RU2588627C1 - Method of refining metallurgical silicon - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to silicon cleaning suitable for making solar cells, semiconductor devices, MEMS devices, as well as use in chemical and pharmaceutical industry. Method of silicon refining in solid phase is performed in graphite crucible by adding of extractant to it, sodium metasilicate and processing of mixture in inert gas atmosphere at temperature approximating to silicon fusion temperature (1,390±10 °C).

EFFECT: invention ensures production of high-purity silicon.

3 cl, 1 dwg, 1 tbl

Description

The invention relates to the field of purification of silicon, suitable for the manufacture of solar cells, semiconductor devices, MEMS devices, as well as use in the chemical industry.

Currently, one of the main materials used in the production of solar cells, semiconductor devices, microelectromechanical systems (MEMS), and the chemical industry is mono- and polycrystalline silicon (Si) of high purity, the so-called silicon of "solar quality" (99.99-99 , 9999%).

Currently used industrial methods for producing silicon of "solar quality" relate to methods of pyrolysis or reduction from its volatile compounds. Previously, they were chosen from a large number of thermodynamically possible reactions, since it was assumed that they would provide a minimum cost. The most widespread reactions are the chemical synthesis of silanes from industrial silicon with their subsequent distillation and pyrolysis or reduction. Varieties of such reactions, which account for the bulk of solar silicon produced in the world, form the basis of such processes as the Siemens process, Union Carbide Corp. process, Schmid process. The economic efficiency of the industrial methods used to produce high-purity silicon is relatively low, which is due, firstly, to the complex technological cycle of converting the initial silicon into compounds, their subsequent separation and reduction of silicon to the initial one, and secondly, to environmental safety restrictions, since in their technological cycle Hazardous substances such as hydrogen chloride, chlorine, chlorosilanes of various compositions and explosive monosilane are used and formed. The most common Siemens process, which makes up about 75% of the total solar energy market, is also the most economically dangerous, as according to [1], in the production of 1 m ^{2 of} solar cells based on multicrystalline silicon, emissions of chlorine into the atmosphere (including in the form of hydrogen chloride ) amount to about 2 g.

In this regard, researchers are working to find alternative cost-effective and environmentally less hazardous methods for purifying silicon from impurities.

So, the international patent No. WO89 / 02415 A1, “Method for the purification of silicon” [2], in which silicon raw materials are processed by molten slag consisting of chlorine-containing solid components and at least one component from the oxide group, is known hydroxides or carbonates of alkali or alkaline earth metals. Chlorides of alkali and / or alkaline earth metals can be used as solid chlorine compounds.

Also known is RF patent No. 2154606 "Method for the production of silicon for use in solar cells" [3], which is as follows. The molten silicon oxide is reduced with carbon, and metallurgical-grade silicon is obtained in the form of a melt. It is poured into a mold and gradually cooled to a solid state. When cooling silicon, the surface of the liquid is heated or insulated to slow down solidification. Metallurgical silicon is pre-refined. The resulting silicon is again melted and refined. Phosphorus is removed by melting at a pressure below atmospheric. Boron and silicon are removed by contacting with a gas mixture of acidic and inert gases. Oxygen is removed by deoxidation. Refined silicon is cast into a bar. The bar is purified by zone melting of Fe, Al, Ti and Ca.

The problem to which the claimed invention is directed, is to implement a method for producing silicon of "solar" quality, i.e. purity of 99.99 - 99.9999%, with the achievement of a technical result with regard to the environmental safety of the process, as well as with respect to reducing the content of impurities in silicon and reducing its cost.

The closest is the method of purification of silicon (see application N PCT - WO 9703922 A1 from 07/14/95) [4], which consists in:

1) melting the original crude silicon together with calcium silicate at a temperature not lower than 1544 ° C, during which boron, present as an impurity in silicon, passes into slag;

2) holding the melt under an inert gas atmosphere to separate it into the lower slag layer and the upper silicon layer, followed by temperature control in the range of 1430-1544 ° C to coagulate the slag, and silicon does not undergo any changes at this time;

3) immersion of the cooling element in the silicon melt, as a result of which high purity silicon is deposited on its surface. Then this element is removed from the melt and the mass of solidified silicon is removed from it;

4) remelting and vacuum processing of high-purity silicon to evaporate the phosphorus contained in it. In the same place, a device for its implementation is proposed consisting of a fixed crucible with a melt and a rotating element of pure silicon that is removed and cooled from the inside and lowered into the melt. However, this method requires a sufficiently large energy consumption to maintain high temperature, serious technological equipment and is time-consuming. Silicon phase transitions also occur.

The aim of this invention is a method of purification of silicon in solid phase and placed in a graphite crucible by adding to it an extractant - sodium metasilicate, capable of removing impurities, in particular aluminum, iron, chromium, boron, characterized in that the mixture is processed in the atmosphere inert gas, such as argon, at a temperature close to the melting point of silicon (1390 ± 10 ° C).

In FIG. 1 presents: 1 - bed, 2 - graphite (quartz) crucible, 3 - crucible heater, 4 - graphite furnace body, 5 - heat-insulating gasket, 6 - chamber body, 7 - viewing window, 8 - chamber cooling circuit.

In practice, the implementation of the proposed method is as follows. Metallurgical silicon is crushed on a jaw crusher (for example, ЩД-6) to a particle size of 2-3 mm, crushed to a particle size of 20-80 microns (for example, in a ball mill or a Titan-MD mill), and fractionated (for example, air classifier of particles "Golf - 2"). Use silicon with a particle size of 20-60 microns, while the content of the fraction of 20 microns should be at least 50%. Then, silicon is mixed with sodium metasilicate pre-dehydrated and crushed to 50-100 μm in a ratio of not less than 1 to 4 by volume and loaded into a quartz (graphite) crucible placed in a graphite support, which is installed on the heater platform (3), which can rotate around its axis at a speed of 0 to 30 revolutions per minute. Next, the chamber (6) is closed and air is pumped out through the hole to a pressure of 10 ^-5 mm Hg. Art. Next, using a graphite heater (3), the loaded mixture of silicon and extractant together with the crucible is heated to a temperature of 500 ° C and held for about 30 minutes. Then, the inert gas supply begins with a flow rate of at least 3 l / min, without stopping the pumping of the chamber, and the caliper is rotated at a constant speed of 20-30 rpm to ensure uniform heating of the crucible. Then gradually raise the temperature to 1390 ± 10 ° C for 1 hour and hold it for at least 4 hours. Starting from 1090 ° C, sodium metasilicate begins to melt, and from 1250 ° C it begins to boil [5]. At the same time, due to the influence of high temperature, there is an effective diffusion of impurities (aluminum, iron, chromium, boron, etc.) with a high pressure of saturated vapors. Vapors of impurities and extractant are removed through a pumping system. Also, due to the boiling of the extractant, there is some mechanical mixing of the mixture of silicon and extractant, as a result of which the process of removing impurities from the mass of the extractant melt to the surface and their subsequent removal is accelerated.

Next, reduce the power of the heater, stop the gas supply and slowly cool the system until it cools completely, leaving it under vacuum. After complete cooling, the chamber is opened and the crucible with the obtained mixture of silicon and extractant is removed. Silicon and extractant are separated in layers, silicon on top, and extractant on the bottom. Next, the extractant layer is removed mechanically, and then silicon is washed from the residual extractant in hot (about 80 ° C) deionized water in two doses.

At the next stage, vacuum remelting of silicon is carried out in order to remove the impurity of phosphorus contained in it. The result is a silicon purity of 99.99 (table 1)

Then, if it is necessary to obtain a single crystal, a silicon ingot with directed crystallization is grown according to the Czochralski method.

Information sources

[1] A.F.B. Braga, S.P. Moreira, P.R. Zampieri, J.M.G. Bacchin, P.R. Mei. New processes for the production of solar-grade polycrystalline silicon: A review // Solar Energy, Materials and Solar Cells, 2008, vol. 92, pp. 418-424.

[2] Patent No. WO89 / 02415 A1 "Method for the purification of silicon".

[3] RF patent No. 2154606 "Method for the production of silicon for use in solar cells."

[4] Shingu Hideo, Ishihara Keiichi, Fujiwara Hiroyasu, Otsuka Ryotatsu, Zhang Jin. Process for producing high-purity silicon. Patent WO9703922 (A1). http: //worldwide.espacenet.corn/publicationDetails/originalDocument? CC = WO & NR = 9703922A1 & KC = A 1 & FT = D & ND = 2 & date = 19970206 & DB = & locale = en_EP

[5] Patent for invention No. 2473465. "A method of obtaining crystalline hydrates of sodium metasilicate pentahydrate, six-water, nine-water."

http://www.findpatent.ru/patent/247/2473465.html

Claims (3)

1. The method of purification of silicon in the solid phase and placed in a graphite crucible by adding to it an extractant - sodium metasilicate, able to remove impurities, in particular aluminum, iron, chromium, boron, characterized in that in order to reduce the content of impurities in silicon To reduce the time of its cleaning process, silicon is crushed to particle sizes of 20-60 μm, and the content of 20 μm fractions is at least 50%, then mixed with the extractant in a ratio of at least 1: 4 by volume, placed in a crucible, and the mixture is processed at temperature close to the melting temperature of silicon, equal to 1390 ± 10 ° C, for at least 4 hours in an inert gas atmosphere. 2. The method according to p. 1, characterized in that to ensure uniform heating, the crucible is rotated at a speed of 20-30 rpm 3. The method according to p. 1, characterized in that to ensure that the boiling extractant inside the crucible, the ratio of the diameter and height of the crucible is at least 1: 3.

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