JP2012046394A - Electromagnetic casting device for silicon ingot - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electromagnetic casting device for a silicon ingot capable of increasing a melting speed while restraining generation of a side arc during continuous casting by an electromagnetic casting method.SOLUTION: In the electromagnetic casting device, a silicon raw material 11 is charged in a bottomless cooling crucible 7 through a raw material introduction pipe 10, the silicon raw material 11 is melted by electromagnetic induction heating from an induction coil 8 and plasma arc heating from a plasma torch 13 inserted into an upper part of the crucible 7, and the molten silicon 12 is solidified while pulling down the molten silicon 12 from the crucible 7, and the ingot 3 is continuously cast. The raw material introduction pipe 10 is supported through an insulation member 21, and an end of the raw material introduction pipe 10 is arranged in a position of the same height as an end of the plasma torch 13 in the crucible 7, or is arranged in a position higher than the end of the plasma torch 13 and lower than an upper end of the crucible 7, and the silicon raw material 11 is charged in the center of a surface of the molten silicon 12 through the raw material introduction pipe 10.

Description

  The present invention relates to an electromagnetic casting apparatus for continuously casting a silicon ingot that is a material of a substrate for a solar cell.

  The mainstream of the solar cell substrate is a polycrystalline silicon wafer. The polycrystalline silicon wafer is manufactured by slicing a unidirectionally solidified silicon ingot. Therefore, in order to promote the spread of solar cells, it is necessary to secure the quality of the silicon wafer and reduce the cost. Therefore, it is required to manufacture the silicon ingot at a high quality and at a low cost in the previous stage. As a method that can meet this requirement, for example, as disclosed in Patent Document 1, a continuous casting method using electromagnetic induction (hereinafter also referred to as “electromagnetic casting method”) has been put into practical use.

  FIG. 3 is a diagram schematically showing a configuration of a conventional typical electromagnetic casting apparatus used in the electromagnetic casting method. As shown in FIG. 1, the electromagnetic casting apparatus includes a chamber 1. The chamber 1 is a water-cooled container having a double wall structure in which the inside is isolated from the outside air and maintained in an inert gas atmosphere suitable for casting. A raw material supply hopper 2 is connected to the upper wall of the chamber 1. The chamber 1 is provided with an inert gas inlet 5 at the top and an exhaust port 6 at the lower side wall.

  In the chamber 1, a bottomless cooling crucible 7, an induction coil 8, and an after heater 9 are disposed. The cooling crucible 7 functions not only as a melting container but also as a mold, is a rectangular tube made of metal (for example, copper) having excellent thermal conductivity and conductivity, and is suspended in the chamber 1. . The cooling crucible 7 is formed with a plurality of slits (not shown) in the vertical direction, leaving the upper and lower portions, and is divided into a plurality of strip-shaped pieces in the circumferential direction by the slits, and is forced by cooling water flowing through the inside. To be cooled.

  The induction coil 8 is provided around the cooling crucible 7 so as to surround the cooling crucible 7 and is connected to a power supply device (not shown). A plurality of after-heaters 9 are concentrically connected to the cooling crucible 7 below the cooling crucible 7 and heat the silicon ingot 3 pulled down from the cooling crucible 7 to provide an appropriate temperature gradient in the axial direction thereof.

  In the chamber 1, a raw material introduction pipe 10 is disposed below the raw material supply hopper 2. Granular or lump silicon raw material 11 is supplied from the raw material supply hopper 2 to the raw material introduction pipe 10 and is introduced into the cooling crucible 7 from above the cooling crucible 7 through the raw material introduction pipe 10.

  On the bottom wall of the chamber 1, a drawer port 4 for extracting the ingot 3 is provided directly below the after heater 9, and this drawer port 4 is sealed. The ingot 3 is pulled down while being supported by a support base 14 that descends through the drawer opening 4.

  A plasma torch 13 is provided directly above the cooling crucible 7 so as to be movable up and down. The plasma torch 13 is connected to one pole of a plasma power supply device (not shown), and the other pole is connected to the ingot 3 side. The plasma torch 13 is inserted into the upper part of the cooling crucible 7 by lowering.

  In the electromagnetic casting method using such an electromagnetic casting apparatus, the silicon raw material 11 is charged into the cooling crucible 7, an alternating current is applied to the induction coil 8, and the plasma torch 13 inserted at the top of the cooling crucible 7 is energized. Do. At this time, since the strip-shaped pieces constituting the cooling crucible 7 are electrically divided from each other, an eddy current is generated in each piece due to electromagnetic induction by the induction coil 8, and the cooling crucible 7 The eddy current on the inner wall side generates a magnetic field in the cooling crucible 7. As a result, the silicon raw material 11 in the cooling crucible 7 is melted by electromagnetic induction heating to form molten silicon 12. Further, a plasma arc is generated between the plasma torch 13 and the molten silicon 12, and the silicon raw material 11 is also heated and melted by the plasma arc heating, and the burden of electromagnetic induction heating is reduced, so that the molten silicon 12 is efficiently formed. It is formed.

  The molten silicon 12 has a force (pinch force) in the inner normal direction of the surface of the molten silicon 12 due to the interaction between the magnetic field generated along with the eddy current on the inner wall of the cooling crucible 7 and the current generated on the surface of the molten silicon 12. ) Is held in a non-contact state with the cooling crucible 7. When the support 14 that supports the molten silicon 12 is gradually lowered while the silicon raw material 11 is dissolved in the cooling crucible 7, the induction magnetic field decreases as the distance from the lower end of the induction coil 8 decreases. Further, the molten silicon 12 is solidified from the outer peripheral portion by cooling from the cooling crucible 7. Then, as the support 14 is lowered, the silicon raw material 11 is sequentially introduced from above the cooling crucible 7 through the raw material introduction pipe 10 and is continuously melted and solidified, whereby the molten silicon 12 is solidified in one direction, and the ingot 3 can be continuously cast.

  According to such an electromagnetic casting apparatus, contact between the molten silicon 12 and the cooling crucible 7 is reduced, so that impurity contamination from the cooling crucible 7 due to the contact is prevented, and a high-quality ingot 3 can be obtained. it can. And since it is continuous casting, it becomes possible to manufacture the ingot 3 at low cost.

International Publication WO02 / 053496 Pamphlet

  In the conventional electromagnetic casting apparatus described above, the upper end portion of the cooling crucible 7 is fixed to the metal frame 20 attached to the chamber 1, and the cooling crucible 7 is suspended and supported in the chamber 1 by the metal frame 20. The raw material introduction tube 10 is supported by a metal member 121 fixed on the metal frame 20. And the front-end | tip of the raw material introduction tube 10 is arrange | positioned in the position away from the front-end | tip (lower end) of the plasma torch 13 inserted in the upper part of the cooling crucible 7 10 cm or more upwards. Usually, the arrangement height of the tip of the raw material introduction pipe 10 is higher than the upper end of the cooling crucible 7 as shown in FIG.

  The reason why the tip of the raw material introduction tube 10 and the tip of the plasma torch 13 are separated as described above is as follows. The ground of the plasma torch 13 is connected to a support base 14, which is in conduction with the ingot 3 and the molten silicon 12 and also with the chamber 1. For this reason, the potential of the molten silicon 12 becomes the same ground potential as that of the chamber 1. At this time, a member having a low potential equal to or lower than the potential (ground potential) of the molten silicon 12 is disposed in the vicinity of the plasma space formed between the tip of the plasma torch 13 and the molten silicon 12. An arc is generated between the low potential member and the tip of the plasma torch 13. This arc is an abnormal arc that does not contribute to the melting of the raw material, but rather makes the continuous casting operation unstable, and is a normal arc that occurs between the plasma torch 13 and the molten silicon 12 and contributes to the melting of the raw material. This is referred to as a side arc.

  Here, since the potential of the raw material introduction tube 10 becomes the same ground potential as that of the chamber 1 through the metal member 121 that supports the raw material introduction tube 10, if the tip of the raw material introduction tube 10 and the tip of the plasma torch 13 are close, A side arc is generated between them. Therefore, in order to prevent the occurrence of this side arc, it is necessary to separate the tip of the raw material introduction tube 10 from the tip of the plasma torch 13.

  Under such circumstances, in the conventional electromagnetic casting apparatus, the tip of the raw material introduction pipe 10 is disposed at a position higher than the upper end of the cooling crucible 7, but in this case, the following problems occur.

  Since the silicon raw material 11 charged from the raw material introduction pipe 10 has a long drop distance to the molten metal surface of the molten silicon 12, the silicon raw material 11 spreads greatly in the process of dropping and is scattered widely on the molten metal surface of the molten silicon 12. Deposited in a region outside the plasma space formed between the tip of the plasma torch 13 and the molten silicon 12. For this reason, the melting of the silicon raw material 11 by plasma arc heating does not proceed effectively, and the melting rate does not increase, so that the operation of setting the pulling speed of the ingot 3, that is, the casting speed, to be low is forced. As a result, productivity cannot be improved.

  The present invention has been made in view of the above problems, and when silicon ingots are continuously cast by an electromagnetic casting method in which plasma arc heating is used in combination with raw material melting, the generation of silicon raw materials is suppressed while suppressing the occurrence of side arcs. It is an object of the present invention to provide an electromagnetic casting apparatus for a silicon ingot that can increase the speed and increase the casting speed accordingly to improve the productivity.

  As a result of intensive studies to achieve the above object, the present inventors have obtained the findings shown in the following (a) to (c).

  (A) The plasma space formed between the tip of the plasma torch and the molten silicon is narrowed down by Ar gas blown from the tip of the plasma torch, and the temperature reaches 10,000 ° C. or more. Immediately below this plasma space corresponds to the central portion of the molten silicon surface, and if the silicon material can be concentrated and charged here, the silicon material is instantaneously dissolved and the dissolution rate is increased. In order to concentrate the molten silicon at the center of the molten metal surface without scattering the silicon raw material, it is effective to extend the raw material introduction tube so that the tip is disposed in the cooling crucible.

  (B) As a measure shown in the above (a), when the raw material introduction tube is simply extended and its tip is disposed in the cooling crucible, the tip of the raw material introduction tube and the tip of the plasma torch are close to each other. A side arc occurs.

  (C) In order to suppress the occurrence of the side arc shown in (b) above, it is effective to make the potential of the raw material introduction pipe higher than the potential of the chamber (ground potential), that is, the potential of molten silicon. This can be realized by supporting the raw material introduction pipe in an electrically insulated state.

  This invention is completed based on the knowledge of said (a)-(c), The summary exists in the electromagnetic casting apparatus of the silicon ingot shown below. That is, a silicon raw material is introduced into a conductive bottomless cooling crucible disposed in the chamber through a raw material introduction tube, and electromagnetic induction heating from an induction coil surrounding the bottomless cooling crucible is inserted into the top of the bottomless cooling crucible. In an electromagnetic casting apparatus for continuously casting a silicon ingot by melting a silicon raw material by plasma arc heating from a plasma torch and then solidifying the molten silicon while pulling it down from a bottomless cooling crucible, the raw material introduction pipe passes through an insulating member. In addition to being supported, the tip of the raw material introduction tube is arranged at the same height as the tip of the plasma torch in the bottomless cooling crucible, or higher and lower than the top of the bottomless cooling crucible, The silicon material is introduced into the center of the molten silicon melt surface through this material introduction pipe. An electromagnetic casting apparatus of the ingot.

  In said electromagnetic casting apparatus, it is preferable that the said insulation member is comprised with an insulator.

  In the electromagnetic casting apparatus described above, it is preferable that the raw material introduction tube is made of a paramagnetic metal and the inner surface of the raw material introduction tube is covered with silicon. In this case, the paramagnetic metal is preferably Mo.

  In the above electromagnetic casting apparatus, the raw material introduction tube may be made of insulating ceramics.

  According to the electromagnetic casting apparatus for a silicon ingot of the present invention, when the silicon ingot is continuously cast, the tip of the raw material introduction tube is disposed in the cooling crucible, so that the center of the molten silicon surface corresponding to the plasma space is directly below. Since the silicon raw material can be concentrated and introduced into the part, the melting of the silicon raw material by plasma arc heating proceeds instantaneously, resulting in an increase in the melting rate, thereby increasing the casting speed and improving the productivity. It becomes possible. In addition, since the raw material introduction tube is supported in an electrically insulated state by the insulating member, the potential of the raw material introduction tube can be made higher than the potential of the molten silicon. It is possible to suppress the occurrence of side arcs between the tips of each other.

It is a figure which shows typically the structure of the electromagnetic casting apparatus of this invention. It is a figure which shows the measurement result of the lifetime in the silicon ingot by the test of an Example. It is a figure which shows typically the structure of the conventional typical electromagnetic casting apparatus used with an electromagnetic casting method.

  Below, the embodiment is described in full detail about the electromagnetic casting apparatus of the silicon ingot of this invention.

  FIG. 1 is a diagram schematically showing the configuration of the electromagnetic casting apparatus of the present invention. The electromagnetic casting apparatus of the present invention shown in the figure is based on the configuration of the electromagnetic casting apparatus shown in FIG. 3, and the same components are denoted by the same reference numerals, and redundant description is omitted as appropriate.

  As shown in FIG. 1, in the electromagnetic casting apparatus of the present invention, an insulating member 21 is fixed on a metal frame 20 that supports the cooling crucible 7 in a chamber 1, and the raw material introduction pipe 10 is supported by the insulating member 21. Has been. Since the insulating member 21 is required to have heat resistance and mechanical strength so that the raw material introduction tube 10 can be stably supported in a high temperature atmosphere in the chamber 1 in addition to electrical insulation, an insulator is preferable. . Thereby, the raw material introduction tube 10 is in a state of being electrically insulated from the metal frame 20 and further to the chamber 1, and its potential is made higher than the ground potential of the chamber 1, that is, the potential of the molten silicon 12. Can do.

  The raw material introduction pipe 10 is extended as compared with that shown in FIG. 3, and the tip thereof is disposed in the cooling crucible 7. That is, the tip of the raw material introduction tube 10 is arranged at the same height as the tip of the plasma torch 13 inserted in the upper part of the cooling crucible 7 or at a position higher than that and lower than the upper end of the cooling crucible 7. Yes. Thereby, the arrangement height of the tip of the raw material introduction tube 10 is 10 cm or less upward from the tip of the plasma torch 13. FIG. 1 shows an example in which the tip of the raw material introduction tube 10 is arranged at the same height as the tip of the plasma torch 13.

  Furthermore, the tip of the raw material introduction tube 10 is arranged close to the plasma torch 13 so that the silicon raw material 11 can be put into the center of the molten silicon 12.

  In such an arrangement relationship between the raw material introduction tube 10 and the plasma torch 13, the tip of the raw material introduction tube 10 and the tip of the plasma torch 13 are close to each other, but by supporting the raw material introduction tube 10 with the insulating member 21, Since it is electrically insulated from any other member and the potential of the raw material introduction tube 10 can be made higher than the potential of the molten silicon 12 (earth potential), the tip of the raw material introduction tube 10 and the tip of the plasma torch 13 The occurrence of side arcs can be suppressed.

  In the electromagnetic casting apparatus of the present invention, the raw material introduction tube 10 is made of a paramagnetic metal such as Mo, W, or stainless steel, and a silicon plate cut out from an ingot is attached to the inner surface, or a silicon film is applied. For example, the inner surface may be covered with silicon. The reason why the raw material introduction tube 10 is made of a paramagnetic metal is to prevent the electromagnetic induction by the induction coil 8 from being affected. As the paramagnetic metal, Mo which is excellent in heat resistance and workability is suitable. On the other hand, the reason why the inner surface of the raw material introduction tube 10 is covered with silicon is that the silicon raw material 11 is in contact with the inner surface of the raw material introduction tube 10 to prevent contamination of the silicon raw material 11 due to this contact. is there.

  Moreover, in the electromagnetic casting apparatus of the present invention, the raw material introduction tube 10 can be composed of insulating ceramics. Here, the raw material introduction pipe 10 is made of insulating ceramic so as not to affect the electromagnetic induction by the induction coil 8, and at the same time, the raw material introduction pipe 10 itself is made an insulator to further suppress the side arc. It is. As the insulating ceramic, porcelain is suitable.

  According to the electromagnetic casting apparatus having such a configuration, when the silicon ingot is continuously cast by the electromagnetic casting method in which the raw material melting is combined with the plasma arc heating, the tip of the raw material introduction tube 10 is disposed in the cooling crucible 7. The drop distance of the silicon raw material 11 to the molten silicon 12 surface is shortened. Accordingly, the silicon raw material 11 is not scattered, and the molten silicon 12 is located in the central portion of the molten silicon 12 immediately below the plasma space. It can be concentrated. For this reason, the melting of the silicon raw material 11 by the plasma arc heating proceeds instantaneously and the melting rate is increased. Accordingly, the casting speed can be increased, and the productivity can be improved.

  Furthermore, even if the tip of the raw material introduction tube 10 is disposed near the tip of the plasma torch 13, the raw material introduction tube 10 is supported in an electrically insulated state by the insulating member 21. The potential can be made higher than the potential of the molten silicon 12 (earth potential), whereby the generation of side arcs between the tips of the raw material introduction tube 10 and the plasma torch 13 can be suppressed.

  In order to confirm the effect of the electromagnetic casting apparatus of the present invention, as an example of the present invention, a silicon ingot having a square section of 345 mm on one side and a total length of 7000 mm was continuously cast using the electromagnetic casting apparatus shown in FIG. As a comparative example, a silicon ingot having the same dimensions was continuously cast using the conventional electromagnetic casting apparatus shown in FIG. At that time, in the continuous casting of the example of the present invention, the casting speed (ingot pulling speed) was set to a speed higher than that of the comparative example.

  Each continuous casting was carried out in batches of 3 batches, samples were taken from each of the obtained ingots, and a test for measuring the lifetime was performed. The sample was collected from the center of the ingot in a cross section corresponding to a length of 3600 mm from the lower end of the ingot (the position at the beginning of continuous casting). The lifetime was measured by a microwave photo conductivity decay method (Microwave Photo Conductivity Decay: μ-PCD method), and the recombination lifetime of carriers in the sample with respect to the irradiation time when the laser beam was irradiated was investigated.

  FIG. 2 is a diagram illustrating a measurement result of lifetime in the silicon ingot by the test of the example. As shown in the figure, compared with the comparative example, in the present invention example, the casting heat was increased in accordance with the increase in the dissolution rate of the silicon raw material, thereby increasing the latent heat per unit time accompanying solidification. Since the ingot can be gradually cooled by using the latent heat as a heat source, the crystal grains of the ingot are enlarged and uniform, and as a result, the lifetime is improved.

  According to the electromagnetic casting apparatus for a silicon ingot of the present invention, when a silicon ingot is continuously cast by an electromagnetic casting method in which plasma arc heating is used in combination with raw material melting, a side arc is generated between the respective tips of the raw material introduction tube and the plasma torch. It is possible to increase the dissolution rate of the silicon raw material while suppressing the above, and to increase the casting speed accordingly. Therefore, the electromagnetic casting apparatus of the present invention is extremely useful in that productivity can be improved.

1: chamber, 2: raw material supply hopper, 3: silicon ingot,
4: Drawer port, 5: Inert gas inlet port, 6: Exhaust port,
7: bottomless cooling crucible, 8: induction coil, 9: after heater,
10: Raw material introduction pipe, 11: Silicon raw material, 12: Molten silicon,
13: Plasma torch, 14: Support base,
20: Metal frame, 21: Insulating member

Claims (5)

The silicon raw material was introduced into the conductive bottomless cooling crucible placed in the chamber through the raw material introduction tube, and was inserted into the top of the bottomless cooling crucible, and electromagnetic induction heating from the induction coil surrounding the bottomless cooling crucible. In an electromagnetic casting apparatus for continuously casting a silicon ingot by melting a silicon raw material by plasma arc heating from a plasma torch, solidifying the molten silicon while pulling it down from a bottomless cooling crucible,
The raw material introduction pipe is supported via an insulating member, and the tip of the raw material introduction pipe is located at the same height as the tip of the plasma torch in the bottomless cooling crucible or higher than the top of the bottomless cooling crucible. The silicon ingot electromagnetic casting apparatus is characterized in that the silicon raw material is placed in the center of the molten silicon surface through the raw material introduction pipe.   2. The silicon ingot electromagnetic casting apparatus according to claim 1, wherein the insulating member is made of an insulator.   3. The silicon ingot electromagnetic casting apparatus according to claim 1, wherein the raw material introduction tube is made of a paramagnetic metal, and an inner surface of the raw material introduction tube is covered with silicon.   The electromagnetic casting apparatus for a silicon ingot according to claim 3, wherein the paramagnetic metal is Mo.   3. The silicon ingot electromagnetic casting apparatus according to claim 1, wherein the raw material introduction pipe is made of an insulating ceramic.

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