A comparison of the oxidation of sodium-implanted CVD Si3N4 with the oxidation of sodium-implanted SiC-crystals

• Published in: Corrosion science 1992-04, Vol.33 (4), p.569-580
• Main Authors: Zheng, Z., Tressler, R.E., Spear, K.E.
• Format: Article
• Language: English
• ISSN: 0010-938X; 1879-0496
• DOI: 10.1016/0010-938X(92)90205-H

The effects of sodium contamination on the oxidation kinetics of CVD Si3N4 were examined at temperatures of 1100–1300°C. Sodium (50 ppm and 1000 ppm, respectively) was introduced into the material by using a multilayer ion implantation technique. The results of oxidation studies of sodium implanted CVD Si3N4 were compared with those of sodium-implanted SiC-crystals reported in an earlier paper. Sodium contamination accelerates the oxidation of both SiC and Si3N4 materials. The dependence of the parabolic rate constant on sodium concentration for oxidation of Si3N4 is much greater than that for the oxidation of SiC. While the parabolic rate constants for the oxidation of sodium-implanted SiC samples are no more than twice that of unimplanted SiC samples at all temperatures investigated, the parabolic rate constant for the oxidation of the 1000 ppm sodium implanted Si3N4 samples is 14 times that of unimplanted Si3N4 samples at 1100°C. The apparent activation energy for the oxidation of sodium-implanted SiC-crystals is similar to that of unimplanted SiC-crystals (∼130 kJ mol −1). The apparent activation energy values decrease with increasing sodium contamination level for the oxidation of Si3N4, i.e. the activation energy decreases from 460 kJ mol−1 for the oxidation of unimplanted Si3N4 samples to 404 kJ mol−1 for the oxidation of 50 ppm sodium implanted Si3N4 samples, and to 384 kJ mol−1 for the oxidation of 1000 ppm sodium implanted Si3N4 samples. The measured sodium concentrations in the oxides by atomic absorption spectrophotometry are close to calculated sodium concentrations in oxides assuming that all of the sodium from the SiC and Si3N4 consumed by oxidation remains in the oxide. Step etch-back experiments and a double oxidation experiments using 18O2/16O2 gas mixture were performed to explore the mechanism through which sodium accelerates the oxidation of Si3N4. Accelerated oxidation of CVD Si3N4 is suggested to be caused by accelerated transport of oxidants through the thin silicon oxynitride oxidation product layer which is sandwiched between the silica formed during the oxidation and the silicon nitride substrate. This accelerated rate is presumably due to the dissolution of single bonded sodium into the oxynitride layer, which would be expected to facilitate the diffusion of oxidants.