3b and rise to the original position, be simultaneously driven in a phase-shifted manner.

The drawer opening 4 provided at the lower part of the chamber 1 is a removable member of the electromagnetic casting means described above, and has a labyrinth packing 15 as a sealing member. The drawer opening 4 is opened and closed by a gate valve 16 below the labyrinth packing 15. A shielding plate 17 is provided below the gate valve 16.

The labyrinth packing 15 has an inner diameter slightly larger than the outer diameter of the silicon ingot 13b, so that the gas flowing through the gap between the seal and the silicon ingot 13b passing through it can pass through several stages of different sizes. While passing through the gap, the flow velocity energy is lost, and the gas pressure eventually reaches O, sealing the gap with the silicon ingot 13b without contact. The amount of gas flowing out from the labyrinth packing 15 is given by the following equation.

Q is the amount of gas outflow, A is the narrow gap area, g is the gravitational acceleration, and R is the gas constant, which is 2.15 cra for argon.
/' K, n is the number of stages in the gap, To is the gas temperature, Po is the gas pressure on the high pressure side, P, l is the gas pressure after n stages, α is the flow coefficient within the range of 0.6 to 0.9 It is in.

Outside the chamber l below the shielding plate 17 are a diamond cutter 18 as a mechanical cutting means and an ingot support member 19.
is provided. These are designed to be able to descend in synchronization with the pulling down speed of the silicon ingot 13b, and cut the silicon ingot 13b drawn out of the chamber l from the draw-out port 4 while following its movement.

The specific means of the casting method using the apparatus shown in FIG. 1 will be explained below.

A cylindrical seed ingot made of polycrystalline silicon is placed in the chamber 1 by the supporting and pulling device 14 so that the upper end of the seed ingot is in the center of the bottomless crucible 6 in the height direction.
Completely replace the inside with argon gas as an inert gas by vacuum displacement.

The charging duct 11 above the bottomless crucible 6 is laterally retracted, and with the gate valve 16 closed, the heating element 12 made of graphite or the like is lowered and inserted into the bottomless crucible 6 to perform seed casting. It is set up close to just above the lump, and the induction coil 7 is started to be energized.

When the upper end surface of the seed ingot in the induction coil 7 is melted, the heating element 12 is raised and returned to its original position, and the seed ingot is raised in the crucible 6, molten silicon 13a is initially formed.

Immediately after the initial formation of the molten silicon 13a, granular polycrystalline silicon material 1o is added from the charging duct 11 to the molten surface of the molten silicon 13a.
While melting, with the slow cooling device 8 operating,
By actuating the support and extraction device 14,
The silicon ingot 13b containing the seed ingot is pulled out from the bottomless crucible 6 and the slow cooling device 8.

As a result, the molten silicon 13a is sequentially pulled away from the area where the electromagnetic force acts, and is continuously solidified starting from the part that is in contact with the silicon ingot 13b. An axial temperature gradient of .

When the lower end of the silicon ingot 13b has passed through the labyrinth shield 15, the gate valve 16 is opened while the chamber 1 is pressurized slightly above atmospheric pressure. chamber 1
Since the inside is pressurized, the argon gas in the chamber 1 may flow out from between the silicon ingot 13b and the labyrinth packing 15, but external air will not flow into the chamber 1. do not have.

Casting is continued while maintaining the pressure inside the chamber 1 slightly higher than atmospheric pressure. When the length of the silicon ingot 13b pulled out of the chamber 1 reaches a predetermined length, the silicon ingot 13b is cut by the diamond cutter 18 while being supported by the ingot support means 19. The diamond cutter 18 and the ingot support means 19 cut the silicon ingot 13.
It follows the movement of b and does not interfere with its movement. Water as a lubricant used during cutting and chips generated from the cutting section are shielded by the shielding plate 17 and do not adversely affect the gate valve 16 and the labyrinth packing 15. When the cutting is completed, the diamond cutter 18 and the ingot support means 19 are raised to their original positions to prepare for the next cut.

By continuing casting while reversing the cutting process, 1
A large amount of silicon ingots 13b can be produced in one operation. When the operation is stopped, the supply of material into the bottomless crucible 6 is stopped, and the silicon ingot 13b is directly lowered. The raw material hopper 9 is appropriately replenished with polycrystalline silicon material lO from the raw material charging chamber 3.

The results of actual casting using the above method will be explained below. The target diameter of the silicon ingot 13b is 100 mm.

The bottomless crucible 6 has an inner diameter of 100 mm, and the inner circumferential surface thereof is tapered downward to the outer circumferential side at an angle of 0.5°. The induction coil 7 is a 4-turn coil with an inner diameter of 140 mm and a height of 50 m, with a maximum output of 120 kW and a frequency of 30 kHz.
H, is connected to a high frequency ffi source. The slow cooling device 8 is a furnace housing a resistance heating element with an inner diameter of 110 mm and a length of 200 mm, and is configured to provide a temperature gradient of about 1200°C at the upper part and about 600°C at the lower part on the inner peripheral side.

Labyrinth packing 15 is 1o1 in the narrow gap part.
The wide gap part was 110 mm wide, and these were stacked in 15 tiers at a pitch of 3.5 m.

When a silicon ingot 13b with a diameter of 100 mm passes through the labyrinth seal 15, if argon gas is supplied into the chamber so that the pressure in the chamber 1 becomes 0.1 Torr higher than atmospheric pressure, the silicon ingot 13b passes through the labyrinth seal 15. Argon gas flows out from between the spacer and the labyrinth seal 15 at a rate of about 701/min.

The oxygen and nitrogen concentrations of the silicon ingot 13b were determined by varying the amount of argon gas supplied into the chamber 1 and performing casting at a casting speed of 1.5 an/min and a high frequency power supply ratio cuff of 5 kW. It is shown in Table 1. The oxygen concentration in the polycrystalline silicon material 9 is 1.5×10 ”atoms/cc,
Nitrogen concentration cannot be analyzed.

Table 1 As is clear from Table 1, 101/
If argon gas is supplied for more than 1 minute, silicon ingots with no quality problems can be manufactured.

The cutting conditions for the diamond cutter 18 were, for example, a canter diameter of 450 mm, a rotational speed of 11,000 rpm, and a cutting speed of 25 mm/min.

In the present invention, the term "substantially non-contact sealing member" includes a contact type sealing member that contacts the ingot with a pressure that does not inhibit the withdrawal of the ingot. If the pressure inside the chamber is reduced, a seal like the one shown in the second example cannot prevent outside air from entering the chamber.
If the pressure inside the chamber is positive with respect to atmospheric pressure, such a sealing member can also prevent intrusion of outside air.

If casting is started with the seed ingot inserted into the draw-out port 4 in advance, there is no need to open and close the gate valve 16 during casting.

[Effects of the Invention] The silicon casting apparatus of the present invention allows the ingot to be cast out of the chamber during casting without degrading its quality. Therefore, a large amount of ingots can be continuously produced without being limited by the capacity of the chamber. As a result, it is possible to downsize the chamber and improve manufacturing efficiency.

Furthermore, both ends of the ingot are of low quality, which greatly reduces the yield in the case of the punch type, but by making continuous casting possible with the silicon casting equipment of the present invention, this decrease in yield will be minimized. . In addition, the ingot can be cut outside the chamber without adversely affecting the quality of the ingot, and continuous casting can be continued for a long time without being restricted by the space below the chamber, reducing casting time and casting length. The effect of continuity can be further increased by increasing

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of a silicon casting apparatus showing an embodiment of the present invention, and FIGS. 2(a) and 2(b) are a plan view and a longitudinal sectional view showing the basic principle of electromagnetic casting. In the figure, l: chamber, 4: drawer opening, 6: bottomless crucible, 7: induction coil, 15: labyrinth seal (sealing member). Figure (a) Figure

Claims (1)

[Scope of Claims] 1. A chamber equipped with an electromagnetic casting means inside and equipped with a drawer opening for the ingot cast by the electromagnetic casting means at the bottom, and an inert gas supply means connected to the chamber;
A silicon casting apparatus characterized by comprising: a sealing member attached to the drawer port and sealing the ingot drawn out from the drawer port in a substantially non-contact manner. 2. The ingot according to claim 1, further comprising a mechanical cutting means for cutting the ingot, which is provided below the outlet and descends following the movement of the ingot drawn out from the outlet. Silicon ingot equipment.