AEROSOL FORMATION BY RAPID NUCLEATION DURING THE PREPARATION OF SIO2 THIN-FILMS FROM SICL4 AND O2 GASES BY CVD PROCESS

The formation of aerosol particles and molecular clusters by rapid nucleation in the semi-conductor thin-film preparation process by atmospheric pressure chemical vapor deposition (APCVD) or low pressure chemical vapor deposition (LPCVD) is evaluated by a vapor nucleation and diffusion model. Such a model is applicable for situations corresponding to very high saturation ratios in which the evaporation of monomers from clusters is insignificant as compared to the growth by coagulation. The physical processes taken into account include simultaneous generation of vapor monomers by chemical reaction, aerosol coagulation, and convective diffusion of various species onto the thin-film surface. SiO2 thin-film preparation by thermal decomposition of SiCl4 vapor in O2 gas using a horizontal-type CVD reactor is simulated as an illustration of the model. The mass and number concentrations of various species are computed as functions of temperature, pressure, input SiCl4 concentration, and position in the reactor; the individual contributions of monomers, clusters and particles to the thin-film growth rate are then evaluated. Based on two dimensionless parameters representing the importance of diffusive deposition of monomers relative to their generation and coagulation, the most suitable conditions under which the thin-film growth is dominated by monomer diffusive deposition are discussed.