Method for producing purified trichlorosilane

Abstract

The purpose of the present invention is to highly reduce the concentration of dimethyldichlorosilane, isopentane or the like, which is an impurity having a boiling point close to that of trichlorosilane, in trichlorosilane that is a raw material for producing polycrystalline silicon or the like. A method for producing purified trichlorosilane, which is characterized in that crude trichlorosilane containing at least isopentane and methyldichlorosilane is supplied to a first distillation column, and distillation purification is performed while discharging a low-boiling-point fraction in which the concentration of isopentane is 150 times or more higher than the concentration in the crude trichlorosilane. The trichlorosilane having a reduced isopentane concentration is extracted from the bottom of the tower, then the obtained trichlorosilane having a reduced isopentane concentration is supplied to a second distillation tower, and distillation purification is performed in a state where a high-boiling-point fraction having a methyldichlorosilane concentration 1.5 times or more higher than the concentration in the crude trichlorosilane is discharged, and the trichlorosilane having a reduced isopentane concentration is extracted from the bottom of the second distillation tower. And distilling off the purified trichlorosilane from the top of the tower after the concentration of the methyl dichlorosilane is reduced.

Description

Technical Field

[0001] The present invention relates to a method for producing purified trichlorosilane, and more particularly to a method for purifying crude trichlorosilane containing at least isopentane and methyldichlorosilane by distillation to obtain purified trichlorosilane. Background Art

[0002] Conventionally, high-purity trichlorosilane (SiHCl3) has been used as a raw material for the production of polycrystalline silicon (Si). Polycrystalline silicon is used as a raw material for semiconductors or solar power generation wafers, for example.

[0003] As a method for producing trichlorosilane, a method of reacting a gas containing hydrogen chloride or silicon tetrachloride and hydrogen with metallurgical grade silicon is known. Specifically, the following methods are known: a method of hydrochlorinating metallurgical grade silicon with a gas containing hydrogen chloride at a temperature of 250°C or above, generally at 250 to 450°C; or a method of hydrochlorinating metallurgical grade silicon with a mixed gas of silicon tetrachloride and hydrogen at a temperature of 400°C or above, generally at 400 to 600°C. In addition, trichlorosilane is known to be produced as a by-product by hydrochlorinating metallurgical grade silicon with a mixed gas of chlorinated hydrocarbons and hydrogen.

[0004] The trichlorosilane produced by these methods is crude trichlorosilane mixed with various by-product impurities generated by side reactions accompanying the production of the trichlorosilane and side reactions caused by hydrochlorination of carbon impurities contained in the metallurgical grade metallic silicon as a raw material. Representative examples of by-product impurities include: impurities with lower boiling points than trichlorosilane (boiling point 31.8°C) to be purified, such as dichlorosilane (H2SiCl2: boiling point 8.4°C) and tetramethylsilane (Si(CH3)4: boiling point 27°C); impurities with higher boiling points than trichlorosilane, such as silicon tetrachloride (SiCl4: boiling point 57.7°C), trimethylchlorosilane ((CH3)3SiCl: boiling point 57°C), and dimethyldichlorosilane ((CH3)2SiCl2: boiling point 70°C); and impurities with boiling points close to trichlorosilane, such as isopentane (CH3CH2CH(CH3)2: boiling point 27.7°C) and methyldichlorosilane (CH3SiHCl2: boiling point 42°C). It should be noted that, in the present specification, the boiling point refers to the value at normal pressure (one atmosphere). Therefore, in order to remove the by-product impurities contained in such crude trichlorosilane, purification by distillation has been conventionally performed. (For example, paragraph 0028 of Patent Document 1, paragraph 0007 of Patent Document 2, and Figure 1 ).

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2018-052765

[0008] Patent Document 2: Japanese Patent Application Publication No. 2014-152093 Summary of the invention

[0009] Problems to be solved by the invention

[0010] According to the purification by distillation, among the by-product impurities contained in the crude trichlorosilane, the low-boiling-point impurities and the high-boiling-point impurities having a large boiling point difference with trichlorosilane can be removed without much difficulty in the form of the distillate and the bottom liquid from the distillation tower, respectively. However, it is obvious that the distillation purification is a separation method using the boiling point difference between substances, so if the substance has a small boiling point difference, that is, the by-product impurities contained in the crude trichlorosilane, the separation becomes insufficient for the substances shown as the impurities having a similar boiling point (for example, paragraph 0028 of Patent Document 1 and paragraph 0012 of Patent Document 2). In particular, the boiling point of isopentane is 27.7°C, and the boiling point of trichlorosilane is 31.8°C, which are very close, and the isopentane may also form an azeotrope with the chlorosilanes including the trichlorosilane, so it is quite difficult to purify them to a high degree.

[0011] In detail, in the patent document 2, the ( Figure 1 and paragraph 0045), a crude trichlorosilane having a high methyldichlorosilane content is subjected to a chlorine atom redistribution reaction including a specific step to obtain a fraction having a low methyldichlorosilane content. Then, the fraction having a low methyldichlorosilane content is distilled again to obtain trichlorosilane from which a large amount of the methyldichlorosilane has been removed (paragraph 0050). In this method, it is not clear whether the crude trichlorosilane contains the isopentane. In this method, it is true that the methyldichlorosilane may be removed to some extent, but if the isopentane is contained, it will basically not be removed but will remain in the purified trichlorosilane obtained.

[0012] In recent years, it has been increasingly desired to reduce the amount of carbon impurities in silicon single crystals, which are raw materials for manufacturing semiconductor devices, as they have an adverse effect on electrical properties. In this context, it is also required to reduce the content of hydrocarbons that cause such carbon impurities in trichlorosilane, which is a raw material for manufacturing polycrystalline silicon used to manufacture the silicon single crystals, as much as possible. However, it is difficult to fully remove the impurities with similar boiling points, especially isopentane, and this has become a major technical problem.

[0013] Solutions for solving problems

[0014] The present inventors have continued intensive research in view of the above technical problems and have found that the above technical problems can be solved by subjecting crude trichlorosilane containing at least isopentane and methyldichlorosilane to a specific two-stage distillation, thereby completing the present invention.

[0015] That is, the present invention is a method for producing purified trichlorosilane, characterized in that crude trichlorosilane containing at least isopentane and methyldichlorosilane is supplied to a first distillation tower, purified by distillation in a state where a low boiling point fraction having an isopentane concentration 150 times or more higher than that in the crude trichlorosilane is discharged, and trichlorosilane with a reduced isopentane concentration is extracted from the bottom of the tower. Subsequently, the obtained trichlorosilane with a reduced isopentane concentration is supplied to a second distillation tower, purified by distillation in a state where a high boiling point fraction having a methyldichlorosilane concentration 1.5 times or more higher than that in the crude trichlorosilane is discharged, and purified trichlorosilane with a reduced methyldichlorosilane concentration is distilled from the top of the tower.

[0016] Effects of the Invention

[0017] According to the method of the present invention, in crude trichlorosilane containing at least isopentane and methyldichlorosilane, impurities having similar boiling points and being difficult to separate from trichlorosilane, especially isopentane, can be reduced to a high degree by a simple method. As a result, the carbon impurity content of polycrystalline silicon produced using the obtained purified trichlorosilane as a raw material is extremely low, and high-purity silicon single crystals as raw materials for manufacturing semiconductor devices can be produced, which is extremely useful in industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a distillation flow chart showing a typical embodiment of the method for producing purified trichlorosilane in the present invention.

[0019] Figure 2 1 is a distillation flow chart showing the method for producing purified trichlorosilane carried out in Comparative Example 1. DETAILED DESCRIPTION

[0020] In the method of the present invention, the crude trichlorosilane used to produce purified trichlorosilane contains at least isopentane and methyldichlorosilane. Usually, the crude trichlorosilane is obtained by reacting the gas containing hydrogen chloride or silicon tetrachloride and hydrogen with metallurgical grade silicon.

[0021] Specifically, crude trichlorosilane obtained by the following methods is preferred: a method of subjecting metallurgical grade silicon and a gas containing hydrogen chloride to a hydrochlorination reaction at a temperature of 250° C. or higher, generally 250 to 450° C.; or a method of subjecting metallurgical grade silicon and a mixed gas of silicon tetrachloride and hydrogen to a hydrochlorination reaction at a temperature of 400° C. or higher, generally 400 to 600° C., etc. These crude trichlorosilanes are generally prepared by cooling the reaction gas generated by the above reaction to form a condensate, feeding the condensate to a crude trichlorosilane recovery tower, and separating and removing impurities having a higher boiling point than silicon tetrachloride, specifically metal chlorides such as ferric chloride, to obtain crude trichlorosilane.

[0022] Alternatively, crude trichlorosilane obtained by subjecting metallurgical grade silicon to a hydrochlorination reaction with a mixed gas of a chlorinated hydrocarbon and hydrogen may be preferably used as the crude trichlorosilane.

[0023] The crude trichlorosilane involved generally contains 10 to 40 mol%, preferably 20 to 30 mol% of trichlorosilane. On the other hand, the crude chlorosilane generally contains 30 to 200 ppb mol, more specifically 50 to 100 ppb mol of the isopentane as an impurity, and 1000 to 20000 ppb mol, more specifically 6000 to 10000 ppb mol of methyldichlorosilane. In addition, in addition to these impurities with similar boiling points, the crude trichlorosilane generally contains 0.1 to 10 mol%, more specifically 0.2 to 2 mol% of dichlorosilane as a low boiling point impurity, and generally contains 50 to 89.9 mol%, more specifically 70 to 80 mol% of silicon tetrachloride as a high boiling point impurity. It should be noted that the content of the above-mentioned components refers to the concentration of the above-mentioned components. Furthermore, tetramethylsilane and the like as low-boiling-point impurities and trimethylchlorosilane, dimethyldichlorosilane and the like as high-boiling-point impurities may be contained.

[0024] It should be noted that in the present specification, the concentrations of impurities such as silanes such as methyldichlorosilane, silicon tetrachloride, and dichlorosilane contained in the trichlorosilane and isopentane are values ​​measured by gas chromatography.

[0025] In the method of the present invention, the crude trichlorosilane containing at least isopentane and methyldichlorosilane is purified by a specific distillation in which the first distillation tower and the second distillation tower described below are combined. Each distillation tower used may be any of a plate type distillation tower and a packed type distillation tower. In the plate type, the actual number of layers is not particularly limited, and is, for example, 10 or more and 150 or less, and more preferably 20 or more and 100 or less. In the packed type, as fillers, for example, Raschig rings, Lessing rings, etc. can be cited. The operation of the distillation tower can be carried out in a batch type or a continuous type.

[0026] In the method of the present invention, crude trichlorosilane is first supplied to a first distillation column for purification. During the purification in the first distillation column, isopentane is distilled out of the impurities having similar boiling points. The greatest feature of the present invention is that the isopentane is distilled out before the methyldichlorosilane, which is another component of the impurities having similar boiling points, is distilled out. In other words, the isopentane is distilled out in the presence of the methyldichlorosilane. Thus, during the distillation in the first distillation column, a low boiling point fraction having an isopentane concentration 150 times or more higher than that in the crude trichlorosilane can be discharged, and trichlorosilane with a reduced isopentane concentration can be extracted from the bottom of the column. In the low boiling point fraction, it is particularly preferred that the isopentane concentration be 200 times or more and 300 times or less higher than that in the crude trichlorosilane.

[0027] Here, as described above, isopentane has a boiling point difference that is particularly small with respect to trichlorosilane even among impurities having a boiling point close to that of trichlorosilane, and also exhibits the property of forming an azeotrope with chlorosilanes, making separation in distillation particularly difficult. In view of this property, if distillation is carried out in the coexistence of methyldichlorosilane as described above, the removal of isopentane can be carried out at a higher level. Specifically, in crude trichlorosilane, even if the isopentane content (isopentane concentration) is 30 ppb mol or more, more specifically, 50 to 100 ppb mol, the isopentane content can be reduced to 20 ppb mol or less, more preferably to 10 to 19 ppb mol.

[0028] The difference in boiling point between isopentane and trichlorosilane is 4.1° C., and the difference in boiling point between methyldichlorosilane and trichlorosilane is 10° C. The concentration of isopentane in crude trichlorosilane is about 30 to 200 ppb mol, and the concentration of methyldichlorosilane in crude trichlorosilane is about 2000 to 20000 ppb mol, as described above.

[0029] When a substance containing a component to be purified, a component having a lower boiling point than the component to be purified, and a component having a higher boiling point than the component to be purified is distilled to obtain the component to be purified, which component to remove first (the order in which the distillation operation is performed) takes into consideration the difference in boiling points between the components to be removed (the low-boiling-point component and the high-boiling-point component) and the component to be purified and the content of the components to be removed.

[0030] In consideration of the efficiency and cost of the distillation operation, generally, among the components to be removed, a distillation operation for removing a component having a large boiling point difference with the component to be purified is performed, followed by a distillation operation for removing a component having a small boiling point difference. Similarly, generally, among the components to be removed, a distillation operation for removing a component having a large content is performed, followed by a distillation operation for removing a component having a small content. In particular, in a distillation column for separation into a column top component and a column bottom component, it is preferred that a distillation operation for removing a component having a large content is performed first.

[0031] Therefore, when crude trichlorosilane is distilled to separate isopentane and methyldichlorosilane from the crude trichlorosilane, methyldichlorosilane is first removed by distillation, and then isopentane is removed by distillation, both from the viewpoint of boiling point difference and from the viewpoint of content.

[0032] However, when the distillation removal of methyldichlorosilane is first performed and then the distillation removal of isopentane is performed, the isopentane concentration in trichlorosilane is obtained as a value that is reduced to a certain extent in the analysis by the gas chromatography. However, the removal effect of carbon impurities derived from the isopentane is not actually sufficient, and the following phenomenon occurs: the content of carbon impurities in polycrystalline silicon produced using the purified trichlorosilane is not sufficiently low. That is, when the distillation removal of methyldichlorosilane is performed first, the isopentane reduction effect obtained by performing the distillation removal operation in the coexistence of the above-mentioned isopentane and methyldichlorosilane cannot be obtained.

[0033] The reason has not been completely determined, but the inventors of the present invention speculate that methyldichlorosilane has the effect of preventing isopentane from forming an azeotrope with the chlorosilanes. That is, if the methyldichlorosilane coexists, isopentane exists independently in trichlorosilane, and if the isopentane is distilled off, it can be purified to a higher degree. On the other hand, when the methyldichlorosilane is removed first, it can be considered that the isopentane forms an azeotrope with various silanes headed by the trichlorosilane. Among them, as for the azeotrope of isopentane and trichlorosilane, its boiling point is closer to the boiling point of trichlorosilane than the boiling point of isopentane.

[0034] Here, it is generally difficult to detect such an azeotrope in analysis using gas chromatography used for purity measurement of trichlorosilane. Therefore, it is considered that even if purified trichlorosilane is judged to have a significantly reduced isopentane concentration by analysis using gas chromatography, a large amount of isopentane actually remains in the form of such an azeotrope. Thus, even if polycrystalline silicon is produced using such purified trichlorosilane, the obtained polycrystalline silicon contains a large amount of carbon impurities derived from the isopentane azeotrope, resulting in that the carbon content (carbon concentration) cannot be sufficiently reduced.

[0035] The reason for this is that, as described above, in the gas chromatography column used for the purity measurement of trichlorosilane, although isopentane can be detected, isopentane azeotropes are difficult to detect. Therefore, it is considered that the isopentane azeotropes contained in the purified trichlorosilane and not detected are converted into carbon in the process of producing polycrystalline silicon using the purified trichlorosilane, and are detected as carbon impurities in the polycrystalline silicon.

[0036] In the distillation in the first distillation column, the discharge of the low boiling point fraction having an isopentane concentration 150 times or more higher than that in the crude trichlorosilane can be carried out by heating the crude trichlorosilane stored at the bottom and appropriately setting the distillation conditions to discharge the low boiling point fraction at the top or upper side of the column. For example, the temperature of the top or upper side of the column is set to about 50 to 53°C.

[0037] In the first distillation column, it is preferred that a part of the distillate from the top of the column is cooled and condensed and refluxed. The larger the reflux ratio at this time, the better the separation efficiency. However, the reflux ratio is appropriately set in consideration of productivity.

[0038] Next, in the first distillation tower, the trichlorosilane with a reduced isopentane concentration extracted from the bottom of the tower is supplied to a second distillation tower for purification. In the purification in the second distillation tower, the remaining methyldichlorosilane among the impurities with similar boiling points is removed by distillation. Specifically, the distillation purification is performed in a state where a high boiling point fraction having a methyldichlorosilane concentration 1.5 times or more higher than that in the crude trichlorosilane is discharged, and purified trichlorosilane with a reduced methyldichlorosilane concentration is distilled from the top of the tower. In this embodiment, the methyldichlorosilane concentration in the high boiling point fraction is 2 to 5 times higher than that in the crude trichlorosilane.

[0039] In the distillation in the second distillation column, the discharge of the high boiling point region fraction having a methyldichlorosilane concentration 1.5 times or more higher than that in the crude trichlorosilane can be carried out by heating the trichlorosilane in which the isopentane concentration has been reduced and supplied from the first distillation column and stored at the bottom, and appropriately setting the distillation conditions to extract the high boiling point region fraction at the bottom or the lower side of the column. For example, the temperature of the bottom or the lower side of the column is set to about 64 to 67°C.

[0040] In the second distillation column, as in the first distillation column, it is preferred that a portion of the distillate from the top of the column is cooled and condensed and refluxed. The larger the reflux ratio at this time, the better the separation efficiency. However, the reflux ratio is appropriately set in consideration of productivity.

[0041] Specifically, the removal of methyldichlorosilane in the second distillation column can reduce the content of methyldichlorosilane to 100 ppb mol or less, more preferably 50 to 90 ppb mol, even if the content of methyldichlorosilane (methyldichlorosilane concentration) in the trichlorosilane after isopentane reduction is 1000 ppb mol or more, more specifically 2000 to 20000 ppb mol. Thus, according to the present invention, purified trichlorosilane can be obtained as a pure substance having an isopentane content (isopentane concentration) of 20 ppb mol or less and a methyldichlorosilane content (methyldichlorosilane concentration) of 100 ppb mol or less. In addition, in the purified trichlorosilane, not only the content of isopentane that can be detected is extremely small, but also the content of azeotropes of isopentane and trichlorosilane that are difficult to detect is extremely small. Therefore, polycrystalline silicon produced using the purified trichlorosilane is obtained as a substance having a highly reduced carbon impurity content (carbon concentration).

[0042] In the distillation in the second distillation column, the high boiling point fraction discharged from the second distillation column and having a methyldichlorosilane concentration 1.5 times or more higher than that in the crude trichlorosilane is supplied to a third distillation column, and distillation purification is again carried out in the third distillation column while discharging the high boiling point fraction having a methyldichlorosilane concentration (preferably 10 times or more) higher than that in the crude trichlorosilane, to obtain purified trichlorosilane having a reduced methyldichlorosilane concentration distilled from the top of the column. The purified trichlorosilane obtained in the third distillation column can also have an isopentane content (isopentane concentration) and a methyldichlorosilane content (methyldichlorosilane concentration) equivalent to those of the purified trichlorosilane discharged from the top of the second distillation column.

[0043] As described above, crude trichlorosilane generally contains dichlorosilane as a low boiling point impurity in addition to impurities (isopentane and methyldichlorosilane) with similar boiling points, and silicon tetrachloride as a high boiling point impurity. It is preferable to remove these low boiling point impurities and high boiling point impurities to a high degree. In order to remove these low boiling point impurities and high boiling point impurities to a high degree, in this embodiment, before the crude trichlorosilane is supplied to the first distillation column, that is, before the distillation operation for removing impurities with similar boiling points is performed, it is desirable to perform the following pre-purification.

[0044] That is, the [pre-purification] is carried out by supplying the crude trichlorosilane to a preliminary first distillation tower, performing distillation purification in a state where a high boiling point fraction having a silicon tetrachloride concentration 1.1 times or more higher than the concentration in the crude trichlorosilane is discharged, distilling crude trichlorosilane with a reduced silicon tetrachloride concentration from the tower top, then supplying the obtained crude trichlorosilane with a reduced silicon tetrachloride concentration to a preliminary second distillation tower, performing distillation purification in a state where a low boiling point fraction having a dichlorosilane concentration 100 times or more higher than the concentration in the crude trichlorosilane is discharged, extracting crude trichlorosilane with a reduced dichlorosilane concentration from the tower bottom, and supplying it to the first distillation tower. In this specification, the crude trichlorosilane with a reduced dichlorosilane concentration extracted from the tower bottom of the preliminary second distillation tower is also referred to as pre-purified trichlorosilane.

[0045] In the above-mentioned pre-purification, in the purification in the preliminary first distillation tower, silicon tetrachloride, which is a high-boiling impurity and has a high content (high concentration) in the crude trichlorosilane, is removed by distillation. Specifically, the distillation purification is performed in a state where a high-boiling-point region fraction having a silicon tetrachloride concentration 1.1 times or more higher than that in the crude trichlorosilane is discharged, and trichlorosilane with a reduced silicon tetrachloride concentration is distilled from the tower top. Here, it is particularly preferred that the silicon tetrachloride concentration in the high-boiling-point region fraction is 1.2 to 2 times higher than that in the crude trichlorosilane.

[0046] In the distillation in the preliminary first distillation tower, the discharge of the high boiling point region fraction having a silicon tetrachloride concentration 1.1 times or more higher than that in the crude trichlorosilane can be carried out by heating the crude trichlorosilane stored at the bottom, appropriately setting the distillation conditions, and extracting the high boiling point region fraction at the bottom or the lower side of the tower. For example, the temperature of the bottom or the lower side of the tower is set to about 90 to 93° C. In addition, the high boiling point region fraction may contain solid components such as unreacted silicon fine powder.

[0047] In the preliminary first distillation column, as in the first distillation column, it is preferred that a portion of the distillate from the top of the column is cooled and condensed and refluxed. The larger the reflux ratio at this time, the better the separation efficiency. However, the reflux ratio is appropriately set in consideration of productivity.

[0048] According to the distillation purification in the preliminary first distillation column, even if the content of silicon tetrachloride (silicon tetrachloride concentration) in the crude trichlorosilane is 50 mol% or more, more specifically, 50 to 89.9 mol%, it can be reduced to 0.1 mol% or less, more preferably to 10 ppm mol or less (TCD detection limit value).

[0049] In the pre-purification, dichlorosilane, which is a low-boiling impurity and has a high content (high concentration) in the crude trichlorosilane, is removed by distillation in the subsequent purification in the pre-second distillation column. Specifically, distillation purification is performed in a state where a low-boiling-point region fraction having a dichlorosilane concentration 100 times or more higher than that in the crude trichlorosilane is discharged, and trichlorosilane with a reduced dichlorosilane concentration is extracted from the bottom of the column. In this regard, it is particularly preferred that the dichlorosilane concentration in the low-boiling-point region fraction is 100 to 200 times higher than that in the crude trichlorosilane.

[0050] In the distillation in the pre-second distillation column, the discharge of the low-boiling-point fraction having a dichlorosilane concentration 100 times or more higher than that in the crude trichlorosilane can be carried out by heating the trichlorosilane after the silicon tetrachloride stored at the bottom is reduced, and the distillation conditions are appropriately set to discharge the low-boiling-point fraction at the top or upper side of the column. For example, the temperature of the top or upper side of the column is set to about 40 to 43°C.

[0051] In the pre-second distillation column, as in the first distillation column, it is preferred that a portion of the distillate from the top of the column is cooled and condensed and refluxed. The larger the reflux ratio at this time, the better the separation efficiency. However, the reflux ratio is appropriately set in consideration of productivity.

[0052] According to the distillation purification in the pre-second distillation column, even if the content (concentration) of dichlorosilane in the trichlorosilane after the silicon tetrachloride concentration is reduced is 0.1 mol% or more, more specifically, 0.2 to 2 mol%, the content (concentration) can be reduced to less than 0.1 mol%, more preferably to 10 ppm mol (TCD detection limit value) or less. In this way, according to the pre-purification, trichlorosilane with a reduced dichlorosilane concentration extracted from the bottom of the pre-second distillation column, that is, pre-purified trichlorosilane can be obtained. In the pre-purified trichlorosilane, each content (concentration) of silicon tetrachloride and dichlorosilane is 10 ppm mol (TCD detection limit value) or less, and the pre-purified trichlorosilane can be supplied to the first distillation column. Therefore, in the pre-purified trichlorosilane, the content (concentration) of trichlorosilane is preferably 99 mol% or more, more preferably 99.5 mol% or more, and even more preferably 99.9 mol% or more.

[0053] It should be noted that, in the pre-purification, even if the removal operation of the silicon tetrachloride, dichlorosilane, etc., which are impurities with a large boiling point difference from trichlorosilane, is carried out, the content (concentration) of isopentane and methyldichlorosilane, which are impurities with similar boiling points, contained in the crude trichlorosilane does not substantially decrease. Therefore, in general, the concentration of these impurities with similar boiling points in the crude trichlorosilane containing silicon tetrachloride and dichlorosilane (crude trichlorosilane before pre-purification) is substantially the same as the concentration of these impurities with similar boiling points in the pre-purified trichlorosilane. However, in the case where they are intentionally changed, in the subsequent removal of these impurities with similar boiling points by combining the first distillation tower and the second distillation tower, the reference concentration of isopentane and methyldichlorosilane before discharge can be obtained based on the concentration in the pre-purified trichlorosilane just before the removal process.

[0054] The purified trichlorosilane obtained by the method of the present invention may be further subjected to further purification treatment such as adsorption removal, or may be mixed with trichlorosilane recovered from exhaust gas after polycrystalline silicon is precipitated using purified trichlorosilane.

[0055] The method for producing purified trichlorosilane described above is as follows: Figure 1 To illustrate more specifically. Figure 1 In the embodiment, a liquid stream of crude trichlorosilane produced by reacting a gas containing hydrogen chloride or silicon tetrachloride and hydrogen with metallurgical grade silicon flows through a crude trichlorosilane flow pipe 1. The crude trichlorosilane contains, in addition to isopentane and methyldichlorosilane, silicon tetrachloride and the like as high-boiling impurities and dichlorosilane and the like as low-boiling impurities.

[0056] The crude trichlorosilane is first supplied to a pre-purification process, and silicon tetrachloride as the high-boiling impurity and dichlorosilane as the low-boiling impurity are removed. That is, a crude trichlorosilane flow pipe 1 is connected to the side of a pre-first distillation tower 2, and the crude trichlorosilane liquid flow is supplied to the pre-first distillation tower 2. Then, in the pre-first distillation tower 2, the crude trichlorosilane liquid stored at the bottom is heated, and the distillation purification is carried out under the above conditions. By the distillation purification, a bottom liquid corresponding to a high-boiling-point fraction having a silicon tetrachloride concentration 1.1 times or more higher than that in the crude trichlorosilane is extracted from the bottom of the tower to a silicon tetrachloride concentration tower bottom liquid extraction pipe 3, and on the other hand, a gas phase of trichlorosilane after the silicon tetrachloride concentration is reduced is distilled from the top of the tower to a trichlorosilane distillation pipe 4 after silicon tetrachloride is reduced.

[0057] The gas phase of the trichlorosilane after silicon tetrachloride reduction distilled from the trichlorosilane after silicon tetrachloride reduction distillation pipe 4 is liquefied by passing through the condenser 5, and a part of the liquid flow is refluxed from the trichlorosilane after silicon tetrachloride reduction reflux pipe 6 to the pre-first distillation tower 2. On the other hand, another trichlorosilane after silicon tetrachloride reduction supply pipe 7 branched from the trichlorosilane after silicon tetrachloride reduction reflux pipe 6 is connected to the side of the pre-second distillation tower 8, and the remaining trichlorosilane after silicon tetrachloride reduction refluxed to the pre-first distillation tower 2 is supplied to the pre-second distillation tower 8. Then, in the pre-second distillation tower 8, the trichlorosilane after silicon tetrachloride reduction liquid stored at the bottom is heated, and its distillation purification is carried out under the above conditions. By this distillation purification, a gas phase corresponding to a low boiling point fraction having a dichlorosilane concentration 100 times or more higher than that in the crude trichlorosilane is distilled from the top of the tower to a dichlorosilane concentrate distillation pipe 10, while a liquid stream of pre-purified trichlorosilane having a reduced dichlorosilane concentration is extracted from the bottom of the tower to a pre-purified trichlorosilane extraction pipe 9.

[0058] The gas phase of the dichlorosilane concentrate distilled from the dichlorosilane concentrate distillation pipe 10 is liquefied by passing through a condenser 11, and a part of the liquid flow is refluxed from a dichlorosilane concentrate reflux pipe 12 to the pre-second distillation tower 8. On the other hand, the remaining dichlorosilane concentrate is sent to the outside of the system through another dichlorosilane concentrate flow pipe 13 branched from the dichlorosilane concentrate reflux pipe 12.

[0059] After the above pre-purification, the pre-purified trichlorosilane from which the silicon tetrachloride and dichlorosilane are removed and which flows through the pre-purified trichlorosilane extraction pipe 9 is subjected to the removal of isopentane and methyldichlorosilane. That is, the pre-purified trichlorosilane extraction pipe 9 is connected to the side of the first distillation tower 14, and the liquid stream of the pre-purified trichlorosilane is supplied to the first distillation tower 14. Then, in the first distillation tower 14, the pre-purified trichlorosilane liquid stored at the bottom is heated, and the distillation purification is carried out under the above conditions. By the distillation purification, a gas phase corresponding to a low boiling point region fraction having an isopentane concentration 150 times or more higher than that in the crude trichlorosilane is distilled from the top of the tower to the isopentane concentrate distillation pipe 16, and on the other hand, a liquid stream of trichlorosilane having a reduced isopentane concentration is extracted from the bottom of the tower to the isopentane reduced trichlorosilane extraction pipe 15.

[0060] The gas phase of the isopentane concentrate distilled from the isopentane concentrate distillation pipe 16 is liquefied by passing through a condenser 17, and a part of the liquid flow is refluxed to the first distillation column 14 through an isopentane concentrate reflux pipe 18. On the other hand, the remaining isopentane concentrate is sent to the outside of the system through another isopentane concentrate flow pipe 19 branched from the isopentane concentrate reflux pipe 18.

[0061] The isopentane-reduced trichlorosilane extraction pipe 15 is connected to the side of the second distillation tower 20, and the isopentane-reduced trichlorosilane liquid stream is supplied to the second distillation tower 20. Then, in the second distillation tower 20, the isopentane-reduced trichlorosilane liquid stored at the bottom is heated, and distillation purification is performed under the above conditions. By this distillation purification, a bottom liquid corresponding to a high boiling point fraction having a methyldichlorosilane concentration 1.5 times or more higher than that in the crude trichlorosilane is extracted from the bottom of the tower to a methyldichlorosilane concentration tower bottom liquid extraction pipe 21, while a gas phase of trichlorosilane having a reduced methyldichlorosilane concentration is distilled from the top of the tower to a purified trichlorosilane distillation pipe 22.

[0062] The gas phase of the purified trichlorosilane distilled from the purified trichlorosilane distillation pipe 22 is liquefied by passing through the condenser 23, and a part of the liquid flow is refluxed from the purified trichlorosilane reflux pipe 24 to the second distillation column 20. In this way, the purified trichlorosilane obtained by the production method of the present invention is taken out as the remaining liquid from another purified trichlorosilane flow pipe 25 branched from the purified trichlorosilane reflux pipe 24. The purified trichlorosilane obtained in this way can be used usefully as a raw material for the production of polycrystalline silicon.

[0063] Example

[0064] Hereinafter, in order to explain the present invention in detail, examples are given for description, but the present invention is not limited to these examples. It should be noted that the measurements and evaluations performed in the examples and comparative examples were obtained by the following methods.

[0065] (1) Analysis methods for each component

[0066] The concentration of chlorosilanes was measured by gas chromatography equipped with a thermal conductivity detector (TCD detector) (the detection limit of chlorosilanes was 10 ppm mol). The concentration of carbon-containing compounds was measured by gas chromatography mass spectrometry (GCMS) equipped with a mass spectrometer (MS) as a detector (the detection limit was 1 ppb mol), and the concentration of metal chlorides was measured by inductively coupled plasma (ICP) emission spectrometry (the detection limit was 1 ppb mol).

[0067] (2) Measurement of Carbon Concentration in Polycrystalline Silicon Produced Using Purified Trichlorosilane

[0068] The gas obtained by gasifying trichlorosilane and mixing it with hydrogen was thermally decomposed on a 10 mm square silicon core wire by the Siemens method, and silicon was precipitated on the silicon core wire to make it grow. The obtained 30 mm diameter polycrystalline silicon rod was crystallized by the floating zone method (FZ method), and the carbon concentration in the single crystal silicon was measured by a low temperature Fourier transform infrared spectrometer (FT-IR) device (the detection limit is 10 ppb-atom).

[0069] (3) Production conditions of crude trichlorosilane

[0070] A mixed gas of silicon tetrachloride and hydrogen is brought into contact with metallurgical grade silicon in a reactor at about 480 to 520°C to hydrochlorinate the metallurgical grade silicon and generate a gas containing trichlorosilane (SiHCl3). The gas contains silicon tetrachloride (SiCl4), dichlorosilane (SiH2Cl2), etc. Moreover, impurities other than silicon in the metallurgical grade silicon are also hydrochlorinated in the reactor and are contained in the generated gas as trace components in the form of iron chloride (FeCl3), aluminum chloride (AlCl3), organochlorosilane, and various organic substances. The composition of the raw materials used is: the silicon concentration in the metallurgical grade silicon is 99 mass%, the iron concentration is 0.7 mass%, the aluminum concentration is 0.3 mass%, and the carbon concentration is 300 ppmwt, and the silicon tetrachloride and hydrogen are 99 mass% or more pure.

[0071] The reaction product gas containing trichlorosilane is cooled within a temperature range of 0 to 20° C. to obtain a cooling condensate. The cooling liquid is sent to a crude trichlorosilane recovery tower to separate metal chlorides such as ferric chloride having a higher boiling point than silicon tetrachloride to obtain a crude trichlorosilane liquid.

[0072] The composition of the obtained crude trichlorosilane liquid is as follows: trichlorosilane concentration is 27.2 mol%, silicon tetrachloride concentration is 72.3 mol%, dichlorosilane concentration is 0.5 mol%, methyldichlorosilane concentration is 15000 ppb mol, isopentane concentration is 85 ppb mol, ferric chloride concentration and aluminum chloride concentration are less than 1 ppb-wt respectively.

[0073] Example 1

[0074] The crude trichlorosilane solution is supplied to the Figure 1 The distillation process shown is carried out.

[0075] 〔Each distillation column〕

[0076] exist Figure 1In the distillation process shown, the pre-first distillation tower 2 and the pre-second distillation tower 8 each use a distillation tower with a tower diameter of 150 mm and a theoretical number of 30 layers. The first distillation tower 14 uses a distillation tower with a tower diameter of 150 mm and a theoretical number of 80 layers, and the second distillation tower 20 uses a distillation tower with a tower diameter of 150 mm and a theoretical number of 85 layers.

[0077] [Operation conditions of each distillation column]

[0078] The pre-first distillation tower 2 is operated at a tower bottom pressure of 200 kPa (gauge pressure) and a tower bottom temperature of 91 to 92° C. The pre-second distillation tower 8 is operated at a tower top pressure of 100 kPa (gauge pressure) and a tower top temperature of 41 to 42° C. The first distillation tower 14 is operated at a tower top pressure of 100 kPa (gauge pressure) and a tower top temperature of 51 to 52° C. The second distillation tower 20 is operated at a tower bottom pressure of 200 kPa (gauge pressure) and a tower bottom temperature of 65 to 66° C.

[0079] [Confirmation of impurity removal results in each distillation column]

[0080] After the operation under the above-mentioned conditions was stabilized, samples were taken at various locations in the distillation process and their compositions were analyzed.

[0081] (Pre-1st distillation tower 2)

[0082] The bottom liquid flowing through the silicon tetrachloride concentration tower bottom liquid extraction pipe 3 was sampled and analyzed for composition. As a result, the silicon tetrachloride concentration was 99.9 mol%, and the trichlorosilane concentration was less than 10 ppm mol, which is the detection limit. The silicon tetrachloride concentration in the bottom liquid was 1.38 times higher than that in the crude trichlorosilane. From this, it was confirmed that the silicon tetrachloride as an impurity was highly removed in the trichlorosilane distilled from the top of the tower and flowing through the trichlorosilane distillation pipe 4 after silicon tetrachloride reduction by the distillation operation of the preliminary first distillation tower 2.

[0083] (Pre-second distillation tower 8)

[0084] The dichlorosilane concentrate flowing through the dichlorosilane concentrate flow pipe 13 was sampled and analyzed for composition. As a result, the dichlorosilane concentration was 81 mol%, the trichlorosilane concentration was 19 mol%, and the silicon tetrachloride concentration was 10 ppm mol or less, which was the detection limit. The dichlorosilane concentration of the dichlorosilane concentrate was 162 times higher than that of the crude trichlorosilane. From this, it was confirmed that the dichlorosilane as an impurity was highly removed in the trichlorosilane extracted from the bottom of the tower and flowing through the pre-purification trichlorosilane extraction pipe 9 by the distillation operation of the pre-second distillation column 8. In fact, the composition was sampled from the pre-purification trichlorosilane extraction pipe 9 and analyzed for composition. As a result, dichlorosilane and silicon tetrachloride were 10 ppm mol or less, which was the detection limit, trichlorosilane was 99.9 mol%, methyldichlorosilane was 15000 ppb mol, and isopentane was 85 ppb mol.

[0085] (First distillation tower 14)

[0086] The isopentane concentrate flowing through the isopentane concentrate flow pipe 19 was sampled and analyzed for composition. As a result, the trichlorosilane concentration was 99.9 mol%, the isopentane concentration was 23000 ppb mol, and the methyldichlorosilane concentration was 3 ppb mol. The isopentane concentration of the isopentane concentrate was 224 times higher than that of the crude trichlorosilane (pre-purified trichlorosilane). From this, it was confirmed that the isopentane as an impurity was highly removed from the trichlorosilane extracted from the bottom of the column and flowing through the isopentane reduction post-trichlorosilane extraction pipe 15 by the distillation operation of the first distillation column 14. In fact, the trichlorosilane concentration was 99.9 mol%, the isopentane concentration was 17 ppb mol, and the methyldichlorosilane concentration was 15000 ppb mol when the isopentane concentration was sampled and analyzed for composition.

[0087] (Second distillation tower 20)

[0088] The bottom liquid flowing through the methyldichlorosilane concentration tower bottom liquid extraction pipe 21 was sampled and analyzed for composition. As a result, the trichlorosilane concentration was 99.9 mol%, the isopentane concentration was 17 ppb mol, and the methyldichlorosilane concentration was 33000 ppb mol. The methyldichlorosilane concentration of the bottom liquid was 2.2 times higher than that in the crude trichlorosilane (pre-purified trichlorosilane). From this, it was confirmed that the methyldichlorosilane as an impurity was highly removed in the trichlorosilane distilled from the top of the tower and flowing through the purified trichlorosilane distillation pipe 22 by the distillation operation of the second distillation column 20. In fact, the composition of the purified trichlorosilane obtained by sampling from the purified trichlorosilane flow pipe 25 and analyzing the composition was a high-purity substance with a trichlorosilane concentration of 99.9 mol%, an isopentane concentration of 17 ppb mol, and a methyldichlorosilane concentration of 72 ppb mol.

[0089] [Confirmation of Carbon Concentration in Polycrystalline Silicon Produced Using Trichlorosilane]

[0090] Polycrystalline silicon was produced using the purified trichlorosilane obtained as described above, and its carbon concentration was measured to be a low content of 30 ppb mol.

[0091] Comparative Example 1

[0092] In Example 1, the distillation process of the crude chlorosilane liquid was changed from Figure 1 The process shown is changed to Figure 2 The process shown in FIG. 1 is carried out in the same manner as above except for the above, and purified trichlorosilane is produced. Figure 1 In the process of the present invention, the distillation of crude trichlorosilane (pre-purified trichlorosilane) is carried out by distilling an isopentane concentrate from the top of the first distillation tower 14, and obtaining isopentane-reduced trichlorosilane from the bottom. The isopentane-reduced trichlorosilane is supplied to the second distillation tower, and the methyldichlorosilane-concentrated bottom liquid is extracted from the second distillation tower 20, and purified trichlorosilane is distilled from the top of the tower. Figure 2 The distillation process of the present invention is different in that the removal targets are replaced, and the methyldichlorosilane concentrated bottom liquid is extracted in the second distillation tower 20, and the trichlorosilane after the methyldichlorosilane reduction is distilled from the top of the tower. The trichlorosilane after the methyldichlorosilane reduction is supplied to the first distillation tower. In the first distillation tower 14, the isopentane concentrate is distilled from the top of the tower, and the purified trichlorosilane is obtained from the bottom of the tower. The processes are different in this respect. The designs of the distillation towers and the operating conditions thereof other than these relationships are substantially the same.

[0093] [Confirmation of impurity removal results in each distillation column]

[0094] The results up to (preliminary first distillation tower 2) and (preliminary second distillation tower 8) were the same steps and the same results as those in Example 1.

[0095] (Second Distillation Tower 20)

[0096] The bottom liquid flowing through the methyldichlorosilane concentration bottom liquid extraction pipe 21 was sampled and analyzed for composition. As a result, the trichlorosilane concentration was 99.9 mol%, the isopentane concentration was 85 ppb mol, and the methyldichlorosilane concentration was 33000 ppb mol. The methyldichlorosilane concentration of the bottom liquid was 2.2 times higher than that in the crude trichlorosilane (pre-purified trichlorosilane). Therefore, even when methyldichlorosilane was removed first, it was confirmed that methyldichlorosilane was highly removed from the trichlorosilane distilled from the top of the second distillation column 20 and flowing through the trichlorosilane distillation pipe 22 after methyldichlorosilane reduction. In fact, the trichlorosilane concentration was 99.9 mol%, the isopentane concentration was 85 ppb mol, and the methyldichlorosilane concentration was 73 ppb mol when the trichlorosilane concentration was sampled from the trichlorosilane supply pipe 21 after methyldichlorosilane reduction was analyzed for composition.

[0097] (First distillation tower 14)

[0098] The isopentane concentrate flowing through the isopentane concentrate flow pipe 19 was sampled and analyzed for composition. As a result, the trichlorosilane concentration was 99.9 mol%, the isopentane concentration was 13000 ppb mol, and the methyldichlorosilane concentration was 73 ppb mol. The isopentane concentration of the isopentane concentrate was 152 times higher than that of the crude trichlorosilane (pre-purified trichlorosilane), and it was confirmed that isopentane was removed at a high concentration in the trichlorosilane extracted from the bottom of the first distillation column 14 and flowing through the purified trichlorosilane extraction pipe 15. In fact, sampling was performed from the purified trichlorosilane extraction tube 15 and the composition was analyzed. As a result, the trichlorosilane concentration was 99.9 mol%, the isopentane concentration was 18 ppb mol, and the methyldichlorosilane concentration was 73 ppb mol. In such a measurement using GCMS, the isopentane concentration and the methyldichlorosilane concentration were both reduced to a level close to that of the purified trichlorosilane produced in Example 1.

[0099] [Confirmation of Carbon Concentration in Polycrystalline Silicon Produced Using Trichlorosilane]

[0100] Polycrystalline silicon was produced using the purified trichlorosilane obtained above, and its carbon concentration was measured. The result was 610 ppb mol, which was significantly higher than that of the polycrystalline silicon produced using the purified trichlorosilane produced in Example 1.

[0101] This is because, in Example 1, the distillation of isopentane in the first distillation tower 14 is performed in the presence of methyldichlorosilane, and in this state, the isopentane is removed as an azeotrope, whereas in Comparative Example 1, the distillation of isopentane in the first distillation tower 14 is estimated to be caused by the fact that almost all of the methyldichlorosilane is removed in the previous second distillation tower 20, and therefore the removal of the isopentane azeotrope is insufficient. That is, it is estimated that in the absence of methyldichlorosilane, most of the isopentane forms an azeotrope with such trichlorosilane, and the boiling point of the azeotrope is closer to the boiling point of trichlorosilane than that of isopentane, and therefore, the isopentane is not sufficiently removed in the first distillation tower 14. Therefore, even if the purified trichlorosilane having an isopentane concentration measured to be as low as that in Example 1 is used, carbon derived from the azeotrope exists in the polycrystalline silicon produced thereby, resulting in a high carbon concentration.

[0102] Description of Reference Numerals

[0103] 1: crude trichlorosilane circulation pipe; 2: pre-first distillation tower; 3: extraction pipe for bottom liquid of silicon tetrachloride concentration tower; 4: distillation pipe for trichlorosilane after silicon tetrachloride reduction; 5, 11, 17, 23: condenser; 6: reflux pipe for trichlorosilane after silicon tetrachloride reduction; 7: supply pipe for trichlorosilane after silicon tetrachloride reduction; 8: pre-second distillation tower; 9: extraction pipe for pre-purification trichlorosilane; 10: dichlorosilane concentrate distillation pipe; 12: dichlorosilane concentrate reflux pipe; 13: dichlorosilane concentrate flow pipe; 14: first distillation tower distillation tower; 15: trichlorosilane extraction pipe after isopentane reduction (purified trichlorosilane extraction pipe); 16: isopentane concentrate distillation pipe; 18: isopentane concentrate reflux pipe; 19: isopentane concentrate flow pipe; 20: second distillation tower; 21: methyldichlorosilane concentration tower bottom liquid extraction pipe (trichlorosilane supply pipe after methyldichlorosilane reduction); 22: purified trichlorosilane distillation pipe (trichlorosilane distillation pipe after methyldichlorosilane reduction); 24: purified trichlorosilane reflux pipe; 25: purified trichlorosilane circulation pipe.

Claims

1. A method for producing purified trichlorosilane, characterized in that: supplying crude trichlorosilane containing at least isopentane and methyldichlorosilane to a first distillation column, performing distillation purification while discharging a low boiling point fraction having an isopentane concentration 150 times or more higher than that in the crude trichlorosilane, and extracting trichlorosilane having a reduced isopentane concentration from the bottom of the column, Next, the trichlorosilane with a reduced isopentane concentration is supplied to a second distillation column, and distilled and purified while discharging a high boiling point fraction having a methyldichlorosilane concentration that is 1.5 times or more higher than that in the crude trichlorosilane, and purified trichlorosilane with a reduced methyldichlorosilane concentration is distilled from the top of the column.

2. The method for producing purified trichlorosilane according to claim 1, wherein: The crude trichlorosilane has an isopentane content of 30 ppb mol or more, and the trichlorosilane after the isopentane concentration is reduced has an isopentane content of 20 ppb mol or less.

3. The method for producing purified trichlorosilane according to claim 1 or 2, wherein: The crude trichlorosilane has a content of methyldichlorosilane of 1 ppm or more, and the purified trichlorosilane after the methyldichlorosilane concentration is reduced has a content of methyldichlorosilane of 100 ppb or less.

4. The method for producing purified trichlorosilane according to claim 1, wherein: The crude trichlorosilane also contains silicon tetrachloride and dichlorosilane. Before the crude trichlorosilane is supplied to the first distillation column, the following pre-purification is performed: Pre-purification: supplying the crude trichlorosilane to a preliminary first distillation tower, performing distillation purification in a state where a high boiling point fraction having a silicon tetrachloride concentration that is 1.1 times or more higher than the concentration in the crude trichlorosilane is discharged, and distilling crude trichlorosilane having a reduced silicon tetrachloride concentration from the top of the tower, Next, the obtained crude trichlorosilane with a reduced silicon tetrachloride concentration is supplied to a pre-second distillation tower, and distillation purification is performed in a state where a low boiling point fraction having a dichlorosilane concentration 100 times or more higher than that in the crude trichlorosilane is discharged, and the crude trichlorosilane with a reduced dichlorosilane concentration is extracted from the bottom of the tower and supplied to the first distillation tower.

5. The method for producing purified trichlorosilane according to claim 4, wherein: The content of silicon tetrachloride in the crude trichlorosilane is 70 mol % or more, and the content of silicon tetrachloride in the crude trichlorosilane after the silicon tetrachloride concentration is reduced is 10 ppm or less.

6. The method for producing purified trichlorosilane according to claim 4 or 5, wherein: The dichlorosilane content in the crude trichlorosilane is 0.1 mol % or more, and the dichlorosilane content in the crude trichlorosilane after the dichlorosilane concentration is reduced is 10 ppm by mole or less.

7. The method for producing purified trichlorosilane according to claim 1, wherein: The crude trichlorosilane is produced by reacting a gas containing hydrogen chloride or silicon tetrachloride and hydrogen with metallurgical grade silicon.

8. A method for producing purified trichlorosilane, which comprises purifying crude trichlorosilane containing isopentane and methyldichlorosilane and having a trichlorosilane concentration of 99 mol% or more to produce purified trichlorosilane, wherein: Before the distillation operation for removing the methyldichlorosilane from the crude chlorosilane, a distillation operation for removing the isopentane is performed.

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