We have studied the composition and physical properties of silicon oxynitride (SiOxNy) films grown on silicon membranes by means of plasma-enhanced chemical vapour deposition (PECVD process). The influence of these properties on the mechanical behaviour of PECVD deposited films is investigated by an original "point-wise" deflection method. This technique, particularly appropriate for determining the residual stress in the case of prestressed membranes, is operating at the first order of buckling. It combines the classical interferometry with the nanoindentation technique. The interferometry measures the out-of-plane displacements of membranes with SiOxNy layers of different optical quality, while the nanoindentation pen-nits the extraction of micromechanical properties such as hardness and Young's modulus. From the "point-wise" deflection technique, the distribution of residual stress was monitored as a function of the refractive index of SiOxNy films, establishing the correlation between the optical and micromechanical properties of deposited thin films. High measuring accuracy and resolution have been demonstrated, allowing the measurements to be used to enhance PECVD process control. (C) 2004 Elsevier B.V. All rights reserved.