We have performed a calibration process in order to reduce the uncertainty in the radio-frequency (rf)-plasma preparation of silicon oxynitride. Two typical curves. were built : the first one gives atomic concentration ratios versus plasma conditions and the second one gives deposition rates versus the atomic concentration ratios. Prior to the calibration process, we determined an optimum plasma working point. Infrared reflection experiments performed on two multilayered devices prepared under identical conditions give similar spectra, and confirm the improvement obtained in the reproducibility of the rf-plasma process. The composition depth profile of another multilayered structure demonstrates the in-depth stoichiometric homogeneity.