Ellipsometry and transmission electron microscopy study of MoSi2 coatings after oxidation at high temperature in air

MoSi₂ coatings were deposited on Si (100) substrates by means of magnetron sputtering. The structural and optical properties of the coatings were then investigated through transmission electron microscopy (TEM) and UV-visible-near IR ellipsometry after heat treatments in air at two temperatures (1100 K and 1600 K). After annealing at 1100 K, no silica layer could be observed by TEM at the surface of the MoSi₂ coating, whereas annealing at 1600 K gave rise to a protective silica layer. As observed by TEM, this silica layer contained nanometric molybdenum silicide crystals. Increasing the annealing time at 1600 K from 20 min to 2 h resulted in both a higher concentration and growth of these molybdenum silicide crystals in the silica layer. An optical model, taking into account the observed microstructure and allowing a good fit of the ellipsometry data, was developed. It is also shown that the dielectric constants of the non-oxidized part of the MoSi₂ layer became modified when the annealing temperature (1100 K versus 1600 K) and the annealing time at 1600 K (20 min versus 2 h) were increased.