WO2010024310A1 - Method for purifying silicon - Google Patents

Method for purifying silicon Download PDF

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Publication number
WO2010024310A1
WO2010024310A1 PCT/JP2009/064918 JP2009064918W WO2010024310A1 WO 2010024310 A1 WO2010024310 A1 WO 2010024310A1 JP 2009064918 W JP2009064918 W JP 2009064918W WO 2010024310 A1 WO2010024310 A1 WO 2010024310A1
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silicon
plasma
gas
boron
phosphorus
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PCT/JP2009/064918
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French (fr)
Japanese (ja)
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素行 山田
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信越化学工業株式会社
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/02—Silicon
    • C01B33/037—Purification
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B11/00—Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/02—Elements
    • C30B29/06—Silicon

Definitions

  • the present invention relates to a method for highly purifying industrially produced silicon (silicon), and more particularly to a method for purifying silicon useful for solar cells.
  • a solar battery in which battery cells are formed using a silicon wafer is the most popular solar power generation method.
  • the concentration of the impurity component of silicon used for this solar cell silicon wafer is not required to be as low as the impurity concentration level of silicon for semiconductors.
  • silicon for semiconductors should have impurities as low as possible, and its required purity is 99.99999999% (10N), whereas silicon for solar cells has 99.999%. (5N) to 99.9999% (6N) purity is required.
  • the main impurities of metallic silicon are metallic elements such as iron, aluminum, calcium and titanium, and nonmetallic elements such as boron and phosphorus which act as dopants.
  • the metal element has a very small solidification distribution coefficient with silicon.
  • the solidification distribution coefficient of iron which is often present in the largest amount as an impurity component in metal silicon, is at most 8 ⁇ 10 ⁇ 6 . Therefore, the iron concentration in the solid silicon at the start of solidification is low, and the iron concentration in the solid silicon gradually increases from the middle stage to the last stage of solidification. If a portion having a desired iron concentration is selected from the cast ingot using this solidification segregation phenomenon, silicon having a low iron concentration can be obtained.
  • impurity metal elements other than iron low impurity concentration silicon can be obtained by the same method.
  • Patent Document 1 Japanese Patent Laid-Open No. 04-193706
  • silicon containing an impurity element such as boron (B), carbon (C), phosphorus (P), iron (Fe), aluminum (Al), or the like is formed at the bottom.
  • an impurity element such as boron (B), carbon (C), phosphorus (P), iron (Fe), aluminum (Al), or the like is formed at the bottom.
  • a silicon purification method in which a gas blown tuyere is melted in a container mainly composed of silica and argon (Ar), helium (He), or a mixed gas thereof is blown from the tuyere.
  • Patent Document 2 Japanese Patent Application Laid-Open No. 05-3308157 discloses a method for effectively removing B contained as an impurity when manufacturing silicon used for solar cells and the like, in a container having a gas blowing tuyere at the bottom.
  • the silicon purification method is disclosed in which silicon is kept in a molten state and nitrogen (N 2 ) is mixed and blown into Ar, hydrogen (H 2 ), or a mixed gas thereof from this tuyere at 1 vol% or less. .
  • Patent Document 3 Japanese Patent Application Laid-Open No. 2003-238138
  • an inert gas, water vapor, or carbon monoxide is added to a melt of silicon dioxide (SiO 2 ) and calcium oxide (CaO) and silicon. It is said that impurities can be efficiently removed by supplying at least one kind of gas.
  • Patent Document 4 Japanese Patent Publication No. 2004-5353564 discloses a method for producing medium-purity silicon, and this method includes the following steps. That is, a step of refining silicon with a low boron content by carbothermal reduction of silica in a submerged arc electric furnace, a step of refining liquid silicon with oxygen or chlorine, and a neutral gas injection to reduce pressure of 10 to 100 Pa The process of processing the refined silicon below, and the separation and solidification process.
  • Patent Documents 1 to 4 each of the techniques of Patent Documents 1 to 4 has a problem that it becomes a large-scale facility such as blowing gas into molten silicon.
  • Patent Document 5 Japanese Patent Publication No. 2004-537491 discloses a method for refining silicon, particularly for the production of solar cells, in an electric furnace equipped with a high-temperature crucible in a neutral atmosphere.
  • refining under plasma transferring the molten silicon into an electric furnace equipped with a high temperature crucible, argon, chlorine, fluorine, hydrogen chloride (HCl) and fluorine.
  • HF hydrogen fluoride
  • a step of refining under this plasma a step of casting into a casting mold to be separated and solidified in a controlled atmosphere, etc.
  • Patent Document 6 Japanese Patent Laid-Open No. 05-262512 proposes a technique for irradiating a silicon surface with thermal plasma and adding water vapor and hydrogen chloride to a plasma gas. Is not touched at all.
  • Patent Document 7 Japanese Patent Laid-Open No. 06-227808 discloses a technique for melting metal silicon in a non-oxidizing atmosphere and maintaining it in a reduced pressure atmosphere of 10 Pa or less. However, a heating method or boron removal is disclosed. Is not disclosed at all.
  • Patent Document 8 Japanese Patent Laid-Open No. 04-228414
  • a gas obtained by adding 0.1 to 10% of water vapor to an inert gas in a plasma jet flow is blown onto the molten silicon surface, in particular, boron and carbon.
  • the processing atmosphere is preferably in the range of 1 ⁇ 10 ⁇ 3 to 1 atm so that the loss of silicon as SiO is not excessive. Since this plasma is a plasma generated by a general arc discharge, its working atmosphere pressure is limited, and plasma is not generated or generated at a pressure lower than the atmosphere in the range of 1 ⁇ 10 ⁇ 3 to 1 atm. Became unstable and had operational problems. Moreover, in patent document 8, about the refinement
  • Patent Document 9 Japanese Patent Application Laid-Open No. 2007-5104 discloses a process of removing impurities having a vapor pressure higher than that of silicon from molten silicon using an electron beam, and reacting with impurities having a vapor pressure lower than or equal to that of silicon. Then, a technique is disclosed that includes a step of introducing a compound-generating substance that generates an evaporable compound into a chamber to generate a compound, and evaporating and removing the compound. In this technique, an electron beam, particularly a cold cathode glow discharge type electron beam, is used as a heating source. However, the energy of the electron beam is directly applied to the object to be heated in a high vacuum atmosphere.
  • Japanese Patent Laid-Open No. 04-193706 Japanese Patent Laid-Open No. 05-330815 JP 2003-238138 A Special Table 2004-535354 JP-T-2004-537491 JP 05-262512 A Japanese Patent Laid-Open No. 06-227808 Japanese Patent Laid-Open No. 04-228414 JP 2007-51047 A
  • the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a silicon purification method capable of efficiently reducing the contents of boron, phosphorus, and the like contained as impurities in metallic silicon.
  • the present inventor heated and melted silicon containing impurities with a plasma flame under a reduced-pressure atmosphere, and then sprayed an oxidizing gas on the silicon to thereby dissolve the silicon in the molten silicon.
  • the present invention has found that boron and phosphorus can be efficiently removed at the same time because the oxides formed by the reaction of boron, particularly boron, with an oxidizing gas can be removed by evaporation and phosphorus in silicon can also be removed by evaporation. It came to make.
  • the present invention provides the following method for purifying silicon.
  • a method for purifying silicon wherein silicon containing impurities is melted by a plasma flame in a reduced-pressure atmosphere, and then the impurities are removed by blowing an oxidizing gas onto the molten silicon.
  • FIG. 1 is a schematic view showing an example of a holo-cathode plasma apparatus used in the present invention.
  • This holocathode plasma apparatus includes a cathode (hollow cathode) 2 made of a conductive heat-resistant material having fine holes (cylindrical cavities) in a chamber 1 and an opposing anode.
  • a cathode glass cathode
  • a crucible 3 that is a receiver of a melt or silicon 10 that is a melt
  • oxidizing gas introduction nozzles 4 and 5 are installed in the chamber 1, and gas introduction pipes 6 are connected to the fine holes (holocathode) 2 and the oxidizing gas introduction nozzles 4 and 5, respectively.
  • the nozzle 4 is disposed so that the oxidizing gas ejected from the tip thereof is directly jetted to the silicon 10, and the nozzle 5 directs the oxidizing gas ejected from the tip toward the plasma gas from the cathode 2.
  • the nozzle 4 is disposed so that the oxidizing gas ejected from the tip thereof is directly jetted to the silicon 10, and the nozzle 5 directs the oxidizing gas ejected from the tip toward the plasma gas from the cathode 2.
  • a plasma is generated by applying a voltage between the cathode (holocathode) and the anode while introducing a gas that is a plasma gas such as argon from the gas introduction tube 6 into the microhole (holocathode) 2. .
  • a gas that is a plasma gas such as argon from the gas introduction tube 6 into the microhole (holocathode) 2.
  • the holocathode 2 is red-hot by the heat of the plasma or the passing current, and thermionic electrons are emitted from the cathode.
  • the thermoelectrons turn the gas such as argon passing through the pores of the cathode into plasma.
  • the plasma flame 20 is formed.
  • a vacuum exhaust device 7 is connected to the chamber 1, and the plasma is ignited by generating an arc by bringing the cathode and anode into contact with each other after reducing the pressure appropriately by the vacuum exhaust device 7. Note that either one or both of the nozzles 4 and 5 may be disposed. When the oxidizing gas is introduced from the holocathode, the nozzles 4 and 5 may be omitted.
  • the plasma gas passes through the holocathode and is then released into the chamber.
  • the holocathode has fine holes (cylindrical cavities), and the plasma passes through the holes with a considerable amount of gas. Since it passes, the internal pressure in the pores is a pressure that can maintain the plasma.
  • the plasma gas is released into the chamber, but the inside of the chamber becomes a reduced pressure atmosphere due to the balance between the capacity of the chamber and the capacity of the connected vacuum exhaust apparatus. For this reason, a stable thermal plasma can be maintained even in a reduced pressure atmosphere.
  • metal silicon to be purified is placed in the crucible 3 in the chamber 1.
  • the crucible 3 is mainly made of carbon, which is a heat-resistant conductive material, or a water-cooled metal material, because it is brought into contact with the molten silicon 10 or is electrically connected as an anode.
  • the silicon used in the present invention is usually silicon having a purity of about 98 to 99.5%, which is said to be metallic silicon, and iron, aluminum, calcium, titanium and the like as main impurity elements are 10 to 5,000 ppm.
  • An industrially produced silicon containing about 2 to 20 ppm of boron as a dopant and about 5 to 50 ppm of phosphorus can be used.
  • the impurity concentration can be measured by an ICP-AES method (high frequency plasma emission spectroscopy) or the like.
  • the inside of the chamber 1 is depressurized by an evacuation device 7 connected thereto.
  • a sufficiently reduced pressure of about 1 ⁇ 10 ⁇ 6 to 1 ⁇ 10 ⁇ 8 Pa is reached, a voltage is applied between the cathode and the anode, and an arc is generated by contacting or approaching both electrodes in that state.
  • the applied voltage is preferably 5 to 50V, particularly 15 to 35V. If the voltage is too low, plasma may not be generated or the plasma may become unstable. If it is too high, the electrode may be consumed quickly.
  • the holocathode electrode material used in the present invention is required to have heat resistance, corrosion resistance, chemical stability, etc., depending on the type of oxidizing gas, heat resistant materials such as tungsten, molybdenum, tantalum, carbon, iridium, A material having heat resistance, corrosion resistance, and chemical stability such as rhodium is used. Furthermore, a composite of these materials and a laminate structure can be listed as options.
  • argon or helium which is mainly a monoatomic gas, is preferable, and argon is usually used from the viewpoint of cost.
  • Thermal plasma is generated between the holocathode cathode and the crucible as the anode or silicon set therein, but the plasma generation position can be moved by adjusting the positional relationship between the crucible and silicon relative to the cathode.
  • the cathode it is more practical to use a structure in which the cathode can be moved than a structure in which the crucible position is variable.
  • the silicon in the crucible is melted by this thermal plasma. Melting is started around the part directly irradiated with plasma and its periphery. After the start of melting, it is influenced by the characteristics of the crucible as well as the size and amount of silicon. That is, when water-cooled copper is used as a crucible, the amount of heat is actively deprived by water-cooled copper. Therefore, when expanding the melting range, it is necessary to actively move the cathode and change the plasma irradiation position. However, when melting in a crucible made of a heat-resistant material such as carbon, since the heat removal is less likely to proceed than water-cooled copper, the melting range tends to be widened.
  • the plasma irradiation time (melting time) is appropriately adjusted under the above conditions so that silicon is completely melted.
  • an oxidizing gas is introduced into the chamber.
  • a method of injecting into the molten silicon from the nozzle in the chamber (4 in FIG. 1) or a method of injecting into the plasma gas (5 in FIG. 1) can be taken. It is also possible to introduce an oxidizing gas directly (2 in FIG. 1).
  • the method of directly introducing the oxidizing gas into the cathode electrode has the advantage that heating with a high-temperature plasma flame and supply of the oxidizing gas are possible at the same position.
  • Oxidizing gases include oxygen-containing gases such as oxygen, ozone, water vapor, carbon dioxide, and nitrogen oxide, which can be oxygen emitters in high-temperature atmospheres, and halogen-containing gases such as hydrogen chloride, chlorine, bromine, and carbon tetrachloride. Thus, there are those that become halogen emitters at high temperatures, and one or more of these gases can be used. Among them, oxygen gas and water vapor gas are preferable gases for carrying out the method of the present invention because they are inexpensive, non-toxic and rich in activity.
  • the amount of oxidizing gas is determined by the correlation between the amount of plasma gas and the capacity of the vacuum evacuation device, and the setting state of boron and phosphorus removal capacity.
  • an oxidizing gas is mixed with an inert gas such as nitrogen or argon as a method for controlling the gas flow direction and linear velocity in the chamber. May be.
  • the concentration of the oxidizing gas mixed with the plasma gas is high, the plasma may disappear, so the concentration is 1 to 40% by volume, particularly 5 to 15%. It is preferable that it is volume%.
  • the internal pressure of the chamber is a degree of decompression determined by the balance between the gas introduced into the exhaust gas and the exhaust gas to the vacuum evacuation apparatus, but 0.1 to 200 Pa at which the thermal plasma by the present method is stably and efficiently generated. In particular, it is preferable to adjust the pressure to 5 to 150 Pa. If the internal pressure of the chamber is higher than 200 Pa, the holocathode plasma may disappear. If it is lower than 0.1, the arc discharge is dominant instead of the plasma, but the supply amount of oxidizing gas involved in boron removal is sufficient. There is a risk that removal of boron and phosphorus cannot be achieved.
  • Boron in silicon is oxidized by oxidizing gas. Since this oxide has a high vapor pressure at the silicon melting temperature (1,410 ° C.), it can be estimated that boron is evaporated and removed as an oxide.
  • phosphorus in silicon does not form a compound whose evaporation is accelerated by oxidizing gas, but the vapor pressure of phosphorus alone is sufficiently higher than that of silicon, and the vapor pressure of phosphorus is atmospheric pressure.
  • the temperature to become is about 453 ° C for black phosphorus. Accordingly, the concentration of phosphorus in the molten silicon is gradually reduced only by the melting of silicon.
  • the pressure of the molten atmosphere is a reduced pressure atmosphere of 0.1 to 200 Pa. Is faster than atmospheric pressure.
  • the holocathode plasma used in the present invention has a high temperature of about several thousand to several tens of thousands of degrees C, and a portion of the molten silicon in direct contact with the plasma flame is also partially in a high temperature state (1,800 to 2,2). About 200 ° C.). This is the most different point from silicon melted by ordinary resistance heating or induction heating (usually about 1,500 to 1,700 ° C.).
  • both boron and phosphorus can be removed by evaporation. Due to the presence of the part, the removal of boron and phosphorus acts efficiently and efficiently.
  • boron and phosphorus in silicon melted in a reduced-pressure oxidizing atmosphere are gradually removed as the melting time elapses, and after the concentration is lowered to a desired concentration, the dissolution treatment is terminated.
  • Melting may be performed at once in the same crucible as a batch process from the beginning to the end of melting, or two or more crucibles are prepared and used as a multi-stage crucible, and the molten silicon is moved sequentially as the purification proceeds.
  • Various purification methods such as increasing efficiency can be taken.
  • boron and phosphorus are removed by different technical elements.
  • the timing of this removal is the concentration of boron and phosphorus in silicon, although boron removal and phosphorus removal occur simultaneously.
  • the removal rate is not necessarily the same for boron removal and phosphorus removal. Therefore, it is preferable to appropriately change the removal conditions depending on whether the substance to be removed is mainly boron, phosphorus, or both.
  • the silicon to be purified contains 2 to 20 ppm of boron and / or 5 to 50 ppm of phosphorus as impurities
  • the respective initial contents can be obtained by performing purification under appropriate purification conditions at a temperature or the like.
  • the amount can be reduced to about 1/30 to 1/2, particularly about 1/20 to 1/5.
  • unidirectional solidification or the like is performed on the metal silicon obtained before or after the purification method of the present invention.
  • metal impurities other than boron and phosphorus are removed, and high purity such as solar cells, sputtering target materials, secondary battery active materials, silicon for thermoelectric materials, ceramic materials, and glass materials is required.
  • Metal silicon suitable for the silicon material to be used can be obtained.
  • Example 1 20 g of metallic silicon containing 10 ppm of boron and 20 ppm of phosphorus was put into a water-cooled copper container in the chamber of the holocathode plasma generator, and the chamber was deaerated to 0.1 Pa or less by a vacuum evacuation apparatus. .
  • a potential difference of 40V DC is applied between the holocathode electrode in the chamber and the copper container and an arc is generated when they are brought close to each other, argon is immediately vented to the holocathode at 100 cc / min while the distance between the electrodes is increased above the silicon.
  • plasma was generated between the electrode and silicon, and silicon began to be dissolved by the plasma flame.
  • the chamber internal pressure at this time was 10 Pa.
  • Plasma irradiation was continued for 30 minutes while adding water vapor 15 cc / min to argon introduced into the holocathode. Although the chamber internal pressure fluctuated, it was finally 8 Pa.
  • the silicon was analyzed after the plasma irradiation, the boron concentration was 0.8 ppm and the phosphorus concentration was reduced to 1.2 ppm.
  • Example 2 The same metallic silicon as in Example 1 was used, and in the apparatus of Example 1, an oxidizing gas introduction nozzle was provided separately, and the direction of the nozzle was directed toward the site where the plasma flame and molten silicon contacted. Silicon was melted in the same manner as in Example 1, but the plasma gas was 50 cc / min. In a state in which a mixed gas of argon 50 cc / min and water vapor 50 cc / min is introduced from an oxidizing gas introduction nozzle, and the direction of the nozzle is adjusted so that the oxidizing gas hits the contact position between the plasma flame and the molten silicon. Melting was continued for 30 minutes. The chamber internal pressure was finally 12 Pa.
  • the boron concentration was 1.2 ppm and the phosphorus concentration was 1.8 ppm.
  • the method of introducing an oxidizing gas into the holocathode is more effective for removing boron and phosphorus. I found out.
  • Example 3 Using the same metallic silicon as in Example 1, plasma was generated by the apparatus and method of Example 1 to melt the silicon. Argon 10 cc / min and water vapor 2 cc / min were introduced into the holocathode, and dissolution was performed for 30 minutes as a plasma flame in an oxidizing atmosphere. The pressure in the chamber was 0.8 Pa. When the silicon was analyzed after the plasma irradiation, the boron concentration was 5 ppm and the phosphorus concentration was reduced to 0.8 ppm. It was found that the element to be removed can be selected by selecting the removal conditions.
  • Example 1 Using the same metallic silicon as in Example 1, plasma was generated by the apparatus and method of Example 1 to dissolve the silicon. When the entire amount of silicon was dissolved, only 100 cc / min of argon as a plasma gas was introduced into the holocathode. The pressure in the chamber was 9 Pa. After this dissolution was continued for 30 minutes, the energization was terminated. When the dissolved silicon was analyzed, the boron concentration was 11 ppm and the phosphorus concentration was 1.0 ppm. Although removal of phosphorus was achieved, removal of boron could not be confirmed.

Abstract

A method for purifying silicon, which is characterized in that a silicon containing impurities is melted by a plasma flame in a reduced pressure atmosphere and then an oxidizing gas is sprayed onto the thus-obtained silicon melt, so that the impurities are removed therefrom.  The method can remove impurities, particularly boron and phosphorus from silicon more efficiently at lower cost when compared with conventional methods for purifying silicon.

Description

珪素の精製方法Method for purifying silicon
　本発明は、工業的に生産される珪素（シリコン）の高純度化方法に関し、特に、太陽電池用として有用な珪素の精製方法に関する。 The present invention relates to a method for highly purifying industrially produced silicon (silicon), and more particularly to a method for purifying silicon useful for solar cells.
　炭酸ガスを排出する化石エネルギーが地球温暖化を促進するとして、化石エネルギーに代替するエネルギーが種々提案され、実用化されている。そのなかでも、太陽光発電は、地球上に遍く分布するエネルギーによって作り出されること、比較的小規模の設備でも可能であること、実用化の歴史が長いことなどから、年々その設備発電量が増加している。 As fossil energy that emits carbon dioxide promotes global warming, various alternatives to fossil energy have been proposed and put into practical use. Among them, the amount of power generated by photovoltaic power generation increases year by year because it is created by energy distributed evenly on the earth, it can be used with relatively small equipment, and has a long history of practical application. is doing.
　太陽光発電には種々の方法があるが、なかでもシリコンウェハーを使用して電池セルを形成した太陽電池は、最も普及している太陽光発電法である。この太陽電池用シリコンウェハーに使用する珪素の不純物成分の濃度は、半導体用の珪素ほどの低い不純物濃度レベルまでは必要とされない。即ち、半導体用珪素は、不純物を極力低レベルとすることがよいとされ、その必要純度が９９．９９９９９９９９％（１０Ｎ）とされるのに対し、太陽電池用の珪素には、９９．９９９％（５Ｎ）ないし９９．９９９９％（６Ｎ）純度が必要とされる。 There are various methods for solar power generation, and among them, a solar battery in which battery cells are formed using a silicon wafer is the most popular solar power generation method. The concentration of the impurity component of silicon used for this solar cell silicon wafer is not required to be as low as the impurity concentration level of silicon for semiconductors. In other words, silicon for semiconductors should have impurities as low as possible, and its required purity is 99.99999999% (10N), whereas silicon for solar cells has 99.999%. (5N) to 99.9999% (6N) purity is required.
　従来、太陽電池用珪素をこの不純物レベルとするために、その原料には半導体用の９９．９９９９９９９９％（１０Ｎ）純度品に加え、半導体珪素製造工程中で不純物濃縮や異物付着品として廃棄されるいわゆるオフグレード品を再処理又は精製した珪素が使用されてきた。このように太陽電池用珪素は、半導体用の珪素又はその派生品が原料であることから、その流通量は半導体産業の盛衰の影響を受けてしまい、太陽電池用珪素の需要に対応できないことがしばしば起こる状態となっていた。このため、工業的に十分な製造量を持つ金属珪素の純度を向上させて、太陽電池用の珪素として使用することが検討されてきた。 Conventionally, in order to bring the silicon for solar cells to this impurity level, in addition to a 99.99999999% (10N) purity product for semiconductors, the raw material is discarded as an impurity concentration or foreign matter adhering product in the semiconductor silicon manufacturing process. Silicon that has been reprocessed or purified from so-called off-grade products has been used. Thus, since silicon for solar cells is made from silicon for semiconductors or derivatives thereof, the distribution amount is affected by the rise and fall of the semiconductor industry, and it may not be able to meet the demand for silicon for solar cells. It was a frequent occurrence. For this reason, it has been studied to improve the purity of metallic silicon having an industrially sufficient production amount and use it as silicon for solar cells.
　ところで、金属珪素の主な不純物は、鉄、アルミニウム、カルシウム、チタンなどの金属元素と、ドーパントとして作用する硼素、リンなどの非金属元素である。このうち、金属元素は、珪素との凝固分配係数が非常に小さく、例えば金属珪素中に不純物成分として最も多量に存在することが多い鉄の凝固分配係数は、たかだか８×１０-6である。従って、凝固開始時の固体珪素中には鉄濃度が低く、凝固中期から末期にかけて固体珪素中の鉄濃度が徐々に増加することとなる。この凝固偏析現象を利用して所望の鉄濃度の部分を鋳造塊より選択すれば、低鉄濃度の珪素が得られる。鉄以外の不純物金属元素についても同様の方法で低不純物濃度珪素が得られる。 By the way, the main impurities of metallic silicon are metallic elements such as iron, aluminum, calcium and titanium, and nonmetallic elements such as boron and phosphorus which act as dopants. Among them, the metal element has a very small solidification distribution coefficient with silicon. For example, the solidification distribution coefficient of iron, which is often present in the largest amount as an impurity component in metal silicon, is at most 8 × 10 −6 . Therefore, the iron concentration in the solid silicon at the start of solidification is low, and the iron concentration in the solid silicon gradually increases from the middle stage to the last stage of solidification. If a portion having a desired iron concentration is selected from the cast ingot using this solidification segregation phenomenon, silicon having a low iron concentration can be obtained. For impurity metal elements other than iron, low impurity concentration silicon can be obtained by the same method.
　しかし、硼素は、珪素中でドーパント物質として作用するので、太陽電池用の珪素中濃度については濃度制御すべき物質であるにもかかわらず、凝固分配係数が０．８と１に近く、この凝固偏析現象ではほとんど偏析しない。また、同じドーパント物質のリンについても凝固分配係数が０．３５と十分小さい数字ではないので、硼素と同様に、凝固偏析現象だけでは偏析による分離精製を十分に達成することはできない。このため、硼素及びリンを凝固偏析以外の方法で除去すべく種々の方法が提案されている。 However, since boron acts as a dopant substance in silicon, the solidification distribution coefficient is close to 0.8 and 1 despite the fact that the concentration in silicon for solar cells should be controlled. The segregation phenomenon hardly segregates. In addition, since the solidification distribution coefficient of phosphorus of the same dopant substance is not a sufficiently small number of 0.35, separation and purification by segregation cannot be sufficiently achieved only by the solidification segregation phenomenon as in the case of boron. For this reason, various methods have been proposed to remove boron and phosphorus by a method other than solidification segregation.
　例えば、特許文献１（特開平０４－１９３７０６号公報）では、硼素（Ｂ）、炭素（Ｃ）、リン（Ｐ）、鉄（Ｆｅ）、アルミニウム（Ａｌ）等の不純物元素を含む珪素を、底部にガス吹き込み羽口を有するシリカを主成分とする容器内で溶融し、この羽口からアルゴン（Ａｒ）、ヘリウム（Ｈｅ）又はこれらの混合ガスを吹き込む珪素の精製方法が提案されている。 For example, in Patent Document 1 (Japanese Patent Laid-Open No. 04-193706), silicon containing an impurity element such as boron (B), carbon (C), phosphorus (P), iron (Fe), aluminum (Al), or the like is formed at the bottom. There has been proposed a silicon purification method in which a gas blown tuyere is melted in a container mainly composed of silica and argon (Ar), helium (He), or a mixed gas thereof is blown from the tuyere.
　特許文献２（特開平０５－３３０８１５号公報）は、太陽電池等に用いる珪素を製造する際に、特に不純物として含まれるＢを有効に除去する方法として、底部にガス吹き込み羽口を有する容器内で珪素を溶融状態に保持し、この羽口からＡｒ、水素（Ｈ2）あるいはこれらの混合ガスに、窒素（Ｎ2）を１容量％以下混合させて吹き込む珪素の精製方法を開示している。 Patent Document 2 (Japanese Patent Application Laid-Open No. 05-330815) discloses a method for effectively removing B contained as an impurity when manufacturing silicon used for solar cells and the like, in a container having a gas blowing tuyere at the bottom. The silicon purification method is disclosed in which silicon is kept in a molten state and nitrogen (N 2 ) is mixed and blown into Ar, hydrogen (H 2 ), or a mixed gas thereof from this tuyere at 1 vol% or less. .
　特許文献３（特開２００３－２３８１３８号公報）に記載の方法は、二酸化珪素（ＳｉＯ2）及び酸化カルシウム（ＣａＯ）の混合物と珪素との融液に、不活性ガス、水蒸気、一酸化炭素の少なくとも１種類以上のガスを供給することで、不純物が効率良く除去できるとされている。 In the method described in Patent Document 3 (Japanese Patent Application Laid-Open No. 2003-238138), an inert gas, water vapor, or carbon monoxide is added to a melt of silicon dioxide (SiO 2 ) and calcium oxide (CaO) and silicon. It is said that impurities can be efficiently removed by supplying at least one kind of gas.
　特許文献４（特表２００４－５３５３５４号公報）には、中純度の珪素を製造する方法が開示されており、この方法は以下の工程を含んでいる。即ち、硼素含有率が低い珪素を、サブマージアーク電気炉でのシリカの炭素熱還元によって精錬する工程、酸素又は塩素で液体珪素を精錬する工程、中性ガスを注入して、１０～１００Ｐａの減圧下で精錬された珪素を処理する工程、及び分離凝固工程。 Patent Document 4 (Japanese Patent Publication No. 2004-535354) discloses a method for producing medium-purity silicon, and this method includes the following steps. That is, a step of refining silicon with a low boron content by carbothermal reduction of silica in a submerged arc electric furnace, a step of refining liquid silicon with oxygen or chlorine, and a neutral gas injection to reduce pressure of 10 to 100 Pa The process of processing the refined silicon below, and the separation and solidification process.
　しかし、これら特許文献１～４の技術は、どれも溶融した珪素中にガスを吹き込むなどの大がかりな設備になってしまうという問題点があった。 However, each of the techniques of Patent Documents 1 to 4 has a problem that it becomes a large-scale facility such as blowing gas into molten silicon.
　また、特許文献５（特表２００４－５３７４９１号公報）には、特に太陽電池セルの製造のための珪素を精製する方法が開示されており、高温るつぼを備えた電気炉で、中性雰囲気下で精錬珪素を再溶融する工程、プラズマ下での精錬を実現するために、高温るつぼを備えた電気炉内に溶融珪素を移送する工程、アルゴンと、塩素、フッ素、塩化水素（ＨＣｌ）及びフッ化水素（ＨＦ）の少なくとも１種の気体との混合物をプラズマ発生気体として、このプラズマ下で精錬する工程、分離凝固させる鋳造鋳型内へ制御雰囲気下で鋳込みを行う工程等により、硼素やリンの低減を達成し、太陽電池用のセル用の珪素を製造するとされるが、減圧雰囲気での精錬については言及されていない。 Patent Document 5 (Japanese Patent Publication No. 2004-537491) discloses a method for refining silicon, particularly for the production of solar cells, in an electric furnace equipped with a high-temperature crucible in a neutral atmosphere. In order to realize refining of refined silicon in the furnace, refining under plasma, transferring the molten silicon into an electric furnace equipped with a high temperature crucible, argon, chlorine, fluorine, hydrogen chloride (HCl) and fluorine. By using a mixture of hydrogen fluoride (HF) with at least one gas as a plasma generating gas, a step of refining under this plasma, a step of casting into a casting mold to be separated and solidified in a controlled atmosphere, etc. Although reduction is achieved and silicon for cells for solar cells is produced, refining in a reduced pressure atmosphere is not mentioned.
　特許文献６（特開平０５－２６２５１２号公報）には、珪素表面に熱プラズマを照射すると共に、プラズマガスに水蒸気及び塩化水素を添加する技術が提案されているが、雰囲気を減圧にする技術については全く触れられていない。 Patent Document 6 (Japanese Patent Laid-Open No. 05-262512) proposes a technique for irradiating a silicon surface with thermal plasma and adding water vapor and hydrogen chloride to a plasma gas. Is not touched at all.
　特許文献７（特開平０６－２２７８０８号公報）では、金属珪素を非酸化性雰囲気下で溶融し、１０Ｐａ以下の減圧雰囲気に保つ技術が開示されているが、加熱の方法や硼素を除去することについては全く開示がない。 Patent Document 7 (Japanese Patent Laid-Open No. 06-227808) discloses a technique for melting metal silicon in a non-oxidizing atmosphere and maintaining it in a reduced pressure atmosphere of 10 Pa or less. However, a heating method or boron removal is disclosed. Is not disclosed at all.
　特許文献８（特開平０４－２２８４１４号公報）には、プラズマジェット流の不活性ガスに水蒸気を０．１～１０％添加したガスを、溶融珪素溶湯面に吹き付けることで、特に、ボロン及び炭素を除去する方法が提案されており、処理雰囲気は、ＳｉＯとしての珪素のロスを過大にしない１×１０-3～１ａｔｍの範囲が望ましいことが開示されている。このプラズマは、一般的なアーク放電によるプラズマであるので、その作動雰囲気圧力には制約があり、上記１×１０-3～１ａｔｍの範囲の雰囲気より低い圧力ではプラズマが発生しないか、発生しても不安定となって操業上問題があった。また、特許文献８では、リンの精製については全く想定されていなかった。 In Patent Document 8 (Japanese Patent Laid-Open No. 04-228414), a gas obtained by adding 0.1 to 10% of water vapor to an inert gas in a plasma jet flow is blown onto the molten silicon surface, in particular, boron and carbon. It has been disclosed that the processing atmosphere is preferably in the range of 1 × 10 −3 to 1 atm so that the loss of silicon as SiO is not excessive. Since this plasma is a plasma generated by a general arc discharge, its working atmosphere pressure is limited, and plasma is not generated or generated at a pressure lower than the atmosphere in the range of 1 × 10 −3 to 1 atm. Became unstable and had operational problems. Moreover, in patent document 8, about the refinement | purification of phosphorus, it was not assumed at all.
　特許文献９（特開２００７－５１０４７号公報）には、電子ビームを用いて、蒸気圧が珪素より高い不純物を溶融珪素から除去する工程と、蒸気圧が珪素より低いか同程度の不純物と反応し、蒸発可能な化合物を生成させる化合物生成物質をチャンバー内に導入して化合物を生成し、これを蒸発除去する工程を含む技術が開示されている。この技術では、加熱源に電子ビーム、特には冷陰極グロー放電型の電子ビームを使用するとされる。しかし、電子ビームは、高真空雰囲気ではビームのエネルギーがそのまま被加熱物体に印加されるが、低真空度とされる１～１００Ｐａの真空度では、ビーム電子が被加熱物体に衝突する前に雰囲気ガス分子と衝突してしまうので、電子ビームのエネルギーの多くが無駄となり、被加熱物体を加熱するエネルギーはわずかとなるという欠点がある。
　このように、これまでに金属珪素から、硼素、リン等の不純物を効率良く除去できる珪素の精製方法は報告されていない。 Patent Document 9 (Japanese Patent Application Laid-Open No. 2007-51047) discloses a process of removing impurities having a vapor pressure higher than that of silicon from molten silicon using an electron beam, and reacting with impurities having a vapor pressure lower than or equal to that of silicon. Then, a technique is disclosed that includes a step of introducing a compound-generating substance that generates an evaporable compound into a chamber to generate a compound, and evaporating and removing the compound. In this technique, an electron beam, particularly a cold cathode glow discharge type electron beam, is used as a heating source. However, the energy of the electron beam is directly applied to the object to be heated in a high vacuum atmosphere. However, in a vacuum degree of 1 to 100 Pa, which is a low degree of vacuum, the atmosphere before the electron beam collides with the object to be heated. Since it collides with gas molecules, much of the energy of the electron beam is wasted, and there is a drawback that the energy for heating the object to be heated is small.
Thus, no silicon purification method that can efficiently remove impurities such as boron and phosphorus from metallic silicon has been reported so far.
特開平０４－１９３７０６号公報Japanese Patent Laid-Open No. 04-193706 特開平０５－３３０８１５号公報Japanese Patent Laid-Open No. 05-330815 特開２００３－２３８１３８号公報JP 2003-238138 A 特表２００４－５３５３５４号公報Special Table 2004-535354 特表２００４－５３７４９１号公報JP-T-2004-537491 特開平０５－２６２５１２号公報JP 05-262512 A 特開平０６－２２７８０８号公報Japanese Patent Laid-Open No. 06-227808 特開平０４－２２８４１４号公報Japanese Patent Laid-Open No. 04-228414 特開２００７－５１０４７号公報JP 2007-51047 A
　本発明は、上記事情に鑑みなされたもので、金属珪素中に不純物として含まれる硼素、リン等の含有量を効率良く低減することのできる珪素の精製方法を提供することを目的とする。 The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a silicon purification method capable of efficiently reducing the contents of boron, phosphorus, and the like contained as impurities in metallic silicon.
　本発明者は、上記目的を達成するために鋭意検討を重ねた結果、不純物を含む珪素を減圧雰囲気下でプラズマ炎により加熱・溶融した後、これに酸化性ガスを吹き付けることで、溶融珪素中の不純物、特に硼素が酸化性ガスと反応して形成された酸化物を蒸発除去し、併せて珪素中のリンも蒸発除去できるため、硼素とリンを同時に、効率良く除去できることを見出し、本発明をなすに至った。 As a result of intensive studies to achieve the above object, the present inventor heated and melted silicon containing impurities with a plasma flame under a reduced-pressure atmosphere, and then sprayed an oxidizing gas on the silicon to thereby dissolve the silicon in the molten silicon. The present invention has found that boron and phosphorus can be efficiently removed at the same time because the oxides formed by the reaction of boron, particularly boron, with an oxidizing gas can be removed by evaporation and phosphorus in silicon can also be removed by evaporation. It came to make.
　即ち、本発明は、下記の珪素の精製方法を提供する。
〔１〕　不純物を含む珪素を減圧雰囲気下でプラズマ炎により溶融した後、この溶融珪素に酸化性ガスを吹き付けることで上記不純物を除去することを特徴とする珪素の精製方法。
〔２〕　減圧雰囲気が０．１～２００Ｐａである〔１〕記載の精製方法。
〔３〕　プラズマ炎がホロカソード型プラズマ源から発せられる〔１〕又は〔２〕記載の精製方法。
〔４〕　酸化性ガスの供給をプラズマガスと混合して行う〔１〕乃至〔３〕のいずれかに記載の精製方法。
〔５〕　酸化性ガスの濃度が１～４０体積％である〔１〕乃至〔４〕のいずれかに記載の精製方法。
〔６〕　酸化性ガスが、酸素、オゾン、水蒸気、二酸化炭素、酸化窒素、塩素、塩化水素、四塩化炭素及び臭素から選ばれるガスである〔１〕乃至〔５〕のいずれかに記載の精製方法。
〔７〕　不純物が硼素及び／又はリンである〔１〕乃至〔６〕のいずれかに記載の精製方法。 That is, the present invention provides the following method for purifying silicon.
[1] A method for purifying silicon, wherein silicon containing impurities is melted by a plasma flame in a reduced-pressure atmosphere, and then the impurities are removed by blowing an oxidizing gas onto the molten silicon.
[2] The purification method according to [1], wherein the reduced-pressure atmosphere is 0.1 to 200 Pa.
[3] The purification method according to [1] or [2], wherein the plasma flame is emitted from a holocathode type plasma source.
[4] The purification method according to any one of [1] to [3], wherein the oxidizing gas is mixed with the plasma gas.
[5] The purification method according to any one of [1] to [4], wherein the concentration of the oxidizing gas is 1 to 40% by volume.
[6] The purification according to any one of [1] to [5], wherein the oxidizing gas is a gas selected from oxygen, ozone, water vapor, carbon dioxide, nitrogen oxide, chlorine, hydrogen chloride, carbon tetrachloride, and bromine. Method.
[7] The purification method according to any one of [1] to [6], wherein the impurity is boron and / or phosphorus.
　本発明によれば、従来の珪素の精製方法に比べ、珪素中の不純物、特に硼素とリンを安価に効率良く除去することができる。 According to the present invention, it is possible to efficiently and efficiently remove impurities in silicon, particularly boron and phosphorus, as compared with conventional silicon purification methods.
本発明で用いるホロカソードプラズマ装置の一例を示す概略図である。It is the schematic which shows an example of the holo cathode plasma apparatus used by this invention.
　以下、本発明に係る珪素の精製方法の一実施形態について、図面を参照しながら説明するが、本発明は、下記構成に制限されるものではなく、本発明の要旨を逸脱しない範囲で種々変更して差し支えない。 Hereinafter, an embodiment of a method for purifying silicon according to the present invention will be described with reference to the drawings. However, the present invention is not limited to the following configurations, and various modifications can be made without departing from the gist of the present invention. There is no problem.
　図１は、本発明に使用されるホロカソードプラズマ装置の一例を示す概略図である。このホロカソードプラズマ装置は、チャンバー１内に、微細孔（筒状の空洞）を有する導電性耐熱材料製の陰極（ホロカソード：ｈｏｌｌｏｗ　ｃｈｔｈｏｄｅ）２と、対向する陽極とを備える。陽極としては、通常、溶融物の受器であるるつぼ３、或いは溶融物である珪素１０が用いられる。チャンバー１内には、更に、酸化性ガス導入ノズル４，５が設置されており、微細孔（ホロカソード）２及び酸化性ガス導入ノズル４，５には、それぞれガス導入管６が接続されている。ここで、ノズル４は、その先端から噴出する酸化性ガスが珪素１０に直接噴射されるように配設され、ノズル５は、その先端から噴出する酸化性ガスが陰極２からのプラズマガスに向けて噴射されるように配設されている。 FIG. 1 is a schematic view showing an example of a holo-cathode plasma apparatus used in the present invention. This holocathode plasma apparatus includes a cathode (hollow cathode) 2 made of a conductive heat-resistant material having fine holes (cylindrical cavities) in a chamber 1 and an opposing anode. As the anode, usually, a crucible 3 that is a receiver of a melt or silicon 10 that is a melt is used. Further, oxidizing gas introduction nozzles 4 and 5 are installed in the chamber 1, and gas introduction pipes 6 are connected to the fine holes (holocathode) 2 and the oxidizing gas introduction nozzles 4 and 5, respectively. . Here, the nozzle 4 is disposed so that the oxidizing gas ejected from the tip thereof is directly jetted to the silicon 10, and the nozzle 5 directs the oxidizing gas ejected from the tip toward the plasma gas from the cathode 2. Are arranged so as to be injected.
　このホロカソードプラズマ装置では、ガス導入管６から微細孔（ホロカソード）２中にアルゴン等のプラズマガスとなるガスを導入しつつ、陰極（ホロカソード）と陽極間に電圧を印加してプラズマを発生させる。プラズマ発生中、ホロカソード２は、プラズマの熱或いは通過電流で赤熱しており、このカソードから熱電子が放出されるが、この熱電子がカソードの細孔を通過するアルゴン等のガスをプラズマ化することでプラズマ炎２０が形成される。また、チャンバー１には、真空排気装置７が接続されており、この真空排気装置７にて適度に減圧してから陰極と陽極を接触させてアークを発生させることでプラズマを点火させる。なお、ノズル４，５はいずれか一方を配設しても、両方を配設してもよく、またホロカソードから酸化性ガスを導入する場合には、ノズル４，５は省略しうる。 In this holocathode plasma apparatus, a plasma is generated by applying a voltage between the cathode (holocathode) and the anode while introducing a gas that is a plasma gas such as argon from the gas introduction tube 6 into the microhole (holocathode) 2. . During the plasma generation, the holocathode 2 is red-hot by the heat of the plasma or the passing current, and thermionic electrons are emitted from the cathode. The thermoelectrons turn the gas such as argon passing through the pores of the cathode into plasma. Thus, the plasma flame 20 is formed. A vacuum exhaust device 7 is connected to the chamber 1, and the plasma is ignited by generating an arc by bringing the cathode and anode into contact with each other after reducing the pressure appropriately by the vacuum exhaust device 7. Note that either one or both of the nozzles 4 and 5 may be disposed. When the oxidizing gas is introduced from the holocathode, the nozzles 4 and 5 may be omitted.
　プラズマガスは、ホロカソード中を通過した後、チャンバー中に放出されることになるが、ホロカソードは微細な孔（筒状の空洞）を有しており、プラズマはその孔を相当量のガス量で通過するので、細孔内の内圧はプラズマを維持できる圧力である。プラズマガスはチャンバー中に放出されるが、チャンバーの容量と、接続された真空排気装置の能力とのバランスによって、チャンバー内は減圧雰囲気となる。このために安定した熱プラズマが減圧雰囲気においても維持できる。 The plasma gas passes through the holocathode and is then released into the chamber. The holocathode has fine holes (cylindrical cavities), and the plasma passes through the holes with a considerable amount of gas. Since it passes, the internal pressure in the pores is a pressure that can maintain the plasma. The plasma gas is released into the chamber, but the inside of the chamber becomes a reduced pressure atmosphere due to the balance between the capacity of the chamber and the capacity of the connected vacuum exhaust apparatus. For this reason, a stable thermal plasma can be maintained even in a reduced pressure atmosphere.
　本発明では、まず精製する金属珪素をチャンバー１内のるつぼ３中に置く。るつぼ３は、溶融珪素１０と接触させることや、陽極として電気導通することなどから、耐熱性の導電材料である炭素や、水冷された金属材料などが主に使用される。また、本発明で使用する珪素は、通常、金属珪素と言われている純度９８～９９．５％程度の珪素で、主要不純物元素として、鉄、アルミニウム、カルシウム、チタンなどを１０～５，０００ｐｐｍ程度含有し、更に珪素中ではドーパントとなる硼素を２～２０ｐｐｍ程度、リンを５～５０ｐｐｍ程度含有する工業的に生産されているものを用いることができる。なお、不純物濃度は、ＩＣＰ－ＡＥＳ法（高周波プラズマ発光分光分析法）等により測定することができる。 In the present invention, first, metal silicon to be purified is placed in the crucible 3 in the chamber 1. The crucible 3 is mainly made of carbon, which is a heat-resistant conductive material, or a water-cooled metal material, because it is brought into contact with the molten silicon 10 or is electrically connected as an anode. The silicon used in the present invention is usually silicon having a purity of about 98 to 99.5%, which is said to be metallic silicon, and iron, aluminum, calcium, titanium and the like as main impurity elements are 10 to 5,000 ppm. An industrially produced silicon containing about 2 to 20 ppm of boron as a dopant and about 5 to 50 ppm of phosphorus can be used. The impurity concentration can be measured by an ICP-AES method (high frequency plasma emission spectroscopy) or the like.
　チャンバー１内は、これに接続された真空排気装置７で減圧する。１×１０-6～１×１０-8Ｐａ程度の十分な減圧状態となったら、陰極と陽極間に電圧を印加し、その状態で両極を接触或いは接近することによりアークが発生する。印加電圧は、５～５０Ｖ、特に１５～３５Ｖが好ましい。電圧が低すぎるとプラズマが発生しなかったり、プラズマが不安定となったりする場合があり、高すぎると電極の消耗が早くなる場合がある。 The inside of the chamber 1 is depressurized by an evacuation device 7 connected thereto. When a sufficiently reduced pressure of about 1 × 10 −6 to 1 × 10 −8 Pa is reached, a voltage is applied between the cathode and the anode, and an arc is generated by contacting or approaching both electrodes in that state. The applied voltage is preferably 5 to 50V, particularly 15 to 35V. If the voltage is too low, plasma may not be generated or the plasma may become unstable. If it is too high, the electrode may be consumed quickly.
　本発明で用いられるホロカソード電極材には、耐熱性、耐食性、化学的安定性等が要求されるが、酸化性ガスの種類に応じてタングステン、モリブデン、タンタル、炭素などの耐熱材料や、イリジウム、ロジウムなどの耐熱性と耐食性、化学的安定性を具備した材料が用いられる。更にはこれらの材料を複合したものやラミネート構造なども選択肢として挙げられる。 The holocathode electrode material used in the present invention is required to have heat resistance, corrosion resistance, chemical stability, etc., depending on the type of oxidizing gas, heat resistant materials such as tungsten, molybdenum, tantalum, carbon, iridium, A material having heat resistance, corrosion resistance, and chemical stability such as rhodium is used. Furthermore, a composite of these materials and a laminate structure can be listed as options.
　また、プラズマガスとしては、主として単原子ガスであるアルゴンやヘリウムが好ましく、コストの点からアルゴンが通常使用される。 Further, as the plasma gas, argon or helium, which is mainly a monoatomic gas, is preferable, and argon is usually used from the viewpoint of cost.
　熱プラズマは、ホロカソード陰極と、陽極であるるつぼ或いはその中にセットした珪素間で発生するが、プラズマの発生位置は、陰極に対するるつぼ或いは珪素との位置関係を調整することで移動でき、実際の設備では、るつぼ位置を可変とする構造よりも、陰極を動かせる構造とする方が実用的である。 Thermal plasma is generated between the holocathode cathode and the crucible as the anode or silicon set therein, but the plasma generation position can be moved by adjusting the positional relationship between the crucible and silicon relative to the cathode. In the equipment, it is more practical to use a structure in which the cathode can be moved than a structure in which the crucible position is variable.
　るつぼ中の珪素は、この熱プラズマにより溶融する。溶融は、プラズマが直接照射されている部分と、その周囲を中心に開始される。溶融開始後は、珪素のサイズ、量と共に、るつぼの特性にも影響される。即ち、るつぼとして水冷銅を使用した場合は、加熱の熱量が水冷銅によって積極的に奪われるために、溶融範囲を広げる場合は、積極的に陰極を移動させてプラズマの照射位置を変化させる必要があるが、カーボンなどの耐熱性材料製のるつぼにて溶解する場合は、水冷銅よりも奪熱が進みにくいので溶解範囲が広くなる傾向にある。 The silicon in the crucible is melted by this thermal plasma. Melting is started around the part directly irradiated with plasma and its periphery. After the start of melting, it is influenced by the characteristics of the crucible as well as the size and amount of silicon. That is, when water-cooled copper is used as a crucible, the amount of heat is actively deprived by water-cooled copper. Therefore, when expanding the melting range, it is necessary to actively move the cathode and change the plasma irradiation position. However, when melting in a crucible made of a heat-resistant material such as carbon, since the heat removal is less likely to proceed than water-cooled copper, the melting range tends to be widened.
　プラズマの照射時間（溶融時間）は、珪素が完全に溶融するよう上記条件下で適宜調整される。 The plasma irradiation time (melting time) is appropriately adjusted under the above conditions so that silicon is completely melted.
　珪素が溶融したら、チャンバー内に酸化性ガスを導入する。酸化性ガスの導入方法は、チャンバー内のノズルから溶融珪素に噴射する方法（図１中の４）や、プラズマガスに噴射する方法（図１中の５）をとることができ、カソード電極に直接酸化性ガスを導入する（図１中の２）こともできる。カソード電極に直接酸化性ガスを導入する方法は、高温のプラズマ炎による加熱と酸化性ガスの供給が同一位置で可能であるとの利点を持つ。 When the silicon is melted, an oxidizing gas is introduced into the chamber. As the method for introducing the oxidizing gas, a method of injecting into the molten silicon from the nozzle in the chamber (4 in FIG. 1) or a method of injecting into the plasma gas (5 in FIG. 1) can be taken. It is also possible to introduce an oxidizing gas directly (2 in FIG. 1). The method of directly introducing the oxidizing gas into the cathode electrode has the advantage that heating with a high-temperature plasma flame and supply of the oxidizing gas are possible at the same position.
　酸化性ガスとしては、酸素、オゾン、水蒸気、二酸化炭素、酸化窒素などの酸素含有ガスで、高温雰囲気で酸素放出体となるものや、塩化水素、塩素、臭素、四塩化炭素などのハロゲン含有ガスで、高温ではハロゲン放出体となるものなどが挙げられ、これらの１種又は２種以上のガスが使用できる。なかでも酸素ガスと水蒸気ガスは、安価なこと、毒性が無いこと、活性に富んでいることなどから本発明方法の実施には好ましいガスである。 Oxidizing gases include oxygen-containing gases such as oxygen, ozone, water vapor, carbon dioxide, and nitrogen oxide, which can be oxygen emitters in high-temperature atmospheres, and halogen-containing gases such as hydrogen chloride, chlorine, bromine, and carbon tetrachloride. Thus, there are those that become halogen emitters at high temperatures, and one or more of these gases can be used. Among them, oxygen gas and water vapor gas are preferable gases for carrying out the method of the present invention because they are inexpensive, non-toxic and rich in activity.
　酸化性ガスの量は、プラズマガスの量と真空排気装置の能力との相互関係や、硼素やリンの除去能力の設定状態で決定するものである。また、硼素やリンの除去が効率的に実施できることなどのために、チャンバー内のガス流の方向や線速を制御する方法として、酸化性ガスを窒素、アルゴン等の不活性ガスと混合供給してもよい。 The amount of oxidizing gas is determined by the correlation between the amount of plasma gas and the capacity of the vacuum evacuation device, and the setting state of boron and phosphorus removal capacity. In addition, for the purpose of efficiently removing boron and phosphorus, an oxidizing gas is mixed with an inert gas such as nitrogen or argon as a method for controlling the gas flow direction and linear velocity in the chamber. May be.
　カソード陰極に直接酸化性ガスを導入する際は、プラズマガスに混合する酸化性ガスの濃度が高いとプラズマが消滅してしまうことがあるので、その濃度は１～４０体積％、特に５～１５体積％であることが好ましい。 When the oxidizing gas is directly introduced into the cathode and cathode, if the concentration of the oxidizing gas mixed with the plasma gas is high, the plasma may disappear, so the concentration is 1 to 40% by volume, particularly 5 to 15%. It is preferable that it is volume%.
　チャンバー内圧は、これに導入されるガスと、真空排気装置への排気ガスとのバランスで決定される減圧度となるが、本方法による熱プラズマが安定して効率良く発生する０．１～２００Ｐａ、特に５～１５０Ｐａになるように調節するのがよい。チャンバー内圧が２００Ｐａより高いと、ホロカソードプラズマは立ち消えてしまう場合があり、０．１より低いとプラズマではなくアーク放電が支配的となるものの、硼素除去に関与する酸化性ガスの供給量を十分取れなくなり、硼素とリンの除去が達成できなくなるおそれがある。 The internal pressure of the chamber is a degree of decompression determined by the balance between the gas introduced into the exhaust gas and the exhaust gas to the vacuum evacuation apparatus, but 0.1 to 200 Pa at which the thermal plasma by the present method is stably and efficiently generated. In particular, it is preferable to adjust the pressure to 5 to 150 Pa. If the internal pressure of the chamber is higher than 200 Pa, the holocathode plasma may disappear. If it is lower than 0.1, the arc discharge is dominant instead of the plasma, but the supply amount of oxidizing gas involved in boron removal is sufficient. There is a risk that removal of boron and phosphorus cannot be achieved.
　珪素中の硼素は、酸化性ガスにより酸化される。この酸化物は、珪素溶融温度（１，４１０℃）での蒸気圧が高いことから硼素は酸化物として蒸発除去されると推測できる。一方、珪素中のリンは、酸化性ガスで蒸発が促進されるような化合物を形成することはないが、リン単体としての蒸気圧は珪素に比較して十分高く、リンの蒸気圧が大気圧になる温度は黒リンで４５３℃といわれているほどである。従って、珪素が溶融することだけで、溶融珪素中のリンの濃度は徐々に低減するが、本発明においては、溶融雰囲気の圧力が０．１～２００Ｐａと減圧雰囲気であるので、リンの除去速度は大気圧よりも速くなる。 Boron in silicon is oxidized by oxidizing gas. Since this oxide has a high vapor pressure at the silicon melting temperature (1,410 ° C.), it can be estimated that boron is evaporated and removed as an oxide. On the other hand, phosphorus in silicon does not form a compound whose evaporation is accelerated by oxidizing gas, but the vapor pressure of phosphorus alone is sufficiently higher than that of silicon, and the vapor pressure of phosphorus is atmospheric pressure. The temperature to become is about 453 ° C for black phosphorus. Accordingly, the concentration of phosphorus in the molten silicon is gradually reduced only by the melting of silicon. However, in the present invention, the pressure of the molten atmosphere is a reduced pressure atmosphere of 0.1 to 200 Pa. Is faster than atmospheric pressure.
　本発明で用いるホロカソードプラズマは、それ自体の温度が数千～数万℃程度の高温であり、そのプラズマ炎と直接接触する部分の溶融珪素も部分的に高温状態（１，８００～２，２００℃程度）となる。これが通常の抵抗加熱や誘導加熱によって溶融している珪素（通常、１，５００～１，７００℃程度）と最も異なる点であり、本発明においては、硼素もリンも蒸発除去できるので、この高温部分が存在することで、硼素とリンの除去が高効率で有効に作用する。 The holocathode plasma used in the present invention has a high temperature of about several thousand to several tens of thousands of degrees C, and a portion of the molten silicon in direct contact with the plasma flame is also partially in a high temperature state (1,800 to 2,2). About 200 ° C.). This is the most different point from silicon melted by ordinary resistance heating or induction heating (usually about 1,500 to 1,700 ° C.). In the present invention, both boron and phosphorus can be removed by evaporation. Due to the presence of the part, the removal of boron and phosphorus acts efficiently and efficiently.
　本発明によれば、減圧酸化雰囲気で溶融した珪素中の硼素とリンは、溶融時間の経過と共に徐々に除去され、所望の濃度に低下してから溶解処理を終了する。溶融は、溶融初期から終了までをバッチ処理として同じるつぼ内で一度に処理してもよいし、るつぼを２つ以上用意して多段のるつぼとして、精製の進行に従って溶融珪素を順次移動して精製効率を上げるなど、種々の精製方法をとることができる。 According to the present invention, boron and phosphorus in silicon melted in a reduced-pressure oxidizing atmosphere are gradually removed as the melting time elapses, and after the concentration is lowered to a desired concentration, the dissolution treatment is terminated. Melting may be performed at once in the same crucible as a batch process from the beginning to the end of melting, or two or more crucibles are prepared and used as a multi-stage crucible, and the molten silicon is moved sequentially as the purification proceeds. Various purification methods such as increasing efficiency can be taken.
　本発明においては、硼素とリンの除去は、それぞれ異なる技術要素によって除去するが、この除去のタイミングは、硼素の除去とリンの除去が同時に起こるものであるものの、珪素中の硼素及びリンの濃度や除去条件に関係する真空度、酸化性ガス濃度、溶融温度などに依存してその除去速度は必ずしも硼素除去とリン除去は等速ではない。従って、除去対象物質が主に硼素であるか、リンであるか、その両方であるかによって適宜除去条件を変更するのがよい。 In the present invention, boron and phosphorus are removed by different technical elements. The timing of this removal is the concentration of boron and phosphorus in silicon, although boron removal and phosphorus removal occur simultaneously. Depending on the degree of vacuum, oxidizing gas concentration, melting temperature and the like related to the removal conditions, the removal rate is not necessarily the same for boron removal and phosphorus removal. Therefore, it is preferable to appropriately change the removal conditions depending on whether the substance to be removed is mainly boron, phosphorus, or both.
　本発明においては、精製すべき珪素が不純物として硼素を２～２０ｐｐｍ及び／又はリンを５～５０ｐｐｍ含有するものである場合、温度等において適切な精製条件下で精製を行えば、それぞれの初期含有量の１／３０～１／２程度、特に１／２０～１／５程度まで低減させることができる。
　本発明によれば、金属珪素中の不純物、特に硼素とリンを低減した精製珪素を得ることができるが、本発明の精製方法を実施する前又は実施後得られる金属珪素に、一方向凝固等の公知の精製方法を行うことで、硼素、リン以外の金属不純物等を除去し、太陽電池、スパッタリングターゲット材、二次電池活物質、熱電材料用珪素、セラミック原料、ガラス原料など高純度が要求される珪素材料用として好適な金属珪素を得ることができる。 In the present invention, when the silicon to be purified contains 2 to 20 ppm of boron and / or 5 to 50 ppm of phosphorus as impurities, the respective initial contents can be obtained by performing purification under appropriate purification conditions at a temperature or the like. The amount can be reduced to about 1/30 to 1/2, particularly about 1/20 to 1/5.
According to the present invention, it is possible to obtain purified silicon in which impurities in metal silicon, in particular boron and phosphorus, are reduced. However, unidirectional solidification or the like is performed on the metal silicon obtained before or after the purification method of the present invention. By using known purification methods, metal impurities other than boron and phosphorus are removed, and high purity such as solar cells, sputtering target materials, secondary battery active materials, silicon for thermoelectric materials, ceramic materials, and glass materials is required. Metal silicon suitable for the silicon material to be used can be obtained.
　以下、実施例及び比較例を示し、本発明をより具体的に説明するが、本発明は下記の実施例に制限されるものではない。なお、下記例において、不純物濃度の測定は、ＩＣＰ－ＡＥＳ法（（株）Ｐｅｒｋｉｎ　Ｅｌｍｅｒ製）により行った。 Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated more concretely, this invention is not restrict | limited to the following Example. In the following examples, the impurity concentration was measured by ICP-AES method (manufactured by Perkin Elmer Co., Ltd.).
　　［実施例１］
　ホロカソードプラズマ発生装置のチャンバー内にある水冷された銅製の容器中に、硼素を１０ｐｐｍ、リンを２０ｐｐｍ含有する金属珪素２０ｇを入れ、チャンバーを真空脱排気装置にて０．１Ｐａ以下まで脱気した。チャンバー内のホロカソード電極と銅製容器間に直流４０Ｖの電位差をつけ、両者を接近させるとアークが発生したので直ぐにホロカソードにアルゴンを１００ｃｃ／ｍｉｎ通気しながら両電極の距離を遠ざけながら電極を珪素上方に移動することで電極と珪素間にプラズマが発生し、そのプラズマ炎によって珪素が溶解し始めた。この状態で電極を移動させることで珪素を全面溶解させた。このときのチャンバー内圧は１０Ｐａだった。 [Example 1]
20 g of metallic silicon containing 10 ppm of boron and 20 ppm of phosphorus was put into a water-cooled copper container in the chamber of the holocathode plasma generator, and the chamber was deaerated to 0.1 Pa or less by a vacuum evacuation apparatus. . When a potential difference of 40V DC is applied between the holocathode electrode in the chamber and the copper container and an arc is generated when they are brought close to each other, argon is immediately vented to the holocathode at 100 cc / min while the distance between the electrodes is increased above the silicon. By moving, plasma was generated between the electrode and silicon, and silicon began to be dissolved by the plasma flame. By moving the electrode in this state, the entire surface of silicon was dissolved. The chamber internal pressure at this time was 10 Pa.
　ホロカソードに導入しているアルゴンに水蒸気１５ｃｃ／ｍｉｎを添加しながらプラズマの照射を３０分継続した。チャンバー内圧は変動したものの最終的には８Ｐａであった。プラズマ照射終了後、この珪素を分析したところ、硼素濃度は０．８ｐｐｍで、リン濃度は１．２ｐｐｍに低下していた。 Plasma irradiation was continued for 30 minutes while adding water vapor 15 cc / min to argon introduced into the holocathode. Although the chamber internal pressure fluctuated, it was finally 8 Pa. When the silicon was analyzed after the plasma irradiation, the boron concentration was 0.8 ppm and the phosphorus concentration was reduced to 1.2 ppm.
　　［実施例２］
　実施例１と同じ金属珪素を用い、実施例１の装置で、酸化性ガス導入ノズルを別個に設け、そのノズルの向きはプラズマ炎と溶融珪素が接触する部位に向かっている様にした。実施例１と同様に珪素を溶融したが、プラズマガスは５０ｃｃ／ｍｉｎとした。
　酸化性ガス導入ノズルよりアルゴン５０ｃｃ／ｍｉｎと水蒸気５０ｃｃ／ｍｉｎの混合ガスを導入し、ノズルの方向を調節してプラズマ炎と溶融珪素との接触位置にこの酸化性ガスが当たる様にした状態で３０分溶融を継続した。チャンバー内圧は最終的に１２Ｐａであつた。
　プラズマ照射後、この珪素を分析したところ、硼素濃度は１．２ｐｐｍで、リン濃度は１．８ｐｐｍであり、酸化性ガスをホロカソードに導入する方法の方が硼素とリンの除去には効果的であることがわかった。 [Example 2]
The same metallic silicon as in Example 1 was used, and in the apparatus of Example 1, an oxidizing gas introduction nozzle was provided separately, and the direction of the nozzle was directed toward the site where the plasma flame and molten silicon contacted. Silicon was melted in the same manner as in Example 1, but the plasma gas was 50 cc / min.
In a state in which a mixed gas of argon 50 cc / min and water vapor 50 cc / min is introduced from an oxidizing gas introduction nozzle, and the direction of the nozzle is adjusted so that the oxidizing gas hits the contact position between the plasma flame and the molten silicon. Melting was continued for 30 minutes. The chamber internal pressure was finally 12 Pa.
When this silicon was analyzed after plasma irradiation, the boron concentration was 1.2 ppm and the phosphorus concentration was 1.8 ppm. The method of introducing an oxidizing gas into the holocathode is more effective for removing boron and phosphorus. I found out.
　　［実施例３］
　実施例１と同じ金属珪素を用い、実施例１の装置・方法でプラズマを発生させ、珪素を溶融した。ホロカソードにはアルゴン１０ｃｃ／ｍｉｎと水蒸気２ｃｃ／ｍｉｎを導入し、酸化雰囲気のプラズマ炎として３０分間の溶解を行った。チャンバーの圧力は０．８Ｐａであった。
　プラズマ照射終了後、この珪素を分析したところ、硼素濃度は５ｐｐｍで、リン濃度は０．８ｐｐｍに低下しており、除去条件の選択で除去する元素を選択できることがわかった。 [Example 3]
Using the same metallic silicon as in Example 1, plasma was generated by the apparatus and method of Example 1 to melt the silicon. Argon 10 cc / min and water vapor 2 cc / min were introduced into the holocathode, and dissolution was performed for 30 minutes as a plasma flame in an oxidizing atmosphere. The pressure in the chamber was 0.8 Pa.
When the silicon was analyzed after the plasma irradiation, the boron concentration was 5 ppm and the phosphorus concentration was reduced to 0.8 ppm. It was found that the element to be removed can be selected by selecting the removal conditions.
　　［比較例１］
　実施例１と同じ金属珪素を用い、実施例１の装置・方法でプラズマを発生させ、珪素を溶解した。珪素が全量溶解したところでホロカソードにプラズマガスであるアルゴン１００ｃｃ／ｍｉｎだけを導入した。チャンバーの圧力は９Ｐａであった。
　この溶解を３０分間継続したのちに通電を終了した。溶解した珪素を分析したところ、硼素濃度は１１ｐｐｍ、リン濃度は１．０ｐｐｍとなり、リンの除去は達成されたが硼素の除去は確認できなかった。 [Comparative Example 1]
Using the same metallic silicon as in Example 1, plasma was generated by the apparatus and method of Example 1 to dissolve the silicon. When the entire amount of silicon was dissolved, only 100 cc / min of argon as a plasma gas was introduced into the holocathode. The pressure in the chamber was 9 Pa.
After this dissolution was continued for 30 minutes, the energization was terminated. When the dissolved silicon was analyzed, the boron concentration was 11 ppm and the phosphorus concentration was 1.0 ppm. Although removal of phosphorus was achieved, removal of boron could not be confirmed.
　１　チャンバー
　２　ホロカソード
　３　るつぼ
　４　酸化性ガス導入ノズル
　５　酸化性ガス導入ノズル
　６　ガス導入管
　７　真空排気装置
１０　溶融珪素
２０　プラズマ炎 DESCRIPTION OF SYMBOLS 1 Chamber 2 Horo cathode 3 Crucible 4 Oxidizing gas introduction nozzle 5 Oxidizing gas introduction nozzle 6 Gas introduction pipe 7 Vacuum exhaust apparatus 10 Molten silicon 20 Plasma flame

Claims (7)

  1. 　不純物を含む珪素を減圧雰囲気下でプラズマ炎により溶融した後、この溶融珪素に酸化性ガスを吹き付けることで上記不純物を除去することを特徴とする珪素の精製方法。 A method for purifying silicon, wherein silicon containing impurities is melted by a plasma flame in a reduced-pressure atmosphere, and then the impurities are removed by blowing an oxidizing gas onto the molten silicon.
  2. 　減圧雰囲気が０．１～２００Ｐａである請求項１記載の精製方法。 The purification method according to claim 1, wherein the reduced-pressure atmosphere is 0.1 to 200 Pa.
  3. 　プラズマ炎がホロカソード型プラズマ源から発せられる請求項１又は２記載の精製方法。 The purification method according to claim 1 or 2, wherein the plasma flame is emitted from a holocathode plasma source.
  4. 　酸化性ガスの供給をプラズマガスと混合して行う請求項１乃至３のいずれか１項記載の精製方法。 The purification method according to any one of claims 1 to 3, wherein the oxidizing gas is supplied by mixing with the plasma gas.
  5. 　酸化性ガスの濃度が１～４０体積％である請求項１乃至４のいずれか１項記載の精製方法。 The purification method according to any one of claims 1 to 4, wherein the concentration of the oxidizing gas is 1 to 40% by volume.
  6. 　酸化性ガスが、酸素、オゾン、水蒸気、二酸化炭素、酸化窒素、塩素、塩化水素、四塩化炭素及び臭素から選ばれるガスである請求項１乃至５のいずれか１項記載の精製方法。 The purification method according to any one of claims 1 to 5, wherein the oxidizing gas is a gas selected from oxygen, ozone, water vapor, carbon dioxide, nitrogen oxide, chlorine, hydrogen chloride, carbon tetrachloride and bromine.
  7. 　不純物が硼素及び／又はリンである請求項１乃至６のいずれか１項記載の精製方法。 The purification method according to any one of claims 1 to 6, wherein the impurity is boron and / or phosphorus.
PCT/JP2009/064918 2008-08-29 2009-08-27 Method for purifying silicon WO2010024310A1 (en)

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DE102011100884A1 (en) * 2011-05-08 2012-11-08 Centrotherm Photovoltaics Ag METHOD AND DEVICE FOR REMOVING CONTAMINATION FROM METALLURGICAL SILICON
DE102011112662A1 (en) * 2011-05-08 2012-11-08 Centrotherm Photovoltaics Ag Process for treating metallurgical silicon
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CN111807372A (en) * 2020-07-21 2020-10-23 昆明理工大学 Method for top-blown refining of silicon wafer cutting waste
CN111807372B (en) * 2020-07-21 2022-08-26 昆明理工大学 Method for top-blown refining of silicon wafer cutting waste

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