CVD PROCESS FOR Si-N-O FIBER GROWTH CONTROLLED BY IN-SITU FTIR SPECTROSCOPIC MONITORING

摘要

High-performance amorphous Si-N-O fibers are grown by a high temperature chemical vapor deposition (CVD) process. The fibers are grown at 1450 deg C by exposing a stoichiometric precursor powder mixture of SiO_2 + SiC to flowing NH_3. The reaction has been found to occur by a two stage reaction mechanism, the first involving the production of SiO(g) by the reduction of SiO_2 by SiC, and the second the ammonolysis of SiO(g) by NH_3 or related radicals NH_2 or NH. Nucleation occurs on the surface of the SiO_2 precursor by the reaction between adsorbed NH_x molecules and SiO(g). The fibers grow initially as thin filaments from the fiber tip and continue to grow in the longitudinal direction, provided the supply of reactants is sufficient. Simultaneous deposition of material on the surface occurs, resulting in radial growth. The overall volume growth rate was found to be controlled by a surface adsorption reaction.Measurements of the reaction gases have been carried out by mass spectrometry and gas chromatography. In order to improve CVD process control, an in-situ Fourier Transformed Infrared (FTIR) monitoring system has been developed. The set-up allows the measurement of the hot gas atmosphere directly above the precursor powder mixture. In addition to the decomposition of NH_3, different reaction products have been identified, such as CO, HCN and CH_4. Gaseous SiO species could be detected which are proposed to be responsible for the silicon transport in the gas phase from the solid SiO_2 precursor powder to the fiber growth location. The interpretation of the high temperature spectra has been supported by spectra calculations.