Silane is actually an abbreviation for monosilane. Silane is easy to thermally decompose, and high-purity polysilicon can be obtained by decomposing at 800~900℃, and the reduction energy consumption is low. In addition, monosilane is easy to purify and is a gas at room temperature, so it can be effectively removed by adsorption purification methods. The first is the preparation of silane. There are many methods for preparing monosilane. For example, the ratio of silicon powder and electrolytic magnesium chips is 7:12, and liquid ammonia is used as the medium to react in the reactor at about -33°C to generate silane gas. The generated silane gas passes through a reflux condenser to separate and remove ammonia and magnesium chloride. The separated silane gas is adsorbed by molecular sieves (or activated carbon, silica gel, etc.) to purify the silane gas. Since various metal impurities cannot generate similar hydrides or other volatile compounds, the impurities in the crude silicon are first removed in a large amount during the process of silane generation. Silane is a gas at room temperature, and rectification must be carried out at low temperature or under low temperature and very pressure.
In addition, it also includes decomposition. In the thermal decomposition furnace, silane gas is decomposed to obtain pure silicon and hydrogen. The decomposition temperature of silane is low, and good polycrystalline crystals can be obtained at 850°C, and the yield of silicon can reach more than 90%. However, monosilane is easily decomposed into amorphous silicon when the temperature is above 500℃, and amorphous silicon is easy to adsorb impurities. It is also difficult to maintain the purity of amorphous silicon that has reached high purity. Therefore, the production of amorphous silicon cannot be allowed during the thermal decomposition of silane. . To improve the quality of polycrystalline by the silane method, techniques such as hydrogenation dilution thermal decomposition can be used. When monosilane is decomposed, polycrystalline silicon is deposited on a thin silicon rod (silicon core) heated to 850°C.
Silane gas is a toxic and flammable gas with a low boiling point. The reaction equipment should be sealed, and safety measures such as fire prevention, antifreeze, and explosion prevention should be taken. The disadvantage of this method is that the crystalline state of polycrystals during thermal decomposition is not as good as other methods, and it is easy to form amorphous substances.
Other chemical methods for preparing solar-grade polycrystalline silicon include: ① Tokuyama’s Vapor to Liquid Deposition method, which uses SiHCl3 as a raw material for gas phase reaction in a barrel reactor to directly precipitate liquid silicon: The precipitation rate of the method is 10 times faster than that of the Siemens method, which greatly improves the production efficiency and reduces the cost; ②The improved fluidized bed method of Wacker and SGS uses SiHCl3 and SiH4 as raw materials for reduction and thermal decomposition processes.
In short, the chemical production of polysilicon has a huge investment, a complicated process, and serious pollution hazards.