JP2630417B2 - Silicon casting equipment - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a continuous casting apparatus for silicon by electromagnetic induction and a continuous casting method using the same.

[Conventional technology]

As one of the continuous casting methods by electromagnetic induction, as shown in FIG. 4, a conductive bottomless crucible 6 divided into a plurality in the circumferential direction is installed in an induction coil 7 and a material 20 is placed therein.
There is known a method of solidifying by successively pulling downward while solidifying (hereinafter, continuous casting by electromagnetic induction means this method).

According to this method, since the bottomless crucible 6 is divided into a plurality in the circumferential direction, a current is generated in each crucible divided piece 16 by the current flowing through the induction coil 7, and the current is generated in the crucible 6.
An electric current is generated in the material 20 inside, and the material 20 is heated and melted. At the same time, a repulsive force is generated between the electric current flowing through the crucible segment 16 and the electric current flowing through the material 20, and the material 20
Can be maintained in a non-contact state.

However, in the actual operation, as shown in FIG. 5A, the molten material 20 in the crucible 6 is divided into crucible pieces.
It enters the gap 17 of 16 and solidifies. This phenomenon is called insertion, which makes it difficult to pull down the solidified material 20 from inside the crucible 6, and causes the solidified material inserted between the crucible pieces 16 to come into contact with the charged crucible surface to generate a discharge. For this reason, this method is practically used only in a form in which a slag is interposed between the crucible 6 and the material 20 in the crucible, and charging, melting, pulling down, and solidifying are continuously performed.

The continuous casting method by electromagnetic induction in which a slag is sandwiched between a crucible and a material in the crucible is called an induct slag melting method (Note), and is mainly used for melting and casting of an active metal such as titanium. Acts as a cushioning and insulating material between the crucible and the material in the crucible.

(Note) PGCLITES and RAREALL: Proc.the Fifth Inte
rnational Conf.on Electro-slag and Special Meltin
g Technology (1975) P477 On the other hand, the directionally solidified ingot of silicon used as a material for solar cells, etc., conventionally dissolves silicon in a bottomed crucible under an atmosphere inert to molten silicon. Is solidified by pouring into a mold provided with a predetermined temperature gradient in the vertical direction and solidifying.

However, in this method, silicon contamination due to impurities from the crucible and the mold is inevitable. Further, in order to suppress this contamination, the crucible and the mold require particularly high-purity materials, and the cost of casting is significantly increased due to the complicated heating equipment for the mold. Further, since crystallization proceeds simultaneously from the bottom of the mold and the side of the mold, it is not preferable from the viewpoint of crystallography.

Therefore, introduction of the above-described continuous casting method by electromagnetic induction to silicon casting has been considered.

The continuous casting method by electromagnetic induction is a method that has been considered for a long time, but until the development of the indext slag melting method, power supply equipment and the balance with the power cost have hardly been implemented on an industrial scale. Was. However,
Recent significant technological advances have provided small, high-capacity power supplies and reduced power costs. Against this background, continuous casting by electromagnetic induction has attracted attention again as a silicon casting method.

As described above, the continuous casting method using electromagnetic induction has no material contact with the bottomless crucible, and completely prevents impurity contamination of silicon when used for casting silicon. If there is no contamination from the crucible, the material of the crucible can be reduced in grade, and the cost of equipment will be significantly reduced due to the absence of the need for a mold. Can be continuously manufactured at low cost. Further, it is very preferable because crystallization from the side wall of the crucible can be suppressed in crystallography.

Based on this idea, the United States filed a patent application in Japan for a continuous casting method of silicon by electromagnetic induction in Japanese Patent Application Laid-Open No. 61-52962.

[Problems to be solved by the invention]

However, the method disclosed in Japanese Patent Application Laid-Open No. Sho 61-52962 does not solve any problem of insertion which is a problem in continuous casting by electromagnetic induction. Theoretically, large power supply is effective in solving the insertion. However, the supply of large electric power departs from the gist of producing large silicon ingots at low cost.

From the above, at present, it is necessary to rely on the above-described inductive slag melting method to implement the continuous casting method by electromagnetic induction on an industrial scale. It is not suitable for casting silicon because it contaminates the material in the crucible.

In view of such a situation, the present invention solves the problem associated with insertion or insertion without using the inductive slag melting method and also without requiring the supply of large power,
It is an object of the present invention to provide a continuous silicon casting apparatus by electromagnetic induction and a continuous silicon casting method using the same, which can produce a large silicon ingot at a low cost while guaranteeing a smooth ingot reduction. .

[Means for solving the problem]

The present inventors have determined that it is important to suppress insertion without supplying large electric power in order to industrially perform silicon casting by a continuous casting method using electromagnetic induction. Even if the insertion cannot be suppressed, if the smoothness of the ingot can be guaranteed to be reduced, the supply of large power is not required. As a result of continuing research on specific measures from such a viewpoint,
From the former point of view, it was found that the management of the dividing gap in the circumferential direction of the bottomless crucible and the control of the induction frequency were effective, and from the latter point of view, the angle control of the inner wall surface of the crucible was found to be effective.

The present invention has been made based on such knowledge, and has the following three devices and one method.

The first apparatus of the present invention is a continuous casting apparatus for silicon by electromagnetic induction, in which a circumferentially divided gap of a bottomless crucible is reduced to 0.
3 to 1.0 mm.

A second apparatus of the present invention is an apparatus for continuously casting silicon by electromagnetic induction, in which an inner wall surface of a bottomless crucible is provided with a slope that extends outward at an angle of 0.4 to 2.0 degrees downward.

According to a third device of the present invention, in the first device, the inner wall surface of the bottomless crucible is provided with a slope that extends outward at an angle of 0.4 to 2.0 ° downward.

In the method of the present invention, in the silicon casting method using any of the first to third devices, the induction frequency is 0.5 to 200 KHz.
It is assumed that.

[Action]

By setting the circumferential division gap of the bottomless crucible to 1.0 mm or less,
Since the current flowing on the inner wall surface of the crucible and the current flowing on the surface of the melted silicon in the crucible are in opposite directions, the repulsive force generated between the two works seamlessly in the divided gap, thereby effectively suppressing insertion.

Fig. 3 shows the relationship between the induction frequency when silicon with diameters of 50mm and 120mm is continuously cast by electromagnetic power at a standard power (45 to 95kw) and the depth of insertion of molten silicon into the split gap. Is a graph in which is represented as a parameter.

When the division gap is 1.5 mm, the insertion depth exceeds 0.6 mm, but when the division gap is 1.0 mm or less, only the standard power mainly considering heat melting of silicon is given. Regardless, the insertion depth is 1/3 of 1.5mm
It is greatly suppressed to about 0.2 mm or less.

If the insertion depth is suppressed to 0.2 mm or less, there is a gap due to the repulsive force generated because the current flowing on the inner wall surface of the crucible and the current flowing on the skin portion of the molten silicon in the crucible are in the opposite direction. The lump can be lowered smoothly.

However, when the division gap is less than 0.3 mm, the current generated in the crucible divided piece rapidly reduces the efficiency of the current induced in the molten silicon in the crucible, causing unnecessary power consumption and economical melting. Can not be performed.

As described above, according to the first apparatus of the present invention, the ingot can be smoothly lowered without supplying a large amount of power.

On the other hand, according to the investigation by the present inventors, the insertion becomes prominent below the induction coil 7 as shown in FIG. This is probably because the lower part of the molten silicon 13 in the crucible 6 receives a stronger vertical force due to its own weight.
Therefore, if the inner wall surface of the crucible 6 is provided with a slope that expands downward and outward, even if insertion occurs,
Lowering is possible.

Table 1 shows the relationship between the inclination angle when silicon with a diameter of 50 mm is continuously cast with a standard electric power (45 to 55 kw) by crucible division gap 1.0 mm by electromagnetic induction and the judgment result of whether or not the ingot can be lowered. It is shown.

As is clear from the table, when the inclination angle is 0.4 ° or less, the ingot can be lowered.

However, if the inclination angle exceeds 2.0 °, the diameter below the crucible becomes excessively large.If the ingot pulling speed is increased, the molten silicon melts from the solid-liquid interface on the side of the ingot before the molten silicon solidifies. Silicon is dropped, and continuous ingot solidification cannot be performed.

From the above, according to the second apparatus of the present invention, the ingot can be smoothly lowered.

Further, the third apparatus of the present invention combines the management of the divided gap and the angle management of the inner wall surface, and makes it possible to more smoothly lower the silicon ingot from the bottomless crucible.

On the other hand, regarding the electrical conditions, the oscillation frequency during induction heating is important in suppressing insertion and reducing power consumption. The induction frequency is a factor that determines the penetration depth of the current flowing through the inner wall surface of the crucible and the current flowing through the surface of the molten silicon in the crucible. The relationship between the induction frequency and the insertion depth is as shown in FIG. It is.

The lower the frequency, the greater the insertion depth, but under the condition that the dividing gap is controlled to 0.3 to 1.0 mm and / or the condition that the inclination angle of the inner wall of the crucible is controlled to 0.4 to 2.0 °, the induction frequency is 0.5. In the range of ~ 200KHz, the continuous lowering of the ingot is further smoothed and the power consumption is reduced.

If the induction frequency is less than 0.5 KHz, the frequency characteristics of the induction coil, the bottomless crucible and the molten silicon do not match, and the insertion of the molten silicon cannot be suppressed unless excessive power is consumed. Similarly, the frequency characteristics among the induction coil, the bottomless crucible, and the molten silicon deteriorate, causing unnecessary power consumption.

From the above, according to the method of the present invention, it is possible to lower the ingot more smoothly.

〔Example〕

FIG. 1 shows an example of a bottomless crucible which is a main part of the first to third devices of the present invention.

The bottomless crucible 6 is a copper cylinder and is divided into a plurality of parts in the circumferential direction except for the lower part. The inside of the crucible is partitioned into the inside and the outside by the partition plate 2, and the cooling water introduced into the crucible from the introduction pipe 18 circulates in the circumferential direction, ascends the outer portion, descends the inner portion, and cools the inner wall surface. Thereafter, the liquid is discharged out of the crucible 6 collected in the outlet pipe 19.

The bottomless crucible 6 may have a gap of 0.3 to 1.0 mm between the crucible divided pieces 16 in the circumferentially divided portion, or may have an inner wall surface extending outward at an angle of 0.4 to 2 ° downward.
Alternatively, it has a structure satisfying both conditions.

In the illustrated example, the bottomless crucible 6 is divided in the circumferential direction while leaving the lower part, but may be left in the upper part or may be divided in the circumferential direction in the entire axial direction.

Further, the material is not limited to a cylindrical shape, and may be a rectangular tube shape, and the material may be a conductive material other than copper.

FIG. 2 is a longitudinal sectional view showing the entire structure of one example of the first to third devices of the present invention.

Numeral 1 denotes an airtight container, which is a water cooling container having a double structure to protect the container from internal heat generation. Furthermore, it is connected to a vacuum exhaust pump via a duct so that the inside of the container can be evacuated, and also connected to an inert gas supply pipe so that the inside of the container can be controlled to an arbitrary pressure of the inert gas. .

The hermetic container 1 has a raw material charging container 4 and an ingot take-out chamber 5 separated by vacuum shut-off devices 2 and 3 at an upper portion and a lower portion, respectively. This can be performed while maintaining an inert atmosphere with respect to silicon.

An induction coil 7 for generating an electromagnetic force for forming and holding the molten silicon 13 is installed slightly above the center of the hermetic container 1, and the bottomless crucible 6 shown in FIG. 1 is installed therein.

A raw material hopper 8 is provided below the raw material charging container 4 in the airtight container 1, and granular and massive raw material silicon 9 charged in the hopper 8 passes through a revolving charging duct 10 in a bottomless crucible. The molten silicon 13 is supplied.

Immediately above the bottomless crucible 6, a resistance heating element 11 made of graphite or the like is provided so as to be able to move up and down, and is inserted into the bottomless crucible 6 in a lowered state.

Below the bottomless crucible 6, there is provided a support and pull-out device 14 for pulling out the silicon ingot 12 while supporting it.

The support and pull-out means 14 may be of a clamp roll type, but in consideration of the influence on the silicon ingot 12, the silicon ingot 12 is gripped without slip, and after descending a predetermined distance, the silicon ingot 12 is released. Then, it is preferable that a plurality of clamp bodies which are raised to the original position be simultaneously driven with a phase shift.

Further, for the silicon ingot 12, a cutting device using a laser beam or the like can be provided in the container 1.

The specific procedure of the method of the present invention using the apparatus shown in FIG. 2 will be described below.

First, the inside of the airtight container 1 is evacuated and then filled with argon as an inert gas. Thereafter, the cylindrical seed ingot made of silicon is supported and drawn by the support layer 14 so that the upper end of the seed ingot is a bottomless crucible 6. So that it is located at the center in the height direction.

Next, with the charging duct 10 above the bottomless crucible 6 retracted in the horizontal direction, the heating element 11 made of graphite or the like is used.
Is heated and then lowered into the bottomless crucible 6, charged and set so as to approach immediately above the seed ingot, and energization of the induction coil 7 is started.

When the upper end surface of the seed ingot in the induction coil 7 is melted, the heating element 11 is raised and returned to its original position, and when the seed ingot is raised in the crucible 6, molten silicon 13 is initially formed.

Immediately after the molten silicon 13 is initially formed, granular raw silicon 9 is immediately added to the molten surface of the molten silicon 13 through the charging duct 10 to melt the raw silicon 9 and the seed casting is performed by operating the support and drawing device 14. The silicon ingot 12 containing the lump is pulled out of the crucible 6 without bottom, and the molten silicon 13 is sequentially separated from the region where the electromagnetic force acts, so that the molten silicon 13 is continuously solidified from the portion in contact with the silicon 12.

By continuing the continuous raw material charging, melting and solidifying operations, a directional solidified ingot of silicon can be produced.

The manufactured silicon ingot 12 is moved to the discharge chamber 5 with the shut-off device 3 released, and is extracted outside the discharge chamber 5 after the shut-off device 3 is closed.

As for the raw material silicon 9, the shut-off device 2 is released,
A predetermined amount is supplied from the charging container 4 into the hopper 8, and then the shutoff device 2 is closed again to prepare for the next casting.

The results of actual casting according to the above procedure are described below, including comparative examples.

The induction frequency used two bands of 0.5 to 20 KHz and 100 to 200 KHz.

For 100 to 200 KHz, a copper bottomless crucible with an inner diameter of 50 mm, an outer diameter of 80 mm, a circumferential division number of 12, a division gap of 1.5, 1.0, 0.3 mm, and a crucible division piece height of 150 mm as shown in FIG. used. The inclination angle of the crucible inner wall surface is basically 1.0 °, and 1.0 ° for the one with a division gap of 1.0 mm, and 0.1 °.
There are five types: 2, 0.4, 2.0, and 5.0 mm.

The induction coil combined with this bottomless crucible has an outer diameter of 90 mm,
It is a water-cooled copper-cooled 4-turn 45mm high. The electric power was 45-55kw, mainly considering the heating and melting of silicon.

For 0.5 to 20 KHz, the first of 120 mm in inner diameter, 150 m in outer diameter, 24 circumferential divisions, 1.5, 1.0, 0.3 mm disassembly gap, 150 mm crucible split piece height, and 1.0 ° inclination angle of crucible inner wall surface A bottomless crucible having the shape shown in the figure was used.

The induction coil combined with this bottomless crucible has an outer diameter of 160m
Power of 70 to 95 kw was supplied by a water-cooled 4-turn copper-made 45 m high, 45 mm high.

The ingot lowering speed was 1.5 mm / min in each case.

FIG. 3 and Table 1 described above summarize the results of casting under the above conditions.

〔The invention's effect〕

The first device of the present invention regulates the division gap of the bottomless crucible, thereby suppressing the insertion of silicon into the decomposition gap portion to a level that does not hinder the lowering without supplying large power.

The second device of the present invention allows the inner wall surface of the bottomless crucible to be inclined smoothly by lowering it even if silicon is inserted.

The third device of the present invention further facilitates the lowering of the silicon ingot by regulating the division gap and regulating the angle of the inner wall surface.

As a result, any of the devices enables economical and efficient continuous silicon casting on an industrial scale by electromagnetic induction, and exhibits a great effect in reducing the cost of casting silicon.

In addition, the method of the present invention further facilitates lowering of the silicon ingot by combining frequency management with the first to third devices.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an example of the structure of a bottomless crucible which is a main part of the apparatus of the present invention, FIG. 2 is a longitudinal sectional view illustrating the entire structure of the apparatus of the present invention, and FIG. FIGS. 4 (a) and 4 (b) are a plan view and a longitudinal sectional view showing the basic principle of continuous casting by electromagnetic induction, and FIGS. (A) and (b) are a cross-sectional view and a vertical cross-sectional view showing the insertion phenomenon. In the figure, 6: bottomless crucible, 7: induction coil, 16: crucible split piece, 17: split gap.

Claims (4)

(57) [Claims] 1. A conductive bottomless crucible, at least part of which is divided in the axial direction into a plurality of sections in a circumferential direction, is installed in an induction coil, and silicon is brought into contact with the bottomless crucible without contacting the inner wall of the crucible. A continuous casting apparatus for silicon by electromagnetic induction, which is melted by electromagnetic induction and pulled down and solidified, wherein the circumferentially divided gap of the bottomless crucible is 0.3 to 1.0 mm. 2. The continuous casting apparatus for silicon by electromagnetic induction according to claim 1, wherein the inner wall surface of the bottomless crucible is provided with a slope extending downward and outward at an angle of 0.4 to 2.0 °. Silicon casting equipment. 3. The silicon casting apparatus according to claim 1, wherein the inner wall surface of said bottomless crucible has a thickness of 0.4 to 2.0.
A silicon casting apparatus characterized by having a slope extending outward at an angle of ゜. 4. A silicon casting method using the silicon casting apparatus according to claim 1, wherein the induction frequency is 0.5 to 200 KHz.