RU2129984C1 - Method of preparing high-purity monosilane - Google Patents

Abstract

FIELD: organosilicon compounds and semiconductor engineering. SUBSTANCE: invention aims at preparing high-purity monosilane suitable for fabrication of thin-layer semiconductor products and also at preparing high-purity poly- and monocrystalline silicon suitable for semiconductor and solar engineering. Monosilane is prepared by interaction of silicon metal with an alcohol in presence of catalyst at elevated temperature followed by isolation of desired alkoxysilanes, purification thereof and their catalytic disproportionation to form gaseous monosilane, which is further purified by absorbing impurities and liquid disproportionation products including active catalytic materials, teraalkoxysilane and traces of compounds with Si-H bonds. Resultant monosilane is freed of unreacted alcohol which is recycled into reaction with silicon. Concentrated mixture of alkoxysilanes is subjected to catalytic disproportionation at temperature from -20 to +40 C and reaction time from 1 to 50 hr to release gaseous monosilane, which is further purified by preliminary absorption, whereas, to liquid disproportionation products, alcohol is added and tetraalkoxysilane isolated. The latter is completely hydrolyzed to yield commercial silica sol and alcohol, which is dried with tetraalkoxysilane and returned into reaction with silicon. EFFECT: increased yield of product and its purity. 5 cl, 2 tbl, 4 ex

Description

 The invention relates to a method for producing high-purity monosilane suitable for the formation of thin semiconductor and dielectric layers, as well as high-purity poly- and single-crystalline silicon for various purposes (electronics, solar energy).

The production of high purity silicon can be carried out in various ways, of which the most preferred, even compared with the most common method of trichlorosilane reduction with hydrogen, is the thermal decomposition of monosilane. The basis for such a statement is as follows:
- thermal decomposition of monosilane requires less energy, as it occurs at a temperature of about 850 ^o C instead of 1100 ^o C;
- in the reaction products there are no aggressive agents (hydrogen chloride, chlorosilanes, etc.) that reduce the purity of the resulting silicon;
- purification of monosilane from most harmful impurities is more effective due to a significant difference in the physical and chemical properties of monosilane and compounds of impurity elements;
- along with silicon, the commercial product is monosilane and its gas mixtures, which are necessary for thin-film technology for the manufacture of semiconductor products.

 However, many existing methods for producing silicon through monosilane are complex, and the cost of their implementation is more substantial in comparison with trichlorosilane. For this reason, monosilane is used only to obtain polycrystalline silicon of particularly high purity, the processing of which into monocrystalline silicon is carried out by the method of zoneless melting. Such silicon is necessary for the manufacture of infrared radiation detectors and nuclear radiation detectors, etc.

 To reduce the cost of monosilane and silicon while maintaining their purity, intensive searches are being conducted in the world for new, more efficient methods for producing high purity monosilane.

Процесс идет при температуре 400-600^oC и давлении 0,7-41,4 бар с последующим выделением ди- и трихлорсилана.A known method for the catalytic disproportionation of trichlorosilane (German patent N 3311650, publ. 13.10.83,). The first stage of this process is the catalytic hydrogenation of silicon metal, proceeding by reactions

The process proceeds at a temperature of 400-600 ^o C and a pressure of 0.7-41.4 bar, followed by the allocation of di - and trichlorosilane.

В качестве катализатора используются анионообменные смолы с третичными аминами, а диспропорционирование проводят при температуре до 150^oC. Далее моносилан подвергают очистке от примесей.The second stage is the catalytic disproportionation of chlorosilanes, performed in several stages according to the following resulting reaction:

Anion exchange resins with tertiary amines are used as a catalyst, and the disproportionation is carried out at a temperature of up to 150 ^o C. Next, monosilane is subjected to purification from impurities.

 In this method, the hydrogenation of silicon tetrachloride formed as a by-product during the production of monosilane by the catalytic disproportionation of chlorosilanes is carried out, which ensures high efficiency of the use of the initial metal silicon and significantly reduces the cost of monosilane and polycrystalline silicon.

However, this method is not without significant disadvantages, which are as follows:
- the need to create and operate unique equipment that operates at high temperatures and pressures at the stage of hydrogenation of silicon tetrachloride;
- the presence of aggressive chlorine compounds that facilitate the transfer of impurities from the walls of the equipment to monosilane;
- the difficulty of cleaning monosilane to a high degree of purity;
- unsatisfactory environmental characteristics of the method associated with the presence of chlorine compounds.

 The fundamentals of the catalytic disproportionation of alkoxysilanes, in particular triethoxysilane, were developed in the USSR in 1957-59. (Journal of Technical Physics, 1957, v. 27, issue 8, 1645-1548 and Solid State Physics, 1959, v. 1, 999-1001).

 The development of this method, especially in terms of the production of alkoxysilane by direct interaction of ethyl alcohol with metal silicon, is described in Japanese patent N 7427517 from 07/18/74, adopted by us for the prototype of the present invention.

 According to the prototype, the production of high-purity monosilane is carried out by reacting silicon metal with ethyl alcohol in the presence of a catalyst at elevated temperature, after which the obtained interaction products are separated, the purified triethoxysilane is subjected to disproportionation in the presence of a catalyst to obtain the target monosilane, which is isolated and purified from impurities by low-temperature condensation and sorption on various adsorbents.

The method is as follows. The interaction of powdered silicon with ethyl alcohol is carried out at a temperature of 190-200 ^o C using as a catalyst element 1b of the group of the Periodic system (Cu, Ag) in a medium of high boiling solvent, for example, hexaisopropoxysiloxane (boiling point 280 ^o C) or dodecaethoxypentasiloxane (boiling point 290 -300 ^o C).

В идеальном случае n=1, однако обычно n<1, и реакция сдвигается в ту или иную сторону в зависимости от свойств катализатора и среды.The reaction of silicon with alcohol proceeds according to the equation

In the ideal case, n = 1, but usually n <1, and the reaction shifts in one direction or another, depending on the properties of the catalyst and medium.

 The obtained triethoxysilane is isolated, purified from impurities by repeated distillation and disproportionation is carried out in the presence of a catalyst from elements of groups Ia, IIa of the Periodic system (Na, K, Ca, etc.), and the target monosilane is obtained, which is purified from impurities by low-temperature condensation and sorption on adsorbents, the most effective of which are activated carbon, activated alumogel, silica gel, etc.

 The prototype remains unsolved a number of problems associated with ensuring high purity of the obtained monosilane, as well as increasing the efficiency of the process, allowing to reduce the cost and increase the competitiveness of products.

 The use of a large amount of anhydrous alcohol (the consumption coefficient of alcohol per unit of monosilane obtained is 18-20) negatively affects the cost of this product. In addition, the chemistry of the process imposes restrictive requirements for the water content in alcohol (less than 500 ppm). Absolute alcohol produced by industry contains water up to 0.1%, which requires thorough additional purification, which is carried out using special dehydrating agents, such as benzene, alkali metals, etc. The use of alcohol thus obtained is accompanied by the introduction into the process of impurities that pollute the resulting target and by-products, and also increases the cost of alcohol.

 The multiple distillation of the products of the interaction of metallic silicon and alcohol (di-, tri- and tetraethoxysilane, unreacted alcohol, etc.) used to isolate triethoxysilane is not effective enough because it is a very energy-intensive process.

 The use of alkali and alkaline earth metals as a disproportionation catalyst is ineffective, since the surface of the catalyst is covered with a layer of silicon due to the occurrence of side reactions, and the catalyst is deactivated. In addition, the replacement of the catalyst is an extremely dangerous operation, in addition, accompanied by the introduction of a large amount of impurities into the process. The increase in temperature at which the catalyst becomes operational is accompanied by a significant increase in the content of ethoxysilane vapors in the resulting monosilane.

 In the prototype, the question of the use of liquid disproportionation products, which are obtained 25-30 times more than monosilane, is increased, which increases the consumption of raw materials.

 The purification of monosilane by low-temperature condensation is ineffective because of the possible formation of a dispersed liquid phase during supercooling of the gas phase.

 The use of adsorption purification of monosilane with the content of vapors of organosilicon and organoelement compounds in it above 1 vol.% Is also ineffective, because requires large quantities and frequent replacement of the adsorbent.

The objective of the present invention is to increase the yield of monosilane, increasing its purity while reducing consumption rates of raw materials and energy consumption. This problem is solved by the proposed method for producing high purity monosilane, in which metal silicon is reacted with organic alcohol in the presence of a catalyst at an elevated temperature, followed by the isolation of the corresponding alkoxysilanes from the reaction products, their purification, catalytic disproportionation with the formation of monosilane, purified by sorption of impurities, and liquid disproportionation products, including tetraalkoxysilane, the active catalytic part and an admixture of compounds with remnevodorodnoy bond, wherein according to the invention:
- unreacted alcohol is isolated from the reaction products of metallic silicon with an organic alcohol, which is returned to the reaction with silicon, and the resulting concentrated mixture of alkoxysilanes after purification from impurities is subjected to catalytic disproportionation at a temperature of minus 20 - plus 40 ^o C and the residence time of the reaction mixture is 1-50 h with the release of monosilane, pre-purified by absorption of impurities, and organic alcohol is added to the liquid disproportionation products, tetraalkoxysilane is isolated, which they undergo complete hydrolysis with the formation of commercial silica and organic alcohol, which, after drying, return to the reaction with silicon;
- organic alcohol is added to the liquid disproportionation products in an amount that ensures that there are no compounds with a silicon-hydrogen bond in the reaction mass, then tetraalkoxysilane is isolated, and the remaining active catalytic portion is returned to the disproportionation;
- the drying of the organic alcohol formed during the complete hydrolysis of tetraalkoxysilane is carried out using tetraalkoxysilane isolated from the liquid disproportionation products, which is first sent to dry the alcohol, and then to the complete hydrolysis;
- preliminarily, the absorption of impurities from monosilane is carried out with an absorbent cooled to a temperature above its freezing point in countercurrent mode at a contact time of 5-60 minutes and a temperature difference between monosilane and absorbent not more than 5 ^o C;
- as an absorbent take tetraalkoxysilane isolated from liquid disproportionation products.

 The method is as follows. First, to obtain high-purity monosilane, silicon metal is reacted with an organic alcohol (preferably ethyl alcohol) in the presence of a catalyst at an elevated temperature to form alkoxysilanes, of which the target product is trialkoxysilane. This method is carried out according to one of two known options: in the gas phase (US Pat. US N 5260471, publ. 9.11.93 g.) Or in the medium of a high boiling solvent (US Pat. US N 5362897, publ. 8.11.94 g.).

 According to the first embodiment, the interaction of fluidized metal silicon and organic alcohol is carried out in the gas phase in the presence of a halogen (methyl, ethyl chloride) as a catalyst. Lowering the pressure below atmospheric allows you to lower the process temperature and increase the yield of the target product by eliminating the condensation of high-boiling compounds on silicon particles and equipment walls. In this case, a high conversion of metallic silicon and a high selectivity for the formation of trialkoxysilane are achieved.

 In the second embodiment, the interaction of metallic silicon and organic alcohol is carried out in a high-boiling solvent using copper compounds (acetate, oxalate or chloride) as well as aluminum or Al-Si alloy as a catalyst. In this case, a high conversion of silicon metal (up to 99.8%) is achieved, which facilitates the separation of the solid precipitate from the high-boiling solvent reused in the process. The condensed products of the interaction of metallic silicon and organic alcohol according to one of the two options listed above contain trialkoxysilane (about 50 wt.%), Tetraalkoxysilane (about 10 wt.%), Unreacted alcohol (about 25 wt.%), Diethoxysilane impurities (up to 3 wt. %), chlorine compounds (up to 1 wt.%), etc.

It is known that a silicon-hydrogen bond, especially in an acidic environment, is unstable, and hydrogen can easily be replaced by an alkoxy group by the reaction
SiH (OR) ₃ + ROH _ → Si (OR) ₄ + H ₂ , where R is alkyl.
To eliminate this phenomenon, in the first stage of purification of the condensed products, organic alcohol is first isolated. The process is carried out by distillation under reduced pressure (about 100 mm Hg), which allows the separation of organic alcohol at a temperature close to room temperature and practically does not allow the destruction of the silicon-hydrogen bond. The isolated organic alcohol does not contain moisture, and a small admixture of alkoxysilanes and volatile chlorine compounds in it does not prevent its reuse in the reaction with silicon to produce alkoxysilanes. Further purification of alkoxysilanes is carried out according to known technology.

В результате этих реакций происходит осаждение хлорсодержащих соединений в виде объемного твердого осадка с развитой поверхностью, который является прекрасным адсорбентом для примесей металлов и т.п. Далее смесь алкоксисиланов с твердым осадком разделяют (преимущественно фильтрацией). Осадок направляется на биохимическую очистку сточных вод в качестве азотсодержащей добавки. Полученная после отделения твердого осадка смесь алкоксисиланов имеет содержание примесей, представленное в табл. 1.In the case of the production of alkoxysilanes in the presence of chlorine compounds in the condensed products of the interaction of metallic silicon and organic alcohol, mainly chloroesters are formed, of which the most probable is monochlorotriethoxysilane, namely ClSi (OR) ₃ , with an admixture of dissolved HCl, solid aluminum chloride, etc. When processing this mixture of alkoxysilanes with gaseous ammonia, the formation of solid products by reactions

As a result of these reactions, the deposition of chlorine-containing compounds occurs in the form of a bulk solid precipitate with a developed surface, which is an excellent adsorbent for metal impurities, etc. Next, a mixture of alkoxysilanes with a solid precipitate is separated (mainly by filtration). The precipitate is sent to biochemical wastewater treatment as a nitrogen-containing additive. The mixture of alkoxysilanes obtained after separation of the solid precipitate has an impurity content shown in Table 1. 1.

 The proposed technology for the isolation and purification of a mixture of alkoxysilanes from the condensed products of the interaction of metallic silicon and organic alcohol has the following advantages.

 The selection of the most volatile component from condensed products, namely organic alcohol, according to the proposed method allows this process to be carried out with the lowest energy costs, lower the temperature and virtually eliminate the destruction of the silicon-hydrogen bond in di- and trialkoxysilane, and therefore increase the yield of monosilane during disproportionation for example, with multiple distillation, proposed in the prototype.

 Theetraalkoxysilane isolated with di- and trialkoxysilanes does not affect the disproportionation process and will be used according to the proposed method, as shown below.

 The proposed ammonia purification of a mixture of alkoxysilanes is simple in instrumentation and technology and allows you to get products almost free from impurities. There are no unsold waste in the process.

В качестве катализатора используют растворы алкоголятов и триаaлкоксисиланолятов щелочных, щелочноземельных металлов и алюминия в тетралкоксисилане. Подачу алкоксисиланов и катализатора в реактор диспропорционирования осуществляют в периодическом или непрерывном режимах (предпочтительнее в непрерывном режиме) при соотношении 20:1 - 10:1. Использование в качестве катализатора щелочных или щелочноземельных металлов, как уже отмечено выше, нецелесообразно из-за осаждения на поверхности катализатора кремния, который блокирует контакт между алкоксисиланами и катализатором и приводит к его дезактивации. Это происходит при протекании побочных реакций, из которых наиболее вероятной является реакция
SiH(OR)₃+3Na _→ Si+1/2H₂+3C₂H₅ONa.
Выделяющийся моносилaн насыщается парами алкоксисиланов, причем при увеличении температуры диспропорционирования увеличивается концентрация паров алкоксисиланов в моносилане. Поэтому диспропорционирование проводят при возможно более низких температурах в интервале минус 20 - плюс 40^oC. Предпочтительней проводить диспропорционирование при комнатной температуре. Это позволяет упростить конструкцию реактора и исключить подачу хладагента.Next, a mixture of purified alkoxysilanes consisting of di-, tri- and tetraalkoxysilanes is sent to catalytic disproportionation proceeding by reactions

As a catalyst, solutions of alkoxides, alkaline earth metal alkali metals and aluminum trialoxoxysilanolates and aluminum in tetralkoxysilane are used. The alkoxysilanes and catalyst are supplied to the disproportionation reactor in batch or continuous modes (preferably in a continuous mode) at a ratio of 20: 1 to 10: 1. The use of alkali or alkaline earth metals as a catalyst, as already noted above, is impractical due to deposition of silicon on the surface of the catalyst, which blocks the contact between alkoxysilanes and the catalyst and leads to its deactivation. This occurs when adverse reactions occur, of which the reaction is most likely
SiH (OR) ₃ + 3Na _ → Si + 1 / 2H ₂ + 3C ₂ H ₅ ONa.
The emitted monosilane is saturated with vapors of alkoxysilanes, and with increasing temperature of disproportionation, the concentration of vapors of alkoxysilanes in monosilane increases. Therefore, the disproportionation is carried out at the lowest possible temperature in the range of minus 20 - plus 40 ^o C. It is preferable to carry out the disproportionation at room temperature. This simplifies the design of the reactor and eliminates the flow of refrigerant.

 The most important characteristic of the disproportionation process is the conversion of alkoxysilanes containing a silicon-hydrogen bond to monosilane. Among other conditions, the reaction time plays a significant role to achieve maximum conversion. In the case of disproportionation in continuous mode, the necessary contact time is selected in the range of 1-50 hours, preferably 10-25 hours. In this case, taking into account the fire and explosion hazard of monosilane, as well as the possibility of contamination with environmental impurities, the disproportionation is carried out in sealed equipment, not requiring mixing devices, driven by engines located outside the reactor. Subject to the proposed modes of disproportionation, the conversion of alkoxysilanes reaches 98%.

It should be noted that the disproportionation of alkoxysilanes with the existing silicon-hydrogen bond is a selective process. Of the most undesirable electroactive elements in semiconductor silicon are the elements of the third and fifth groups of the Periodic system, the so-called acceptors (boron, etc.) and donors (phosphorus, arsenic, etc.) that do not have a ready-made element-hydrogen bond in the organoelement compounds (B (OR) ₃ , P (OR) ₃ , As (OR) ₃ , etc.), and therefore, are unable to disproportionate with the formation of volatile hydrides (diborane, arsine, phosphine, etc.). Due to this fact, the purification of monosilane from compounds of these harmful impurities is greatly simplified. In practice, the transition of these impurities into monosilane can be carried out only in the form of vapors of organoelement compounds, examples of which are given above.

In the present invention, the purification of monosilane has as its goal the separation of organoelement and organic impurities in the most effective ways. If the disproportionation of alkoxysilanes is carried out without cooling, then the possible temperature increase due to the release of the heat of the disproportionation reaction is small, and the actual temperature of the disproportionation is 30-40 ^o C. In this case, the maximum elasticity of saturated vapors of organosilicon compounds in monosilane will be 1-2 vol.%. For the purification of monosilane from significant concentrations of impurities of organoelement compounds, preliminary absorption is proposed.

 When using chilled tetraethoxysilane as an absorbent in which practically all organoelement impurities are unlimitedly soluble, it is possible to reduce their concentration in monosilane to a level corresponding to the saturated vapor pressure at the temperature of the absorbent. The conditions for such cleaning are a sufficient contact time between monosilane and a cooled adsorbent, as well as a small temperature difference between them at any point of contact, excluding the formation of a dispersed liquid phase (condensation of vapor of impurities in the volume).

In reality, the absorption of impurities from monosilane is carried out with tetraalkoxysilane, preferably tetraethoxysilane, cooled to a temperature above its freezing point, in countercurrent mode at a contact time of 5-60 minutes and a temperature difference between monosilane and tetraalkoxysilane of not more than 5 ^o C, preferably 2 ^o C. conditions, the concentration of organoelement impurities in monosilane after absoorption purification decreases to 0.001 vol.%.

 Further purification of monosilane from impurities is carried out by adsorption on activated carbon at room temperature and atmospheric pressure. The concentration of organoelement impurities reduced due to absorption can be easily reduced from 0.001 to 0.00001-0.00005 vol.% Even in a one-stage adsorber filled with activated carbon.

 The obtained monosilane of high purity can be used in gaseous form at 100% concentration or in the form of gas mixtures for the formation of any thin-layer structures of semiconductor products, including its use for the manufacture of the most critical structures of integrated circuits.

 By thermal decomposition of the obtained high-purity monosilane, high-purity polycrystalline silicon is obtained, which can be used in various fields of electronic technology.

 When rods are formed on cylindrical substrates heated by electric current, the purest polycrystalline silicon is obtained, which, after processing into single crystal silicon by crucibleless zone melting, can be used for the manufacture of infrared detectors and nuclear radiation detectors.

 When thermal decomposition of monosilane is carried out in more productive and less energy-intensive devices, for example, devices with a fluidized bed of silicon germ particles, polycrystalline silicon of the solar class is obtained, which, due to its low cost, can be used to create solar cells.

The liquid reaction products formed after the disproportionation of alkoxysilanes and the isolation of monosilane contain tetraalkoxysilane, the active catalytic part is a solution of alcoholates or trialkoxysilanolates, as well as an admixture of unreacted alkoxysilanes with a silicon-hydrogen bond. At the first stage of processing of these products, alkoxysiliation of the silicon-hydrogen bond is carried out
SiH (OR) ₃ + ROH _ → Si (OR) ₄ + H ₂ .
To do this, add the calculated amount of organic alcohol and carry out stirring at a temperature of 50 ^o C. In the presence of a catalyst, the alkoxysilane reaction proceeds with a degree of conversion close to 100%.

At the second stage, distillation of tetraalkoxysilane is carried out. This process is carried out at a residual pressure of 100 mm Hg, which allows to reduce the temperature to 90 ^o C. The release of tetraalkoxysilane is continued until the concentration of catalyst in tetraalkoxysilane reaches the desired. The active catalytic part remaining after the isolation of tetraalkoxysilane is returned again to the disproportionation of alkoxysilanes. The isolated tetraalkoxysilane with an admixture of organic alcohol is sent to the top of the packed absorption reaction column to dry the organic alcohol formed after the complete hydrolysis of tetraalkoxysilane with water.

Partially hydrolyzed tetraalkoxysilane after passing the packed column with a temperature of 90-100 ^o C sent for complete hydrolysis and the formation of a colloidal dispersion of silicon dioxide, which is accompanied by the desorption of organic alcohol. The obtained colloidal dispersion of silicon dioxide (silica sol) is sent for sale as an additional commercial product.

 Desorbed organic alcohol in the form of vapors is fed into the lower part of the packed absorption-reaction column, in which, as a result of contact with tetraalkoxysilane, absorption and interaction of water vapor with the formation of hydrolysis products occurs. Dried organic alcohol is returned to the alkoxysilane production step for reaction with silicon metal.

 Thus, the proposed method allows to obtain a valuable commercial product - silica sol and return almost all of the dried organic alcohol to the reaction with metallic silicon. As a result, the need for dried organic alcohol used as raw material is reduced to 3% of the total demand.

 All this allows us to reduce the cost of high purity monosilane and increase its competitiveness in the world market. In particular, the preparation of polycrystalline silicon by thermal decomposition of monosilane is appropriate for its widespread use in the creation of solar cells.

 The proposed method is illustrated by the following examples.

 Example 1

Silicon metal powder with a content of the latter of 98.2 wt.% And a dispersion of not more than 100 mesh, mixed with copper chloride (5% by weight of silicon metal) and heat-treated in a neutral atmosphere, is dispersed in dodecaethoxypentasiloxane and, after addition of aluminum powder, react with anhydrous ethyl alcohol at a temperature of 200 ^o C. Vapors formed ethoxysilanes and unreacted alcohol are condensed under cooling and condensation of the resulting product was distilled ethyl alcohol, which is recycled and the reaction with silicon.

A mixture of ethoxysilanes containing 87 wt.% Triethoxysilane, cleaned of impurities by bubbling ammonia gas, followed by filtration separation of solid products. Purified ethoxysilanes are subjected to continuous catalytic disproportionation in the presence of a one percent solution of sodium triethoxysilanolate in tetraethoxysilane as a catalyst. The weight ratio of catalyst and ethoxysilanes fed to the disproportionation is 1:15. The process temperature is 20 ^o C, and the residence time of the reaction mass in disproportionation is 20 hours. Under these conditions, the conversion of triethoxysilane is 98.5%.

The released monosilane is cleaned of impurities of organoelement compounds by absorption of tetraethoxysilane cooled to a temperature of minus 80 ^o C in a continuous mode at a contact time of 30 minutes. In the absorption process, a countercurrent of gaseous monosilane and chilled tetraethoxysilane is carried out. The temperature of monosilane decreases from 20 ^o C to minus 80 ^o C, and tetraethoxysilane rises from minus 80 ^o C to 20 ^o C when the temperature difference between monosilane and tetraethoxysilane is not higher than 5 ^o C at any point of contact.

 The content of organoelement compounds in monosilane, controlled by the chromatographic method according to the carbon content in terms of methane, after absorption purification is 0.001 vol.%. After the final adsorption treatment on activated carbon, the quality of monosilane was evaluated by the specific resistance of the poly- and single-crystal silicon rods obtained from it. The specific resistance of single-crystal silicon during crucible-free zone melting after one and seven passes of the zone is 900 and 28000 Ohm • cm, respectively.

 Ethyl alcohol is added to the liquid disproportionation products in an amount of 0.85% by weight of the liquid products to break the silicon-hydrogen bond in unreacted ethoxysilanes, after which tetraethoxysilane is isolated from them until the remaining active catalytic part reaches the desired composition, i.e. a one percent solution of sodium triethoxysilane in tetraethoxysilane, after which it is returned to disproportionation.

The isolated tetraethoxysilane with an admixture of ethyl alcohol (0.43%) is sent to the top of the packed absorption-reaction column to dry the ethyl alcohol formed after the complete hydrolysis of tetraethoxysilane with water. After this, the partially hydrolyzed tetraethoxysilane is completely hydrolyzed at a temperature of 95 ^° C. and a colloidal dispersion of silicon dioxide is formed, which is accompanied by the desorption of ethyl alcohol.

 The obtained colloidal dispersion of silica sol is sold as a commercial product, and ethyl alcohol is fed to the lower part of the absorption reaction column for drying, after which it is returned to the stage of interaction with metallic silicon. The amount of alcohol returned to the process is 96% of the reaction of the alcohol with metallic silicon during the production of ethoxysilanes. The conditions of the experiment and its results are presented in table. 2.

 Example 2

The process is carried out in the same manner as in example 1, except that a one percent solution of sodium ethyl acetate in tetraethoxysilane is used as a disproportionation catalyst for triethoxysilane, and the disproportionation is carried out at a temperature of minus 20 ^o C and a contact time of 50 hours. Absorption of impurities from monosilane is carried out at the contact time between monosilane and tetraethoxysilane for 60 minutes

 The conditions of the experiment and its results are given in table. 2.

 Example 3

The process is carried out in the same manner as in example 1, 2, except that the disproportionation is carried out at a temperature of 40 ^° C and a contact time of 1 hour. Absorption of impurities from monosilane by chilled tetraethoxysilane is carried out at a contact time between monosilane and tetraethoxysilane for 5 minutes. The conditions of the experiment and its results are presented in table. 2.

 Example 4

 The process is carried out in the same manner as in example 1, except that the interaction of metallic silicon is carried out with methyl alcohol, resulting in a mixture of methoxysilanes and unreacted methyl alcohol. The conditions of the experiment and its results are presented in table. 2.

 As can be seen from the examples and description of the invention, the proposed method in comparison with the prototype has the following technical advantages.

1. The method allows to increase the purity of the resulting target monosilane due to:
- separation from the products of the interaction of silicon with alcohol of unreacted (anhydrous) alcohol at a temperature close to room temperature and its return to the process;
- return of the active catalytic part to the disproportionation of alkoxysilanes;
- the implementation of preliminary absorption purification of monosilane with a cooled absorbent followed by adsorption purification;
- return of anhydrous organic alcohol formed during the complete hydrolysis of tetraalkoxysilane and its subsequent purification to the reaction with silicon.

2. The method allows to reduce the consumption rates of the raw materials used due to:
- return of alcohol from all stages to the beginning of the process;
- return of the active catalytic part;
- increase the yield of the target monosilane by reducing the formation of by-products at all stages.

3. The method provides a reduction in energy consumption by:
- separation of unreacted alcohol from the products of the interaction of silicon with alcohol;
- carrying out catalytic disproportionation at a temperature close to room temperature in a continuous mode;
- the introduction of the stage of preliminary absorption purification of monosilane.

 4. The proposed method is non-waste and environmentally friendly.

 Reducing the consumption of raw materials, energy consumption, increasing the yield of the target monosilane and obtaining, along with monosilane, a valuable commercial product - silica sol - can significantly reduce the cost of monosilane, which makes it possible to obtain polycrystalline silicon of the required quality suitable for the manufacture of the entire spectrum of semiconductor devices, including solar panels.

 Thus, the combination of the proposed techniques and modes of the method for producing monosilane of high purity allows us to solve the problem.

Claims (5)

1. The method of producing high purity monosilane by the interaction of metallic silicon with an organic alcohol in the presence of a catalyst at elevated temperature, the separation of the corresponding alkoxysilanes from the reaction products, their purification, catalytic disproportionation with the formation of gaseous monosilane purified by sorption of impurities, and the formation of liquid disproportionation products, including tetraalkoxysilane, active catalytic part and an admixture of compounds with a silicon-hydrogen bond, characterized in that unreacted alcohol is isolated from the reaction products of metallic silicon with an organic alcohol, which is returned to the reaction with silicon, and the resulting mixture of trialkoxysilane with other alkoxysilanes is subjected to catalytic disproportionation at a temperature of minus 20 - plus 40 ^o С and the residence time of the reaction mixture 1 - 50 h with the release of gaseous monosilane, pre-cleaned by absorption of impurities, and organic alcohol is added to the liquid disproportionation products, tet ralkoxysilane, which is subjected to complete hydrolysis with the formation of commercial silica sol and organic alcohol, which, after drying, are sent to react with silicon.  2. The method according to claim 1, characterized in that the organic alcohol is added to the liquid disproportionation products in an amount ensuring the absence of compounds with a hydrogen-hydrogen bond in the reaction mass, after which tetraalkoxysilane is isolated, and the remaining active catalytic portion is returned to disproportionation.  3. The method according to p. 1 or 2, characterized in that the drying of the organic alcohol formed during the complete hydrolysis of tetraalkoxysilane is carried out isolated from the liquid disproportionation products of tetraalkoxysilane, which is first sent to dry the alcohol, and then to complete hydrolysis. 4. The method according to claim 1, characterized in that the preliminary absorption of impurities from monosilane is carried out by an absorbent cooled to a temperature above its freezing point in countercurrent mode at a contact time of 5-60 minutes and a temperature difference between monosilane and absorbent not more than 5 ^o C .  5. The method according to claim 1 or 4, characterized in that as the absorbent take tetraalkoxysilane isolated from liquid disproportionation products.

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