RU2078034C1 - Method for production of high-purity polycrystalline silicon - Google Patents

Abstract

FIELD: silicon production. SUBSTANCE: alkali or alkali-earth metal silicofluoride is thermally decomposed to yield silicon tetrafluoride, which is subjected to absorption purification in organic solution followed by converting silicon tetrafluoride into silicon dioxide and then into silicon monoxide. The latter is reduced with hydrogen at 500-600 C into polycrystalline silicon. EFFECT: increased yield and improved environmental condition.

Description

 The invention relates to methods for physicochemical production of a substance and can be used in the national economy in the production of high-purity polycrystalline silicon for semiconductor technology.

Various methods for producing silicon are known (US Pat. Nos. 4,529,576, Dec. 27.12.82; N 453457, published. 07.16.85, CL C 01 B 33/02 and N 4446120, CL C 01 B33 / 02) using as the original compound of the waste production of phosphate fertilizers containing hydrofluoric acid (H ₂ SiF ₆ ). A solution of hydrofluoric acid is treated with alkali metal fluoride, for example sodium fluoride, precipitated with sodium silicofluoride (Na ₂ SiF ₆ ) by reaction
H ₂ SiF ₆ + 2NaF __ → Na ₂ SiF ₆ + 2HF
the precipitate is washed, dried and then decomposed with the release of gaseous silicon tetrafluoride (SiF ₄ ) at 650 ^o C by reaction
Na ₂ SiF ₆ __ → SiF ₄ + 2NaF
A silicon melt of metal sodium and a gas mixture containing 10% chilled SiF ₄ and 90% argon heated to 300 600 ^o C are simultaneously fed to the silicon production reactor in a reactor for the production of silicon. The process is carried out by reaction
SiF ₄ + 4Na __ → Si + 4NaF
According to this method, silicon is purified by extraction of impurities in the sodium fluoride melt.

However, a single purification from impurities in the melt does not provide silicon semiconductor purity. In the applications of Japan N 57-11815, stated. 06/24/80, class C 01 B 33/02); N 55-84649, published. 01/21/82, cl. C 01 B 33/02 describes a method for producing 99.9 purity silicon, which consists in mixing powdered sodium silicofluoride (containing 95 basic substance) with aluminum metal granules (99.9 basic substance content) and heating the mixture at 560 ^o C for 1 h in an electric furnace. The process proceeds according to the reaction.
3Na ₂ SiF ₆ + 4Al __ → 3Si + 4AlF ₃ + 6NaF
Next, the reaction products are treated with an aqueous 10% hydrochloric acid solution, the residues are heated to 1200 ^o C in order to extract AlF ₃ .

 The main disadvantage of this method is the low purity of the resulting silicon. Silicon is not suitable for use in semiconductor technology.

According to the method described in Japanese application N 63-74910 "Obtaining high purity silicon tetrafluoride", cl. C 01 B 33/10 metal silicofluoride, for example Na ₂ SiF ₆ , is subjected to heat treatment at a temperature below the decomposition temperature of the salt at a pressure of ≅ 20 mm RT. Art. Thus, additional purification of silicon tetrafluoride from volatile impurities occurs.

 This method does not provide cleaning from hardly volatile impurities. The preparation of silicon is carried out by reducing silicon tetrafluoride with sodium metal.

By the method described in Japanese application N 61-28604 "Method for producing high purity silicon", published. 1986, cl. C 01 B 33/10, carry out the reduction of silicon halides (mainly chlorides) and halogen-substituted silanes with gaseous hydrogen preheated to a temperature of more than 500 ^o C. In this way it is possible to obtain silicon of high purity when using in the process pre-purified from impurities halides (chlorides ) silicon and halogen-substituted (chloride-substituted) silanes.

 Obtaining silicon by reducing silicon tetrafluoride with hydrogen is thermodynamically impossible.

The method described in the application of Germany N 053206766, published. 01/09/83, "A method of producing silicon used in solar panels", cl. G 01B 33/02), consists in the reduction of SiO ₂ in a flame of an electric arc. To obtain silicon dioxide, sodium silicofluoride (Na ₂ SiF ₆ ) is subjected to thermal decomposition to obtain NaF and SiF ₄ , and the latter is hydrolyzed in an alkaline solution. To reduce the boron content, SiF _{4 is} passed through a solution of C ₄ H ₉ O ₂ dioxide.

However, the hydrolysis of SiF ₄ in an alkaline solution does not purify SiO ₂ to semiconductor purity. In addition, during the reduction of SiO ₂ in an electric arc flame, silicon is contaminated with impurities due to erosion of the electrodes. In the applications of Germany N 3228177 (declared. 28.07.82, class C 01 B 33/02), NP 3228177.3 (published, 09.02.84, class C 01 B 33/02) to obtain metallic silicon, pure SiF ₄ , which is converted to Na ₂ SiF _{6 by} dissolution in water and precipitation with NaF according to the reactions
3SiF ₄ + 2H ₂ O __ → SiO ₂ + 2H ₂ SiF ₆
H ₂ SiF ₆ + NaF __ → Na ₂ SiF ₆ + HF
By recrystallization at 26 ^° C, Na ₂ SiF _{6 is} purified from choice and its compounds, and SiO _{2 is} obtained by thermal decomposition with air
Na ₂ SiF ₆ __ → SiF ₄ + 2NaF
SiF ₄ + O ₂ __ → SiO ₂ + 2F ₂
The resulting SiO _{2 is} reduced with pure carbon to silicon by the reaction
SiO ₂ + 2C __ → Si + 2CO
This method has two disadvantages:
the purity of the resulting silicon is determined by the purity of the carbon used for reduction;
the difficulty of separating silicon from excess carbon.

In application EPO (EP) N 0087732, published. 03/07/83 c. C 01 B 33/01 obtain silicon by reduction of SiO ₂ carbon. SiO _{2 is} obtained by thermal decomposition of sodium hexafluorosilicate to sodium fluoride and silicon tetrafluoride, which is then treated in an alkaline (ammonia) solution
Na ₂ SiF ₆ __ → SiF ₄ + 2NaF SiF ₄ + 4NaOH __ → SiO ₂ + 4NaF + 2H ₂ O
or
SiF ₄ + 4NH ₄ OH __ → SiO ₂ + 4NH ₄ F + 2H ₂ O SiO ₂ + 2C __ → Si + 2CO
This method has the same disadvantages as the previous one.

A widespread laboratory method for producing silicon (B. Nekrasov. General chemistry course. M. 1952) is the reduction of silicon dioxide by magnesium
2Mg + SiO ₂ __ → 2MgO + Si
In the case of excess magnesium, silicon forms magnesium silicide Mg ₂ Si. In order to avoid the formation of silicide for the reaction, metallic magnesium and silicon dioxide are taken in quantities that strictly correspond to the stoichiometric level. So that the reaction proceeds without explosion, 25 magnesium oxide is added to the control mixture. To liberate from MgO and unreacted SiO _{2, the} reaction product is successively treated with hydrochloric and hydrofluoric acids.

 In addition to magnesium, other alkaline earth and alkali metals can be used as a reducing agent, and silicon halides can be used instead of silicon dioxide.

US Pat. No. 4,208,978, class C 01 B 33/02, describes a method for producing polycrystalline silicon of semiconductor purity from silicofluoride compounds (H ₂ SiF ₆ , SiF ₄ ), which are by-products of the production of phosphoric fertilizers, elemental phosphorus or phosphoric acids. This method was developed by two American firms "Allied Chem. Corp." and Ethyl Corp.
The technology for producing polycrystalline silicon of semiconductor purity is based on the reaction of converting SiF ₄ into monosilane SiH ₄ with the subsequent decomposition of monosilane into silicon and hydrogen.

Known processes of Conversion SiH ₄ by hydride-NaH, NaAlH ₄ or LiAlH ₄ as described in (Belov E. P. N. Lebedev E. Grigorash YP Monsilan in semiconductor technology material. NIITEKHIM M., 1989). A hydride, for example NaH, is suspended in a diphenyl ether type solvent, and SiF ₄ or its mixture with nitrogen is bubbled through the solution in a 1: 1 ratio. Complete conversion occurs at 520 531K. It has been established that SiF ₄ can react with NaF to form Na ₂ SiF ₆ only after all hydride is consumed.

A similar process for Ethyl Corp. based on reaction
SiF ₄ + NaAlH ₄ __ → SiH ₄ + NaF + AlF ₃
The solvent for sodium aluminum hydride can be dimethyl ether or diethylene glycol. The reaction proceeds at 323 333K.

The process of producing monosilane by the interaction of silicon tetrafluoride with lithium aluminum hydride was also investigated.
SiF ₄ + LiAlH ₄ __ → SiH ₄ + LiF + AlF ₃
It was found that for this process it is necessary to take a 20% excess of lithium aluminum hydride with respect to stoichiometry. The reaction proceeds at 323 333> K and a pressure of 3.9 • 10 ⁵ Pa. The output of monosilane is 97
The disadvantage of this method is the explosiveness of silicon compounds used in the technology, for example SiH ₄ , which burns upon contact with oxygen, as well as organic solvents of diphenyl or dimethyl ethers used as solvents for hydrides to produce monosilane. The disadvantage of this method is the process of producing hydrides, for example sodium hydride, by the interaction of sodium and hydrogen in xylene, a high fire and explosion hazard process.

A known method for the production of polycrystalline silicon for semiconductor technology from technical silicon (Project of the Tash-Kumyrsky Semiconductor Materials Plant. Developed by the State Scientific Research and Design Institute (GIREDMET) and approved by the Ministry of Color and Metrology on 31.12.82, protocol No. 166). In this method, lump technical silicon is initially crushed, pulverized, then chlorinated with anhydrous hydrogen chloride. Anhydrous hydrogen chloride is obtained by reacting previously thoroughly dried hydrogen and chlorine, which is taken in a twofold excess with respect to hydrogen. The yield of hydrogen chloride is 35
Trichlorosilane is isolated from the vapor-gas phase of various silicon chlorides and chlorine-substituted silanes, subjected to distillation purification, and then reduced with hydrogen to elemental silicon in the condensed phase by reaction
SiHCl + H ₂ __ → Si + 3HCl
The described method is the most widespread in the production of polycrystalline silicon for semiconductor technology. This method, as proposed, involves the preparation of polycrystalline silicon from its compounds by reducing the silicon compound (SiNCl ₃ ) with hydrogen, therefore, it is accepted as the closest in technical essence and achieved positive effect as a prototype.

 The disadvantages of the prototype method are low technical and economic indicators and due to the low corrosion resistance of equipment in chloride environments, high environmental environmental pollution by elemental chlorine, hydrogen chloride, silicon chloride compounds and hexachlorobutadiene.

 The purpose of the invention is to increase the direct yield of polycrystalline silicon and to ensure environmental cleanliness of the industrial production of polycrystalline silicon.

To do this, carry out the restoration of silicon monoxide pre-purified hydrogen at 500 600 ^o C.

 Further, the prototype method and the proposed method are considered with specific examples, starting with raw materials, which are substances derived in industrial quantities. In the prototype method, the initial raw material is technical silicon, and in the inventive method, silicofluoride compounds, for example sodium silicofluoride, obtained as waste in the extraction of apatites and their processing into phosphoric acid and mineral fertilizers.

 Example 1 (prototype method). Raw materials: lump technical silicon of grades Кр 1, Кр 0 or Кр 00; liquid chlorine, hexachlorobutadiene, soda; hydrogen gas.

В процессе синтеза протекают и другие реакции с образованием полисиланхлоридов кремния, хлоридов металлов примесей технического кремния и ряда других соединений. Реактор кипящего слоя выводится на ремонт через ≈ 700 ч работы, так как он прогорает. Шлаки периодически выводятся в кюбель и в отвал. Парогазовая смесь, содержащая H₂,HCl, SiHCl₃, SiH₂Cl₂, SiH₃Cl, SiCl₄, полисиланхлориды, хлориды металлов, кремниевую пыль направляется на три последовательных циклотронных пылеосодителя, работающие при 180^oC, а затем на рукавные фильтры с несколькими слоями алюмомагнезиальной стеклоткани (при 110 150^oC).1 kg of lump technical silicon of grades Кр 1, Кр 0 or Кр 00 is crushed in a jaw, cone or ball crusher to a dust-like state. Anhydrous hydrogen chloride is prepared from liquid chlorine and gaseous hydrogen at a furnace wall temperature of 430 ^° C, which is ground chlorinated at 290-350 ^° C. The process takes place according to the reactions

In the synthesis process, other reactions also occur with the formation of silicon polysilane chlorides, metal chlorides, impurities of industrial silicon and a number of other compounds. The fluidized bed reactor is brought out for repair after ≈ 700 hours of operation, since it burns out. Slags are periodically discharged into the cube and dump. A gas-vapor mixture containing H ₂ , HCl, SiHCl ₃ , SiH ₂ Cl ₂ , SiH ₃ Cl, SiCl ₄ , polysilane chlorides, metal chlorides, silicon dust is sent to three successive cyclotron dust separators operating at 180 ^o C, and then to bag filters with several layers of alumomagnesia fiberglass (at 110 150 ^o C).

The vapor-gas mixture from bag filters is sequentially cleaned in planting columns irrigated with 90 SiCl ₄ mixture and polychloride condensate. Next, the gas-vapor mixture is sent to the condensation unit of the chloride mixture. The non-condensed vapor-gas mixture is captured by hexachlorobutadiene and neutralized by a soda solution.

 A mixture of chlorosilanes and chlorides, after preliminary sedimentation and separation, undergoes a deep distillation treatment.

 Then, hydrogen reduction of silicon from purified trichlorosilane is carried out to obtain polycrystalline silicon.

 In the prototype method, the output of polycrystalline silicon from technical silicon is not more than 30; the consumption of liquid chlorine is not less than 200 to stoichiometry.

 Example 2 (the proposed method). Raw materials: technical sodium silicofluoride; highly purified steam; highly purified hydrogen.

 To solve the problem of the invention, it is proposed to use sodium silicofluoride obtained as waste during the extraction of apatites and their processing into phosphoric acid and mineral fertilizers as a feedstock.

The initial powder of technical sodium silicofluoride through a shutter of powder is fed using an auger feeder into an inclined rotary kiln at a furnace wall temperature of 550 600 ^o C. Before the start of the process, the furnace is purged with nitrogen to remove moist air, since silicon tetrafluoride formed during thermal decomposition of silicon silicofluoride in the presence of moisture is hydrolyzed. The resulting gaseous silicon tetrafluoride is discharged from the furnace to the receiver, and the second product of the process is discharged from the furnace into a sealed hopper. The yield of silicon in tetrafluoride is 99.9 99.95
Silicon tetrafluoride is purified from impurities by absorption into ethylene glycol using a plate-type sorption column. The direct yield of silicon to purified tetrafluoride was 99.90 99.99%
Purified to a content of impurities 10 ^-6 10 ^-7 silicon tetrafluoride is fed into the receiver or pumped into cylinders. Then, purified silicon tetrafluoride is contacted with steam preliminarily purified from impurities to form silicon dioxide by reaction
SiF ₄ + 2H ₂ O _ → SiO ₂ + 4HF
For completeness of the process, steam is supplied with a 5-10-fold excess compared with stoichiometry. The temperature of the process is 250 -350 ^o C. The resulting silicon dioxide powder is separated from anhydrous hydrogen fluoride and 5 10% excess steam (relative to stoichiometry) on a sintered metal filter at a temperature of 150 -200 ^o C. The impurity content in the obtained silicon dioxide does not exceed 10 ^-6 -10 ^-7 % Silicon dioxide yield is 99.7 99.9
Then, the silicon dioxide separated from the gas phase is restored by pre-purified hydrogen in a plasma jet to obtain silicon monoxide (SiO) by thermal decomposition of SiO ₂ at 1 atm in the temperature range 4400 4800 ^o C. Quick quenching of the decomposition products of SiO ₂ from a temperature of 4800 ^o C to temperatures below the beginning of the disproportionation of solid silicon monoxide (≈400 ^o C) will allow it to be obtained. The impurity content in the resulting silicon monoxide does not exceed 10 ^-6 10 ^-7 %
To obtain a high-purity polycrystalline silicon powder, silicon monoxide is reduced by previously purified and dried hydrogen. The content of impurities in hydrogen is not more than 10 ^-7 %. The process proceeds by reaction
SiO + H ₂ __ → Si + H ₂ O
The process is carried out in an airtight vessel in which silicon monoxide powder is placed, air is pumped out and the vessel is filled with hydrogen. Hydrogen is supplied with a 5-10th excess relative to stoichiometry. Then the vessel is heated to a temperature of 550 ^o C. the Process is carried out for 30 minutes. After the end of the process, the reaction vessel is cooled to 120 ^o C, the resulting vapor and 5 10-nd excess hydrogen is pumped out of the reaction vessel. The impurity content in the resulting polycrystalline silicon did not exceed 10 ^-6 10 ^-7 The output of polycrystalline silicon was 99.5 99.9%. The total silicon yield from sodium silicofluoride was 99.4 99.5
The expected effect of the use of the present invention is to obtain cheap polycrystalline silicon for semiconductor technology using an unlimited source of cheap raw materials (not more than 300 rubles / t) obtained in the form of waste from the extraction of apatites and their processing into phosphoric acid and mineral fertilizers. The inventive method allows for waste-free, environmentally friendly technology for the industrial production of polycrystalline silicon for semiconductor technology.

Claims (1)

A method of obtaining high-purity polycrystalline silicon, including the purification and recovery of an inorganic compound of silicon with hydrogen, characterized in that the recovery of silicon monoxide at 500 - 600 ^o C.