The production technology of polysilicon is mainly to improve the Siemens method and the silane method. The Siemens method produces columnar polycrystalline silicon by vapor deposition. In order to improve the utilization of raw materials and environmental friendliness, the closed-loop production process is adopted on the basis of the former, that is, the improved Siemens method. The process reacts industrial silicon powder with HCl, processes it into SiHCI3, and then reduces and deposits SiHCl3 in a reduction furnace of H2 atmosphere to obtain polycrystalline silicon. The off-gas H2, SiHCl3, SiCl4, SiH2Cl2 and HCl discharged from the reduction furnace are separated and recycled. In the silane method, silane is introduced into a fluidized bed in which polycrystalline silicon seeds are used as fluidized particles, and silane is cleaved and deposited on the seed crystals to obtain granular polycrystalline silicon. The modified Siemens method and silane method mainly produce electronic grade crystalline silicon, and can also produce solar grade polycrystalline silicon.
Siemens law
The Siemens method was invented by the German company Siemens and applied for a patent in 1954 to achieve industrialization around 1965. After decades of application and exhibition, Siemens Law has been continuously improved, and the first, second and third generations have appeared successively. The third generation polysilicon production process is the improved Siemens method, which has been added on the basis of the second generation. The reduction tail gas dry recovery system and the SiCl4 recovery hydrogenation process realize complete closed-loop production. It is the latest technology for the production of high-purity polysilicon technology by Siemens. The recycling of silicon in the Siemens process polysilicon production process.
Silane method
The silane method is to pass silane into a fluidized bed in which polycrystalline silicon seeds are used as fluidized particles, which are silane-cracked and deposited on the seed crystals, thereby obtaining granular polycrystalline silicon. Due to the different preparation methods of silane, there is a magnesium silicide method invented by Komatsu of Japan. The specific process is shown in Fig. 2, the disproportionation method invented by Union Carbide in the United States, and the NaAlH4 and SiF4 reaction method used in MEMC in the United States.
The magnesium silicide method uses Mg2Si to react with NH C1 in liquid ammonia to form a silane. Due to the large consumption of raw materials, high cost and high risk, the method has not been promoted. Currently, only Komatsu of Japan uses this method. Modern silane is prepared by disproportionation method, which uses metallurgical grade silicon and SiC14 as raw materials to synthesize silane. Firstly, SiCl3 is formed by SiCl4, Si and H2 reaction, then SiHCl3 is disproportionated to form SiH2Cl2, and then SiH2Cl2 is used for catalytic disproportionation to form SiH4. 3SiCl4+ Si+ 2H2=4SiHCl3, 2SiHC13=SiH2Cl2+ SiC14, 3SiH2C12=SiH4+ 2SiHC13. Since the conversion efficiency of each step is relatively low, the material needs to be circulated many times, and the whole process is repeatedly heated and cooled, so that the energy consumption is relatively high. The obtained silane is purified by rectification, passed through a fixed-bed reactor similar to the Siemens method, and thermally decomposed at 800 ° C, and the reaction is as follows: SiH4 = Si + 2H2.
The silane gas is a toxic and flammable gas, the boiling point is low, the reaction equipment should be sealed, and safety measures such as fire prevention, antifreeze and explosion protection should be provided. Silane is also known for its unique self-ignition and explosive properties. Silane has a very wide spontaneous ignition range and extremely high combustion energy, which determines it is a high-risk gas. The application and promotion of silanes is largely limited by their high-risk properties in engineering or experiments involving silanes. Improper design, operation or management can cause serious accidents and even disasters. However, practice shows that excessive fear and improper precautions do not provide the safety guarantee for the application of silane. Therefore, how to use silane safely and effectively has always been a concern of production lines and laboratories.
Compared with the Siemens method, the silane thermal decomposition method has the following advantages: silane is easy to purify, silicon content is high (87.5%, decomposition rate is fast, decomposition rate is as high as 99%), decomposition temperature is low, and energy consumption of polycrystalline silicon is generated. Only 40 kW · h / kg, and the product purity is high. However, the shortcomings are also prominent: silane is not only expensive to manufacture, but also flammable, explosive, and safe. There have been accidents in the silane plant that have exploded in foreign countries. Therefore, in industrial production, the application of the silane thermal decomposition method is not as good as the Siemens method. Although the improved Siemens method currently has the largest market share, its operating risks are also the biggest due to its inherent shortcomings in technology: low yield, high energy consumption, high cost, large capital investment, slow capital recovery, etc. Only by introducing advanced technologies such as plasma enhancement and fluidized bed, and strengthening technological innovation, it is possible to improve market competitiveness. The advantages of the silane method are conducive to serving the chip industry. At present, its production safety has been gradually improved, and its production scale may rapidly expand, even replacing the improved Siemens method. Although the modified Siemens method is widely used, the silane method has a promising future.
Similar to the Siemens method, fluidized bed technology has also been introduced into the thermal decomposition process of silane in order to reduce production costs. The fluidized bed decomposition furnace can greatly increase the decomposition rate of SiH4 and the deposition rate of Si. However, the purity of the obtained product is not as good as that of the fixed bed decomposition furnace technology, but it can fully meet the solar grade silicon quality requirements, and the safety of silane still exists.
MEMC Corporation of the United States has realized mass production using fluidized bed technology. It uses NaA1H4 and SiF4 as raw materials to prepare silane. The reaction formula is as follows: SiF4+NaAlH4=Sil4+4NaAlF4. The silane is purified and decomposed in a fluidized bed type decomposition furnace at a reaction temperature of about 730 ° C to obtain a granular polycrystalline silicon having a size of 1000 μm. The method has low energy consumption, and the decomposition power consumption of granular polycrystalline silicon production is about 12 kW·h/kg, which is about 1/10 of the modified Siemens method, and the primary conversion rate is as high as 98%, but there is a large amount of dust in the micrometer scale in the product, and Granular polycrystalline silicon has a large surface area and is easily contaminated. The product has a high hydrogen content and must be dehydrogenated.
Metallurgy
The metallurgical preparation of solar grade polycrystalline silicon (Solar Grade Silicon for short) is referred to as metallurgical grade silicon (MetallurgicalGrade Silicon MG-Si) (98.5%~99.5%). A method for producing a polycrystalline silicon raw material for a solar cell with a purity of 99.9999% or more by metallurgical purification. In the service of solar photovoltaic power generation industry, metallurgical technology has the advantages of low cost, low energy consumption, high output rate and low investment threshold. By developing a new generation of energy-carrying high vacuum metallurgy technology, the purity can reach 6N or more. In a few years, it has gradually developed into a mainstream preparation technology for solar grade polysilicon.
Different metallurgical grade silicon contain different impurity elements, but the main impurities are basically the same, mainly including impurity elements such as Al, Fe, Ti, C, P, B. Moreover, some effective removal methods have been studied for different impurities. Since Wacker's casting method used polysilicon materials in 1975, the metallurgical preparation of solar grade polysilicon is considered to be a production method that effectively reduces production costs and is specifically positioned in solar multi-level polysilicon to meet the rapid development needs of the photovoltaic industry. A technical route for preparing solar grade polysilicon for different impurity properties.
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