Abstract The influence of the deposition parameters temperature, gas pressure, gas phase composition and r.f. frequency on the mechanical properties of silicon nitride layers prepared by plasma-enhanced deposition was investigated. The results are related to film composition and density, as derived from Rutherford backscattering spectrometry, Auger electron spectroscopy and IR measurements. In addition, etch rate measurements (in buffered hydrofluoric acid at 25°C) were performed. It appears that layers deposited at low temperatures (less than 550°C) and at low frequencies (less than 4 MHz) show a compressive stress while layers deposited at higher temperatures exhibit a tensile stress. Layers deposited at low temperatures and high frequencies (more than 4 MHz) show a tensile stress behaviour. A simplified deposition model is proposed in order to explain the experimental data. In this model the large difference in hydrogen content and density between layers deposited at different r.f. frequencies and temperatures is attributed to the influence of ion bombardment on the hydrogen desorption rate and on the short-range order in the plasma nitride layer.