JP3571507B2 - Method for manufacturing polycrystalline silicon ingot - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a method for producing a polycrystalline silicon ingot used for a solar cell, and more specifically, granulates a silicon powder produced in the production of high-purity silicon, and uses this compact as a melting raw material. The present invention relates to a method for manufacturing a polycrystalline silicon ingot.
[0002]
[Prior art]
With the recent spread of photovoltaic power generation, solar cell manufacturing techniques have been applied in various fields such as silicon for semiconductors, amorphous silicon, polycrystalline silicon, and the like, and technological development is remarkable in any field. The characteristics required of solar cells at the beginning of development were high performance with a focus on improving photoelectric conversion efficiency. However, with the spread of solar cells, price reduction has come to be aimed at. For this reason, for example, in the production of a polycrystalline silicon substrate, an extraordinary product of polycrystalline silicon produced as a raw material silicon for a semiconductor or a residual material of single crystal silicon is used after being crushed and washed. However, there is a limit to the cost reduction because the variety of silicon for semiconductors is so diverse that a product of a certain quality cannot be secured, or a quantity corresponding to the production amount of the solar cell cannot be secured stably. For this reason, in the future development of a solar cell, it is necessary to establish a silicon production technology that meets high-purity quality that exhibits a predetermined photoelectric conversion efficiency and that is cost-effective.
[0003]
Conventionally, as a method for producing high-purity polycrystalline silicon, there is a Siemens Method (Siemens Method). In this method, trichlorosilane (SiHCl ₃ ), which is an intermediate compound, is reduced with hydrogen (H ₂ ), and vaporized high-purity trichlorosilane is introduced into a reaction furnace together with high-purity hydrogen to form Chlorosilane is decomposed according to the following reaction formula (A), and polycrystalline silicon is vapor-phase grown on the surface of a polycrystalline silicon mandrel supported at both ends by graphite electrodes and heated to about 1100 ° C.
[0004]
SiHCl ₃ + H ₂ → Si + 3HCl (A)
However, in the Siemens method, a large amount of power is consumed for heating the polycrystalline silicon mandrel, so that the power consumption is deteriorated. Furthermore, since only a small percentage of the trichlorosilane introduced into the reactor contributes to the production of polycrystalline silicon, the production efficiency is extremely low. For this reason, this manufacturing method is not suitable as a method for manufacturing a silicon raw material for a solar cell, which is aimed at reducing costs.
[0005]
Therefore, a fluidized-granulation method has been developed as a process for producing granular polycrystalline silicon completely different from the Siemens method. In this manufacturing method, a fluidized bed reactor is used, silicon fine powder serving as a seed is fluidized in the reactor, and a mixed gas of monosilane (SiH ₄ ) and hydrogen is introduced into the reactor. Monosilane decomposes in a flowing atmosphere heated to ℃. At this time, granular polycrystalline silicon is generated through the following reaction formula (B).
[0006]
SiH ₄ → Si + 2H ₂ (B)
Since the total area of the granular silicon is overwhelmingly larger than the silicon mandrel in the Siemens method, the production efficiency of polycrystalline silicon in the fluidized bed reactor is high. Furthermore, the purity of polycrystalline silicon produced by the fluidized-granulation method is equivalent to that of the Siemens method, but the energy consumption can be reduced from 1/5 to 1/10 and the capital investment is reduced by half. Can be done. From such a viewpoint, it can be said that the fluidized granulation method is an effective method as a method for producing a silicon raw material for a solar cell.
[0007]
Further, as a method for producing polycrystalline silicon having excellent reaction efficiency, a so-called “zinc reduction method of silicon tetrachloride” (hereinafter, simply referred to as “zinc reduction method”) is adopted as a method for producing silicon raw materials for solar cells. It has become. This production method is a method of producing polycrystalline silicon by using silicon tetrachloride (SiCl ₄ ) as an intermediate compound and reducing it with molten zinc (Zn) as shown by the following reaction formula (C). It is.
[0008]
SiCl ₄ + 2Zn → Si + 2ZnCl ₂ (C)
As shown in the above formula (C), the zinc reduction method is a method of producing an intermediate compound by subjecting metallic silicon to a chlorination treatment (halogenation reaction), and then reducing and decomposing the intermediate compound without secondary contamination. Therefore, it is suitable for manufacturing high-purity silicon. On the other hand, since almost all silicon tetrachloride (SiCl ₄ ) is a reaction system that proceeds to silicon (Si), it is a production method having excellent reaction efficiency and good production efficiency, and is also advantageous in terms of production cost. is there.
[0009]
FIG. 1 is a diagram illustrating an example of a process for producing polycrystalline silicon by a zinc reduction method. As is clear from the figure, this zinc reduction method is largely divided into a supply step of a reaction raw material (SiCl ₄ and Zn), a reduction step, a separation step of by-products, and a removal step of high-purity silicon (Si). .
[0010]
In the step of supplying the reaction raw material, silicon tetrachloride (SiCl ₄ ) obtained by subjecting metal silicon to chlorination and metal zinc (Zn) as a reducing agent are prepared as raw materials. Metallic zinc (Zn) is heated to a temperature in the range of 450 to 550 ° C. to be in a molten state in order to facilitate supply to the reaction furnace. On the other hand, silicon tetrachloride (SiCl ₄ ), which is an intermediate compound, is stored at room temperature and kept in a liquid state (melting point -70 ° C.).
[0011]
In the reduction step, polycrystalline silicon and by-product zinc chloride (ZnCl ₂ ) are obtained by reducing silicon tetrachloride (SiCl ₄ ), which is an intermediate compound, with molten zinc (Zn). At this time, since the boiling point of zinc chloride is 732 ° C., zinc chloride (ZnCl ₂ ) generated by the reduction reaction becomes a vapor, and the polycrystalline silicon powder generated at the same time is very fine, and zinc chloride (ZnCl ₂ ) ₂ ) It is discharged outside the reduction reactor together with the vapor.
[0012]
In the by-product separation step, an evaporative separation method or a liquid filtration method is used to separate by-product zinc chloride (ZnCl ₂ ). When the evaporative separation method is used, a mixture composed of polycrystalline silicon and zinc chloride (ZnCl ₂ ) is introduced into a separation vessel, and then heated together with evacuation to evaporate and separate zinc chloride (ZnCl ₂ ). , Polycrystalline silicon is recovered. When the liquid filtration method is used, a porous fine ceramic filter having high temperature characteristics and low contamination is provided in the separation vessel, and the introduced mixture is subjected to pressure filtration.
[0013]
In the step of extracting high-purity silicon, impurities are completely removed by repeating the above separation, and polycrystalline silicon with sufficiently high purity is extracted. In the zinc reduction method, the steps from the supply of the reaction raw materials to the removal of high-purity silicon can be continuously performed, so that a production method with higher production efficiency can be provided.
[0014]
As described above, both the fluidized-granulation method and the zinc reduction method are high-efficiency production methods corresponding to low costs, and are not likely to cause secondary contamination, and are therefore effective as methods for producing silicon materials for solar cells. Is a great way.
[0015]
[Problems to be solved by the invention]
Usually, granular polycrystalline silicon produced by the fluid granulation method has a diameter of 100 to 1500 μm and an average diameter of about 700 μm. Fine particles generated at this time, for example, powdered silicon having a diameter of 3 μm or less are generated at about 15%. On the other hand, as described above, in the zinc reduction method, as shown in the above formula (C), the polycrystalline silicon powder produced by reducing silicon tetrachloride with molten zinc is very fine, and All of the silicon taken out after the separation are fine particles having a diameter of 3 μm or less.
[0016]
FIG. 3 is a diagram showing a schematic configuration of a continuous casting apparatus by electromagnetic induction used for manufacturing a polycrystalline silicon ingot described later. At the center of the device, a metal water-cooled crucible (cold crucible) cut into segments so as to be electrically insulated from each other is installed in an induction coil. Then, by infiltrating the magnetic field into the crucible, the molten silicon is held in a non-contact manner by the magnetic pressure to be melted. In order to continuously manufacture the ingot, the upper end surface of the silicon ingot in the induction coil is melted, and the granular molten material is supplied to the molten silicon formed on the upper surface, and the melting and solidification of the material are repeated to form the silicon ingot. Pulled down. During the melting of the raw material, the atmosphere in the apparatus is maintained in an inert gas atmosphere after evacuation.
[0017]
At this time, if the granular dissolved raw material supplied to the molten silicon is fine, for example, 1 mm or less in diameter, it only floats on the surface of the molten silicon, does not melt into the molten silicon, and cannot be sufficiently dissolved. In addition, the inert gas is scattered due to the blowing or the gas flow, which causes gas suction failure in the apparatus. Therefore, when using granular polycrystalline silicon produced by the fluidized-granulation method as a melting raw material, it is necessary to remove fine-grain silicon having a diameter of 1 mm or less. For this reason, an increase in the cost of the dissolved raw material is inevitable. Further, since the polycrystalline silicon powder produced by the zinc reduction method is all fine particles having a diameter of 3 μm or less, it cannot be used in a continuous casting apparatus as shown in FIG.
[0018]
The present invention, in the production of ingots of polycrystalline silicon in the conventional continuous casting method, in view of the problems that silicon powder has, in order to improve the particle size of fine-grained silicon, promote reuse, high production efficiency It is an object of the present invention to provide a method for manufacturing high-purity polycrystalline silicon corresponding to a reduction in cost.
[0019]
[Means for Solving the Problems]
The gist of the present invention is the following method for producing a polycrystalline silicon ingot.
[0020]
That is, it is "a method for producing a polycrystalline silicon ingot, which comprises granulating silicon powder by high-pressure molding, and then casting an ingot by using the granulated silicon compact as a melting raw material."
[0021]
In the above-mentioned production method, it is desirable to dry and / or sinter the granulated silicon compact, and it is further desirable to set the particle size of the silicon compact as a melting raw material to 1 mm to 10 mm.
[0022]
Here, the silicon powder includes a silicon powder of less than 1 mm generated by a fluid granulation method and a polycrystalline silicon powder generated by a zinc reduction method. The target is fine-grain silicon having a diameter of less than 1 mm that satisfies the purity.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
In the production of silicon for solar cells, it is premised that the silicon has the performance to exhibit a predetermined photoelectric conversion efficiency. For this reason, polycrystalline silicon used for solar cells and silicon raw materials for producing the same are required to have high purity, specifically, 6N (99.9999%) and 7N (99.999999), respectively. %) It is required to ensure a considerable purity. In the present invention, silicon powder is granulated so that even high-purity polycrystalline silicon can be efficiently produced even with a silicon powder having a small particle diameter, and an ingot is cast using the granulated silicon powder. Further, the method is characterized in that the granulated silicon is dried and sintered as required. Hereinafter, the contents will be described.
[0024]
(1) Granulated silicon powder Examples of the silicon powder include fine-grained polycrystalline silicon produced by a flow granulation method and polycrystalline silicon powder produced by a zinc reduction method, each having a diameter of 3 μm or less. Fine-grained. One or more silicon powders can be used alone or in combination. In either case, it is desirable to achieve complete uniform mixing. This is because if there is partial variation, when compacts having the same shape, size and density ratio are manufactured, the size and characteristics after granulation or sintering are different.
[0025]
FIG. 2 is a diagram showing a schematic configuration of a high-pressure roll forming machine used for granulating silicon powder in the present invention. As shown in the figure, the uniformly mixed silicon powder 1a is subjected to high pressure molding by a pressure roll 2 to form a granulated silicon compact 1b.
[0026]
High pressure molding is suitable for granulation of silicon powder. By subjecting the silicon powder to high-pressure molding, the irregularities on the particle surface collapse due to mutual friction and pressing, or solid contact is created between the particles due to meshing, and the density ratio of the green compact Because it can be raised.
[0027]
In the high-pressure roll forming machine shown in FIG. 2, the sufficiently mixed silicon powder 1a is formed while a pressure of 1T or more is applied by the pressure of the pressure roll 2. It is desirable that the particle size of the formed silicon compact 1b be 1 mm to 10 mm. This is because the green compact has many voids and it is difficult to conduct heat, so the upper limit of the particle size is set to 10 mm in order to ensure solubility, while if the dissolved raw material is too fine, it floats on the molten surface. This is because the lower limit of the particle size is desirably set to 1 mm.
[0028]
At this time, if the moldability (density ratio, etc.) of the silicon compact is not sufficient, a granulating binder, for example, polyvinyl alcohol (PVA) is added as an aqueous solution. Although the addition ratio depends on the moldability of the silicon compact, it is desirable that the volume ratio of the 10% PVA solution to the silicon powder be 10%.
[0029]
After being granulated, the silicon compact is supplied to a molten raw material. However, if a binder is added during granulation, it must be removed by drying. Usually, drying of the green compact for debinding is by a combination of low-temperature pre-drying and high-temperature heating under vacuum or reduced pressure.
[0030]
When the strength of the granulated green compact is not sufficient and there is a problem in handling the molten raw material, it is desirable to further add a sintering step. As described above, when silicon powder is molded under high pressure at room temperature to produce a green compact, the bonding between the powder particles in the green compact is mainly based on mechanical meshing, so the strength is limited. There is. Therefore, the strength of the green compact can be ensured by further heating the green compact to generate sufficient atomic bonds between the particles.
[0031]
In addition, since the sintering further promotes the adhesion phenomenon between the particle surfaces, the bulk density increases and the thermal conductivity improves, so that the solubility of the silicon raw material can be improved.
[0032]
(2) Casting of Ingot FIG. 3 is a view for explaining a schematic configuration of an apparatus for continuously forming and casting an ingot using silicon compact as a melting raw material in the present invention. In manufacturing a silicon ingot for a solar cell, continuous casting is not limited as a device to be used, and a batch or semi-continuous casting method using a mold may be used. In this case, it is desirable to use a continuous casting apparatus.
[0033]
The apparatus shown in FIG. 3 employs a continuous casting method by electromagnetic induction. In this method, the crucible 7 and the molten silicon 11 can be held and melted in a non-contact state by magnetic pressure, so that secondary contamination can be avoided. The appearance of the casting apparatus is composed of a water-cooled airtight container 3. The vacuum port 4 is connected to a vacuum pump so that the inside of the airtight container 3 can be evacuated to a vacuum, and an inert gas port 5 is provided so that the inert gas can be controlled at an arbitrary pressure in the airtight container 3. The upper and lower portions of the airtight container 3 are partitioned by a vacuum valve 6 so that the raw material can be charged and the ingot can be taken out in an inert atmosphere.
[0034]
At the center of the device, a water-cooled crucible 7 made of metal and having no bottom is provided, around which an induction coil 8 is wound, and a heat insulating furnace 9 is installed below the induction coil 8. The upper end of the ingot 10 in the induction coil 8 is melted to form molten silicon 11. A raw material hopper is provided below the raw material charging device 12, and the charged granular molten raw material 1b is supplied to the molten silicon through a swirling charging duct. In this case, the molten raw material can be sufficiently dissolved in the molten silicon and uniformly dissolved without floating on the surface of the molten silicon by introducing the silicon compact compacted in the present invention. In addition, it is not scattered by blowing out an inert gas or a gas flow. A drawing device 13 is provided below the heat retaining furnace 9 so that the silicon ingot 10 is continuously drawn at a predetermined speed. Thus, a silicon ingot can be manufactured efficiently.
[0035]
【Example】
The effects of the present invention will be specifically described based on Embodiments 1 and 2.
[0036]
(Example 1)
Silicon powder having a purity of 7N and an average particle diameter of 0.35 μm was granulated to produce a silicon ingot as a melting raw material.
[0037]
a. For granulation of the granulated silicon powder, a high-pressure roll forming machine shown in FIG. 2 was used, and the reduction by a pressure roll was set to 1T. In order to secure the moldability of the silicon compact, an aqueous solution having a PVA concentration of 10% was added as a binder to the silicon powder at a volume ratio of 10%, and the mixture was uniformly mixed with a mixer. Thereafter, briquettes (compacts) having a diameter of 3 to 5 mm were produced by high-pressure molding using a high-pressure roll molding machine.
[0038]
b. The briquettes were dried for drying and sintering. The drying conditions were as follows: pre-drying was performed at 200 ° C. × 10 hours with a dryer, and then 600 ° C. × 10 hours in a vacuum furnace (the degree of vacuum was 0.2 Torr).
[0039]
After drying, sintering was performed at 1300 ° C. for 2 hours at a degree of vacuum of 10 ⁻⁴ Torr using a vacuum sintering furnace in order to secure strength as a melting raw material.
[0040]
c. An ingot was produced by the continuous casting apparatus shown in FIG. After the inside of the airtight container was evacuated to vacuum, argon gas was sealed therein and the pressure was adjusted to +30 Torr. With the charging duct of the raw material charging device above the crucible retracted in the horizontal direction, a heating element (not shown) is lowered into the crucible, placed close to the ingot, and energized to the induction coil. The upper end surface of the ingot in the induction coil is melted, and molten silicon is initially formed on the upper surface. After the initial formation of molten silicon, the granulated silicon compact is supplied as a melting raw material, and is melted as a silicon raw material, and is gradually solidified in a warm furnace, and a drawing device is operated to pull out a silicon ingot. .
[0041]
In charging the raw material during melting, the strength of the green compact is ensured, there is no problem such as chipping of the raw material due to handling, collapse, etc., it can be sufficiently melted, continuously from below the casting device, High-purity granular silicon was taken out. When the taken out ingot was analyzed, the purity was 7N (99.999999%), and it was confirmed that the ingot could be used for a solar cell.
[0042]
(Example 2)
A polycrystalline silicon powder produced by reducing silicon tetrachloride by a zinc reduction method was granulated to produce a silicon ingot as a melting raw material. All of the produced silicon powders were fine particles having a diameter of 3 μm or less, and the purity was 7N.
[0043]
a. Briquettes (compacts) having a diameter of 3 to 5 mm were produced by high-pressure molding using a high-pressure roll molding machine without mixing a binder into the granulated silicon powder. Other granulation conditions were the same as in Example 1.
[0044]
b. In the charging of raw materials during melting, some raw materials were chipped or collapsed due to handling, but they could be sufficiently dissolved in the molten silicon, and high-purity powder particles were continuously obtained from below the casting equipment. The body silicon was taken out. When the taken out ingot was analyzed, the purity was 7N (99.999999%), and it was confirmed that the quality was satisfactory for a solar cell.
[0045]
【The invention's effect】
According to the method for producing polycrystalline silicon of the present invention, even fine-grain silicon powder can be used without impairing the solubility as a dissolving raw material, without increasing the cost of the dissolving raw material, It is possible to stably produce high-purity polycrystalline silicon optimal for a solar cell substrate with high efficiency in terms of quality.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating an example of a process for producing polycrystalline silicon by a zinc reduction method.
FIG. 2 is a diagram showing a schematic configuration of a high-pressure roll forming machine used for granulating silicon powder in the present invention.
FIG. 3 is a diagram illustrating a schematic configuration of an apparatus for continuously forming and casting an ingot using silicon compact as a melting raw material according to the present invention.
[Explanation of symbols]
1a: Silicon powder, 1b: Silicon green compact 2: Pressure roll, 3: Airtight container 4: Vacuum outlet, 5: Inert gas port 6: Vacuum valve, 7: Crucible 8: Induction coil, 9: Heat insulation furnace 10: ingot, 11: molten silicon 12, raw material charging device, 13: drawing device

Claims (3)

A method for producing a polycrystalline silicon ingot, comprising: granulating silicon powder by high pressure molding; and casting an ingot using the granulated silicon compact as a melting raw material. The method for producing a polycrystalline silicon ingot according to claim 1, wherein the granulated silicon compact is dried and / or sintered. The method for producing a polycrystalline silicon ingot according to claim 1 or 2, wherein the particle size of the silicon compact as a melting raw material is 1 mm to 10 mm.

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