The preparation of high-purity silicon generally involves first producing industrial silicon (crude silicon) from silica (SiO2), then producing high-purity polycrystalline silicon, and finally pulling it into semiconductor material silicon single crystal.
Industrially, silica (SiO2) and coke are mixed in a certain proportion and heated to 1600~1800°C in an electric furnace to obtain crude silicon with a purity of 95%~99%. The reaction is as follows: SiO2+2C=Si+2CO
Crude silicon generally contains impurities such as iron, aluminum, carbon, boron, phosphorus, and copper. These impurities mostly exist in the form of silicate to form silicates. In order to further improve the purity of industrial crude silicon, acid leaching method can be used to remove the impurities. Most of it is dissolved (a small amount of silicon carbide is insoluble). The production process is as follows: after crushing the crude silicon, it is sequentially treated with hydrochloric acid, aqua regia, and (HF+H2SO4) mixed acid, and finally washed with distilled water until neutral. After drying, industrial crude silicon with a content of 10% can be obtained.
There are many methods for preparing high-purity polysilicon. According to Buquan statistics, there are more than a dozen, but all methods start from industrial silicon (or ferrosilicon, because it contains more iron). First of all, it is easy to purify and decompose ( That is, reduction) of silicon-containing intermediate compounds such as SiCl4, SiHCl3, SiH4, etc., and then purifying, decomposing or reducing these intermediate compounds into high-purity polysilicon
At present, my country mainly uses trichlorosilane hydrogenation reduction method, silane pyrolysis method and silicon tetrachloride hydrogenation reduction method to prepare high-purity silicon polysilicon. Generally speaking, because the trichlorosilane reduction method has certain advantages, it is currently widely used. In addition, because SiH4 is easy to purify, the silane thermal decomposition method is a promising method for preparing high-purity silicon. Below we will introduce the chemical principles of preparing high-purity silicon by the above three methods.
1. Trichlorosilane reduction method
(1) Synthesis of trichlorosilane
Step 1: Prepare coarse silicon from silica. Silica (SiO2) is mixed with an appropriate amount of coke and heated to 1600~1800°C in an electric furnace to produce crude silicon with a purity of 95%~99%. The reaction formula is as follows:
SiO2+3C=SiC+2CO (g)↑
2SiC+SiO2=3Si+2CO(g)↑
Overall reaction formula: SiO2+2C=Si+2CO (g)↑
The generated silicon is released from the bottom of the electric furnace and cast into ingots. After the crude silicon produced by this method is treated with acid, its purity can reach %.
Step 2: Synthesis of trichlorosilane Trichlorosilane is synthesized from dry hydrogen chloride gas and coarse silicon powder in a synthesis furnace (250°C). Its main reaction formula is as follows: Si+3HCl=SiHCl3+H2 (g)
(2) Purification of trichlorosilane
Trichlorosilane obtained from the synthesis furnace is often mixed with impurities such as boron, phosphorus, arsenic, and aluminum, and they are harmful impurities that have a great impact on the quality of single crystal silicon and must be removed.
In recent years, the purification method of trichlorosilane has developed rapidly. However, due to the simple process and convenient operation of the distillation method, the distillation method is currently mainly used in industry. Trichlorosilane distillation uses the different boiling points of trichlorosilane and impurity chloride to separate and purify.
Generally synthesized trichlorosilane often contains chlorine such as boron trichloride (BCl3), phosphorus trichloride (PCl3), silicon tetrachloride (SiCl4), arsenic trichloride (AsCl3), aluminum trichloride (Al2Cl3) chemical. The boiling points of most chlorides are quite different from those of trichlorosilane, so these impurities can be removed by distillation. However, the boiling points of boron trichloride and phosphorus trichloride are similar to trichlorosilane, making them difficult to separate. Therefore, efficient distillation is required to remove these two impurities. The boron removal effect of distillation and purification has a certain limit, so the complex method with better boron removal effect is also used in industry.
Trichlorosilane has a low boiling point and is flammable and explosive. All operations must be performed at low temperatures. The general operating environment temperature must not exceed 25°C, and contact with Mars is strictly prohibited during the entire process to avoid explosive combustion.
(3) Hydrogen reduction of trichlorosilane
After purifying trichlorosilane and high-purity hydrogen, they are passed into a reduction furnace at 1150°C for reaction to obtain silicon. The total chemical reaction is: SiHCl3+H2=Si+3HCl
The resulting high-purity polysilicon is deposited on a polysilicon carrier.
