Thread Content
Chemical principles of high-purity silicon production: The production of high-purity silicon generally starts with the conversion of silica (SiO2) into industrial silicon (crude silicon), which is then transformed into high-purity polycrystalline silicon, and finally drawn into silicon single crystals used as semiconductor materials. Industrially, crude silicon with a purity of 95%–99% is produced by mixing silica (SiO2) and coke in a certain ratio and heating them in an electric furnace to 1600–1800°C. 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 exist in the form of silicates resulting from silicidation. To further increase the purity of industrial crude silicon, acid leaching can be used to dissolve most of these impurities (with a small amount of silicon carbide remaining insoluble). The production process involves crushing crude silicon, then treating it successively with hydrochloric acid, aqua regia, and a mixed acid of HF and H2SO4; finally, it is washed with distilled water until it reaches neutrality, and after drying, industrial crude silicon with a purity of 99.9% is obtained. There are many methods for preparing high-purity polycrystalline silicon; according to Bu’s comprehensive statistics, there are over a dozen such methods. All of these methods start with industrial silicon (or ferrosilicon, as it contains a high amount of iron). First, silicon-containing intermediate compounds such as SiCl4, SiHCl3, and SiH4 are produced, as these compounds are easy to purify and reduce. Subsequently, these intermediate compounds are purified, reduced, or transformed into high-purity polycrystalline silicon. The general process flow is shown in Figure 1. At present, the main methods used in China for producing high-purity polycrystalline silicon are the hydroreduction of trichlorosilane, the thermal decomposition of silanes, and the hydroreduction of silicon tetrachloride. Generally speaking, due to the certain advantages of the silicon trichloride hydrogen reduction method, it is currently widely used. Furthermore, since SiH4 is easy to purify, the thermal decomposition of silane is a method with great potential for producing high-purity silicon. Next, we will introduce separately the chemical principles behind the use of the above three methods for preparing high-purity silicon. 1. Silicon trichloride hydrogen reduction method (1) Synthesis of silicon trichloride: Step 1: Production of crude silicon from silica. Silica (SiO2) is mixed with an appropriate amount of coke and heated to 1600–1800°C in an electric furnace, resulting in crude silicon with a purity of 95%–99%. The reaction equations are as follows: SiO2 + 3C = SiC + 2CO(g) ↑ 2SiC + SiO2 = 3Si + 2CO(g) ↑ The overall reaction equation is: SiO2 + 2C = Si + 2CO(g) ↑ The silicon produced is discharged from the bottom of the electric furnace and cast into ingots. The crude silicon produced by this method can achieve a purity of 99.9% after acid treatment. Step 2: Synthesis of trichlorosilane. Trichlorosilane is synthesized by reacting dry hydrogen chloride gas with crude silicon powder in a synthesis furnace at 250°C. Its main reaction equation is as follows: Si + 3HCl = SiHCl3 + H2(g) (2) Purification of trichlorosilane: Trichlorosilane obtained from the synthesis furnace often contains impurities such as boron, phosphorus, arsenic, and aluminum; these are harmful impurities that have a significant impact on the quality of single-crystal silicon, and it is necessary to remove them. In recent years, the purification methods for trichlorosilane have developed rapidly. However, due to the simplicity and ease of operation of the distillation method, it is still the primary technique used in industry today. Silicon trichloride distillation is a method for separating and purifying silicon trichloride by taking advantage of the difference in boiling points between it and impurity chlorides. Typically, synthetically produced trichlorosilane contains chlorides such as boron trichloride (BCl3), phosphorus trichloride (PCl3), silicon tetrachloride (SiCl4), arsenic trichloride (AsCl3), and aluminum trichloride (Al2Cl3). The boiling points of the vast majority of these chlorides differ significantly from that of trichlorosilane; therefore, these impurities can be removed by distillation. However, the boiling points of boron trichloride and phosphorus trichloride are similar to that of silicon trichloride hydroxide, making them difficult to separate; therefore, efficient distillation is required to remove these two impurities. The boron removal effect of distillation purification has certain limitations; therefore, the complexation method, which offers a better boron removal efficiency, is also used in industry. Trichlorosilane has a low boiling point and is flammable and explosive; all operations must be carried out at low temperatures, with the operating environment temperature generally not exceeding 25°C. Moreover, contact with sparks must be strictly avoided throughout the process to prevent explosive combustion. (3) Hydrogen reduction of trichlorosilane: After mixing trichlorosilane with high-purity hydrogen, the mixture is fed into a reduction furnace at 1150°C to carry out the reaction, thereby yielding silicon. The overall chemical reaction is: SiHCl3 + H2 = Si + 3HCl. The resulting high-purity polycrystalline silicon deposits on a polycrystalline silicon substrate. Chemical principles of high-purity silicon preparation (2) 1. Silane pyrolysis method Among the methods for preparing high-purity silicon, the silane pyrolysis method is one with promising prospects. The entire process of this method can be divided into three parts: the synthesis of SiH4, purification, and thermal decomposition. (1) Synthesis of silanes: The magnethermal decomposition of osmanthus to produce silanes is a method widely used in industry at present. Magnesium silicide (Mg2Si) is synthesized by mixing silicon powder and magnesium powder and heating them at 500–550°C in hydrogen (or also in a vacuum or under Ar gas). The reaction equation is as follows: 2Mg + Si = Mg2Si. Subsequently, magnesium silicide is reacted with solid ammonium chloride in liquid ammonia to produce silane. Mg2Si + 4NH4Cl = SiH4↑ + 2MgCl2 + 4NH3↑ Here, liquid ammonia serves not only as a medium but also provides a low-temperature environment. The silane obtained in this way is relatively pure, but in actual production unreacted magnesium is still present, which leads to the following side reaction: Mg + 2NH4Cl = MgCl2 + 2NH3 + H2↑. As a result, the silane gas produced often contains hydrogen gas as well. The ammonium chloride used in production must be dry; otherwise, the product formed when magnesium silicide reacts with water is not silane but hydrogen. The reaction equation is as follows: 2Mg2Si + 8NH4Cl + H2O = 4MgCl2 + Si2H6O3 + 8NH3↑ + 6H2↑. Since silane is flammable in air and can explode at high concentrations, the entire system must be isolated from oxygen, and contact with outside air is strictly prohibited. (2) Purification of silanes Silanes are gaseous at room temperature, and generally, gas purification is easier than that of liquids and solids. Since the formation temperature of silanes is low, most metal impurities do not easily form volatile hydrides at such low temperatures; and even if they do form, their high boiling points prevent them from volatilizing along with the silanes. As a result, the silanes undergo cooling during their formation process, which effectively removes those impurities that cannot form volatile hydrides. Silane is produced in liquid ammonia; at low temperatures, diborane (B2H6) reacts with liquid ammonia to form a non-volatile complex (B2H6•2NH3) that is removed, as a result of which the resulting silane is free from boron impurities. This is one of the advantages of the silane method. However, silanes also contain impurities such as ammonia, hydrogen, and trace amounts of phosphine (PH3), hydrogen sulfide (H2S), arsine (AsH3), antimony hydride (SbH3), methane (CH4), and water. Due to the large difference in boiling points between silanes and these substances, water and ammonia can be removed using low-temperature liquefaction methods, while other impurities can be removed through distillation for purification. In addition, the adsorption method, preheating decomposition method (since the decomposition temperature of silane is as high as 600°C, while the decomposition temperatures of the hydrogen transform gases of other impurities are all below 380°C, controlling the temperature of the preheating furnace at around 380°C can decompose the hydrogen transform compounds of these impurities, thereby achieving the purification of silane), or a combination of various methods can also be used to achieve purification. (3) Thermal decomposition of silanes: Silane gas is introduced into a silane decomposition furnace, where it decomposes on a heated silicon core at 800–900°C, resulting in the deposition of high-purity polycrystalline silicon. The reaction equation is as follows: SiH4 → Si + 2H2 ↑. The thermal decomposition method of silanes has the following advantages: ① No reducing agent is used in the decomposition process, thus there is no risk of contamination by reducing agents. ② The silane has high purity. Metal impurities are effectively removed during the silane synthesis process. It is particularly valuable because ammonia has a strong chelating effect on borohydrides, enabling the removal of boron, the most difficult-to-separate harmful impurity in silicon. Furthermore, silanes can be purified using molecular sieves that have a high adsorption capacity for impurities such as phosphines, arsines, hydrogen sulfide, and boranes, thereby yielding products of high purity; this is another notable advantage of the silane method. ③ The decomposition temperature of silanes is generally 800–900°C, which is much lower than that of other methods; therefore, fewer impurities are introduced through high-temperature volatilization or diffusion. At the same time, the decomposition products of silanes are non-corrosive, thereby preventing corrosion of the equipment as well as contamination of silicon due to corrosion. Both the hydrogen reduction of silicon tetrachloride or silicon trichlorohydride produce highly corrosive hydrogen chloride gas. Since silane gas is a flammable and explosive gas, the entire adsorption system as well as the decomposition chamber must have a high degree of airtightness to prevent air from entering. Two methods are commonly used for storing and transporting silanes: one involves using molecular sieves to adsorb the silanes, which can then be transported using neon gas ； Another method is to press silane into a steel cylinder and then dilute it with hydrogen to reduce its concentration to below 5%, thereby avoiding the risk of explosion and combustion. 2. Silicon tetrachloride hydrogenation reduction method (1) Chlorination of industrial crude silicon to produce silicon tetrachloride: Currently, the industrial method for producing SiCl4 is generally the direct chlorination method, in which industrial crude silicon reacts directly with chlorine under heating conditions to yield SiCl4. In industry, chlorination furnaces made of stainless steel (or quartz) are commonly used. Ferrosilicon is placed in such furnaces, and chlorine gas is introduced from the bottom of the furnace. When the temperature reaches 200–300°C, a reaction begins to produce SiCl4; the chemical reaction is: Si + 2Cl2 = SiCl4. The SiCl4 produced moves in gaseous form from the upper part of the furnace to the condenser, where it is cooled to a liquid state before being sent to a storage tank. In production, the chlorination temperature is generally maintained between 450 and 500°C. This not only improves productivity but also ensures quality, as low temperatures result in a slow reaction rate as well as the formation of by-products such as Si2Cl6 and Si3Cl8, which affect the purity of the product. On the other hand, if the temperature is too high, other volatile impurities present in ferrosilicon will also volatilize along with SiCl4, thereby affecting the purity of SiCl4. (2) Distillation for the purification of silicon tetrachloride: Silicon tetrachloride usually contains impurities such as iron, aluminum, titanium, boron, and phosphorus, but these impurities can be removed by distillation. The principle is that, due to the different boiling points of silicon tetrachloride and impurities, they have varying volatilities; therefore, by controlling the temperature, SiCl4 can be separated from the impurities to achieve purification. (3) Hydrogen reduction of pure silicon tetrachloride: Purified silicon tetrachloride reacts with high-purity hydrogen in a high-temperature reduction furnace to produce high-purity silicon; the reaction is as follows: SiCl4 + 2H2 = Si + 4HCl↑. The actual reaction is rather complex. Since the rate of hydrogen reduction of SiCl4 is lower than that of the SiHCl3 hydrogen reduction method, the SiCl4 hydrogen reduction method is less commonly used to produce high-purity silicon at present.