JP2011173775A - Continuous casting method of silicon ingot - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method of a silicon ingot capable of preventing a molten silicon from being contaminated by metal impurities caused by an atmospheric gas naturally convecting in a chamber at the time of continuous casting by an electromagnetic casting method. <P>SOLUTION: In the continuous casting method of the silicon ingot 3 wherein a silicon raw material 11 is charged in a bottomless cooling crucible 7 arranged in a chamber 1, the silicon raw material 11 is melted by the electromagnetic induction heating from an induction coil 8 and the molten silicon 12 is solidified while pulling down the molten silicon 12 from the cooling crucible 7, a piping 15 opened at an upper part and a lower part of the cooling crucible 7 is connected to a side wall of the chamber 1 and a partition plate 16 is provided between the upper end of the cooling crucible 7 and the side wall of the chamber 1, the casting is performed intercepting the flow of the atmosphere gas flowing into the upper part from the lower part of the cooling crucible 7 by the partition plate 16 while introducing the upper atmosphere gas of the cooling crucible 7 under the cooling crucible 7 through the piping 15. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a continuous casting method of a silicon ingot that is a material of a substrate for a solar cell.

In recent years, the problem of global warming due to CO ₂ emissions and the problem of depletion of energy resources have become serious, and solar power generation utilizing infinitely stored solar energy has attracted attention as one of countermeasures against these problems. . Photovoltaic power generation is a power generation method in which solar energy is directly converted into electric power using a solar cell, and a polycrystalline silicon wafer is mainly used as a substrate of the solar cell.

  A polycrystalline silicon wafer for solar cells is manufactured by slicing a unidirectionally solidified silicon ingot as a raw material. For this reason, in order to promote the spread of solar cells, it is necessary to secure the quality of the silicon wafer and reduce the cost, and it is required to manufacture a high-quality silicon ingot 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. 1 is a diagram schematically showing a configuration of a typical representative 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 device (not shown) is connected to the upper wall of the chamber 1 via an openable / closable shutter 2. The chamber 1 is provided with an inert gas inlet 5 on the upper side wall and an exhaust port 6 on 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 vessel but also as a mold and is a rectangular tube made of metal (for example, copper) excellent in thermal conductivity and conductivity, and is suspended in the chamber 1. The cooling crucible 7 is divided into a plurality of strip-shaped pieces in the circumferential direction, leaving the upper part, and is forcibly cooled by cooling water flowing through the inside.

  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 attached below the shutter 2 connected to the raw material supply device. Along with opening and closing of the shutter 2, granular or lump silicon raw material 11 is supplied from the raw material supply device into the raw material introduction pipe 10 and charged into the cooling crucible 7.

  On the bottom wall of the chamber 1, an extraction port 4 for extracting the ingot 3 is provided immediately below the after heater 9, and the extraction port 4 is sealed with gas. 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 cooling crucible 7 while being lowered.

  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 lowered plasma torch 13 is energized. 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. In addition, a plasma arc is generated between the plasma torch 13 and the silicon raw material 11 and further the molten silicon 12, and the silicon raw material 11 is also heated and melted by the Joule heat, thereby reducing the burden of electromagnetic induction heating. The molten silicon 12 is efficiently 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 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 supporting the molten silicon 12 is gradually lowered while melting the silicon raw material 11 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, the silicon raw material 11 is continuously charged as the support base 14 is lowered, and the melting and solidification are continued, whereby the molten silicon 12 is solidified in one direction, and the ingot 3 is continuously cast. it can.

  During continuous casting, in order to maintain the inside of the chamber 1 in an inert gas atmosphere, the inert gas is sequentially supplied from the inert gas inlet 5 on the upper side wall of the chamber 1 to fill the chamber 1. The inert gas in the chamber 1 is sequentially discharged from the exhaust port 6 on the lower side wall of the chamber 1. At this time, SiO (silicon oxide) is vigorously evaporated from the molten silicon 12 by the plasma arc from the plasma torch 13, and this SiO gas is discharged from the exhaust port 6 together with the inert gas.

  According to such an electromagnetic casting method, since contact between the molten silicon 12 and the cooling crucible 7 is reduced, 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 electromagnetic casting method described above, the atmospheric temperature in the chamber 1 is high in the central portion where the ingot 3 exists, decreases as it approaches the side wall of the chamber 1, and increases in the same central portion toward the upper portion where the molten silicon 12 exists. Due to this temperature difference, natural convection of atmospheric gas occurs in the chamber 1 as shown by the solid line arrow in FIG. Specifically, it rises between the outer periphery of the ingot 3 and the inner periphery of the after heater 9, passes through the gap between the upper end of the uppermost after heater 9 and the lower end of the cooling crucible 7, further outside the cooling crucible 7. After rising, convection of the atmospheric gas descending near the side wall of the chamber 1 occurs. A part of the atmospheric gas that convects and rises outside the cooling crucible 7 also flows directly above the cooling crucible 7 as indicated by a dotted arrow in FIG.

  Then, when a metal impurity is contained in the convection atmosphere gas, the impurity is carried to a position just above the cooling crucible 7 along with the flow of the atmosphere gas, falls into the cooling crucible 7 and falls into the molten silicon 12. May be mixed. In this case, since the molten silicon 12 is contaminated with metal impurities, the quality of the ingot 3 cast from the molten silicon 12 is deteriorated. For example, when a heat-resistant alloy containing Fe or Cr is used as a constituent member of the after heater 9, the metal impurities are easily taken into the atmosphere gas in the process in which the atmosphere gas rises along the after heater 9.

  The present invention has been made in view of the above problems, and when silicon ingots are continuously cast by an electromagnetic casting method, molten silicon is contaminated with metal impurities due to the atmospheric gas that naturally convects in the chamber. An object of the present invention is to provide a continuous casting method of a silicon ingot that can prevent this.

  In order to achieve the above-mentioned object, the present inventors have made extensive studies by paying attention to the flow of atmospheric gas that naturally convects in the chamber during continuous casting, and conducted various tests. As a result, it is possible to prevent impurity contamination of the molten silicon caused by the convection atmosphere gas by controlling the flow of the atmosphere gas so that the atmosphere gas that has once reached the bottom of the cooling crucible does not reach the top of the cooling crucible again. As a result, the present invention was completed.

  The gist of the present invention resides in the following method for continuously casting a silicon ingot. That is, a silicon raw material is charged into a conductive bottomless cooling crucible disposed in the chamber, and the silicon raw material is melted by electromagnetic induction heating from an induction coil surrounding the bottomless cooling crucible. In the method of continuously casting a silicon ingot by solidifying while being pulled down from a cooling crucible, piping that opens above and below the bottomless cooling crucible is connected to the side wall of the chamber, and the atmosphere above the bottomless cooling crucible is connected through this piping. A continuous casting method of a silicon ingot, wherein casting is performed while introducing a gas below the bottomless cooling crucible and blocking a flow of atmospheric gas flowing from the bottom to the bottomless cooling crucible.

  In this continuous casting method, it is preferable that a partition plate is provided between the upper end of the bottomless cooling crucible and the side wall of the chamber.

  According to the silicon ingot continuous casting method of the present invention, the atmospheric gas convects below the cooling crucible, and even if the atmospheric gas contains metal impurities, the atmospheric gas flows above the cooling crucible. Therefore, the metal impurities are not transported directly above the cooling crucible and mixed into the molten silicon, and the impurity contamination of the molten silicon caused by the convective atmosphere gas can be prevented.

It is a figure which shows typically the structure of the conventional typical electromagnetic casting apparatus used with an electromagnetic casting method. It is a figure which shows typically the structure of the electromagnetic casting apparatus which can apply the continuous casting method of the silicon ingot of this invention. It is a figure which shows typically the structure of the electromagnetic casting apparatus used for the comparison. It is a figure which shows the measurement result of Fe density | concentration in the silicon ingot in this invention example and a comparative example.

  Below, the embodiment is explained in full detail about the continuous casting method of the silicon ingot of this invention.

  FIG. 2 is a diagram schematically showing the configuration of an electromagnetic casting apparatus to which the silicon ingot continuous casting method of the present invention can be applied. The electromagnetic casting apparatus in the present invention shown in the figure is based on the configuration of the electromagnetic casting apparatus shown in FIG. 1, and the same components as those in FIG.

  As shown in FIG. 2, the electromagnetic casting apparatus in the present invention has a pipe 15 connected to the upper part and the lower part of the side wall of the chamber 1. The upper and lower ends of the pipe 15 are opened at positions corresponding to the upper side of the cooling crucible 7 and positions corresponding to the lower side of the cooling crucible 7, respectively.

  In the chamber 1, a partition plate 16 is attached between the upper end of the cooling crucible 7 and the side wall of the chamber 1 so as to protrude horizontally from the upper end of the cooling crucible 7 toward the side wall of the chamber 1. The partition plate 16 partitions the space in the chamber 1 up and down with the upper end position of the cooling crucible 7 as a boundary.

  In continuous casting using the electromagnetic casting apparatus having such a configuration, the internal space of the chamber 1 is partitioned vertically by the partition plate 16 and each end of the pipe 15 is opened in each partitioned space. 2, the atmospheric gas existing above the cooling crucible 7 flows into the pipe 15 from the upper end of the pipe 15 opened here, descends in the pipe 15, and then the cooling crucible 7. It is introduced into the lower part of the chamber 1 corresponding to the lower part.

  Although the atmospheric gas introduced into the lower portion of the chamber 1 is finally discharged from the exhaust port 6 to the outside of the chamber 1, most of the atmospheric gas convects naturally in the chamber 1. In other words, the atmospheric gas introduced into the lower portion of the chamber 1 through the pipe 15 enters the inside of the after heater 9 through the gap between the upper and lower adjacent after heaters 9 as indicated by solid arrows in FIG. Then, after ascending between the outer periphery of the ingot 3 and the inner periphery of the after heater 9 as it is, it passes through the gap between the upper end of the uppermost after heater 9 and the lower end of the cooling crucible 7 and reaches the outside of the cooling crucible 7. The atmospheric gas that has reached the outside of the cooling crucible 7 is prevented from rising further by the partition plate 16 and descends in the vicinity of the side wall of the chamber 1. Such natural convection of the atmospheric gas occurs.

  Therefore, according to the continuous casting method of the present invention, even if the convection atmosphere gas contains metal impurities, the atmosphere gas is blocked from flowing upward from below the cooling crucible 7, so that the metal There is no situation in which impurities are carried directly above the cooling crucible 7 and mixed into the molten silicon 12. As a result, impurity contamination of the molten silicon 12 due to the convection atmosphere gas can be prevented, and the ingot 3 having excellent quality can be manufactured.

  In order to confirm the effect of the continuous casting method of the present invention, the ingot produced by continuously casting the silicon ingot using the electromagnetic casting apparatus shown in FIG. 2 has a solidification rate of 0%, 10%, 30%, 50%. Sample wafers were taken from positions corresponding to 70% and 90%, respectively, and a test was performed to measure the Fe concentration in each sample wafer. The solidification rate here represents the ratio of the weight of the solidified ingot to the total weight of the charged silicon raw material, and corresponds to the length from the lower end of the ingot (the first position of continuous casting). For comparison, a silicon ingot was continuously cast using the electromagnetic casting apparatus shown in FIG. 3 below, and a sample wafer was similarly collected from this ingot to measure the Fe concentration.

  FIG. 3 is a diagram schematically showing a configuration of an electromagnetic casting apparatus used for comparison. The electromagnetic casting apparatus used in the comparative example shown in the figure has a pipe 15 connected to the upper part and the lower part of the side wall of the chamber 1 as compared with the electromagnetic casting apparatus used in the example of the present invention shown in FIG. Although it is common, it is different in that it does not have a partition plate 16 that partitions the internal space of the chamber 1 up and down.

  FIG. 4 is a diagram showing the measurement results of the Fe concentration in the silicon ingots in the present invention example and the comparative example. As shown in the figure, when the solidification rate corresponding to the first position of continuous casting is 0%, the Fe concentration of the ingot is the same in the inventive example and the comparative example, but the continuous casting proceeds and the solidification rate. As a result, the Fe concentration in the comparative example significantly increased as compared with the inventive example.

  From this result, in the comparative example using the electromagnetic casting apparatus shown in FIG. 3, the convection atmosphere gas containing metal impurities is cooled in the chamber 1 as in the case of the conventional electromagnetic casting apparatus shown in FIG. In the example of the present invention using the electromagnetic casting apparatus shown in FIG. 2, the molten silicon is contaminated with metal impurities while flowing into the crucible 7 (see the dotted arrow in FIG. 3). It has become clear that the convection atmosphere gas does not flow directly above the cooling crucible 7 and can prevent impurity contamination of the molten silicon.

  In the results shown in FIG. 4, the Fe concentration increases as the solidification rate increases in both the inventive example and the comparative example. This is due to the segregation phenomenon of the impurity element. This is because impurities are concentrated in the molten silicon as the continuous casting progresses. Further, in the example of the present invention, the result is that Fe is contained in the ingot, but this is because Fe is inevitably contained in the silicon raw material to be charged.

  In addition, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, the partition plate 16 shown in FIG. 2 may be provided so as to protrude from the side wall of the chamber 1 toward the upper end of the cooling crucible 7. Further, as long as the flow of the atmospheric gas flowing in from the lower side of the cooling crucible 7 can be interrupted, for example, the inert gas is injected from the inner wall of the chamber 1 over the entire horizontal plane of the outer periphery of the cooling crucible 7 instead of the partition plate 16. A partition such as an air curtain can be formed by the injected gas.

  According to the continuous casting method of the silicon ingot of the present invention, even if the metal gas is contained in the atmospheric gas convection below the cooling crucible, the metal impurity is mixed into the molten silicon in the cooling crucible. In addition, it is possible to prevent the molten silicon from being contaminated with metal impurities due to the convection atmosphere gas. Therefore, the continuous casting method of the present invention is extremely useful in that a silicon ingot for a solar cell excellent in quality can be produced.

1: chamber, 2: shutter, 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, 15: Piping, 16: Partition plate

Claims (2)

A silicon raw material is charged into a conductive bottomless cooling crucible disposed in the chamber, and the silicon raw material is melted by electromagnetic induction heating from an induction coil surrounding the bottomless cooling crucible. In the method of continuously casting a silicon ingot by solidifying while pulling down,
Piping that opens above and below the bottomless cooling crucible is connected to the side wall of the chamber. Through this piping, the atmosphere gas above the bottomless cooling crucible is introduced below the bottomless cooling crucible, A continuous casting method for a silicon ingot, wherein the casting is performed while the flow of the atmospheric gas flowing upward from below is blocked.   2. The silicon ingot continuous casting method according to claim 1, wherein a partition plate is provided between an upper end of the bottomless cooling crucible and a side wall of the chamber.

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