JP5040521B2 - Silicon casting equipment - Google Patents

Description

  The present invention comprises a cooling crucible made of a conductive copper alloy divided into a plurality in the circumferential direction, and is used when continuously casting polycrystalline silicon by solidifying after melting a silicon raw material by electromagnetic induction heating, The present invention relates to a silicon casting apparatus suitable for manufacturing a silicon ingot for solar cells and the like.

  Most of the solar cells currently manufactured use silicon crystals as a substrate material. Silicon crystals are classified into single crystals and polycrystals. In general, when a single crystal is used as a substrate, a solar cell with high conversion efficiency when converting incident light energy into electric energy can be obtained.

  Single crystal silicon is manufactured by the Czochralski method, and high-quality dislocation-free crystals can be obtained. However, since the manufacturing cost is higher than that of polycrystalline silicon, the manufacturing cost of the solar cell is increased. Polycrystalline silicon, on the other hand, is produced by a casting method in which molten silicon is solidified in a mold (hereinafter also referred to as “casting method”) or a continuous casting method by electromagnetic induction (hereinafter also referred to as “electromagnetic casting method”). The substrate material can be manufactured at a lower cost than the single crystal silicon substrate manufactured by the Czochralski method.

  However, since the casting method is an agglomeration method in which molten silicon is solidified with a quartz crucible or a graphite mold, impurity contamination occurs due to contact between the molten silicon and a vessel wall such as a crucible, There is a problem that a release agent used to prevent fusion between the ingot and the mold is mixed into the molten silicon. Furthermore, since continuous casting is difficult with the casting method, a reduction in production efficiency is inevitable.

  The electromagnetic casting method is a method developed to solve such problems. According to this electromagnetic casting method, it is possible to cast a silicon crystal with almost no molten silicon in contact with a crucible or a mold. .

  In this electromagnetic casting method, for example, as described in Patent Document 1, electrical conductivity and heat conduction are electrically insulated from each other in the circumferential direction inside the high frequency induction coil and water-cooled inside. A bottomless cooling crucible in which a good material (usually copper) is arranged in a strip shape is used. The cross-sectional shape of the coil and the cross-sectional shape of the portion surrounded by the strip-shaped object functioning as a crucible may be either cylindrical or rectangular tube.

  When raw material silicon is charged into a copper cooling crucible configured as a melting vessel and an alternating current is passed through a high frequency induction coil, the strip-shaped pieces constituting the cooling crucible are electrically separated from each other. The current forms a loop in each element, and the current on the inner wall side of the cooling crucible forms a magnetic field in the cooling crucible, so that the silicon in the crucible can be heated and melted. The molten silicon in the crucible receives a force in the direction normal to the inner surface of the molten silicon due to the interaction between the magnetic field generated by the current in the inner wall of the cooling crucible and the current in the molten silicon skin, and is melted in a non-contact state with the crucible.

  As the silicon in the crucible is melted in this way, if the support base that holds the molten silicon and the ingot is moved downward, the induced magnetic field decreases as the distance from the lower end of the high-frequency induction coil decreases. The amount of heat generation decreases, and the solidification proceeds upward at the bottom of the molten silicon. As the support is moved downward, the raw material is continuously charged from above the crucible, and melting and solidification are continued, so that silicon polycrystal can be continuously cast while solidifying in one direction. .

  According to this electromagnetic casting method, molten silicon hardly comes into contact with the crucible wall, and impurity contamination can be prevented. Since there is no contamination from the crucible, there is an advantage that it is not necessary to use a high-purity material as the material of the crucible. Moreover, since it can cast continuously, manufacturing cost can be significantly reduced.

  However, in actual operation, the slit shape between the strip-shaped pieces constituting the cooling crucible is distorted and crushed, requiring frequent repairs, and shortening the crucible life and increasing the equipment cost. . This slit distortion or crushing is caused by discharge caused by contact with the surface of the crucible charged with molten silicon (that is, the surface of each strip-shaped element), and is generally referred to as “discharge flaw”. When the discharge flaw increases and the slit deteriorates (slit distortion and crushing become severe), the eddy current generated in the molten silicon becomes weak and the amount of heat for melting the raw material decreases. Moreover, casting becomes unstable and the quality of the ingot obtained also falls.

  FIG. 3 is a photograph illustrating the progress of deterioration of the slit portion of the copper cooling crucible used for comparison in the examples described later. (A) shows the normal slit part before casting implementation. (B) and (c) are slit parts after casting, (b) is the state after the end of the sixth use, and (c) is the slit part after the end of the eighth use. Indicates the state. It can be seen that the deterioration of the slit portion is progressing as the number of times of use in casting increases.

  Non-Patent Document 1 shows heat treatment and various properties of beryllium copper for casting. Later, the relationship between the beryllium content and the strength and hardness of beryllium copper will be described with reference to this.

JP 61-52962 A Standard Metal Engineering Lecture 4 “Nonferrous Metal Materials”, Masataka Hatakeyama, Corona Company (published first edition on June 30, 1965), page 105, Tables 1.56

  The present invention provides a cooling crucible when a silicon raw material is melted by electromagnetic induction heating using a conductive bottomless cooling crucible divided into a plurality in the circumferential direction and then solidified to continuously cast polycrystalline silicon. The present invention provides a silicon casting apparatus having a cooling crucible that can suppress the generation of discharge flaws on the surface of each strip-shaped piece constituting the metal, secure a heat quantity for melting the raw material, and extend the crucible life. It is aimed.

  As one means for solving the above-mentioned problems, the present inventors have a high resistance to the discharge generated when molten silicon contacts the crucible surface (the surface of each strip-shaped piece), in other words, We examined the use of a crucible made of a material that is not easily distorted or crushed in the slit shape. Even if the above discharge occurs, if the resistance to the discharge is large and it is difficult to cause a discharge scratch, it is possible to reduce the distortion and crushing of the slit to extend the life of the cooling crucible and to suppress the increase in the equipment cost. is there.

  As a condition that such a cooling crucible should have, in addition to being excellent in conductivity and thermal conductivity, in order to suppress the occurrence of discharge scratches or to stop the occurrence of slight, hardness at high temperature, strength, It is considered necessary to have excellent wear resistance and the like.

  From such a viewpoint, the present inventors paid attention to a copper alloy containing beryllium. For example, beryllium copper (a copper alloy containing approximately 2% by mass of Be and a small amount of Co, etc.) defined in JIS H 3270 is used as a highly conductive spring, a spot welding electrode, and the like. It is thought that it is excellent in hardness, strength, wear resistance, etc. at high temperature.

FIG. 1 is an extract of tensile strength and Brinell hardness (both values after solution treatment → aging) from Tables 1 and 56 of Non-Patent Document 1 (heat treatment and properties of beryllium copper for casting). It is the figure which displayed content on the horizontal axis. In Tables 1 and 56, all of the Be content, the tensile strength, and the Brinell hardness are represented by numerical values with widths. However, for the sake of convenience in displaying them in FIG. The median value was adopted. The tensile strength of the unit [kg / mm ^2] of the expressed in terms of [N / mm ^2].

  From FIG. 1, it can be seen that both the tensile strength and the Brinell hardness increase almost linearly as the Be content increases. Shown here is test data at room temperature, but it is considered that the beryllium copper is also excellent in hardness, strength, etc. at high temperatures, such as being used for welding electrodes. 1 is presumed to be maintained even at high temperatures.

  Therefore, the material of the cooling crucible was changed from pure copper to beryllium copper to produce a cooling crucible, and when actual electromagnetic casting was performed, it was possible to suppress the occurrence of discharge flaws and greatly extend the life of the cooling crucible. confirmed.

  The present invention has been made on the basis of such examination results, and the gist thereof resides in the following silicon casting apparatus.

  That is, it has a conductive bottomless cooling crucible partly divided in the circumferential direction in the circumferential direction and an induction coil surrounding the cooling crucible, and the silicon melted by electromagnetic induction heating by the induction coil is directed downward In the silicon casting apparatus which is pulled down and solidified, the material of the cooling crucible is a copper alloy containing beryllium.

In silicon casting apparatus of the present invention, der because those the copper alloy contains 0.1 to 5 wt% of beryllium, life prolonging effect of the cooling crucible was observed clearly, also difficulties in the fabrication of the cooling crucible It is not accompanied.

  The silicon casting apparatus of the present invention is an apparatus provided with a bottomless cooling crucible made of copper alloy containing beryllium divided into a plurality of circumferentially as a raw material melting container. As compared with the case of using a copper bottomless cooling crucible, the occurrence of discharge flaws can be effectively suppressed. Thereby, the lifetime of a crucible can be extended significantly and it can contribute to reduction of equipment cost, and also can ensure the calorie | heat amount for raw material melt | dissolution, and can perform stable casting. In addition, since the molten silicon hardly comes into contact with the crucible wall and impurity contamination can be prevented, it is suitable for manufacturing a silicon ingot for a solar cell that requires high quality.

  As described above, the silicon casting apparatus of the present invention uses a conductive bottomless cooling crucible in which a part of the axial direction is divided into a plurality in the circumferential direction, and an apparatus for manufacturing a silicon ingot by an electromagnetic casting method. In the silicon casting apparatus, the material of the cooling crucible is a copper alloy containing beryllium.

  FIG. 2 is a diagram schematically showing a schematic configuration example of the silicon casting apparatus of the present invention. The basic configuration is the same as a conventionally used silicon casting apparatus.

  In FIG. 2, the chamber 1 is a water-cooled container having a double wall structure, the upper part is connected to a raw material charging device partitioned by a blocking means 2, and has a drawer port 4 for extracting an ingot 3 at the bottom. ing. The chamber 1 is provided with an inert gas inlet 5 on the upper side wall and a vacuum suction port 6 on the lower side wall.

  A cooling crucible 7 as an electromagnetic casting means, an induction coil 8 and an after heater 9 are provided at the center of the chamber 1. The cooling crucible 7 is a cooling square cylinder made of copper alloy containing beryllium, and is a bottomless crucible that is divided into a plurality of pieces in the circumferential direction, leaving the upper part, and each forms a strip-shaped piece. The induction coil 8 is concentrically provided on the outer peripheral side of the cooling crucible 7 and is connected to a power source by a coaxial cable (not shown). The after heater 9 is concentrically connected below the cooling crucible 7, heats the ingot 3 pulled down from the cooling crucible 7, and gives a predetermined temperature gradient in the axial direction thereof.

  A raw material introduction pipe 10 is provided below the shielding means 2 provided in the upper part of the chamber 1, and granular and lump silicon raw material 11 charged in the raw material introduction pipe 10 is transferred to the molten silicon 12 in the cooling crucible 7. It comes to be supplied. An auxiliary heater 13 made of graphite or the like is provided directly above the cooling crucible 7 so that it can be raised and lowered, and is inserted into the cooling crucible 7 in a lowered state.

  A gas seal portion 14 is provided below the after heater 9, and a drawing device 15 that pulls out downward while supporting the ingot 3 is attached. A diamond cutting machine 16 as a mechanical cutting means is disposed below the gas seal portion 14 and outside the chamber 1. The diamond cutting machine 16 descends in synchronization with the drawing speed of the ingot 3 and is configured to cut the ingot 3 drawn out of the chamber 1 from the drawing port 4 while following the movement.

  The feature of the silicon casting apparatus of the present invention is that the material of the cooling crucible which is one of electromagnetic casting means is a copper alloy containing beryllium.

  The cooling crucible is made of a copper alloy containing beryllium, as described above, due to discharge flaws (slit distortion and crushing) caused by molten silicon coming into contact with the crucible surface (the surface of each strip-shaped piece). This is to suppress the generation and secure the amount of heat for melting the silicon raw material, and to greatly extend the life of the cooling crucible.

  The beryllium content is not particularly limited. As shown in FIG. 1, the hardness and strength increase almost linearly as the Be content increases. By analogy, if even a small amount of beryllium is included, Accordingly, it is considered that resistance against the generation of discharge scratches is developed, and the resistance increases as the amount of Be increases. The upper limit of the beryllium content is naturally restricted from the viewpoints of melting and processing of the copper alloy.

However, if 0.1% by mass or more of beryllium is contained, the effect of suppressing the occurrence of discharge flaws in the cooling crucible is clearly recognized. If the beryllium content exceeds 5% by mass, Be is reduced during welding repair. This is undesirable because it tends to deposit unevenly and cause local cracking and cracking of Be copper. Therefore, including Yuryou beryllium shall be the 0.1 to 5 wt%.

  As described above, the material of the cooling crucible included in the silicon casting apparatus of the present invention is a copper alloy containing beryllium. However, it has excellent conductivity and thermal conductivity, and has the effect of suppressing the occurrence of discharge flaws. As long as it is not impaired, alloy elements other than beryllium may be included. For example, in addition to beryllium, a small amount of cobalt, nickel, or the like may be included. In addition, about cobalt, when this is contained a little, effects, such as preventing the crystal grain growth by solution treatment, are recognized.

  As the copper alloy containing beryllium, “Beryllium Copper” is defined in JIS H 3270, and “Beryllium Copper for Spring” is defined in JIS H 3130. The chemical composition of the former beryllium copper is: Be: 1.8 to 2.00% (“%” means “mass%”. The same applies hereinafter), Ni + Co: 0.20% or more, Ni + Co + Fe: 0.6% Hereinafter, Cu + Be + Ni + Co + Fe: 99.5% or more. The chemical component of the latter beryllium copper for springs is Be: 1.60 to 1.79%, and other components are the same as the former. In the silicon casting apparatus of the present invention, these beryllium copper equivalent materials and spring beryllium copper equivalent materials may be used as the material for the cooling crucible.

  In order to manufacture a silicon ingot by the electromagnetic casting method using the silicon casting apparatus of the present invention shown in FIG. 2, the silicon raw material 11 is charged into the cooling crucible 7 made of copper alloy containing beryllium, and the induction coil 8 Through alternating current. Thereby, as mentioned above, a magnetic field is formed in the cooling crucible 7, and the silicon raw material 11 is heated and melted. At that time, the molten silicon 12 is melted in a non-contact state with the cooling crucible 7 by receiving a force in the inner normal direction of the surface of the molten silicon 12 and contamination of the ingot 3 due to contact with the cooling crucible 7 is prevented. .

  Subsequently, while melting the silicon raw material 11 in the cooling crucible 7, as described above, the pulling device 15 that holds the molten silicon 12 and the ingot 3 below is moved downward to advance the solidification of the molten silicon 12, By continuously charging the silicon raw material 11 from above the cooling crucible 7 and continuing the melting and solidification, the silicon polycrystal can be continuously cast.

  Thus, if the silicon casting apparatus of the present invention having the cooling crucible whose material is changed from pure copper to a copper alloy containing beryllium is used, the generation of discharge flaws can be suppressed and the life of the cooling crucible can be greatly extended. Can do. In addition, the amount of heat for melting the raw material can be secured, stable casting can be performed, and the molten silicon hardly comes into contact with the crucible wall, so that impurity contamination can be prevented. A high-quality silicon ingot suitable as a material can be manufactured.

  Furthermore, since the mechanical strength of the crucible is remarkably increased, the following secondary effects can be obtained.

A copper alloy containing beryllium is an alloy having high precipitation hardenability, and mechanical strength (tensile strength) is significantly improved by performing an appropriate heat treatment. The tensile strength of pure copper (Cu: 99.9% or more) is a material of alloy number C1100 defined in JIS H 3100, which is about 275 N / mm ² , but the tensile strength after age hardening treatment of the beryllium copper This is a material of alloy number C1720H, which is about 1400 N / mm ² and is generally four times or more.

  Therefore, taking a case of casting a 345 mm square ingot using a cooling crucible made of pure copper (because the crucible is a rectangular cylinder, hereinafter also referred to as “mold”), use a beryllium copper mold. Thus, for example, a 505 mm square ingot can be cast without changing the thickness of the mold. This is made possible by a marked improvement in the mechanical strength of the mold material, and the material saving effect when manufacturing an expensive mold is very large.

  In addition, since the thickness of the cooling mold can be reduced, the magnetic field reduction loss due to this thickness is reduced, and the transmission of the magnetic field from the high-frequency induction coil provided on the outer periphery of the crucible to the inside of the mold is increased. In addition, there is an advantage that the heat generation efficiency can be increased.

  Electromagnetic casting was performed using the silicon casting apparatus of the present invention having the schematic configuration shown in FIG. 2, and the deterioration state of the slit accompanying repeated use of the cooling mold and the life of the mold were investigated. In addition, the lifetime of the mold was evaluated by the number of times of use in casting, assuming that the number of times of use was one when the casting was used from the start of continuous casting to the end of casting.

  As the cooling mold made of copper alloy containing beryllium, a mold manufactured using a beryllium copper equivalent material defined in JIS H 3270 as a material was used.

  FIG. 3 illustrates the progress of deterioration of the slit portion of the copper cooling mold used for comparison. (A) shows the normal slit part before casting, (b) and (c) show the slit part after finishing the 6th use and 8th use in casting, respectively. In the slit portion shown in (c), the deterioration is considerably advanced, and it is determined that the use limit of the mold. That is, the lifetime of the copper cooling mold is 8 times as evaluated by the number of times of use in casting.

  FIG. 4 is a diagram illustrating the progress of deterioration of the slit portion of a copper alloy cooling mold containing beryllium (the beryllium copper mold) used in the silicon casting apparatus of the present invention. (A) is a slit part after finishing the 8th use in casting, (b) is a slit part after finishing the 16th use.

  Comparing FIG. 4 (a) and FIG. 3 (c), the number of times of use is 8 times, but the progress of the deterioration of the slits is significantly different, and cooling of copper alloy containing beryllium is performed. It can be seen that in the mold, the occurrence of discharge flaws is effectively suppressed. However, as shown in FIG. 4B, even in the case of the cooling mold made of beryllium-containing copper alloy, partial slit crushing is noticeable when the number of use reaches 16 times. The life of the mold in this case is evaluated as 16 times.

  From the results shown in FIG. 3 and FIG. 4, by using the silicon casting apparatus of the present invention having a bottomless cooling crucible made of copper alloy containing beryllium, it is possible to suppress the occurrence of discharge flaws when performing electromagnetic casting, It can be seen that the lifetime of the can be greatly extended. The same applies to a cylindrical crucible as well as a mold.

  The silicon casting apparatus of the present invention is an apparatus including a bottomless cooling crucible made of copper alloy containing beryllium, and can more effectively suppress the occurrence of discharge flaws when performing electromagnetic casting. If this apparatus is used, the crucible life can be greatly extended and the equipment cost can be reduced. In addition, the amount of heat for melting the silicon raw material is secured, stable casting is performed, and impurity contamination from the crucible wall can be prevented, so that a high-quality silicon ingot can be manufactured.

  Therefore, the silicon casting apparatus of the present invention can be suitably used for the production of polycrystalline silicon used as a substrate material for solar cells that require high quality.

It is a figure based on Table 1 * 56 of a nonpatent literature 1, and is a figure which shows the relationship of Be content in a Cu-Be alloy, tensile strength, and Brinell hardness. It is a figure which shows typically the example of schematic structure of the silicon | silicone casting apparatus of this invention. In the photograph which illustrates the progress of the deterioration of the slit part of the copper cooling mold used for comparison, (a) is a normal slit part before casting, (b) and (c) are 6 times each in casting. And the slit part after using 8 times is shown. In the photograph which illustrates the progress of the deterioration of the slit portion of the copper alloy cooling mold containing beryllium used in the silicon casting apparatus of the present invention, (a) and (b) were used 8 times and 16 times, respectively, in casting. The rear slit part is shown.

Explanation of symbols

1: Chamber 2: Blocking means 3: Ingot 4: Drawer port 5: Inert gas inlet port 6: Vacuum suction port 7: Cooling crucible 8: Induction coil 9: After heater 10: Raw material inlet tube 11: Silicon raw material 12: Melting Silicon 13: Auxiliary heater 14: Gas seal 15: Pulling device 16: Diamond cutting machine

Claims (1)

A conductive bottomless cooling crucible partly divided in the circumferential direction in the axial direction and an induction coil surrounding the cooling crucible, and the silicon melted by electromagnetic induction heating by the induction coil is pulled down. In the silicon casting machine to solidify,
A silicon casting apparatus, wherein the material of the cooling crucible is a copper alloy containing 0.1 to 5% by mass of beryllium.

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