JP2660477B2 - Silicon casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon casting method for producing a polycrystalline solidified ingot of silicon used for solar cells and the like, and more particularly, to a method of continuously producing the polycrystalline solidified ingot by electromagnetic melting. It relates to a continuous casting method to be manufactured.

[0002]

2. Description of the Related Art As a method for producing a polycrystalline solidified ingot of silicon used as a material for a solar cell or the like, a continuous casting method by electromagnetic melting has been proposed, for example, in Japanese Patent Application Laid-Open No. 2-30698. The continuous casting method of silicon by electromagnetic melting uses an induction coil and a conductive bottomless crucible placed therein. At least a part of the bottomless crucible in the axial direction is divided into a plurality in the circumferential direction. The raw silicon charged in the bottomless crucible is melted in a non-contact state with the inner wall of the crucible by electromagnetic induction by the induction coil.
And, while supplying raw material silicon into the bottomless crucible,
By gradually lowering and solidifying the melt in the bottomless crucible downward, a polycrystalline solidified ingot of silicon is continuously produced.

[0003] In such a continuous casting method of silicon by electromagnetic induction, even if only solid raw material silicon is put into a bottomless crucible, no induced heat is generated. This is because silicon has a large electrical resistivity near room temperature. Therefore, at the time of casting, it is necessary to externally heat the solid raw material silicon in the bottomless crucible to a temperature at which it can be melted only by self-heating.

As a means for externally heating the solid raw material silicon in the bottomless crucible, a self-heating element made of graphite or the like which generates heat by electromagnetic induction is usually used. That is, as shown in FIG. 1, the self-heating element 3 is charged onto the raw silicon 2 in the bottomless crucible 1, and the self-heating element 3 is heated by induction to heat the raw silicon in the bottomless crucible 1.

In the initial melting of the raw silicon 2 in the bottomless crucible 1, a dummy block 4 is used to hold the raw silicon 2. The dummy block 4 is also made of a material capable of inducing heat generation, such as graphite, and is often used as external heating means for the raw silicon 2.

By the way, the upper surface of the dummy block 4 is
Was flat. This is the molten silicon in the bottomless crucible 1
Since there is no contact with the inner surface of the crucible,
The load of the silicon ingot 6 is always
To the upper surface, lower the dummy block 4
Since the upper silicon ingot 6 follows,
Was it thought that it was not necessary to forcibly remove block 6?
It is. However, according to the investigation of the present inventors, the ingot
When the cross-sectional size of the
In the block 4, the silicon ingot 6 and the dummy block
4 is separated and the silicon ingot 6 cannot be pulled out.
It has been found. This is due to the increasing size of ingots
Increase in volume expansion during solidification, dummy block 4
Using a release agent to prevent fusion with
Pinch roll to prevent contamination of silicon ingot 6
It is impossible to directly forcibly remove the ingot by
Which is the cause. Therefore, as shown in FIG.
A recess 5 into which molten silicon flows is provided on the upper surface of
To connect the silicon ingot 6 and the dummy block 4
Tried. That is , as shown in FIG. 2 (A), a part of the initially melted silicon is caused to penetrate into the concave portion 5 and solidify, thereby being bonded to the silicon ingot 6 thereon. As a result, the pulling force of the dummy block 4 is
Conveyed to. In other words, the dummy block 4 will have a function of generating Kakashita force to the silicon ingot 6.

The upper surface of the dummy block 4 is covered with a release agent 7 to prevent the silicon ingot 6 and the dummy block 4 from fusing.

[0008]

The conventional silicon casting method using such a dummy block has the following problems due to the dummy block.

The concave portion 5 provided on the upper surface of the dummy block 4 has a shape such that the silicon 8 solidified in the concave portion 5 does not come out of the concave portion 5. Therefore, when removing the silicon ingot 6 after casting, the silicon ingot 6 is separated from the dummy block 4 by breaking between the solidified silicon 8 and the silicon ingot 6 in the recess 5 as shown in FIG. Is done. In this separation, an excessive force may be applied to the silicon ingot 6 and the silicon ingot 6 may be broken.

When the dummy block 4 is used once, the recess 5 is filled with the solidified silicon 8. Therefore, in the second and subsequent uses, the dummy block 4 and the silicon ingot 6 are connected by connecting the silicon 8 in the recess 5 and the newly melted silicon as shown in FIG. Is achieved. At this time, if the temperature of the dummy block 4 is low, the connection between the silicon 8 and the silicon ingot 6 in the concave portion 5 becomes incomplete, and a crack occurs at the connection portion.
The temperature of the dummy block 4 is increased. However, when the temperature of the dummy block 4 is high, the release agent 7 covering the upper surface of the dummy block 4 is peeled off from the upper surface, and it becomes difficult to separate the dummy block 4 and the silicon ingot 6.

An object of the present invention is to solve these problems,
Without exerting any adverse effect on the produced ingot, the cast
It is an object of the present invention to provide a method for producing silicon that enables stable extraction of a lump .

[0012]

According to a silicon casting method of the present invention, a conductive bottomless crucible having at least a part of an axial direction divided in a circumferential direction is installed in an induction coil, and the bottomless crucible is provided. In the continuous casting of polycrystalline silicon by electromagnetic induction, in which silicon is melted by electromagnetic induction in a state of non-contact with the inner wall of the crucible and the melt is gradually pulled down, the raw silicon in the crucible without bottom Of the dummy block supporting the raw material silicon when initially melting
Dissolved raw material silicon flows into the upper surface and the silicon
The lower cross-section so that the
While forming a recess larger than the cross-sectional area of
Dummy block so that the solidified silicon can be removed.
At least a part of the upper part in the circumferential direction can be removed to the side
And a dummy excluding the divided piece and the divided piece
Both of the main units are made of heat-resistant conductive material capable of induction heating.
The use of prefabricated dummy block cast
Connect the dummy block and silicon ingot inside the dummy block
The silicon ingot is forced downward by the lowering of the lock.
To remove the split piece from the dummy body after punching and casting
It is further characterized in that the silicon ingot is separated from the dummy block .

FIGS. 3 to 6 show examples of dummy blocks used in the silicon casting method of the present invention.

The dummy block 10 shown in FIG. 3 comprises a columnar dummy body 11 and a pair of divided pieces 12 and 12 combined on the upper part thereof. The divided pieces 12, 12 are united to form a cylindrical body having the same outer diameter as the dummy main body 11, the lower part of which is fitted into the small diameter part 11b of the dummy main body 11, and the projections 12a, 12a provided on the inner surface at the lower end have peripheral parts. By fitting into the groove 11c, it is connected to the dummy main body 11.

The divided pieces 12 connected to the dummy body 11
12 has a concave portion 13 formed on the upper inner surface side. And
The projections 12 are formed on the inner surfaces of the upper ends of the divided pieces 12, 12 so that the upper inner diameter D ₁ of the recess 13 is smaller than the lower inner diameter D _2.
b and 12b are provided.

The silicon initially melted flows into the recess 13 and solidifies. Recess 13 the upper inner diameter D ₁ is lower outer diameter D ₂
Since it is smaller, the silicon solidified in the recess 13 does not fall out of the recess 13. Therefore, the dummy block 10 and the silicon ingot 20 are securely connected.

After casting, the divided pieces 12, 12 are disassembled.
Thereby, the recess 13 is opened to the side, and the silicon ingot 20 is separated from the dummy block 10 together with the solidified silicon in the recess 13. Therefore, it is not necessary to forcibly separate the solidified silicon in the concave portion 12 from the silicon ingot 20.
The solidified silicon in the recess 13 can be separated from the silicon ingot 20 without applying excessive force to the silicon ingot 20 by cutting or the like. In addition, since the solidified silicon is removed from the inside of the recess 13 each time it is used, the temperature of the dummy block 10 does not need to be increased more than necessary, and the release agent that covers the upper surface of the dummy block 10 (including the inner surface of the recess) is removed. Is prevented.

Although the dummy block 10 shown in FIG. 3 has a cylindrical shape, when the bottomless crucible is a rectangular tube, the dummy block 10 has a prismatic shape corresponding to the shape.

The divided pieces 12 and 12 in the dummy block 10 shown in FIG. 3 have a configuration in which the cylindrical body is divided into two in the circumferential direction. However, the divided pieces may be divided into three or more parts. As shown, a structure in which a part of the dummy block 10 in the circumferential direction is replaced may be used.
4 is appropriate.

The divided pieces 12, 12 in the dummy block 10 in FIG. 3 are connected to the dummy main body 11 only by fitting, but as shown in FIG. You may make it connect.

The divided pieces 12, which bite into the silicon ingot 20,
As shown in FIG. 5, the protrusions 12b, 12b may have a pointed shape in which the lower surface for transmitting the pulling force to the ingot is inclined, and the angle θ is increased as shown in FIG. Thus, it may be easy to separate from the silicon ingot 20 at the time of division.

The material of the dummy block 10 is preferably graphite or the like for both the dummy body and the divided pieces. That is,
Any heat-resistant conductive material that generates heat by induction may be used, and may be a high melting point metal such as Mo.

[0023]

According to the silicon casting method of the present invention, the material silicon dissolved on the upper surface of the dummy block flows as a dummy block.
So that the silicon does not fall out after solidification
Form a recess with a lower cross-sectional area larger than the upper cross-sectional area
And tie the dummy block to the silicon ingot.
Silicon casting as the dummy block descends.
The lump descends. Therefore, the size of silicon ingot
And without contaminating the ingot,
Cutting can be performed. The silicon that flows into the recess and solidifies
At least around the top of the dummy block so that it can be taken out
The side part is made as a split piece that can be removed to the side
Do not apply excessive force to the silicon ingot after casting.
In addition , the silicon ingot is separated from the dummy block together with the solidified silicon in the recess. Both split pieces and dummy body
Is made of heat-resistant conductive material that can generate induced heat.
The cracks in the silicon ingot near the dummy block
Is prevented. According to the investigation by the present inventors, the split pieces
-Does not generate heat as in the case of the main unit, and the temperature of the dummy block
If it is not uniform, the serial
Cracks occur in the concrete ingot. And this crack
Lowers silicon ingot yield and increases product cost
It becomes a factor to do.

[0024]

Embodiments of the present invention will be described below.

Example 1 Continuous casting of polycrystalline silicon was performed using the dummy block shown in FIG.

The dummy block is made of graphite, and its size is a = 220 mm, b = 110 mm, c = 55 m
m, d = 15 mm, e = 20 mm, D ₁ = 110 mm,
D ₂ = 130 mm. During the initial melting, a silicon block was placed on the dummy block, and a self-heating element made of graphite was placed on the silicon block, and these were heated by induction.

At the place where the silicon block was melted, the granular raw material was charged from above, and the dummy block was continuously lowered by the charged amount so that the silicon melt was always at the same height.

At the time of the initial melting, the melt flows into the concave portion of the dummy block, and as the dummy block is lowered, the melt solidifies from below. The solidification of the silicon in the concave portion allows the dummy block to transmit a pulling-down force to the silicon ingot.

The dummy block was moved to 100 mm / min while the granular material was charged at a rate of 2.0 mm / min at a steady melt volume of 5 liters.
Reduced by 0 mm. The total input amount of the raw materials was 113 kg, and the input time was 500 minutes.

After the completion of the charging, the melting output was continuously reduced to 0% in 100 minutes. After the solidification was completed, the ingot was cooled in a vacuum for 8 hours, and then the ingot was taken out.

In taking out the ingot, the divided pieces are detached from the dummy body by lightly hitting the dummy block with a hammer, and then the dummy body can be separated from the ingot without any resistance. It was confirmed that there was no fusion between the ingot and the dummy block.

Example 2 Using the dummy block shown in FIG. 4, the process from initial melting to cooling was performed in the same manner as in Example 1. The size of the dummy block is D ₁ = 30 mm, D ₂ = 40 mm, d = 40 mm
And As in the case of the dummy block in FIG. 3, the divided pieces and the dummy main body could be separated from the ingot. No peeling of the release agent and no fusion between the ingot and the dummy block were observed, and it was found that there was no effect on the ingot reduction even if the number of divided pieces was one.

Example 3 Using the dummy block shown in FIG. 5, the process from initial melting to cooling was performed in the same manner as in Example 1. The surface transmitting the pulling force of the divided pieces to the ingot was inclined, and the inclination angle was 45 °. Further, the divided piece and the dummy main body are fixed with bolts, and the material of the bolts is molybdenum, but may be ceramic (alumina or the like) or another metal. Also in this case, neither release of the release agent nor fusion of the ingot and the dummy block was observed, and it was found that there was no influence on pulling down the ingot and removing the ingot.

Example 4 Using the dummy block shown in FIG. 6, the process from initial melting to cooling was performed in the same manner as in Example 1. The angle of the sharp protrusion of the divided piece is larger than that of the dummy block in FIG.
0 °. Also in this case, neither release of the release agent nor fusion of the ingot and the dummy block was observed, and it was found that there was no influence on pulling down the ingot and removing the ingot.

[0035]

As is apparent from the above description, the silicon casting method of the present invention can be applied to a silicon ingot and a dummy block.
So that it does not contaminate the silicon ingot,
Enables stable pull-out. Silicon ingot and dummy
Despite joining the blocks, the assembled silicon ingot is separated from the dummy block together with the solidified silicon in the recess using a prefabricated dummy block,
An excessive force is not applied to the silicon ingot at the time of separation, and the silicon ingot can be prevented from being broken. Further, when the dummy block is reused, the molten silicon flows into the concave portion, and the solidified silicon and the silicon ingot in the concave portion are completely connected, so that it is not necessary to increase the heat generation temperature of the dummy block more than necessary.
Thus , the fusion of the dummy block and the silicon ingot due to the release of the release agent can be prevented. Assembled dummy block
Despite the use of the dummy block heating temperature
This prevents the silicon ingot from cracking due to uneven
Wear.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a conventional method.

FIG. 2 is a schematic diagram for explaining a problem of the conventional method.

FIG. 3 is a vertical sectional view and a plan view of a dummy block used in the method of the present invention.

FIG. 4 is a vertical sectional view and a plan view of another dummy block.

FIG. 5 is a vertical sectional view of still another dummy block.

FIG. 6 is a vertical sectional view of still another dummy block.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Dummy block 11 Dummy main body 12 Split piece 13 Depression 20 Silicon ingot

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical indication location C30B 35/00 C30B 35/00 (56) References JP-A-61-52962 (JP, A) JP-A-53-82622 (JP, A) JP-A-2-22118 (JP, A)

Claims (1)

(57) [Claims] 1. A conductive bottomless crucible, at least a part of which is divided in a circumferential direction, is provided in an induction coil, and silicon is placed in the bottomless crucible without contact with an inner wall of the crucible. In the continuous casting of polycrystalline silicon by electromagnetic induction in which the melt is melted by electromagnetic induction in the state and the melt is gradually pulled down and solidified, the raw silicon is supported when the raw silicon in the bottomless crucible is initially melted. Dissolved on the upper surface of the dummy block
Raw material inflows and the silicon solidifies upward after solidification
The lower cross-sectional area is larger than the upper cross-sectional area so that
While forming the concave part, the silicon which flows into the concave part and solidifies
At least at the top of the dummy block so that
Also, the circumferential part is a split piece that can be removed to the side,
Furthermore, both the divided piece and the dummy body excluding the divided piece are guided.
The use of a prefabricated dummy block made of heat-resistant conductive material that can generate heat allows the dummy block to be used during casting.
The silicon block and the silicon ingot to lower the dummy block.
The silicon ingot is forcibly pulled down,
Remove the split pieces from the
A silicon casting method characterized in that a silicon ingot is separated from the steel ingot .