HIGH-TEMPERATURE OXIDATION OF CVD BETA-SIC .2. RELATION BETWEEN OXYGEN DIFFUSION-COEFFICIENTS AND PARABOLIC RATE CONSTANTS

The high temperature (1200-1550-degrees-C) oxidation kinetics of SiC p repared by chemical vapor deposition (CVD) in dry oxygen (3 x 10(-2) - 80 atm) were parabolic after an initial linear step. The parabolic re gime is controlled by two processes: at low temperatures and high pres sures the rate-limiting step is molecular oxygen permeation through th e SiO2 film; at higher temperatures and lower pressures ionic oxygen d iffusion becomes the dominant process. A flux equation relating the pa rabolic rate constants with the molecular and ionic oxygen diffusivity confirms the existence of two parallel processes during the oxidation of CVD beta-SiC. Double oxidation (O-16(2)/O-18(2)) experiments have been performed to obtain additional information about the oxygen diffu sion coefficients through amorphous SiO2 and beta-cristobalite.