Silicon nitride (SiNx) thin films were deposited by RF magnetron sputtering at room temperature in (Ar + N2) atmosphere by tuning the reactive gas flux percentage in the mixture. The samples were analyzed by UV–Vis–NIR and Fourier transform infrared (FTIR) spectrophotometry and by SIMS (Secondary Ion Mass spectroscopy). The effects of the nitrogen content in the gas mixture on the growth process and on SiNx films optical and chemical properties were investigated. A qualitative description of the growth process, which analyzes the role of nitrogen dissociation or ionization in (Ar + N2) atmosphere for the sputtering of a silicon target, is the key factor to interpret the obtained results. It perfectly matches with the thin films physical response. Optical analysis supplied the optical constants (n and k) and optical bandgap values. They increase by rising nitrogen content in the mixture but their values are lower than …