Expanding microwave plasma process for thin molybdenum films nitriding: Nitrogen diffusion and structure investigations
Résumé
Transition metal nitrides exhibit very interesting properties for mechanical and catalytic applications. Thin nitride layers are expected to prevent metal films from oxidizing under working conditions. Molybdenum thin films of about 200 nm thick and deposited on Si (100) wafers are processed in pure N2, (Ar-35%N2) and (Ar-25%N2-30%H2) expanding microwave plasma at 673 K. Secondary neutral mass spectrometry (SNMS) and Raman spectroscopy are both used to make correlations between the composition and the structure of the as-formed compounds. The low nitrogen diffusion up to a depth of about 40 nm is correlated to the formation of well crystallized MoO2 of monoclinic structure acting as a barrier of diffusion for nitrogen, in molybdenum films exposed to pure N2 and (Ar-35%N2) plasma. The large nitrogen diffusion into the film exposed to ternary (Ar-25%N2-30%H2) plasma is correlated to the reduction of MoO2 oxides by hydrogen species such as atomic hydrogen, NHxb3...contained in ternary plasma. The formation of Mo-N phases with defects could take place in molybdenum films processed at 673 K compared to the results obtained at 873 K.