Method for Removing Impurities by Treating Silicon Tetrachloride with Hydrogen Plasma

Abstract

The transformation of organochlorine and organic impurities such as CCl₄, C₂H₂Cl₂, C₂HCl₃, C₂Cl₄, C₂H₂Cl₄, CH₄, C₃H₈, C₄H₁₀, and C₆H₆ in the content range of 10⁻²–10⁻⁶ wt.%, as well as BCl₃ impurities at the level of 3 × 10⁻² wt.%, was considered. A method has been developed for removing limiting impurities of carbon and boron during the process of the hydrogen reduction of silicon tetrachloride in a high-frequency arc gas discharge at atmospheric pressure. The thermodynamic and gas-dynamic analyses of the reduction process of silicon tetrachloride in hydrogen plasma, along with the behavior of organochlorine impurities, organic substances, and boron trichloride, was conducted. These analyses suggest that under equilibrium conditions, the conversion reactions of impurities result in the formation of silicon carbide and boron silicide. Potential chemical reactions for the conversion of the studied impurities into silicon carbide and boron silicide have been proposed. A new potential for plasma chemical processes has been identified, enabling the effective purification of chlorosilanes from both limiting and limited impurities. The results demonstrate the possibility of significantly reducing the concentrations of organochlorine and organic impurities, as well as boron trichloride, during the reduction of silicon tetrachloride in hydrogen plasma. The maximum conversion rates achieved included 99% for the organochlorine impurity CCl₄ to silicon carbide, 91% for benzene impurity to silicon carbide, and 86% for boron trichloride to boron silicide.

1. Introduction

2SiHCl₃ → SiH₂Cl₂ + SiCl₄,

(1)

2SiH₂Cl₂ → SiH₃Cl + SiHCl₃,

(2)

2SiH₃Cl → SiH₂Cl₂ + SiH₄.

(3)

Trichlorosilane is the starting material for the production of silane by the catalytic dismutation reaction.

This process is extensively employed in the industrial production of silane, and interest in its study remains strong [1]. This method relates to low-tonnage chemistry. This method uses organic ion exchange resins on which the disproportionation process takes place. The by-product in this process is silicon tetrachloride. Silicon tetrachloride is contaminated with organochlorine and organic impurities such as CCl₄, HCl₃ C₂H₂Cl₂, C₂HCl₃, C₂Cl₄, C₂H₂Cl₄, C₆H₁₄, and C₆H₆ that come from the ion exchange resin. The resulting silicon tetrachloride is subjected to rectification, as a result of which a stillage residue with a high content of organic and organochlorine impurities, as well as boron trichloride impurities, is formed. The stillage residue can be hydrolyzed, which will create an ecological burden on the environment. It is most expedient to convert waste into trichlorosilane and simultaneously reduce the content of limiting impurities in it. This will allow it to be sent for rectification and create a closed cycle for obtaining silane and silicon tetrachloride.

To achieve the waste-free production of high-purity silane and silicon tetrachloride, it is essential to develop effective methods for disposing of silicon tetrachloride. The most promising approach involves recycling silicon tetrachloride by reducing it to trichlorosilane, which can then be reintegrated into silane synthesis through a dismutation reaction.

Chemical methods for reducing silicon tetrachloride through hydrogenation with highly active reducing agents are well-established [2,3,4], as are high-temperature and catalytic methods by hydrogen reduction [5,6]. An industrial-scale process for the catalytic hydrogenation of silicon tetrachloride at high pressures (up to 4.0 MPa) has been implemented [7]. This method enables the production of trichlorosilane with a yield of up to 38%, which is extensively used in large-scale silicon production via the Siemens process. However, this approach has significant drawbacks, particularly the high pressure and temperature required to process highly corrosive substances.

In this regard, plasma chemical methods for the hydrogenation of silicon tetrachloride are more promising for use in low-tonnage chemistry [8,9,10,11,12,13,14,15]. These methods enable the activation of more effective mechanisms involving active particles such as atoms, radicals, and ions. In [11], a method for obtaining trichlorosilane by reducing silicon tetrachloride with hydrogen in a DC discharge was developed. In [12,13,14], the process of silicon tetrachloride destruction in a microwave discharge with the formation of silicon was studied. In [15], the formation of thin silicon films was studied in low-frequency plasma (880 kHz). A high growth rate and a degree of nanocrystallinity were achieved.

In [16], a method was developed for producing trichlorosilane under conditions of high-frequency arc discharge with a yield of more than 60%.

The basis of this method is the following scheme:

SiCl₄ + H₂ → SiHCl₃ + HCl and SiCl₄ + H₂ → Si(s) + 4HCl.

(4)

In [16], the results of thermodynamic calculations for Reaction (4) are presented, and information is provided on the method for calculating the thermodynamic parameters and on the dependence of the equilibrium composition on temperature and the SiCl₄/H₂ ratio. It is shown that the equilibrium yield of trichlorosilane according to scheme (4), equal to 40–45%, is observed in the temperature range of 1000–1500 K. With this method, it was possible to create a local energy supply area. This made it possible to exclude plasma contact with the walls of the discharge chamber and avoid the ingress of impurities from them. In addition, high-purity silicon was used as the electrode material. This made it possible to exclude the influence of the electrode material on the purity of the resulting trichlorosilane.

In the process of our study in [16] of the possibility of the plasma chemical reduction of silicon tetrachloride to trichlorosilane, we discovered the effect of reducing the concentration of some impurities in the reaction products. This work is devoted to the study of this effect.

The behavior of organochlorine impurities contained in the initial silicon tetrachloride, as well as the main electroactive impurity in the form of boron chloride, has not been studied enough. Therefore, the objective of this study was to investigate the behavior of these impurities during the plasma chemical reduction of silicon tetrachloride with hydrogen using the method outlined in [16].

2. Results

The content of trichlorosilane in the mixture leaving the plasma chemical reactor under the realized experimental conditions was determined to be 40.44%. In plasma, in parallel with the main synthesis reaction of the target substance (4), reactions involving impurities can take place, which lead to changes in their concentration and chemical form.

Table 1. The degrees of conversion of organochlorine impurities in the initial silicon tetrachloride and in a mixture of chlorosilanes. P = 760 Torr.

Impurity	[c] in the Initial SiCl4, wt.%	[c] in a Mixture (SiCl4 + SiHCl3), wt.%	α, %
CCl4	2.4 × 10−4	2.4 × 10−6	99
HCl3	3.1 × 10−5	4.3 × 10−6	86
C2Cl4	7.1 × 10−5	3.9 × 10−5	45
C2HCl3	8.5 × 10−5	3 × 10−5	64
C2H2Cl2	7.3 × 10−5	3 × 10−5	58
C2H2Cl4	2.6 × 10−4	2 × 10−4	23

shows the content of organochlorine impurities in the initial silicon tetrachloride and in a mixture of chlorosilanes (SiCl₄ + SiHCl₃) collected in a cryogenic trap, as well as the degree of conversion α.

It can be seen from the table that a purification effect can be observed for all impurities under consideration. At the same time, the degree of conversion of these impurities decreases depending on the increase in the complexity of the structure and size of the molecules.

Since an increase in the concentration of any of these impurities was not observed, it can be assumed that their interconversion reactions were not present. In this case, the most likely schemes leading to the conversion of these impurities involve their interaction with SiCl₄ to form silicon carbide:

CCl₄ + SiCl₄ + 4H₂ → SiC(s) + 8HCl,       ΔG_r (300 K) = −156.9 kJ/mol
ΔG_r (1200 K) = −441.3 kJ/mol

(5)

CHCl₃ + SiCl₄ + 3H₂ → SiC(s) + 7HCl,       ΔG_r (300 K) = −44.5 kJ/mol
ΔG_r (1200 K) = −299.0 kJ/mol

(6)

C₂Cl₄ + 2SiCl₄ + 6H₂ → 2SiC(s) + 12HCl,        ΔG_r (300 K) = −62.3 kJ/mol
ΔG_r (1200 K) = −458.7 kJ/mol

(7)

C₂HCl₃ + 2SiCl₄ + 5H₂ → 2SiC(s) + 11HCl,        ΔG_r (300 K) = 50.0 kJ/mol
ΔG_r (1200 K) = −319.6 kJ/mol

(8)

C₂H₂Cl₂ + 2SiCl₄ + 4H₂ → 2SiC(s) + 10HCl,       ΔG_r (300 K) = 128.3 kJ/mol
ΔG_r (1200 K) = −224.0 kJ/mol

(9)

C₂H₂Cl₄ + 2SiCl₄ + 5H₂ → 2SiC(s) + 12HCl.       ΔG_r (300 K) = 41.3 kJ/mol
ΔG_r (1200 K) = −467.3 kJ/mol

(10)

The calculation of the Gibbs energy of the reactions was performed in the software system with the IVTANTHERMO ver. 3.0 database for Windows [17]. It is evident that the equilibrium of reaction schemes (5)–(7) is shifted to the right, which indicates that they occur at the indicated temperature values. For reaction schemes (8)–(10) under standard conditions, the Gibbs energy is positive and changes its sign at a temperature of 1200 K. This also indicates the validity of the prediction of these reactions.

Table 2. The contents of organic impurities in the initial silicon tetrachloride and in a mixture of chlorosilanes. P = 760 Torr.

Impurity	The Initial SiCl4, wt.%	Mixture of Chlorosilanes SiCl4+SiHCl3 After Plasma, wt.%	α, %
CH4	˂1.6 × 10−5	4.1 × 10−4	---
C3H8	1.5 × 10−5	2.2 × 10−5	---
C4H10	1 × 10−5	˂5 × 10−6	50
C6H6	0.075	0.0070	91

shows the contents of organic impurities in the initial silicon tetrachloride and in a mixture of chlorosilanes (SiCl₄ + SiHCl₃) at the plasma chemical reactor outlet. It can be seen that unlike organochlorine impurities, interconversion reactions can be observed for organic impurities. This is evidenced by an increase in methane impurity. The propane content does not change significantly. The increase in methane concentration can be explained by the conversion of benzene and butane into methane according to the following schemes:

C₆H₆ + 9H₂ → 6CH₄,        ΔG_r (300 K) = −432.1 kJ/mol
ΔG_r (1200 K) = −51.7 kJ/mol

(11)

C₄H₁₀ + 6H₂ → 4CH₄.        ΔG_r (300 K) = −185.6 kJ/mol
ΔG_r (1200 K) = −190.3 kJ/mol

(12)

The equilibrium of reaction schemes (11) and (12) at the indicated temperature values is shifted towards the formation of reaction products.

The conversion rates of α of these impurities are 91% and 50%, respectively. Due to the fact that the conversion of benzene impurities in terms of the amount of the substance is significantly higher than the conversion of butane, it can also be assumed that most of these impurities pass through intermediate reactions of methane formation via (11) and (12) into silicon carbide according to the following reaction scheme:

CH₄ + SiCl₄ → SiC(s) + 4HCl.        ΔG_r (300 K) = 221.8 kJ/mol
ΔG_r (1200 K) = −5.9 kJ/mol

(13)

The equilibrium of reaction scheme (13) is shifted toward the formation of reaction products at a temperature of 1200 K.

Thermodynamic and Gas-Dynamic Analyses

In [18], we carried out a thermodynamic analysis of the systems SiCl₄-CCl₄-H₂, SiCl₄-CH₄-H₂, and BCl₃-CH₄-H₂. This analysis showed that the formation of the SiC phase in the SiCl₄-CCl₄-H₂ system occurs in the temperature range of 1380–2190 K. Along with this, a carbon phase (Cgr) is formed in the temperature range of 900–2950 K. In the SiCl₄-CH₄-H₂ system, the temperature range of SiC phase formation expands to 800–2610 K, and the temperature range of carbon phase formation (Cgr) is divided into two, namely 985–1440 K and 2310–2800 K. The results of thermodynamic analysis of the BCl₃-CH₄-H₂ system showed the formation of the B₄C phase in the temperature range of 740–3000 K. The formation of the Cgr phase in this case occurs at temperatures of 1000–2800 K. In [19], a thermodynamic analysis of the SiCl₄-BCl₃-H₂ system was performed. It was shown that SiB₆ and SiB₄ compounds can be formed in the temperature range of 1200–1800 K.

Thus, it can be argued that carbon-containing impurities, as well as BCl₃ impurities, are converted under the given experimental conditions into silicon and boron carbides, as well as boron silicide.

In [16], we performed a gas-dynamic analysis of heat fluxes in a plasma chemical reactor. It was found that in the area of a gas discharge fixed between two electrodes, the temperature was maintained in the range of 900–1700 K. Therefore, it can also be assumed that the reactions of the formation of silicon and boron carbides, as well as boron silicide, occur under equilibrium conditions.

For the remaining Reactions (6)–(12), the thermodynamic possibility of their occurrence was assessed. According to the assessment of the change in the standard isobaric potential, the equilibrium of all reactions, except for (8) and (9), is shifted to the right, which indicates the possibility of their occurrence. The assessment of the thermal effect of Reactions (8) and (9) showed that the reactions are endothermic, and in the approximation of the integral form of the isobar equation of a chemical reaction, the equilibrium shifts to the right with an increase in temperature. As follows from the data in [16], a temperature gradient from 293 to 1766 K is realized in the plasma discharge, which creates wide possibilities for the occurrence of endothermic processes.

This assumption was confirmed by an X-ray phase analysis of the powder formed on the electrodes and walls of the plasmatron (see Figure 1). The graph shows the dependence of the intensity of reflected radiation on the Theta-half-width of the swing curve.

It has been experimentally established that in the process of plasma chemical reduction of silicon tetrachloride with hydrogen, not only is the concentration of most of the analyzed impurities reduced but also trichlorosilane is synthesized. The mixture of chlorosilanes can be easily separated via fractional distillation before being recycled into production. Then, silicon tetrachloride and trichlorosilane can be subjected to deep purification, since the concentration of difficult-to-remove impurities in them is significantly reduced, and methane and propane impurities can be easily removed using distillation methods. Since the concentration of impurities of carbon-containing substances in chlorosilanes that are difficult to remove via distillation methods is significantly reduced by the developed plasma chemical method, it is possible to use the plasma chemical reduction of silicon tetrachloride with hydrogen for recycling chlorosilanes at the stage of deep purification and obtain chlorosilanes suitable for use in electronics and optics.

Figure 2 shows the IR spectra of the initial mixture of SiCl₄ + H₂ (1) and the mixture of SiHCl₃ + SiCl₄ + H₂ + HCl released from the plasma chemical reactor after the plasma chemical reduction process (2). The identification of components by the locations of absorption bands was based on data from the literature [20,21].

According to [20], in the IR spectrum of the initial SiCl₄ + H₂ mixture, a characteristic band with a high absorption coefficient in the range of 583–656 cm⁻¹ was observed, belonging to the SiCl₄ molecule. In the range of 2650–3100 cm⁻¹, a very weak band related to the HCl molecule was recorded. At 1427 cm⁻¹, an absorption band corresponding to the fluctuations of the BCl₃ molecule was recorded [21]. The content of the BCl₃ impurity is 3 × 10⁻² wt.%. In the IR spectrum of the gas mixture leaving the reactor, strongly pronounced new bands can be observed at 554, 823, and 2250 cm^−1, which correspond to trichlorosilane SiHCl₃. This observation is consistent with previous gas chromatographic measurements, which indicate that the trichlorosilane content in the gas mixture exiting the reactor is 40%. At the same time, the intensity of the bands associated with HCl increases sharply, which indicates the formation of trichlorosilane via Reaction (4). In addition, in the IR spectrum of the gas mixture leaving the reactor, a significant decrease in the intensity of the band related to the BCl₃ molecule can be observed. The content of this impurity is 4 × 10⁻³ wt.%, and the conversion rate calculated by Formula (5) is 86%.

Therefore, during the synthesis of the target trichlorosilane product, additional purification of the electroactive boron impurity in the form of a BCl₃ occurs.

Since the main recyclable substance in this plasma chemical process is SiCl₄, it can be assumed that the main reaction responsible for the plasma chemical conversion of BCl₃ impurity is the boron silicide formation scheme:

BCl₃ + SiCl₄ + 8H₂ → SiB₄(s) + 16HCl.

(14)

Unfortunately, thermodynamic data for this scheme are not available in the «IVTANTHERMO» database.

Due to the fact that the initial mixture contains a significant amount of carbon-containing impurities, it can be assumed that the conversion of the BCl₃ impurity also occurs through the schemes of boron carbide formation:

4BCl₃ + CCl₄ + 8H₂ → B₄C(s) + 16HCl,       ΔG_r (300 K) = 23.3 kJ/mol
ΔG_r (1200 K) = −386.8 kJ/mol

(15)

4BCl₃ + CHCl₃ + 7H₂ → B₄C(s) + 15HCl.       ΔG_r (300 K) = 135.6 kJ/mol
ΔG_r (1200 K) = −244.5 kJ/mol

(16)

Similar reactions schemes can be written for other organochlorine impurities such as C₂Cl₄, C₂HCl₃, C₂H₂Cl₂, and C₂H₂Cl₄:

8BCl₃ + C₂Cl₄ + 14H₂ → 2B₄C(s) + 28HCl,       ΔG_r (300 K) = 298.1 kJ/mol
ΔG_r (1200 K) = −349.7 kJ/mol

(17)

8BCl₃ + C₂HCl₃ + 13H₂ → 2B₄C(s) + 27HCl,       ΔG_r (300 K) = 410.5 kJ/mol
ΔG_r (1200 K) = −210.6 kJ/mol

(18)

8BCl₃ + C₂H₂Cl₂ + 12H₂ → 2B₄C(s) + 26HCl,        ΔG_r (300 K) = 488.8 kJ/mol
ΔG_r(1200 K) = −115.1 kJ/mol

(19)

8BCl₃ + C₂H₂Cl₄ + 13H₂ → 2B₄C(s) + 28HCl,       ΔG_r (300 K) = 401.7 kJ/mol
ΔG_r (1200 K) = −358.4 kJ/mol

(20)

The assessment of the thermal effect of Reactions (15)–(20) showed that the reactions are endothermic, and in the approximation of the integral form of the isobar equation of a chemical reaction, the equilibrium shifts to the right with an increase in temperature.

For organic impurities, methane is the product of the conversion of benzene and butane via Reactions (11) and (12). In this regard, it is most advisable to record the reaction of the boron chloride formation with methane via the following scheme:

4BCl₃ + CH₄ + 4H₂ → B₄C(s) + 12HCl.        ΔG_r (300 K) = 402.0 kJ/mol
ΔG_r (1200 K) = 48.5 kJ/mol

(21)

As follows from the data in [17], a temperature gradient from 900 to 1700 K is realized in the discharge, which creates wide possibilities for the occurrence of endothermic processes. Therefore, it can be assumed that the equilibrium of Reaction (21) will be shifted to the right when the temperature reaches 1700 K.

When evaluating the thermodynamic parameters of Reactions (5)–(21), it was assumed that silicon carbide and boron carbide form a solid phase.

In [22,23], we have shown that in this type of discharge in macrosystems (SiCl₄ + CCl₄ + H₂) and (ВCl₃ + CH₄ + H₂), the formation of silicon carbide and boron carbide is possible.

3. Materials and Methods

In this work, we used as the initial substances silicon tetrachloride (STC) (99.999%, HORST Ltd., Moskow, Russia) and hydrogen (99.99998%, Monitoring, Saint-Petersburg, Russia). In the plasma chemical reactor, we used Si electrodes with a material purity of 99.999% (Sernia Inc., Moskow, Russia).

Experiments to study the behavior of organochlorine and organic impurities during the reduction of silicon tetrachloride in hydrogen plasma were carried out at an installation, the schematic diagram of which is described in detail in [16]. The power of the RF generator was 340 W and the frequency was 40.68 MHz. The consumption of the plasma-forming gas H₂ + SiCl₄ was 350 cm³/min. The pressure in the plasma chemical reactor during the experiment was maintained equal to atmospheric (760) Torr. The mole ratio H₂/SiCl₄ = 6 was kept constant.

The studies were carried out on a model mixture. The concentration of impurities in the initial silicon tetrachloride in all experiments was constant. The concentrations of organochlorine impurities in the initial silicon tetrachloride were 2.4 × 10⁻⁴ wt.% for CCl₄, 3.1 × 10⁻⁵ wt.% for CHCl₃, 7.3 × 10⁻⁵ wt.% for C₂H₂Cl₂, 7.1 × 10⁻⁵ wt.% for C₂Cl₄, 8.5 × 10⁻⁵ wt.% for C₂HCl₃, and 2.6 × 10⁻⁴ wt.% for C₂H₂Cl₄. Gas chromatography determined the concentration with a relative error of 20%. The concentrations of organic impurities in the initial silicon tetrachloride were less than 1.6 × 10⁻⁵ wt.% for CH₄, 1.5 × 10⁻⁵ wt.% for C₃H₈, 1 × 10⁻⁵ wt.% for C₄H₁₀, and 0.075 wt.% for C₆H₆.

The concentration was also determined by gas chromatography, and the relative error of the method was 10%.

The concentration of BCl₃ impurities in the initial silicon tetrachloride was determined by IR spectroscopy and amounted to 3 × 10⁻² wt.%. The relative error of the method was 20%.

The degree of conversion of α impurities in hydrogen plasma was calculated using the values of the impurity concentration in the initial silicon tetrachloride [c]_init and reaction products [c]_prod according to the equation:

α = ([c]_init − [c]_prod)/[c]_init × 100%.

(22)

3.1. Gas Chromatography Analysis

The analysis of organic impurities was carried out using gas–liquid chromatography on a Chromos GC 1000 chromatograph. Helium was used as the carrier gas, and flame ionization and thermal conductivity detectors were used. The helium consumption was 20 cm³/min, the separation column temperature was 40 °C, and the sorbent was 15 wt.% of silicone elastomer SE-30 on a AW HMDS chromaton. The Figure 3 shows an example of a chromatogram of organic impurities in silicon tetrachloride.

For the gas chromatographic identification of organochlorine impurities, a Chromos GC-1000 gas chromatograph with an electron capture detector and nitrogen as a carrier gas was used. The impurities were separated on fused-silica columns with a nonpolar liquid stationary phase (silicone elastomer SE-30 and a highly polar liquid stationary phase), ether of nitroterephthalic acid, and polyethylene glycol FFAP. The gas chromatographic identification technique for impurities in the SiCl₄ + SiHCl₃ mixture exiting a plasma chemical reactor involves several steps. First, the base undergoes preliminary hydrolysis, followed by the microextraction of organic and organochlorine impurities using n-hexane [24].

3.2. IR Spectroscopy of Exhaust Gases

The identification of boron trichloride impurities in the initial silicon tetrachloride and exhaust gases was performed using IR spectroscopy with an FSM2203 spectrometer with a DTGS detector operating in the range of 500–3400 cm⁻¹. The resolution and aperture were 0.125 cm⁻¹ and 5 mm, respectively. The gas samples were analyzed in a multi-pass cuvette with an optical path length of 5 m. Spectra of the reaction mixture were recorded offline, both before and after the discharge process. Quantitative determination of the components from the obtained spectra was performed using the ASPEC ver. 0.0.11 and FSPEC ver. 0.7.1. software packages.

3.3. Characterization of Solid Deposits

The X-ray diffraction method was used to determine the phase composition of powdered residues. The measurements were conducted using a Bruker D8 Discover X-ray (Billerica, MA, USA) diffractometer with a multilayer parabolic Goebel mirror and a linear tube focus for parallel beam geometry. A linear position-sensitive LynxEYE detector Billerica, MA, USA was used as a detector, capturing an angle of 2 degrees by 2θ with 192 channels. The Diffrac.EVA v. 2.0 program and PDF-2 release 2011 powder diffraction database were used to process the obtained diffractograms.

4. Conclusions

The results demonstrate the potential for a significant reduction in the concentrations of both limited and limiting organochlorine and organic impurities, as well as boron trichloride impurities, during the plasma chemical synthesis of trichlorosilane from SiCl₄. The maximum conversion rate for the organochlorine impurity CCl₄ reached 99%, while benzene showed a conversion rate of 91% and the conversion of the BCl₃ impurity was 86%. The primary chemical reactions responsible for the conversion of these impurities involve the formation of silicon carbide, boron carbide, and boron silicide. Thermodynamic and gas-dynamic analyses of the reduction process of silicon tetrachloride in hydrogen plasma suggest that these impurity conversion reactions occur under equilibrium conditions.