Abstract
Novel technical solutions and ideas for increasing the yield of solar and semiconductor grade polycrystalline silicon processes have been analyzed. The predominant polycrystalline silicon technology is currently still the Siemens process including the conversion of technical grade silicon (synthesized by carbon-thermal reduction of quartzites) to trichlorosilane followed by rectification and hydrogen reduction. The cost of product silicon can be cut down by reducing the trichlorosilane synthesis costs through process and equipment improvement. Advantages, drawbacks and production cost reduction methods have been considered with respect to four common trichlorosilane synthesis processes: hydrogen chloride exposure of technical grade silicon (direct chlorination, DC), homogeneous hydration of tetrachlorosilane (conversion), tetrachlorosilane and hydrogen exposure of silicon (hydro chlorination silicon, HC), and catalyzed tetrachlorosilane and dichlorosilane reaction (redistribution of anti-disproportioning reaction). These processes remain in use and are permanently improved. Catalytic processes play an important role on silicon surface, and understanding their mechanisms can help find novel applications and obtain new results. It has been noted that indispensable components of various equipment and process designs are recycling steps and combined processes including active distillation. They provide for the most complete utilization of raw trichlorosilane, increase the process yield and cut down silicon cost.
Highlights
The production of polycrystalline silicon using the Siemens method or the silane method involves the formation of large amounts of tetrachlorosilane (SiCl4)
Tetrachlorosilane was hydrated in a graphite tube at 600 to 1200 °C with a SiCl4 : H2 = 1 : 1–1 : 50 molar ratio in equilibrium with trichlorosilane and hydrogen and rapid mixture cooling to 300 °C for 50 ms using a cooling liquid
Process simulation in an ideal replacement flow-type reactor [6] taking into account 63 possible reactions and 26 reaction products showed that the highest conversion efficiency (39,34%) is achieved at a SiCl4 : H2 = 1 : 4 molar ratio, T = 1200 °С and a 50,92 K ∙ s−1 cooling rate
Summary
The production of polycrystalline silicon using the Siemens method or the silane method involves the formation of large amounts of tetrachlorosilane (SiCl4). Pressure, H2 : SiCl4 molar ratio, contact duration and silicon powder coarseness, the authors could achieve a trichlorosilane yield of 14 to 38 mol.%. Later on this reaction was studied in laboratory reactors with an immobilized silicon layer [12,13,14,15,16] and in laboratory fluidized bed reactors [17, 18]. The SiCl4 : H2 ratio in the reaction mixture is maintained at 0.25 : 1–4 : 1, the process temperature was maintained at 400–800 °С and pressure at up to 40 MPa. The average silicon grain size should be within 10–1000 mm and the catalyst grains should be 30–100 times smaller [27]. This option reduces sewage water emission, wastes and abgases
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