JP2003238137A - Method of producing polycrystal silicon for solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a production method for a solar cell polycrystal silicon capable of manufacturing an inexpensive solar cell material by using, as a main raw material, a silicon waste sludge winding up conventionally in a landfill. <P>SOLUTION: In a process for refining a molten silicon by a unidirectional solidification method to obtain a silicon ingot meeting a solar cell specification, the molten silicon is formed of only a silicon sludge which is obtained by filtrating and separating waste water from a process for grinding and polishing the silicon ingot or a silicon wafer in the production of the silicon wafer and a semiconductor. In this case, the filtration and separation of waste water from the silicon sludge are preferably performed in such a way that the waste water including the sludge is passed through a first filter and a second filter comprising the sludge is formed on the surface of the first filter. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing polycrystalline silicon for solar cells, and more particularly, to effectively recovering silicon sludge generated when grinding and polishing a silicon wafer in a silicon wafer or semiconductor manufacturing process. use,
The present invention relates to a technique for inexpensively manufacturing polycrystalline silicon for solar cells.

[0002]

2. Description of the Related Art In order to manufacture a silicon wafer or a semiconductor, it is necessary to grind or polish a material such as a silicon ingot or a silicon wafer to a predetermined size. When the grinding and polishing work is performed, a large amount of silicon dust is naturally generated.

The particle size of this silicon waste is 0.1 to 10 μm.
When the silicon wafer is ground to a very small value of m, boron, phosphorus, tungsten, chromium, titanium, arsenic, gallium, and iron, which are implanted as impurities on the wafer surface by the ion implantation method, are contained in addition to silicon. Has been done. In addition, water is used for grinding and polishing to prevent the temperature of the device from rising and to improve lubricity.
Since oil or the like is added to this water, many impurities such as oil are contained. Further, polyaluminum chloride and a sulfuric acid band as a coagulant added when coagulating and sedimenting silicon debris in water are also contained.

As described above, since silicon debris generated when grinding or polishing a silicon ingot or a silicon wafer contains many metal elements and organic / inorganic substances in addition to silicon, it has been appropriately reused. There was no method, and so-called “waste sludge” was used for landfill treatment. In other words, the water was separated and removed from the silicon scrap, and it was buried and discarded.

However, the silicon ingot or the silicon debris of the silicon wafer contained in the separated waste water has not only a very fine particle size of 0.1 to 10 μm, but also a very small content of 50 to 300 ng / liter. Therefore, it takes a lot of cost and time to separate and remove this waste. In addition, the large amount of waste sludge, as described above, has no way to reuse it, and there is no need to do it except for landfill treatment. You have to do it first. Furthermore, in recent years, the problem of depletion of landfill disposal sites has arisen.

[0006] Therefore, the present applicant considers that if a method of reusing this silicon sludge can be developed, it will greatly contribute to society, and has conceived to use the silicon sludge as a raw material of silicon for solar cells. Already, JP-A-9-1652
Japanese Unexamined Patent Publication No. 12 discloses a technique relating to the reuse of silicon sludge. It says, "Silicone sludge containing silicon as a main component generated when slicing a silicon ingot and processing it into a silicon wafer is used as a material,
A method for manufacturing a silicon raw material for a solar cell, which comprises a step of separating a solid content from the material, an acid immersion step of immersing the solid content in an acid solution to remove impurities, and a washing and drying step. And "a pellet obtained from the powder of the silicon raw material for a solar cell is put into a molten high-purity silicon bath to be dissolved, and a solidification step of solidifying the molten silicon by unidirectional solidification to form an ingot. The method of manufacturing a silicon ingot for a solar cell "is mainly used. According to this technique, the conventionally discarded silicon sludge can be effectively used as silicon for solar cells.

However, this Japanese Patent Laid-Open No. 9-16521
Since the technique of using silicon sludge as a raw material for manufacturing silicon for solar cells disclosed in Japanese Patent Publication No. 2 uses an acid solution or a superconducting magnet, it is necessary to provide expensive equipment, and from an economical standpoint, It has not been put to practical use yet. In addition, a method for manufacturing a silicon ingot for a solar cell using pellets molded from the raw material is to put the silicon raw material into molten, high-purity and expensive semiconductor manufacturing silicon. for that reason,
The main material is only high-purity silicon for semiconductor production, and the silicon powder obtained from sludge is merely used as an auxiliary raw material. Therefore, the above-mentioned Japanese Patent Laid-Open No. 9-165
The technique described in Japanese Patent No. 212 cannot process all available silicon sludge. Moreover, even if it is put into practical use, the silicon for solar cells to be manufactured is expected to be expensive.

[0008]

SUMMARY OF THE INVENTION In view of the above situation, the present invention is a solar cell capable of producing an inexpensive solar cell material using waste sludge of silicon, which has been conventionally landfilled, as a main raw material. It is an object of the present invention to provide a method for manufacturing polycrystalline silicon for use.

[0009]

Means for Solving the Problems The inventor has conducted earnest research in order to achieve the above object, and realized the results in the present invention.

That is, according to the present invention, when the molten silicon is refined by directional solidification to obtain a silicon ingot satisfying the solar cell specifications, the molten silicon is
In the production of silicon wafers and semiconductors, a method for producing polycrystalline silicon for solar cells is characterized in that it is formed only by silicon sludge obtained by filtering and separating the silicon ingot or the silicon wafer from wastewater in the steps of grinding and polishing. In this case, filtration separation from the waste water is performed by passing the silicon sludge contained in the waste water through a first filter to form a second filter made of the silicon sludge on the surface of the first filter. Is preferred. Further, to the silicon in the molten state, metallic silicon obtained by reducing silica stone with a carbon-based reducing agent in an electric furnace may be added. At that time, the addition amount of the metallic silicon is It is preferably 0.1 to 20 mass% with respect to a certain silicon. Further, it is preferable that the molten silicon is obtained by drying the silicon sludge and melting it in an inert atmosphere or vacuum.

According to the present invention, it becomes possible to manufacture an inexpensive solar cell material by using, as a main material, waste silicon sludge which has been conventionally landfilled.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below, including the background of the invention. First, the inventor conducted a detailed investigation on silicon sludge and obtained the following results.

When manufacturing a silicon wafer from a silicon ingot, a large amount of silicon grinding / polishing debris is currently discharged in the steps of cutting both ends of the silicon ingot, ingot rough polishing step, slicing step, and chamfering step. Therefore, the yield of silicon wafers from silicon ingots is currently about 40%, and the remaining 6
0% becomes silicon grinding / polishing waste, which is discarded by landfill as described above. Since the silicon purity of this silicon ingot is as high as 99.9999% by mass or more, the silicon purity of the grinding / polishing dust generated from this silicon ingot is also very high, and silicon sludge containing almost no impurities can be obtained.

In addition, the IC manufacturing process in semiconductor manufacturing includes a wafer manufacturing process, an assembly process, and an inspection (test) depending on the technology, equipment and manufacturing environment used.
The process can be roughly divided into three. In the wafer manufacturing process, impurities, thin film formation, and photoetching are repeated on a silicon wafer (silicon substrate) having a diameter of 5 to 8 inches that is cut out from a single crystal silicon ingot and surface-polished to form transistors and wirings. Form and complete the chip. Then, the silicon wafer (IC chip group) that has completed the wafer manufacturing process is cut and separated into individual IC chips in the assembly process. Therefore, the silicon sludge discharged when grinding and polishing a silicon wafer in the manufacture of semiconductors is the silicon sludge discharged in the wafer manufacturing process and the assembling process. In addition, the silicon substrate initially has a thickness of several hundreds of microns, but this is a thickness for ensuring strength, and a silicon wafer actually used is used when it is used in the assembly process. Advantageously, since it is thinned to half its thickness by polishing, about half of the silicon is discarded as polishing debris by this polishing alone. Further, silicon cutting and polishing dust are also generated when the IC chip is cut and separated.

As described above, the currently available silicon sludge is obtained from silicon wafer manufacturing,
There are two types of silicon sludge that can be obtained by semiconductor manufacturing. Silicon sludge obtained by silicon wafer production has high silicon purity and contains almost no impurities, whereas silicon sludge obtained by semiconductor production is not limited to silicon. Impurity elements such as tungsten, chromium, titanium, arsenic, gallium, and iron, and a large amount of inorganic and organic substances such as coagulant and oil.

However, the amount of such silicon sludge generated in Japan alone is 3000 tons per year, which is sufficiently commensurate with the total production of silicon for solar cells. Therefore, the inventor has found that the above-mentioned Japanese Patent Laid-Open No. 9-16521.
Focusing on manufacturing silicon for solar cells at a lower cost than the technology described in Japanese Patent No. 2 publication, silicon sludge to boron,
It was examined to remove the impurity elements such as phosphorus, tungsten, chromium, titanium, arsenic, gallium, and iron, and the inorganic and organic substances such as coagulant and oil by the simplest possible method.

In both of the above two types of silicon sludges, the impurity element can be sufficiently purified by conventional unidirectional solidification, which is sufficient for the specifications of silicon for solar cells (boron content: 0.1 ppm or less). , Phosphorus content: 0.1 ppm or less) was satisfied.
It was also found that a starting material for producing silicon for a solar cell satisfying this specification may have a silicon concentration of 90% or more (anhydrous basis), a boron content of 1 ppm or less, and a phosphorus content of 30 ppm or less. It was However, it was concluded that considerable effort is required to remove inorganic and organic substances such as flocculants and oils. The silicon ingot obtained by this unidirectional solidification is polycrystalline.

Therefore, the inventor has continued to make a diligent effort to find a method for removing a flocculant such as the polyaluminum chloride and the sulfuric acid band and an oil component contained in the silicon sludge, and has conducted extensive research. Moreover, as the method, rather than using an acid solution, it is desirable to remove it more easily. Therefore, attention was paid to a simple water slurry filtration technique, and an appropriate filtration method was searched for. As a result,
It was found that a new filtration technique for separating silicon sludge and water without using any coagulant or oil was proposed in Japanese Patent Laid-Open No. 001-38352, and the availability of the technique was confirmed by experiments. Here, Japanese Patent Laid-Open No. 2001-38352
As shown in FIG. 2, the filtration technique described in Japanese Patent Laid-Open Publication No. 2003-242242 is such that the silicon sludge contained in wastewater is passed through a first filter to form a second filter made of the silicon sludge on the surface of the first filter. It is something that can be done.

Then, many experiments were conducted, and it was confirmed that this technique can separate the silicon sludge from water without using any coagulant or oil. In that case, the first filter is preferably made of a polyolefin-based polymer, and it was good to apply an external force to the surface of the second filter during filtration.

The silicon sludge obtained by this filtration has a silicon concentration of 90% or more (anhydrous basis) and contains no coagulant or oil, and has a silicon concentration of 90% or more (anhydrous basis). Since the boron content was 1 ppm or less and the phosphorus content was 30 ppm or less as elements contained other than the above, the inventor determined that the element can be used as a starting material of polycrystalline silicon for solar cells.

Therefore, in the method for producing polycrystalline silicon for solar cells according to the present invention, as shown in FIG. 1, only the silicon sludge obtained by the filtration as a starting material is dried to further remove water, In an inert atmosphere such as nitrogen or argon, or under a vacuum of 1.33 kPa (10 torr) or less, melting is performed at a temperature of 1500 to 1800 ° C., and purification by unidirectional solidification is performed to remove impurities other than silicon. It will be good. The lower part of the obtained silicon ingot is sufficiently used as polycrystalline silicon for solar cells. By the way, Table 1 shows the average composition of the remainder after cutting and removing the upper part of the ingot where the impurities were concentrated. From Table 1, it is clear that it can be sufficiently used as polycrystalline silicon for solar cells.

[0022]

[Table 1]

Next, from the analysis value of the silicon ingot in Table 1, the inventor considered that the purity of the silicon for the solar cell may be lower. In other words, since silicon sludge is originally good in purity (especially boron and phosphorus),
We thought that raw materials of lower purity could be mixed. Then, another silicon raw material that is relatively inexpensive and easily available was studied. As a result, it was found that metallic silicon obtained by reducing silica stone (SiO ₂ ) with a carbon source such as charcoal, coal, oil, and coke at a high temperature of 2000 ° C. in an electric furnace is suitable, and silicon of this metallic silicon is suitable. The addition to sludge was also decided to be added to the present invention (see FIG. 1). This metallic silicon has a silicon purity of 98% or more, and contains aluminum, iron, titanium, and calcium as impurities in addition to silicon, but these impurities can be sufficiently removed by unidirectional solidification. However, since the amount of boron and phosphorus that cannot be removed only by unidirectional solidification is limited as the silicon for solar cell specifications, it is preferable to determine the amount of metallic silicon mixed depending on their contents in the metallic silicon. In the present invention, it is recommended that the addition amount is 0.1 to 20 mass% with respect to molten silicon.
This is because if it is less than 0.1% by mass, it is too small and there is still room for the silicon quality of the solar cell specification, and if it exceeds 20% by mass, the limit of boron or phosphorus is exceeded.

[0024]

Example 1 In manufacturing a silicon wafer, waste water containing silicon grinding / polishing waste discharged when a silicon ingot was ground / polished was treated with the above-mentioned filtering means according to the present invention to obtain silicon sludge. And water. At that time, the concentration of silicon grinding and polishing debris in the drainage is
As a result of particle size distribution by laser method at 200 ng / liter, the particle size distribution was 0.1 to 20 μm, and the average particle size was 1.
It was 8 μm. The filtration was performed by using a polyolefin-based polymer film for the first filter, and sucking waste water through the polymer film to form a second filter.

Since the silicon sludge obtained by this filtration still had a water content of 80%, the silicon sludge was dried in nitrogen at 90 ° C. for 24 hours to remove water. The analysis results after drying were silicon = 98.3%, iron = 8 ppm, oxygen = 1.5%, and did not contain boron, phosphorus, tungsten, chromium, arsenic, and gallium.

Then, only the silicon sludge was melted at 1600 ° C. in an argon atmosphere and then solidified by a so-called "unidirectional solidification method" to obtain a silicon ingot. The impurity enriched portion on the upper part of the ingot was separated and removed. However, the obtained silicon was 0.1 ppm or less of iron, 0.1 ppm or less of oxygen, 0.1 ppm or less of boron, and 0.1 ppm or less of phosphorus, which satisfied the specifications of the solar cell. Therefore, a solar cell was prepared. The electric energy conversion rate from sunlight of the solar cell was 15%, which was sufficient characteristics. (Example 2) First, only silicon sludge obtained by the same filtration as in Example 1 was melted at 1600 ° C under a vacuum of 0.67 kPa (5 torr). Then, to the molten silicon, 10% by mass of metallic silicon obtained by carbon reduction of silica stone in an electric furnace was added and dissolved. Two
After 0 minutes, it was solidified by the so-called "unidirectional solidification method" to form an ingot, and the impurity concentrated portion on the upper portion was separated and removed. The obtained silicon was 0.1 ppm or less for iron, 0.1 ppm or less for oxygen, 0.1 ppm or less for boron, and 0.1 ppm or less for phosphorus, which satisfied the specifications of the solar cell. Therefore, a solar cell was prepared. The electric energy conversion rate from sunlight of the solar cell was 15%, which was a sufficient characteristic.

These results show that not only the problem of post-treatment of silicon sludge can be solved, but also the wastes conventionally discarded as waste sludge can be reused as a starting material for polycrystalline silicon for solar cells. Is revealed.

[0028]

As described above, according to the present invention, conventionally discarded silicon sludge can be converted into polycrystalline silicon for solar cells, which greatly contributes to the construction of a recycling society. Furthermore, inexpensive solar cells can be manufactured, which greatly contributes to society.

[Brief description of drawings]

FIG. 1 is a process flow chart of a method for producing polycrystalline silicon for solar cells according to the present invention.

FIG. 2 is a diagram illustrating a filtration means for water slurry containing silicon sludge.

[Explanation of symbols]

1 First filter 2 Second filter 3 Water slurry containing silicon sludge 4 Silicon particles 5 Relatively large particles of silicon

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Minoru Nitta             1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture             Within Kuno Research Co., Ltd. (72) Inventor Ryoji Uchimura             1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture             Within Kuno Research Co., Ltd. (72) Inventor Hiroyuki Uesugi             2-3-2 Uchisaiwaicho, Chiyoda-ku, Tokyo             Tetsu Techno Research Co., Ltd. F term (reference) 4G072 AA01 BB01 BB02 BB12 GG01                       GG03 HH01 HH40 MM21 MM38                       UU02                 5F051 AA03 GA04 GA20

Claims (5)

[Claims] 1. When refining molten silicon by unidirectional solidification to obtain a silicon ingot satisfying solar cell specifications, the molten silicon is ground into a silicon ingot or a silicon wafer in the production of silicon wafers and semiconductors. A method for producing polycrystalline silicon for solar cells, which comprises forming only by using silicon sludge obtained by filtering and separating from the waste water of the polishing step. 2. The separation and separation from the waste water is carried out by passing the silicon sludge contained in the waste water through a first filter to form a second filter made of the silicon sludge on the surface of the first filter. The method for producing polycrystalline silicon for solar cells according to claim 1, which is performed. 3. The solar cell according to claim 1, wherein metallic silicon obtained by reducing silica with a carbon-based reducing agent in an electric furnace is added to the molten silicon. Method for manufacturing polycrystalline silicon. 4. The method for producing polycrystalline silicon for a solar cell according to claim 3, wherein the addition amount of the metallic silicon is 0.1 to 20 mass% with respect to silicon in a molten state. 5. The solar cell according to claim 1, wherein the silicon in the molten state is obtained by drying the silicon sludge and melting it in an inert atmosphere or a vacuum. For manufacturing polycrystalline silicon for automobiles.