JP3844856B2 - Manufacturing method of high purity silicon - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a method for producing high-purity silicon by reducing silicon tetrachloride. More specifically, the present invention constitutes a closed cycle from a step of producing polycrystalline silicon by a reduction reaction to a step of collecting and reusing by-products. The present invention relates to a method for producing high purity silicon.
[0002]
[Prior art]
With the recent spread of solar power generation, solar cell manufacturing technology is applied in various fields such as semiconductor silicon, amorphous silicon, polycrystalline silicon, etc., and technological development is remarkable in any field. The characteristics required for solar cells at the beginning of development were to improve the performance with the main purpose of improving the photoelectric conversion efficiency. However, with the widespread use of solar cells, the trend toward lower prices has come to be directed. For this reason, for example, in the manufacture of a polycrystalline silicon substrate, low-priced materials such as an extraordinary product of polycrystalline silicon manufactured as semiconductor silicon as a raw material and a residual material of single crystal silicon have come to be used. It is insufficient in quantity and there is a limit to reducing the price. For this reason, in the future development of solar cells, it is necessary to establish silicon manufacturing technology that is compatible with high-quality and low-cost production that exhibits predetermined photoelectric conversion efficiency.
[0003]
Conventionally, many production methods are known as methods for producing high-purity polycrystalline silicon. Among them, a typical production method is a Siemens Method in which trichlorosilane (SiHCl ₃ ), which is an intermediate compound, is reduced with hydrogen (H ₂ ). In this production method, vaporized high-purity trichlorosilane is introduced into a reaction furnace together with high-purity hydrogen, and the trichlorosilane is decomposed according to the reaction formula (A) below, and both ends are supported by graphite electrodes at about 1100. Polycrystalline silicon is vapor-phase grown on the surface of a polycrystalline silicon mandrel heated to ° C.
[0004]
SiHCl ₃ + H ₂ → Si + 3HCl (A)
As another production method, an ethyl method (Ethy Method) for producing granular polycrystalline silicon is used. In this manufacturing method, a fluidized bed reactor is used, a silicon fine powder as a seed is fluidized in the reactor, and a mixed gas of monosilane (SiH ₄ ) and hydrogen is introduced into the reactor, 600 to 700 Monosilane decomposes in a flowing atmosphere heated to ° C. At this time, granular polycrystalline silicon is produced through the reaction formula (B) below.
[0005]
SiH ₄ → Si + 2H ₂ (B)
However, there are many problems when these high-purity silicon manufacturing methods are applied to solar cells. First, the Siemens method has a low power consumption rate, and furthermore, the production efficiency is extremely low due to the low silicon production ratio of trichlorosilane introduced into the reactor. For this reason, it is not suitable as a manufacturing method of the silicon raw material for solar cells corresponding to cost reduction. On the other hand, the ethyl method has an advantage in terms of production cost compared to the Siemens method, but it is not yet an efficient production method for solar cells, and further cost reduction is necessary.
[0006]
Instead of the above polycrystalline silicon production method, it is a so-called “silicon tetrachloride zinc reduction method”, in which silicon tetrachloride (SiCl ₄ ) is used as an intermediate compound as shown in the reaction formula (C) below. Used, a method of producing silicon (Si) by reducing this with molten zinc (Zn) has come to be used.
[0007]
SiCl ₄ + Zn → Si + ZnCl ₂ (C)
The production method shown in the above formula (C) is a method in which metal silicon is chlorinated (halogenation reaction) to produce an intermediate compound, which is then reduced and decomposed without secondary contamination. This is an effective method for producing high-purity silicon. On the other hand, in terms of manufacturing costs, the reaction system in which almost all silicon tetrachloride (SiCl ₄ ) proceeds to silicon (Si) is excellent in reaction efficiency and, as will be described later, the process can be continued in the manufacturing process. It is a manufacturing method that is easy and has high production efficiency. Therefore, the above manufacturing method is also suitable as a method for manufacturing polycrystalline silicon at a low cost.
[0008]
FIG. 2 is a diagram for explaining an example of the production process of polycrystalline silicon by the so-called “silicon tetrachloride zinc reduction method”, and FIG. 3 shows the reduction from melting of metallic zinc in the production process of polycrystalline silicon by the same method. It is a figure explaining an example of a device configuration which processes a process leading to separation of a reaction by-product after reaction. The steps shown in FIGS. 2 and 3 are: (1) a raw material supply step, (2) a step of reducing silicon tetrachloride, (3) a step of separating the mixture, and (4) a step of taking out high purity silicon. The contents are as follows.
[0009]
(1) Process of supplying raw materials As raw materials, silicon tetrachloride obtained by chlorinating metallic silicon and metallic zinc as a reducing agent are prepared. The metallic zinc is melted to facilitate supply to the reactor. Therefore, the dissolution tank 3 charged with metal zinc is installed in the dissolution tank 2 equipped with a heating device 2a and heated to a temperature range of 450 to 550 ° C. On the other hand, silicon tetrachloride as an intermediate compound is accommodated in the SiCl ₄ tank 1 at room temperature and kept in a liquid state (melting point -70 ° C.).
[0010]
The tanks 1 and 2 of molten zinc and silicon tetrachloride are maintained in an inert gas (for example, argon gas) atmosphere at atmospheric pressure, and this inert gas is added to supply to the reaction vessel 5. The molten zinc and silicon tetrachloride are supplied separately or simultaneously.
[0011]
(2) Step of reducing silicon tetrachloride The reduction reaction is carried out in a sealed reaction vessel 5 installed in the reduction furnace 4, and polycrystalline silicon is obtained by reducing silicon tetrachloride as an intermediate compound with molten zinc. A mixture of silicon and by-produced zinc chloride is obtained. The reduction furnace 4 has a built-in heating device 4a, and zinc inside and by-product zinc chloride are held in a molten state. Silicon tetrachloride is dripped from the top through a nozzle or blown directly into the molten zinc. When blowing directly into the molten zinc, the reaction area can be secured not only in the horizontal direction in the reaction vessel 5 but also in the vertical direction, so that the efficiency of the reaction can be improved.
[0012]
As shown by the formula (C), with the supply of silicon tetrachloride, the silicon tetrachloride is reduced by molten zinc to produce polycrystalline silicon powder and zinc chloride. Since the boiling point of zinc chloride is 732 ° C., the generated zinc chloride is discharged as vapor out of the reaction vessel 5. Moreover, the polycrystalline silicon powder produced | generated simultaneously is very fine, and is discharged | emitted outside the reaction container 5 as the mixture 7 with the vapor | steam of the above-mentioned zinc chloride.
[0013]
(3) Step of separating the mixture The reaction by-product is separated in a separation vessel 10 disposed inside the separation furnace 9. When the evaporative separation method is used, the mixture 7 composed of polycrystalline silicon and zinc chloride is introduced from the upper part of the separation vessel 10, and then the entire separation vessel 10 is heated by the heating device 9 a built in the separation furnace 9. The polycrystalline silicon is recovered by evaporating and separating zinc chloride and zinc.
[0014]
When the liquid filtration method is used, the separation container 10 is provided with a porous fine ceramic filter having high temperature characteristics and little contamination, and the mixture 7 introduced into the separation container 10 is pressure-filtered.
[0015]
(4) Step of taking out polycrystalline silicon By repeating the above separation, impurities are completely removed and purified polycrystalline silicon is manufactured. Such polycrystalline silicon is taken out from the lower part of the separation container 10.
[0016]
As described above, in the method for producing polycrystalline silicon by the “silicon tetrachloride zinc reduction method” shown in FIG. 2, the steps (1) to (4) can be continuously performed, so that the production efficiency is excellent. In addition to the production method, there is no risk of secondary contamination, and high purity silicon can be produced by repeatedly separating the mixture.
[0017]
[Problems to be solved by the invention]
According to the so-called “zinc reduction method of silicon tetrachloride”, the manufacturing method of polycrystalline silicon corresponding to high-quality and low-cost production that exhibits a predetermined photoelectric conversion efficiency as a solar cell substrate. However, the above-described conventional methods are not sufficient for recovering by-products and promoting reuse and for building a closed cycle in the manufacturing process.
[0018]
That is, as shown in FIG. 2, in the conventional “silicon tetrachloride zinc reduction method”, silicon tetrachloride (SiCl ₄ ) and zinc (Zn) as raw materials are mainly purchased from outside. For this reason, zinc chloride (ZnCl ₂ ) produced as a by-product in the reduction process was discharged out of the system and was discarded after hydrochloric acid treatment. In order to recycle zinc (Zn) in part, zinc chloride (ZnCl ₂ ) taken out in the separation process was electrolyzed to collect metal zinc (Zn) individually. Chlorine (Cl ₂ ) generated at the same time could not be used, and it had to be disposed of after neutralization. For this reason, with the manufacture of polycrystalline silicon, capital investment is required for hydrochloric acid treatment, neutralization treatment, and waste treatment, which has been a factor in increasing production costs.
[0019]
Furthermore, by collecting zinc chloride (ZnCl ₂ ) produced as a by-product and promoting the reuse of metallic zinc (Zn) and chlorine (Cl ₂ ), the polycrystalline silicon produced by the “silicon tetrachloride zinc reduction method” Further cost reduction can be achieved in the manufacture of Moreover, if a closed cycle of the manufacturing process from the step of generating polycrystalline silicon to the step of recovering by-products is realized, high-purity silicon can be efficiently manufactured.
[0020]
In view of the problems in the conventional “zinc tetrachloride reduction method”, the present invention promotes the recovery and reuse of zinc chloride (ZnCl ₂ ) by-produced in the process, as well as high-purity polycrystalline. The purpose of the present invention was to provide a method for producing high-purity polycrystalline silicon that is highly productive, stable in quality, and low in price by configuring the silicon manufacturing process in a closed cycle. Is.
[0021]
[Means for Solving the Problems]
As shown in FIG. 1 to be described later, the present invention has a gist of a method for producing high-purity silicon in which a by-product is recovered and reused and a closed cycle of the production process is achieved.
[0022]
That is, silicon tetrachloride in a liquid or gaseous state is reduced with molten zinc in a reaction vessel, and a mixture containing the produced polycrystalline silicon and zinc chloride is taken out of the reaction vessel, and then zinc chloride in the mixture is removed. separated, a process for the preparation of high purity silicon for recovering polycrystalline silicon, to recover the separated zinc chloride (ZnCl ₂₎ and by electrolyzing metal zinc (Zn) and chlorine (Cl _2), the recovery The recovered zinc metal is used again as a reducing agent for the silicon tetrachloride, and the recovered chlorine is synthesized with hydrogen to form hydrogen chloride, which is used for chlorination of the metal silicon to produce the silicon tetrachloride. This is a method for producing high-purity silicon.
[0023]
In the above-described chlorination treatment of metal silicon, hydrogen chloride (HCl) obtained by synthesizing chlorine (Cl ₂ ) recovered by electrolysis of zinc chloride and hydrogen (H ₂ ) recovered by chlorination treatment of metal silicon is synthesized. I am going to use it.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
In the manufacture of silicon for solar cells, it is premised that the solar cell silicon has a performance that exhibits a predetermined photoelectric conversion efficiency. For this reason, polycrystalline silicon provided for solar cells and silicon raw materials for producing the same are required to have a high purity required for this, specifically 6N (99.9999%) and 7N (99.99999%), respectively. It is required to ensure purity. In order to achieve such high purity, the present invention is based on a method for producing polycrystalline silicon by a so-called “silicon tetrachloride zinc reduction method”.
[0025]
FIG. 1 is a diagram for explaining a process for producing high-purity silicon according to the present invention. As shown in the figure, the manufacturing process consists of (1) a raw material supply process, (2) a process for reducing silicon tetrachloride, (3) a process for separating the mixture, and (4) a high-purity silicon. In addition to the step of taking out (5) the step of electrolyzing zinc chloride and (6) the step of chlorinating metallic silicon. These processes (1) to (6) constitute a closed system to achieve a closed cycle. The contents of the steps (1) to (4) which are the basis of the closed cycle process are as described above, but (5) the process of electrolyzing zinc chloride and (6) metal which are newly added in the present invention. The step of chlorinating silicon will be described by classifying its specific contents.
[0026]
(5) Step of electrolyzing zinc chloride Here, zinc chloride (ZnCl ₂ ) taken out in the step of ( ₃ ) separating the mixture is electrolyzed to recover metallic zinc (Zn) and chlorine (Cl ₂ ). As described above, in the conventional manufacturing method, silicon tetrachloride (SiCl ₄ ), which is an intermediate compound, is premised on purchase from the outside, and zinc chloride (ZnCl ₂ ), a reaction byproduct, is also discharged out of the system. ing. In the present invention, in order to promote recovery and reuse of by-products, zinc chloride (ZnCl ₂ ) discharged from the separation vessel, condensed by a heat exchanger and recovered, is recovered by electrolysis. (Zn) is used as a reducing agent, and simultaneously recovered chlorine (Cl ₂ ) is used for chlorination of metal silicon.
[0027]
The electrolysis used in the present invention may be a conventional method in which metal zinc (Zn) is deposited on the electrode and recovered, and chlorine gas (Cl ₂ ) is generated. That is, metallic zinc (Zn) is recovered in a certain amount while zinc chloride (ZnCl ₂ ) condensed and melted is continuously supplied. At this time, the composition of the molten salt is adjusted in order to prevent the recombination reaction between the recovered metal zinc (Zn) and chlorine (Cl ₂ ) and to ensure stable electrolysis efficiency.
[0028]
The bath temperature should be kept near the melting point of zinc (Zn) to prevent excessive volatilization of zinc (Zn) and zinc chloride (ZnCl ₂ ), and is preferably in the range of 450 to 550 ° C. The recovered metallic zinc (Zn) is used again as molten zinc. In order to keep the purity of silicon to be produced above a certain level, it is desirable to manage the purity within a range of 99.9% or more.
[0029]
(6) Step of chlorinating metallic silicon In the present invention, in order to construct a closed cycle, as a preliminary step of supplying raw materials, metallic silicon is chlorinated (halogen reaction) in the silicon production process, and tetrachloride which is an intermediate product. A step of generating silicon (SiCl ₄ ) was provided, and the recovered chlorine (Cl ₂ ) was synthesized with hydrogen and supplied to the chlorination treatment of metal silicon. Thereby, the process of manufacturing polycrystalline silicon constitutes a closed system. For this reason, it is desirable to control the purity of chlorine (Cl ₂ ) recovered by electrolysis to 99%.
[0030]
Formation of silicon tetrachloride (SiCl ₄ ) by chlorination of metallic silicon is based on the reaction of the following formula (D). At this time, the recovered chlorine (Cl ₂ ) is synthesized with hydrogen (H ₂ ) by-produced by the chlorination treatment and supplied to the chlorination treatment as hydrogen chloride (HCl).
[0031]
Si ₂ + 8HCl → 2SiCl ₄ + 4H ₂ (D)
Since the purity of silicon tetrachloride (SiCl ₄ ), which is an intermediate compound, greatly affects the purity of silicon to be produced, usually, the produced silicon tetrachloride (SiCl ₄ ) is purified to be highly purified. For the purification of silicon tetrachloride (SiCl ₄ ), a general distillation method is adopted. Finally, the silicon tetrachloride is purified to have a purity of 99.99% or more, and is used as a raw material compound of high purity silicon.
[0032]
【Example】
High-purity silicon was produced by the production method of the present invention shown in FIG. During the manufacturing process, the configuration of the manufacturing equipment shown in Fig. 3 was used from the process of supplying the raw material to the process of extracting the high-purity silicon from the process of supplying the raw material. And (6) a step of chlorinating metal silicon to provide a closed cycle. The operating conditions in each process are as follows.
[0033]
(1) Step of supplying raw materials The purity of the metal zinc (Zn) used was 4N (99.99%), and it was heated to 500 ° C. to be in a molten state. The zinc zinc (Zn) supplied thereafter uses zinc recovered in the electrolysis process described later. As an intermediate compound, silicon tetrachloride (SiCl ₄ ) produced by the chlorination treatment described later was distilled and used with a purity of 4N (99.99%) and a metal impurity of 20 ppb or less. The supply to these reaction vessels was performed by pressurizing with argon gas to 1 kg / cm ² .
[0034]
(2) Reduction process Molten zinc (Zn) is supplied into the reaction vessel as a reducing agent, heated to 800 ° C. and stabilized, and a supply nozzle is immersed in molten zinc (Zn) from the top of the reaction vessel. Silicon tetrachloride (SiCl ₄ ) was supplied. With the supply of silicon tetrachloride, a mixture of polycrystalline silicon (Si) and zinc chloride (ZnCl ₂ ) is formed. The supply rate of silicon tetrachloride was 1.6 kg-SiCl ₄ / m ² min at maximum when the height from the nozzle tip to the molten zinc layer was 0.5 m. The produced zinc chloride (ZnCl ₂ ) evaporates and becomes a gaseous mixture with the finely divided polycrystalline silicon (Si) produced at the same time, and is transferred to the separation container.
[0035]
(3) Step of separating the mixture The mixture was separated by an evaporation separation method and a liquid filtration method. In the case of the evaporative separation method, the inside of the separation vessel was 500 ° C. and 1 Torr. The end point of the separation was determined by changing the control output of the heating furnace.
[0036]
On the other hand, in the case of the liquid filtration method, the mixture was dissolved in water at room temperature to make zinc chloride as an aqueous solution, and then filtered under pressure using a fine silica porous plate with a pressure of 0.2 kg / cm ² . .
[0037]
(4) Step of removing high-purity silicon After separation was completed, high-purity silicon powder was taken out. When the extracted silicon was analyzed, the purity was 7N (99.99999%), and it was confirmed that it could be applied for solar cells.
[0038]
(5) Electrolysis process of zinc chloride Electrolysis uses zinc chloride (ZnCl ₂ ) aqueous solution discharged in the separation process, bath temperature is 500 ° C, current density is 760A / m ² , bath potential is 3V, bath current is 2.26A. The bath composition was adjusted such that the concentration of the zinc chloride (ZnCl ₂ ) melt was 56 Zn-g / liter. The purity of the metal (Zn) recovered by electrolysis was 99.998%, and the purity was such that it could be used again as molten zinc.
[0039]
(6) Step of chlorinating metallic silicon In order to supply chlorination of metallic silicon, chlorine (Cl ₂ ) recovered by electrolysis is synthesized with hydrogen (H ₂ ) produced as a by-product of chlorination to produce hydrogen chloride ( HCl). The produced hydrogen chloride (HCl) is supplied to the chlorination treatment at 25 liters / min.
[0040]
The metal silicon to be chlorinated was purchased from outside with a particle size of 80-200 mesh and Si purity of 98%. The metal silicon was filled in the chlorination vessel and heated to 600 to 800 ° C., and then hydrogen chloride (HCl) was supplied to produce silicon tetrachloride (SiCl ₄ ). Further, the produced silicon tetrachloride (SiCl ₄ ) was purified by a distillation method to obtain silicon tetrachloride (SiCl ₄ ) having a purity of 4N (99.99%) and a metal impurity of 20 ppb or less.
[0041]
A closed cycle was constituted by the steps (1) to (6) described above, and a silicon powder granular material having a weight of 5.2 kg could be taken out by continuous operation for 3 hours. As described above, the purity of the extracted silicon is 7N (99.99999%), which is of a quality that can be applied for solar cells. In addition, the manufacturing cost of the silicon raw material thus manufactured is compared with the conventional Siemens. Compared with the production cost of polycrystalline silicon produced by the Siemens method, it is 40% of the production cost of the Siemens method.
[0042]
【The invention's effect】
According to the method for producing high-purity silicon of the present invention, after the reduction reaction, it can be carried out continuously until the step of separating the reaction by-product, and the by-product is produced from the step of producing silicon tetrachloride as an intermediate compound. Since it can be configured in a closed cycle up to the recovery step, it is possible to manufacture high-purity silicon that is stable in quality and optimal as a solar cell substrate corresponding to low cost with high efficiency.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram illustrating a process for producing high-purity silicon according to the present invention.
FIG. 2 is a diagram for explaining an example of a process for producing polycrystalline silicon by a so-called “zinc reduction method of silicon tetrachloride”.
FIG. 3 is a diagram for explaining an example of the configuration of an apparatus for processing steps from melting of metallic zinc to separation of reaction by-products after the reduction reaction in the polycrystalline silicon manufacturing process by the “silicon tetrachloride zinc reduction method”. It is.
[Explanation of symbols]
1: SiCl ₄ tank, 2: dissolution tank 3: dissolution tank, 4: reduction furnace 5: reaction vessel, 7: mixture 8: molten Zn layer, 9: separation furnace
10: Separation container
2a, 4a, 9a: Heating device

Claims (2)

Silicon tetrachloride in a liquid or gaseous state is reduced with molten zinc in the reaction vessel, and the mixture containing the generated polycrystalline silicon and zinc chloride is taken out of the reaction vessel, and then zinc chloride in the mixture is separated. A method for producing high-purity silicon that recovers polycrystalline silicon, wherein the separated zinc chloride is electrolyzed to recover metallic zinc and chlorine, and the recovered metallic zinc is again converted into the silicon tetrachloride reducing agent. A method for producing high-purity silicon, characterized in that the recovered chlorine is synthesized with hydrogen to form hydrogen chloride, which is used for chlorination of metallic silicon to produce silicon tetrachloride. 2. High purity characterized by using hydrogen chloride obtained by synthesizing chlorine recovered by electrolysis of zinc chloride and hydrogen recovered by chlorination of metal silicon in chlorination of metal silicon according to claim 1 Silicon manufacturing method.

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