Siemens Production Of Polysilicon Thermodynamics

The complex chemical reactions in the Si-C1-H system with relation to Siemens process were studied by means of calculation based on the selected thermodynamic data. The standard equilibrium constant, equilibrium gas phase composition, yieldη, factor a and others of the ternary system at various conditions were computed by selecting a set of independent chemical reactions. Parts of the important calculations were tabularly displayed, they included theΔ_rG_m^Θand K_p^Θvalue of the fifteen reactions at a variety of temperatures, the yieldηand the factorα(Product ofηwith the initial molar fraction of SiHCl₃) of polycrystalline silicon at different pressure of the system, the equilibrium mole fraction of SiHCl₃, SiCl₄ and SiCl₂ at a series of n_H2⁰/ n_SiHCl3⁰, and so on. Diagrams ofΔ_rG_m^Θand K_p^Θfor the fifteen chemical reactions in the Siemens system have been plotted against thermodynamic temperature T; graphs of equilibrium gas phase composition at a host of temperature and pressure and the residual amount of SiHCl₃ as a function of temperature have been drawn too; the effects of n_H2⁰/n_SiHCl3⁰, pressure, and temperature on the equilibrium yield were also graphically presented; in addition, influence of initial molar fraction of SiHCl₃ upon factorαfor different temperature and total pressure were shown in figure.The results from this thermodynamic study show that:(1) When the temperature is relatively low and the system pressure is higher than 101325Pa, the gas phase products are mainly HCl, SiCl₄ and SiH₂Cl₂. But as the pressure is lower than 101325Pa, the major products in gas phase are HCl and SiCl₄. However, in the case of high temperature, HCl and SiCl₄ are principal product in gas phase no matter how low or high the pressure is.(2) The reaction SiHCl₃(g)+H₂(g)=Si(s)+3HCl(g) is predominant over the others at high hydrogen proportioning while the system pressure is constant and the temperature is fairly low. But under the conditions of high temperature, low pressure, moderate or low hydrogen proportioning, SiHCl₃(g)=SiCl₂(g)+HCl(g) is the primary reaction and is highly sensitive to pressure and temperature of the system.(3) When the ratio of hydrogen to SiHCl₃ is high and the pressure is kept constant, the yields of the polycrystalline silicon increases gradually increase in temperature. What is more, the reduction of SiHCl₃ by hydrogen is more sufficient. However, as the temperature reaches some value, the yield is inversely proportional to temperature.(4) While keeping the hydrogen proportioning or temperature constant, the yield is in inverse proportion to the system pressure. But the abnormality on the yield increasing with increasing the pressure appears at high temperature, little hydrogen proportioning and negative air pressure.(5) The pressure or temperature being held constant, the yieldηof polycrystalline silicon for the system increases but factorαdecreases with increasing hydrogen proportioning. Thereforeηandαfactors should be given a comprehensive survey practically, so that achieve a rather high yield for Si on the basis of guaranteeing the production.