CVD of ceramic coatings in a hot wall and fluidised bed reactor

Abstract

Silicon based ceramic type compounds were chemically deposited by the decomposition of methyltrichlorosilane in a hot wall reactor and a fluidised bed reactor; using nitrogen and/ or hydrogen as a carrier gas. A hot wall reactor was designed and fabricated from a high purity carbon cylinder. This system was later converted to a fluidised bed unit. The coating was effected by injecting methyltrichlorosilane into the reactor at 973, 1073 and 1173 K. The other coating variables included methyltrichlorosilane feed rate and carrier gas (nitrogen and/ or hydrogen) concentration. The effectiveness of varying reaction conditions were evaluated by examining the coatings under Scanning Electron Microscopy for coating thickness, coating uniformity and adherence to the steel plate. From this data, a comparative assessment was made on the efficiency and applicability of hot wall and fluidised bed reactor. The results clearly indicated that fluidised bed technique provided superior coatings in comparison with hot wall reactor. Scanning Electron Microscopy and X-ray diffraction results also indicated that at applied temperatures silicon dioxide, silicon carbide, silicon and carbon were the most prevalent species. Thermodynamic analysis of methyltrichlorosilane/ nitrogen/ hydrogen system under varying reaction conditions was conducted using the CSIRO Thermal Chemistry System computer code CHEMIX, which is based upon the free energy minimisation method. In order to calculate the equilibrium composition of this system, 37 gaseous and S condensed species were taken into consideration. The thermodynamic variables included temperature (500°C-1000°C), mole ratios of nitrogen to methyltrichlorosilane and hydrogen to methyltrichlorosilane. Thermodynamic calculations were in crood 0 agreement with the experimental results except that formation of silicon dioxide was not predicted. Mass spectrometry, however, indicated the presence of hydroxy derivatives of silanes, which probably on decomposition resulted in the formation of silicon dioxide. Kinetics of the reaction were performed using PE DSC 7, and also from the deposition rates. Lower activation energies (14.19kJ /mole) indicate that the reaction is not temperature dependant. Based on the Mass Spectrometry data and the information obtained from thermodynamic calculations, an attempt was made to predict the chemistry of the system. A plausible reaction mechanism is proposed under studied experimental conditions. It is believed that silicon carbide, silicon, and silicon dioxide are the main species deposited. Formation of silicon dioxide is possible, in the presence of water molecules. It is presumed that the water vapour was absorbed by methyltrichlorosilane, a highly hygroscopic material, during feeding. The formation of carbon can be avoided by selecting the operating conditions which favour the formation of silicon carbide, i.e., higher temperature and higher concentrations of hydrogen.