JPH0352407B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for chlorinating silicon or silicon alloys to obtain hexachlorodisilane.
Specifically, the method involves lowering the high-boiling components higher than octachlorotrisilane in the product obtained by chlorinating silicon or silicon alloys in two steps, namely () first, by-product high-boiling components; The present invention relates to a method for producing hexachlorodisilane in good yield by lowering the components through a heating reaction treatment and () further lowering the remaining components with a higher boiling point than octachlorotrisilane with chlorine. In recent years, with the development of the electronics industry, the demand for semiconductor silicon such as polycrystalline silicon or amorphous silicon has increased rapidly.
Trichlorosilane, silicon tetrachloride, and monosilane are mainly used as raw materials for manufacturing silicon for semiconductors. On the other hand, the use of higher chlorinated silicon has also been developed; for example, the production of amorphous silicon or polycrystalline silicon by thermal decomposition of SinCl _2o+2 (n is a number of 2 or more) (Belgium Patent No. 889523);
Furthermore, the production of disilane by reduction of hexachlorodisilane (Journal of the Chemical Physics J.Chem.Phys.) vol.22, P939
(1954)), Journal of Inorganic and Nuclear Chemistry (J.Inorg.
Nucl.Chem.) vol.25, P307 (1963)) etc. have been reported. However, disilane Si ₂ H ₆ can be produced by thermal decomposition (CVD),
When an amorphous silicon film is formed by glow discharge decomposition (GD), the deposition rate of the film formed on the substrate is much higher than that of monosilane _SiH4 , and the film has excellent electrical properties. It is expected that demand will increase significantly in the future as a raw material for semiconductors for solar cells.
-83929). Conventionally, hexachlorodisilane is obtained by chlorinating silicon or a silicon alloy with chlorine. However, in this case, a considerable amount of silicon tetrachloride and higher chlorides higher than octachlorotrisilane are inevitably produced as by-products, so there is a problem that the yield of the target hexachlorodisilane is low. Further, it is possible to dispose of this higher chloride by treating it with alkaline water or the like as it is, but in that case, there is a drawback that the production cost increases because of the provision of disposal treatment equipment. Furthermore, higher chlorides such as octachlorotrisilane or higher easily produce by-product solids that are presumed to be silicooxalics, but since these solids are extremely sensitive to heat or shock, only a small amount of It is an extremely dangerous and troublesome substance that can violently ignite and burn just by being subjected to a shock, which poses a major safety problem. The present inventors have demonstrated that higher chlorides higher than octachlorotrisilane, which are produced as by-products during the production of hexachlorodisilane, are () first subjected to a heat reaction treatment, and () subsequently rechlorinated with chlorine to be lowered in two steps, and a considerable portion of the chlorides are lowered in two steps. The present invention was completed based on the discovery that hexachlorodisilane can be produced in good yield by recovering it as hexachlorodisilane. That is, the present invention provides (1) a method for producing hexachlorodisilane by chlorinating silicon or a silicon alloy at high temperature, in which () a high-boiling component higher than octachlorotrisilane, which is a by-product of the chlorination reaction, is used alone or diluted; A hexachlorodisilane-containing product is obtained by heat reaction treatment at a temperature range of 300 to 700°C in the presence of a reagent, and the hexachlorodisilane is separated and recovered from the obtained product. The remaining high-boiling components higher than octachlorotrisilane are further mixed with chlorine at 200 to 700℃.
The present invention relates to a method for producing hexachlorodisilane, characterized in that the reaction is reduced in a temperature range of 100 to 100%, and the hexachlorodisilane is recovered as hexachlorodisilane. The present invention will be explained in detail below. The method for producing hexachlorodisilane, which is the object of the present invention, is a method known per se, and is obtained by chlorinating silicon or a silicon alloy, particularly calcium silicide, iron silicide, and magnesium silicide, with chlorine. It is. The above chlorination temperature is usually 300 to 500 for silicon.
℃, in the case of silicon alloy it is 150 to 350℃,
In this case, in order to improve the yield of hexachlorodisilane, a method of supplying chlorine along with a diluent such as silicon tetrachloride may also be adopted. The content of hexachlorodisilane in the product solution can be changed to some extent depending on the reaction temperature, chlorine concentration, contact time, etc., but normally, in addition to silicon tetrachloride and hexachlorodisilane, a considerable amount of high-boiling components such as octachlorotrisilane or higher are unavoidable. to generate. High-temperature chlorination products of silicon or silicon alloys such as those mentioned above (the chlorination products leave the chlorination reactor as a mixed gas of silicon tetrachloride, hexachlorodisilane, and other by-product higher chlorides, and are cooled and condensed). silicon tetrachloride and hexachlorodisilane are separated from The most common separation method is distillation separation under normal pressure or reduced pressure, but any other unit operation such as extraction separation with an appropriate solvent or crystallization separation after cooling can be adopted. Of course. In this way, the present invention eliminates by-product high-boiling components such as octachlorotrisilane or higher, which remain after separating hexachlorodisilane and the like from chlorinated products and which have many problems in terms of production costs and safety.
It is converted into hexachlorodisilane (and safe silicon tetrachloride) through a step-down process and recovered. () First, in the first step, a high-boiling component higher than octachlorotrisilane is subjected to a heat reaction treatment in the temperature range of 300 to 700°C, either alone or in the presence of a diluent, to convert at least a portion of it into hexachlorodisilane. A hexachlorodisilane-containing product is obtained by lowering. The higher chloride (high boiling component) of octachlorotrisilane or higher to be heat treated is a higher chlorinated silicon represented by the following formula () SikCl _2k+2 (k is an integer of 3 or more) (), for example: , octachlorotrisilane, decachlorotetrasilane, dodecachloropentasilane, tetradecachlorohexasilane and the like. The heat treatment temperature is preferably from 300 to 700℃
The temperature is 400 to 600°C. Below 300℃, the conversion rate to hexachlorodisilane (lowering rate) is very slow, and above 700℃, silicon tetrachloride is converted to hexachlorodisilane.
This is undesirable because the rate of conversion to SiCl ₄ increases and chlorine gas is produced by decomposition of higher chlorinated silicon. In addition, in order to improve the fluidity of the high-boiling component to be treated and make it easier to handle, it can be diluted with a diluent that is inert to the reaction. As such a diluent, a chlorine compound, particularly silicon tetrachloride, is used, but other materials such as fluorocarbons, methylene chloride, and silicone oil can also be used. In addition, in order to keep the lowering reaction rate within a controllable range and accurately control the reaction heat,
The above reaction may be carried out in an atmosphere of an inert gas such as helium, neon, argon, xenon, nitrogen or the like. Of course, since the silicon tetrachloride is present as a gas under the reaction conditions, it also acts as an inert gas. In addition, since the lowering reaction rate is very fast, 1
The reaction is completed in a reaction time of about seconds to 100 seconds (average residence time in the case of continuous flow reaction). The hexachlorodisilane-containing product obtained through the lowering treatment is obtained as a high-temperature gas, which is cooled and condensed, and the nuclear condensation product is separated, for example, by distillation, to recover the desired hexachlorodisilane together with silicon tetrachloride. . () Next, high-boiling components higher than octachlorotrisilane remaining unreacted or not completely lowered in the condensed product (hereinafter referred to as higher chloride) from which hexachlorodisilane was separated in the first step are further chlorinated. Hexachlorodisilane is obtained by reducing the reaction temperature in a temperature range of 200 to 700°C. The lowering reaction temperature using chlorine is 200 to 700℃.
Preferably it is 300-500°C. Below 200℃, the reaction will not proceed effectively or
When the temperature exceeds 700°C, the conversion rate to silicon tetrachloride increases and the conversion rate of hexachlorodisilane becomes very small. The lowering reaction using chlorine in the second stage is accompanied by a much stronger heat generation than the heating reaction treatment in the first stage. Therefore, in order to accurately control the heat of reaction, it is desirable to dilute the higher chloride to be treated with a diluent such as silicon tetrachloride as used in the first step. Alternatively, it is preferable not to use chlorine gas as it is, but to dilute it with an inert gas such as helium, neon, or nitrogen. Note that the chlorine used in the lowering reaction can be used as is, sold in cylinders, etc., but it is best to use it after thoroughly dehydrating it by treating it with concentrated sulfuric acid, silica gel, etc. More preferred. The amount of chlorine gas supplied to the higher chloride is 0.1 to 10 parts by weight, preferably 0.2 to 1.0 parts by weight, per 1 part by weight of the higher chloride. This is
If it is less than 0.1 part by weight, the lowering reaction will not proceed sufficiently,
Moreover, if it exceeds 10 parts by weight, the conversion rate to silicon tetrachloride becomes large and the recovery rate of hexachlorodisilane decreases, which is not preferable. The reaction rate of the second step of the lowering reaction using chlorine itself in the present invention is thought to be very high, but even when it is a gas-liquid reaction, the solubility of chlorine gas in higher chlorides is quite high (10% It is assumed that the reaction can be generally treated as a homogeneous phase reaction system and the reaction will be completed in a short period of time. In addition, in practice
When the lowering reaction is carried out at 250°C or higher, especially at 350 to 450°C, a considerable portion of the higher chlorides will vaporize at this temperature, so the lowering reaction will essentially be carried out at a temperature of 350 to 450°C. It is presumed that most of the reaction takes place as a gas phase reaction between gas and chlorine gas. As described above, silicon tetrachloride and hexachlorodisilane obtained in the second step of the lowering reaction with chlorine are obtained as a mixture (mixed gas under normal reaction conditions), and this is further separated by distillation etc. to obtain hexachlorodisilane. Disilane (and silicon tetrachloride) is recovered. The amount of distillation residue remaining after recovering and separating hexachlorodisilane in this final stage (second stage) is extremely small, and can be treated with alkaline water or the like and disposed of as is.
It is desirable to mix and circulate at least a part of the treatment liquid in the first or second stage. Next, preferred embodiments for carrying out the present invention will be described with reference to the accompanying drawings. FIG. 1 is a flow sheet showing one example of a preferred embodiment of the present invention, and in the figure, 10 is a chlorination reactor. The chlorination reactor 10 may be a fixed bed type, a fluidized bed type, or a powder type, as long as it can effectively carry out a solid-state reaction, is equipped with cooling and/or heating means, and can control the reaction temperature. It may be of any type, such as a body-stirring bed type. That is, it may be of a vertical tank type, or of various types such as a horizontal groove type, cylindrical type, or U-shape. In particular, the latter type is more preferable in terms of operability and ease of removing the solid residue after the reaction is completed. The stirring means provided in the reaction vessel include propeller type, turbine type, paddle type, spiral type, double spiral type, and screw type stirrers. Since it also has a body transport function, it is particularly suitable for continuous operations in which particles are continuously supplied and continuously extracted. Metal particles F such as calcium silicon, etc., which are raw materials, are charged into the chlorination reactor 10 through the pipe 20.
Normally, sand particles with a specific gravity of 2 to 5 and a particle size of about 5 to 300 mesh are preferably used, but these may be a mixture or a mixture of other metals such as manganese and dissolved. . In the case of large particles, it is preferable to crush them to a suitable particle size before use. The chlorine gas CL is supplied to the reactor through the pipe 30, but the chlorine gas CL to be sent to the reactor is
It is usually sold in cylinders and the like, and after being sufficiently dehydrated, preferably by treatment with concentrated sulfuric acid, silica gel, etc., it can be used as it is without further purification. However, in order to keep the reaction rate within an easily controllable range and accurately control the reaction heat, chlorine gas is not used as it is, and is inert to the reaction system such as helium, neon, argon, xenon, nitrogen, silicon tetrachloride, etc. diluted with a gas containing 1 to 90 mol of chlorine.
%, preferably about 3 to 70 mol% of the chlorine-containing gas. The chlorine-containing gas may be supplied to the lower or middle part of the fixed bed according to a conventional method, and the gas may be allowed to rise in the fixed bed to carry out the solid reaction at once, but the speed of this reaction is very fast. Therefore, for example, if a horizontal reactor is used and the bed height is not that large,
Even if a gas is supplied to the surface of the fixed layer and the reaction is carried out while flowing the gas along the surface of the fixed layer, hexachlorodisilane is produced at a rate sufficiently high for practical use. As mentioned above, when using a silicon alloy, the chlorination reaction temperature is 300 to 600℃, preferably 300℃.
From 130 to 400℃ for silicon alloys.
The temperature is preferably 150 to 350°C. After the reaction, the hexachlorodisilane gas produced flows out of the reactor 10 as a high-temperature gas mixed with by-product gases such as silicon tetrachloride, octachlorotrisilane, and decachlorotetrasilane, and is led to the condenser 50 through the pipe 40, where it is Cooled and condensed. For this purpose, a multi-tubular condenser may be used as the condenser 50, and the produced gas may be cooled and condensed by an indirect heat exchange operation through the multi-tubular walls, or more preferably, the produced gas may be Alternatively, a cooling medium (hereinafter referred to as a refrigerant) may be brought into direct contact to perform a direct heat exchange operation to cause cooling and condensation. The produced condensate passes through a pipe 60 to a distillation column () 7
0, where it is distilled under normal pressure or reduced pressure and separated into higher chlorides such as chlorine, silicon tetrachloride, hexachlorodisilane, and octachlorotrisilane. Chlorine, silicon tetrachloride, and hexachlorodisilane are distilled off from the top of the column, and higher chlorides are taken out as bottom residue from the bottom of the column. Note that a part of silicon tetrachloride is supplied through pipe 80, and chlorine is supplied through pipe 9.
0 are recycled and reused in the reactor 10, respectively. Hexachlorodisilane is extracted through piping 100, and another portion of silicon tetrachloride is extracted through 110 as well. The distillation column may be a packed column filled with Raschig rings, Burr saddle rings, etc., or may be a bubble cap column. High-boiling components higher than octachlorotrisilane, which are discharged as residue from the can, are sent to a heat treatment lowering reactor (converter) 130 via a pipe 120. At that time, it is preferable to dilute with a diluent such as silicon tetrachloride from the pipe 140. The structure of the converter 130 is as follows:
A tube type or a stirred bed type can be used as long as it can effectively perform heat treatment of the raw material gas, is equipped with cooling and/or heating means, and can control the treatment temperature (reaction temperature) within the desired range. It can be of any type, such as a type. For example, in the case of a tubular reactor, it may be used as is as an empty column (empty tube), or it may be filled with packing materials such as α-alumina beads, Raschig rings, and Burl saddle rings for heat exchange operations. It is also preferable to make it more effective. The hexachlorodisilane (and silicon tetrachloride)-containing product thus obtained as a result of the first stage reaction is supplied to a distiller ( ) 160 through a pipe 150, and silicon tetrachloride and hexachlorodisilane are separated and recovered through pipes 170, 180, and 100, respectively. be done. The distillate (higher chloride) from the distillation column is diluted with silicon tetrachloride from pipe 190 and passed through pipe 200 to chlorination lowering reactor 2.
Sent to 10. Here, it is reacted with chlorine fed through the pipe 215 to lower its grade. The chlorination/lowering reactor 210 may be of a tubular type or a stirred bed type as long as it can effectively perform a gas-liquid reaction, is equipped with cooling and/or heating means, and can control the reaction temperature within a desired range. It can be of any type, such as a type. For example, in the case of a tubular reactor, it may be used as is as an empty column (empty tube), or it may be filled with packing materials such as α-alumina beads, Raschig rings, and Burl saddle rings to improve gas-liquid contact. It is also preferable to make it effective. Further, as a modified type of empty tower, it is possible to use a wet wall tower or a bubble tower. As for the mode of gas-liquid contact, both may be brought into contact in cocurrent flow, or may be brought into contact in countercurrent flow. The product from the chlorination-lowering reactor 210 is sent to a distillation column ( ) 230 via a pipe 220, and the produced silicon tetrachloride and hexachlorodisilane are separated and recovered. These are taken out by pipes 240 (and 110) and 250 (and 100), respectively. Note that some of the silicon tetrachloride is in the pipe 190.
for example, to the reactor 210. A small amount of bottoms from the distillation column is sent via line 260 to an alkaline treatment tank 270 and treated with alkaline water supplied via line 290, along with the reaction residue from reactor 10 via line 280. As described above, the present invention () first lowers by-product high-boiling components higher than octachlorotrisilane in the chlorinated product obtained by the chlorination reaction of silicon or silicon alloy by heat treatment, (2) It is characterized by employing a two-step lowering operation in which it is subsequently reacted with chlorine to lower the grade, and is recovered as hexachlorodisilane. That is, in the present invention, () the by-product high-boiling components are first treated by a lowering reaction using only the first-stage heat treatment, which has a relatively small amount of reaction heat; , the reaction heat can be easily controlled and the process can be carried out in a short time. The majority of the total recovered hexachlorodisilane is then recovered in this first stage. () Subsequently, the remaining high-boiling components that could not be lowered by heat treatment alone are lowered using chlorine, but since a considerable amount of by-product high-boiling components have already been treated in the first step. This lowering reaction with chlorine, which is accompanied by significant heat generation, can be treated relatively easily, and a significant portion of the by-product high-boiling components that were not converted and recovered into hexachlorodisilane in the first stage are recovered in the second stage. Therefore, the overall recovery rate of hexachlorodisilane is very high. On the other hand, it is of course possible to lower the by-product high-boiling components higher than octachlorotrisilane in the chlorinated product by directly using chlorine without performing the first-stage heating lowering treatment. In this case, due to the above-mentioned significant heat generation, it becomes difficult to control the lowering reaction, and not only the throughput becomes extremely low, but it is also inevitable that the reaction often goes out of control. In addition, when the grade is lowered using only chlorine, the recovery rate of hexachlorodisilane is essentially much smaller than in the two-step grade lowering of the present invention. Hereinafter, embodiments of the present invention will be explained with reference to Examples, but these are merely illustrative, and Patent Law 70
The technical scope of the present invention as defined in Article 1 shall not be construed as being limited thereby. Example 1 (1) Chlorination reaction of calcium silicide Commercially available calcium silicide (manufactured by Nippon Heavy Chemical Co., Ltd.,
48-200 mesh, Si content 61wt%) 6.0Kg
(Si130.3 g-atm) was charged into a 200 ml fixed bed reactor in an argon atmosphere. Next, the temperature was raised to 180℃, and chlorine gas (manufactured by Tsurumi Soda Co., Ltd., purity 98%, the same hereinafter, chlorine flow rate 25/min) was added to argon (flow rate 20/min).
chlorination reaction was carried out for 12 hours. The produced gas was cooled to about 0°C, and 15.6 kg of produced liquid was collected. The silicon content in the produced liquid is
It was 113.7 gmol, which corresponds to a conversion rate of 87% (based on Si atoms charged, the same applies hereinafter). Next, this product liquid was subjected to simple distillation under normal pressure to obtain 4.0 kg (23.5 g-mole, yield 18.1%) of silicon tetrachloride, and further to simple distillation under reduced pressure (125 mmHg) to obtain 5.8 kg (21.6 g) of hexachlorodisilane. −mole, yield
33.1%) and octachlorotrisilane 4.0Kg
(10.9g-mole, yield 25.0%)
Obtained 5.7Kg. (2) Lowering reaction by heating reaction treatment A mixed solution (residue concentration: 30 wt%) of 5.7 kg of pot residue () diluted with 13.2 kg of silicon tetrachloride was heated at 500°C in advance at a flow rate of 125 g/min under an argon atmosphere.
The mixture was passed through a reaction tube (a glass tube with an inner diameter of 100 mm and a length of 980 mm filled with α-alumina beads with an average particle size of 5 mm) and a heating lowering reaction was carried out for 150 minutes. The generated gas is cooled to 0℃, and the generated liquid is
Collected 18.4Kg. By simple distillation of the product liquid as described above, 13.5 kg of silicon tetrachloride (net production amount 1.8 g-mole, yield 1.4%), 4.7 kg of hexachlorodisilane (17.5 g-mole, yield 26.9%) and octachlorotrisilane were obtained. Silane 0.5Kg (1.4g−mole,
0.7 kg of residue () containing 3.2% yield) was obtained. (3) Lowering reaction using chlorine A mixed solution of 0.7 kg of pot residue () diluted with 6.0 kg of silicon tetrachloride (concentration of pot residue 10.5 wt%) and chlorine,
The mixture was passed through the reaction tube used in (2) at a flow rate of 140 g/min and 25 g/min, respectively, and a chlorination and lowering reaction was carried out at 450°C for 48 minutes. The produced gas was cooled to 0°C, and 7.5 kg of produced liquid was collected. Silicon tetrachloride is obtained by simple distillation of the product liquid as described above.
7.0Kg (net production 5.9g-mole, yield 4.5%),
0.3 kg (1.1 g-mole, yield 1.7%) of hexachlorodisilane and 0.1 kg of residue () were obtained. The yield of hexachlorodisilane obtained by the above three reactions based on the charged calcium silicide is
62% (based on Si charged atoms), and the amount of high-boiling components higher than octachlorotrisilane that ultimately remained was approximately 0.017 g per 1 g of charged calcium silicide. Comparative Example 1 A mixed solution (residue concentration: 30 wt%) obtained by diluting 5.7 kg of pot residue () having the same composition as that obtained in Example 1 (1) with 13.2 kg of silicon tetrachloride (concentration of pot residue 30 wt%) and chlorine were each added under an argon atmosphere. Same as used in (2) and (3) of Example 1 at flow rates of 125 g/min and 64 g/min.
The reaction was conducted through a reaction tube set at 450°C (the flow rate of the bottom () was the same as in (2) of Example 1. The flow rate ratio of the bottom () and chlorine was as in (3) of Example 1). same). Ten minutes after the start of the reaction, the reaction temperature rose rapidly due to reaction heat, so the reaction was stopped. Comparative Example 2 In Comparative Example 1, it was impossible to continue the reaction due to the reaction heat, so next the reaction was carried out by lowering the feed rate of the residue. That is, 5.7 kg of the pot residue () having the same composition as that obtained in (1) of Example 1 was mixed with 48.8 kg of silicon tetrachloride.
Kg diluted mixed solution (pot residual concentration 10.5wt%),
The same conditions as in Example 1 (3) (however, the reaction time was 390
The lowering reaction using direct chlorine was carried out in 20 min). By distilling and separating 61.2 kg of the resulting product liquid, 51.8 kg of silicon tetrachloride (net production amount 17.4 g-mole, yield 13.4%) and 2.5 kg of hexachlorodisilane (9.3 g-mole) were obtained.
0.8 kg of the residue () containing 0.5 kg (1.4 g-mole, yield 3.2%) of octachlorotrisilane were obtained. At this stage, the yield of hexachlorodisilane based on the charged calcium silicide is 47% (based on the charged Si atoms). Next, add 0.8 kg of the above pot residue () to 1.9 kg of silicon tetrachloride.
Kg diluted mixed solution (pot residual concentration 30wt%),
The same conditions as in Example 1 (2) (however, the reaction time was 22
The heating reaction treatment was carried out for 1 minute). Obtained product liquid
Silicon tetrachloride 1.9 by distilling and separating 23g
Kg, hexachlorodisilane 0.6Kg (2.2g−mole,
Yield: 3.4%) and 0.2 kg of pot residue were obtained. The yield of hexachlorodisilane in the final stage was 51% based on the calcium silicide charge (Si
The amount of high-boiling components higher than octachlorotrisilane that ultimately remained was approximately 0.034 g per 1 g of calcium silicide. Example 2 In (2) of Example 1, the heating reaction treatment was carried out at 450°C.
The experiment was conducted in the same manner as in Example 1, except that the experiment was carried out in Example 1. The results are shown in Table 1. Example 3 In (2) of Example 1, 5.7 kg of pot residue having the same composition as the pot residue () was mixed with silicon tetrachloride at a flow rate of 5.7 kg.
An experiment was conducted in exactly the same manner as in Example 1, except that the reaction was carried out at 500° C. for 57 minutes at a rate of 0.1 Kg/min. The results are shown in Table 1.

【table】

【table】 [Brief explanation of drawings]

FIG. 1 is a flow sheet diagram showing a specific embodiment for carrying out the present invention.

Claims (1)

[Claims] 1. A method for producing hexachlorodisilane by chlorinating silicon or silicon alloy at high temperature,
() Hexachlorodisilane-containing product is obtained by heat-reacting a high-boiling component higher than octachlorotrisilane, which is a by-product of the above chlorination reaction, alone or in the presence of a diluent in a temperature range of 300 to 700°C;
The hexachlorodisilane is separated and recovered from the obtained product, and () high-boiling components higher than octachlorotrisilane that still remain in the product are further degraded by reaction with chlorine in a temperature range of 200 to 700°C. A method for producing hexachlorodisilane, which comprises recovering hexachlorodisilane at least as hexachlorodisilane. 2. The method according to claim 1, wherein the silicon alloy is calcium silicide, iron silicide or magnesium silicide. 3. The method according to claim 1, wherein the diluent is silicon tetrachloride.