Introduction

Trichlorosilane is the most important member of the chlorosilane family and a key bridge connecting metallurgical grade silicon (98–99%) and high-purity polysilicon (6N–9N). Whether it is solar cells or semiconductor devices, the high-purity silicon raw materials almost all start from trichlorosilane. In 2020, global polysilicon production capacity is concentrated in 15 companies, most of them are Chinese companies, with a total production capacity of about 660,000 to 675,000 tons/year. Trichlorosilicon accounts for 12%–18% of the cost of polysilicon. Its market has an average annual growth rate of about 6.4%, and it is expected to exceed US$10 billion in 2025 [1][2].

Application of trichlorosilane

Trichlorosilane is mainly used in the Siemens chemical vapor deposition (CVD) process to reduce and deposit high-purity polysilicon on a heated silicon core. In addition, it can also prepare silane through disproportionation reaction for silicon epitaxial growth and synthesis of organic silicon compounds. The purity of trichlorosilane directly determines the quality of the final product, and its impurities (B, P, Fe, Al, etc.) need to be controlled at the ppb level to meet the stringent requirements of solar grade (6N) and electronic grade (9N) [1].

Synthesis method of trichlorosilane

There are four main methods in industry, each with its own applicable scenarios.

Direct Chlorination

Metallurgical grade silica powder reacts with HCl in a fluidized bed reactor:

Si + 3HCl → SiHCl? + H?, side reaction generates SiCl?. The reaction is catalyzed by impurities (activity order Ni > Co > Fe), and the temperature needs to be precisely controlled: the selectivity of Trichlorosilane reaches 95% at 260°C, drops to about 70% at 400°C, and only about 40% at 600°C [1]. Pressure 0.18–0.5 MPa can increase the trichlorosilane content. New reactors can achieve HCl conversion rates close to 100%, and reactors mostly use high-nickel alloys (such as Hastelloy B-2, Incoloy 800H) to resist high-temperature chlorosilane corrosion [1][2].

Silicon tetrachloride hydrogenation method (Conversion)

Convert the by-product SiCl? into Trichlorosilane: SiCl? + H? ? SiHCl? + HCl. The reaction proceeds at 700–1400°C and requires rapid quenching. Under optimized conditions (1100°C, 0.3 MPa, H?:SiCl?=4:1), the single-pass conversion rate is about 25% [2]. Large single-series converters have a processing capacity of up to 15,000 kg/h SiCl? and a conversion rate of approximately 16.5 wt% [2].

Hydrochlorination

SiCl?, H? and silicon powder react at 400–600°C: 3SiCl? + Si + 2H? → 4SiHCl?. The reaction is weakly exothermic and the conversion rate is thermodynamically limited, reaching 38% at high pressure (~30 MPa) [2]. Catalyzed by metals such as Cu, Fe, and Ni, the reaction may occur on the surface of metal silicide [1][2].

Redistribution/anti-disproportionation method (Redistribution)

Utilize the by-product SiH?Cl? to react with SiCl?: SiH?Cl? + SiCl? ? 2SiHCl? (exothermic heat +11 kcal/mol), low temperature is beneficial. Catalyzed using macroporous anion exchange resins such as Dowex M43, conversions of 98–99% can be achieved at 50–80°C, 0.7 MPa [2]. This method has low investment and low energy consumption, but it cannot completely convert all SiCl? and is usually used as an auxiliary process [2].

Detection method

During the production process, gas chromatography (GC) is the main means of analyzing chlorosilane mixtures, and mass spectrometry (MS) can be used to monitor reaction products in real time [3]. The final product needs to control impurities such as B, P, Fe, and Al through trace analysis such as ICP-MS. Surface analysis (SEM, EDX) is used to study the reactivity of silicon particles [1].

Precautions

Safety risk: Trichlorosilane is flammable and explosive. It reacts violently with water to release HCl. Operation must be strictly isolated from air and moisture. Dichlorosilane (SiH?Cl?) is particularly unstable and poses a fire and explosion hazard [2].

Equipment corrosion: The chlorosilane environment is extremely corrosive. Carbon steel corrodes rapidly under trace amounts of moisture. The corrosion rate of high-nickel alloys (Alloy 625, Alloy 617) is only 0.45–0.59 mm/year [1][2].

Surface catalysis and wall effects: Chlorosilanes decomposition is extremely sensitive to reactor wall conditions. Walker et al. [3] studied the thermal decomposition mechanism of trichlorosilane, dichlorosilane and monochlorosilane through a static reactor and shock tube system, and found that the reaction was dominated by silylene intermediates rather than free radical chain reactions. Intermediates such as HSiCl are easily trapped on the vessel wall, and surface pretreatment (such as trimethylchlorosilane passivation) is crucial to inhibit wall catalysis [3]. This study gives the Arrhenius parameters of various species under high temperature and pressure, which has direct guiding significance for understanding side reactions in the reactor and optimizing process temperature [3].

Scrap recycling: 2–27 kg SiCl? is a by-product per kilogram of polysilicon, and its recovery into Trichlorosilane through hydrogenation or redistribution is key to reducing costs and environmental burden [1][2].

References

[1] Jarkin VN, Kisarin OA, Kritskaya TV (2021) Methods of trichlorosilane synthesis for polycrystalline silicon production.Part 1: Direct synthesis. Modern Electronic Materials 7(1): 1–10. https://doi.org/10.3897/j.moem.7.1.64953

[2] Jarkin VN, Kisarin OA, Kritskaya TV (2021) Methods of trichlorosilane synthesis for polycrystalline silicon production.Part 2: Hydrochlorination and redistribution. Modern Electronic Materials 7(2): 33–43. https://doi.org/10.3897/j.moem.7.2.65572

[3] Walker, K. L., Jardine, R. E., Ring, M. A., & O’Neal, H. E. (1998). Mechanisms and kinetics of the thermal decompositions of trichlorosilane, dichlorosilane, and monochlorosilane. International Journal of Chemical Kinetics, 30(1), 69–88. https://doi.org/10.1002/(SICI)1097-4601(1998)30:1<69::AID-KIN8>3.0.CO;2-8