Surface reactivity of molybdenum thin films exposed to (Ar–N2–H2) expanding microwave plasma at low temperature: influence of the addition of H2 gas in the plasma

Abstract

The rapid transfer of a large amount of nitrogen into the surface layers followed by diffusion into the inner layers occurs in thin molybdenum films exposed to expanding microwave plasma using (Ar–35%N2) and (Ar–25%N2–30%H2) gas mixtures. The nitrogen transfer into the surface layers nearly correlates to a Gaussian distribution law, which is explained by the formation of a large number of defects at the film surface in which nitrogen piles up before diffusing into the inner metal layers. Such an effect could be induced by the impinging energetic plasma species as ions, NHx < 3 radicals. The diffusion part is successfully fitted to Fick's second law by introducing diffusion coefficients of about (5–7) × 10−14 cm2 s−1 at 673 K. These values are lower than the diffusion coefficient calculated for a solid solution of nitrogen in molybdenum which is equal to about 4 × 10−13 cm2 s−1. A surprisingly high diffusion coefficient of about 3 × 10−13 cm2 s−1 is found for the molybdenum film exposed to (Ar–25%N2–30%H2) plasma at room temperature. These results highlight the role of plasma hydrogen species on the enhancement of the metal surface reactivity and nitrogen diffusion into the inner metal layers. A modification of the morphology of the film surface exposed to hydrogen species is also seen.The values of electron densities measured in the various (Ar–N2–H2) gas mixtures are consistent with the formation of expanded plasma far from the centre of the discharge, apart from(Ar–30%N2–12%H2) where the plasma expansion is very low. At a distance of 10.5 cm from the centre of the discharge, the electron density is equal to about 1.66 × 1017 m−3 and 1.16 × 1017 m−3 for (Ar–25%N2–30%H2) and (Ar–35%N2) gas mixtures, respectively.