JP2012036067A - Electromagnetic casting device of silicon ingot - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electromagnetic casting device of a silicon ingot, enabling accurate monitoring of temperature variation of the casting surface of the ingot immediately under a cooling crucible even if Si fumes are generated.SOLUTION: In the electromagnetic casting device, a silicon raw material 11 is charged in the bottomless cooling crucible 7 disposed in a chamber 1; the raw material 11 is melted by electromagnetic induction heating from an induction coil 8 surrounding the crucible 7; and the ingot 3 is continuously cast by solidifying the molten silicon 12 while it is pulled down from the crucible 7. A monitoring window 15 is provided to the sidewall of the chamber 1. A heat resisting tube 17 is provided between the monitoring window 15 and the vicinity of the casting surface of the ingot 3 immediately under the crucible 7. The end face, on the ingot 3 side, of the heat resisting tube 17 is closed by a heat resisting plate 18. The temperature of the heat resisting plate 18 is measured through the heat resisting tube 17 by a radiation thermometer 16 outside the monitoring window 15. The variation of the casting surface temperature of the ingot 3 is monitored on the basis of the measured temperature.

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. 4 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 a raw material supply shutter 2 that can be opened and closed. 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 casting 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 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 shutter 2. Along with opening and closing of the raw material supply shutter 2, granular or lump silicon raw material 11 is supplied from the raw material supply device to the raw material introduction pipe 10 and is introduced into the cooling crucible 7 through the raw material introduction pipe 10.

  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 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.

  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 heated and melted by the Joule heat to reduce 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 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, the silicon raw material 11 is sequentially introduced into the cooling crucible 7 as the support base 14 is lowered, and by continuing melting and solidification, the molten silicon 12 solidifies in one direction, and the ingot 3 can be continuously cast. it can.

  In order to maintain the inside of the chamber 1 in an inert gas atmosphere during casting, the inert gas is sequentially supplied from the inert gas inlet 5 at the top of the chamber 1, and the inert gas in the chamber 1 is supplied to the lower side wall of the chamber 1. Are sequentially discharged from the exhaust port 6. At this time, Si (silicon) is vigorously evaporated from the molten silicon 12 by the plasma arc from the plasma torch 13, and this Si vapor aggregates in the chamber 1 to form Si fume, which is discharged from the exhaust port 6 together with the inert gas. Is done.

  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

  Usually, in the electromagnetic casting method, the temperature of the casting surface of the ingot 3 is monitored directly below the cooling crucible 7, and the operation of adjusting the set value of the current applied to the induction coil 8 according to the fluctuation of the casting surface temperature is performed. Done. For this reason, in the conventional electromagnetic casting apparatus described above, as shown in FIG. 4, a monitoring window 15 in which a transparent glass plate is fitted is installed on the side wall of the chamber 1, and a radiation thermometer is provided outside the monitoring window 15. 16 is disposed. The radiation thermometer 16 directly measures the temperature of the casting surface of the ingot 3 immediately below the cooling crucible 7, thereby monitoring the temperature fluctuation of the casting surface of the ingot 3.

  However, in the conventional electromagnetic casting apparatus, during casting, Si fume is remarkably generated in the chamber 1 as the raw material is melted by energizing the plasma torch 13 and using plasma arc heating together, and as shown in FIG. Si fume F floats irregularly between the window 15 and the ingot 3 immediately below the cooling crucible 7. For this reason, even if the substantial temperature of the casting surface of the ingot 3 is constant, the radiant heat energy reaching the radiation thermometer 16 from the casting surface of the ingot 3 changes depending on the amount of floating Si fume F. As a result, the temperature measured by the radiation thermometer 16 changes. That is, the difference between the actual temperature of the casting surface of the ingot 3 and the temperature measured by the radiation thermometer 16 (hereinafter referred to as “measurement error”) is not constant. As a result, it is impossible to accurately monitor temperature fluctuations on the casting surface of the ingot 3.

  The present invention has been made in view of the above problems, and when silicon ingots are continuously cast, Si fumes are generated in the chamber along with the melting of the raw material in combination with plasma arc heating by a plasma torch. Even if it exists, it aims at providing the electromagnetic casting apparatus of the silicon ingot which can make a measurement error constant and can monitor the temperature fluctuation of the casting surface of an ingot correctly just under a cooling crucible.

  As a result of intensive studies to achieve the above object, the present inventors have obtained the following knowledge. To keep the measurement error constant even when Si fume is irregularly floated between the monitoring window and the ingot directly under the cooling crucible, heat resistance is applied between the monitoring window and the vicinity of the casting surface of the ingot. It is effective to provide a tube, close the end surface on the ingot side of the heat resistant tube with a heat resistant plate, and measure the temperature of the heat resistant plate through the heat resistant tube with a radiation thermometer. The heat-resistant plate absorbs the radiant heat from the surface of the ingot's casting surface and red heats in response to the radiant heat. This is because the inside of the heat-resistant tube is prevented from entering Si fume, which becomes an obstacle to temperature measurement.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in an electromagnetic casting apparatus for a silicon ingot described below. That is, a silicon raw material is put into a conductive bottomless cooling crucible disposed in a chamber, and the silicon raw material is melted by electromagnetic induction heating from an induction coil surrounding the bottomless cooling crucible, and this molten silicon is bottomless cooled. In an electromagnetic casting device that continuously casts a silicon ingot by pulling it down from the crucible, a monitoring window is provided on the side wall of the chamber, and heat resistance is provided between the monitoring window and the vicinity of the casting surface of the silicon ingot directly under the bottomless cooling crucible. The end of the heat-resistant tube on the silicon ingot side is closed with a heat-resistant plate, and the temperature of the heat-resistant plate is measured through a heat-resistant tube with a radiation thermometer placed outside the monitoring window. Of the silicon ingot, characterized by monitoring the variation of the casting surface temperature of the silicon ingot based on磁鋳 is forming apparatus.

  In the above electromagnetic casting apparatus, it is preferable that the heat-resistant plate is made of quartz. In this case, it is preferable that the heat-resistant plate is opaque.

  According to the electromagnetic casting apparatus for silicon ingots of the present invention, when silicon ingots are continuously cast, even if the silicon fume is irregularly floated between the monitoring window and the ingot directly under the cooling crucible, In the heat-resistant pipe blocked by the plate, the entry of Si fume is blocked and there is no Si fume, and the heat-resistant plate is heated red by the radiant heat from the casting surface of the ingot, and the casting surface temperature of the ingot Therefore, if the temperature of the heat-resistant plate is measured through the heat-resistant tube with a radiation thermometer, the measurement error can be made constant, and based on the temperature data sequentially output from the radiation thermometer, It becomes possible to accurately monitor temperature fluctuations on the casting surface of the ingot.

It is a figure which shows typically the structure of the electromagnetic casting apparatus of this invention. It is a figure which shows the relationship between the measurement temperature by a radiation thermometer, and the applied electric power to an induction coil with respect to the casting length of an ingot as a test result of Example 1. FIG. It is a figure which shows the relationship between the measurement temperature by a radiation thermometer, and the applied electric power to an induction coil with respect to the casting length of an ingot as a test result of Example 2. FIG. 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. 4, and the same components are denoted by the same reference numerals, and repeated description is omitted as appropriate.

  As shown in FIG. 1, the electromagnetic casting apparatus of the present invention has a heat-resistant tube 17 extending between a monitoring window 15 installed on the side wall of the chamber 1 and the vicinity of the casting surface of the ingot 3 immediately below the cooling crucible 7. Is provided. The heat-resistant tube 17 has an end face on the ingot 3 side closed with a heat-resistant plate 18, and the opposite end face on the monitoring window 15 side is open.

  In addition, a radiation thermometer 16 is disposed on the extended line of the heat-resistant tube 17 outside the monitoring window 15. The radiation thermometer 16 is connected to a control unit (not shown) that controls the operating conditions of the electromagnetic casting apparatus. This control unit monitors temperature fluctuations on the casting surface of the ingot 3 based on temperature data sequentially output from the radiation thermometer 16 and adjusts a set value of the current applied to the induction coil 8.

  In addition to heat resistance, the heat-resistant plate 18 needs to absorb radiant heat from the surface of the casting surface of the adjacent ingot 3 and red heat in accordance with the radiant heat. Transparent quartz, opaque quartz, carbon, PNB (thermal decomposition) Ceramics such as boron nitride and alumina can be used. Among them, the heat-resistant plate 18 using transparent quartz or opaque quartz is preferable because it has a high emissivity and there is no fear of impurity contamination to the ingot 3. In particular, opaque quartz is most suitable as the heat-resistant plate 18 because it has higher emissivity and excellent heat resistance than transparent quartz.

  The material of the heat-resistant tube 17 is not limited as long as it has heat resistance, but it is preferable to use the same material as that of the heat-resistant plate 18 described above. Moreover, the heat-resistant tube 17 and the heat-resistant plate 18 may be integrally molded, or may be individually molded and combined.

  The clearance between the end surface of the heat-resistant tube 17 on the ingot 3 side, that is, the heat-resistant plate 18 and the casting surface of the ingot 3 is preferably about 10 to 30 mm. If the gap is too small, the heat-resistant plate 18 may come into contact with the ingot 3, and if it is too large, a delay in the absorption time of radiant heat from the casting surface of the ingot 3 to the heat-resistant plate 18 becomes significant.

  The clearance between the end face of the heat-resistant tube 17 on the monitoring window 15 side and the monitoring window 15 is preferably about 20 to 50 mm. This gap may be eliminated by contact between the two, but if it is too large, Si fume F that becomes an obstacle to temperature measurement enters the gap.

  According to the electromagnetic casting apparatus having such a configuration, when the silicon ingot is continuously cast, the heat-resistant plate sequentially absorbs radiant heat from the surface of the casting of the ingot and red heats according to the radiant heat. Is one that successively reflects the surface temperature of the casting surface of the ingot. At this time, Si fume is generated in the chamber as the raw material is melted together with the plasma arc heating by the plasma torch, and the Si fume floats irregularly between the monitoring window and the ingot directly under the cooling crucible. Even if there is, Si fume is blocked in the heat-resistant tube blocked by the heat-resistant plate and there is no Si fume, so if you measure the temperature of the heat-resistant plate through the heat-resistant tube with a radiation thermometer, the measurement error will be constant It can be. As a result, it is possible to accurately monitor temperature fluctuations on the surface of the casting surface of the ingot based on the temperature data sequentially output from the radiation thermometer.

  In order to confirm the effect of the electromagnetic casting apparatus of the present invention, a test was conducted in which a silicon ingot having a square cross section of 345 mm on one side and a total length of 4000 mm was continuously cast. In this test, a monitoring window was installed on each side wall of the chamber sandwiching the ingot, and an electromagnetic casting device in which a radiation thermometer was arranged outside each monitoring window. Inside one monitoring window, As shown in FIG. 1, a heat-resistant tube 17 was provided, and nothing was provided inside the other monitoring window as shown in FIG.

Example 1
In the test of Example 1, the ingot was continuously cast by attaching an opaque quartz plate as a heat resistant plate to the end surface of the heat resistant tube on the ingot side. At that time, according to the casting length of the ingot, the measurement temperature by the radiation thermometer on the side where the heat-resistant tube is arranged as an example of the present invention, and the measurement temperature by the radiation thermometer on the side where the heat-resistant tube is not arranged as a comparative example, In addition, the power applied to the induction coil was investigated.

  FIG. 2 is a diagram showing the relationship between the temperature measured by the radiation thermometer and the power applied to the induction coil with respect to the casting length of the ingot as a test result of Example 1. The figure shows the situation at the end of casting when the silicon raw material is stopped being charged into the cooling crucible, and shows the situation before and after the melting of the raw material by plasma arc heating is stopped by switching the energization to the plasma torch.

  As shown in the figure, in spite of the fact that the applied power to the induction coil is substantially constant, in the comparative example, the measured temperature fluctuates greatly, particularly when the melting of the raw material by plasma arc heating is stopped, As the generation of was subsided, the measurement temperature increased rapidly by 100 ° C. or more. On the other hand, in the example of the present invention, the measured temperature became substantially constant with the same tendency as the applied power to the induction coil.

(Example 2)
In the test of Example 2, a transparent quartz plate was attached as a heat-resistant plate to the end face on the ingot side of the heat-resistant tube, and the ingot was continuously cast. At that time, in the same manner as in the test of Example 1 above, according to the casting length of the ingot, the measurement temperature by the radiation thermometer on the side where the heat-resistant tube is arranged as an example of the present invention, and the heat-resistant tube as a comparative example are arranged. The temperature measured by the radiation thermometer on the other side and the power applied to the induction coil were investigated.

  FIG. 3 is a graph showing the relationship between the temperature measured by the radiation thermometer and the power applied to the induction coil with respect to the casting length of the ingot as a test result of Example 2. This figure shows a situation in the middle of the casting in which the silicon raw material is sequentially charged into the cooling crucible, and shows a situation where the plasma torch is energized and the raw material melting is continuously performed by plasma arc heating.

  As shown in the figure, the measured temperature fluctuated greatly in the comparative example, although the power applied to the induction coil was constant. On the other hand, in the example of the present invention, the measurement temperature was almost constant with the same tendency as the applied power to the induction coil, and the fluctuation was extremely small.

  According to the electromagnetic casting apparatus for a silicon ingot of the present invention, Si fume is prevented from entering the heat-resistant pipe closed by the heat-resistant plate, and there is no Si fume, and the heat-resistant plate is the surface of the casting surface of the ingot. The red heat is generated by the radiant heat from the surface, and the temperature of the surface of the ingot casting surface is sequentially reflected. Therefore, if the temperature of the heat-resistant plate is measured through the heat-resistant tube with a radiation thermometer, the measurement error can be made constant, and the ingot It is possible to accurately monitor temperature fluctuations on the surface of the casting surface. Therefore, the electromagnetic casting apparatus of the present invention is extremely useful in that the set value of the current applied to the induction coil can be adjusted with high accuracy in accordance with the fluctuation of the casting surface temperature of the ingot.

1: chamber, 2: raw material supply 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: Monitoring window,
16: Radiation thermometer, 17: Heat-resistant tube, 18: Heat-resistant plate, F: Si fume

Claims (3)

Silicon raw material is put into a conductive bottomless cooling crucible placed in the chamber, and the silicon raw material is melted by electromagnetic induction heating from an induction coil surrounding the bottomless cooling crucible, and this molten silicon is removed from the bottomless cooling crucible. In an electromagnetic casting device that solidifies while pulling down and continuously casts silicon ingots,
A monitoring window is provided on the side wall of the chamber, and a heat-resistant tube is provided between the monitoring window and the vicinity of the casting surface of the silicon ingot immediately below the bottomless cooling crucible, and the end surface on the silicon ingot side of this heat-resistant tube is closed with a heat-resistant plate. Has been
A silicon ingot characterized in that the temperature of the heat-resistant plate is measured through a heat-resistant tube by a radiation thermometer arranged outside the monitoring window, and the fluctuation of the surface temperature of the silicon ingot is monitored based on the measured temperature of the heat-resistant plate. Electromagnetic casting equipment.   2. The silicon ingot electromagnetic casting apparatus according to claim 1, wherein the heat-resistant plate is made of quartz.   The electromagnetic casting apparatus for a silicon ingot according to claim 2, wherein the heat-resistant plate is opaque.

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