Study On Process Of Polysilicon CVD In The Fluidized-Bed Reactor Based On CFD

This paper was focused on the process of polysilicon chemical vapor deposition in a fluidized-bed reactor. A numerical model was established using the CFD source code Mfix, which is coupled with chemical reactions, heat transfer, mass transfer, and momentum transfer. The hydrosilicon thermodynamics database was expanded. The reaction process and the distribution of temperature, pressure, gas velocity, particle size in the fluidized-bed reactor were analyzed, and the mechanism of nucleation of amorphous silicon was studied. The specific work is as follows:1. According to the results of quantum chemistry, the molecular structures of 102 hydrosilicons (Si1-Si10) were determined. The standard formation enthalpy, the standard molar entropy, and the specific heat of hydrosilicons in seven temperature (300 K,400 K,500 K,600 K,800 K,1000 K and 1500 K) were determined by the group-contribution method.15 thermodynamic properties polynomial coefficients of 104 hydrosilicons were calculated by using Matlab software programming. All of the above substances molecular weight was calculated by means of chemical molecular weight calculator. NIST database, the commercial software (Fluent, Chemkin, Mfix etc.), experimental data and simulation results in the literature were compared in this paper to correct properties of H2 and SiH4 in Mfix. The thermodynamic datas of 102 substances(SiH2, Si2H6, Si3H8 etc.) were supplied. The hydrosilicon thermodynamics database in this paper can accurately calculate the specific heat, entropy, enthalpy and other thermodynamic properties of hydrosilicons, and it is the basis of the. numerical simulation of reaction source term in governing equations and amorphous silicon nucleation.2. The kinetic parameters of 2 surface chemical reactions and 220 gas phase reactions in polysilicon CVD process were determined. The source terms of reaction rate, chemical reaction heat, radiation heat transfer, heat transfer coefficient, particle collision dissipation were supplied in governing equations. The chemical reaction, mass transfer, momentum transfer, heat transfer were coupled. Cartesian grid partition method was used to accelerate the calculation speed and accuracy. Two order upwind scheme was used to discrete the governing equations. SIMPLE algorithm was used to solve the pressure coupled equations.3. By simulating of fluidization polysilicon CVD process with experimental conditions of Caussat and Hsu, the polysilicon CVD rate error was 0.8%-30% and 19%-28% respectively. It means that the mathematical model established in this paper can accurately describe the fluidization polysilicon CVD process.Considering the polysilicon CVD reaction rate and inhibiting the formation of amorphous silicon, The optimum fluidizing gas of hydrogen, operating gas ratio of 3/17 (namely the silane inlet concentration 15%), inlet gas velocity of 5.5 times of the minimum fluidization velocity, operating temperature of 963.15 K, and operating pressure of 0.2 MPa were selected.4. The mass, velocity, temperature distribution of gas phase in the reactor outlet were analyzed. The axial and radial gas distribution, and the axial solid fraction distribution in the reactor were also analyzed. The amorphous silicon nucleation mechanism was explored. It reveals that:silane and silylene heterogeneous pyrolysis are the main reactions, gas phase reactions are suppled in the reactor; mass distribution of the gas phase is directly related to the velocity distribution, temperature distribution; the influence of axial gas velocity distribution is higher than the radial; mass fraction of amorphous silicon in the dilute phase is large, and low temperature can inhibit the formation of amorphous silicon.The thermodynamic database can accurately express the hydrosilicons specific heat, enthalpy and entropy of the thermodynamic properties in different pressure and temperature; it is the foundation for the reliability of the results in this paper. The results of this study provide guidance for the design of inner components in polysilicon CVD fluidized-bed reactor, and the simulation results also give the theoretical support for the design of high purity polysilicon production process.