Surface micromachined membranes such as RF-MEMS switches are widely adopted in the field of telecommunication devices and represent nowadays the lowest loss devices for switching and routing RF signals. The knowledge of specific properties of structural materials is crucial for a reliable design of devices with optimized performance. Material properties, like Young's modulus and stress, strongly determine the mechanical behavior of MEMS devices. In this paper, Si x N y /a-Si/Si x N y thin film membranes, of different sizes and porosities, were fabricated by unconventional low temperature PECVD, using surface micromachining approach. The tri-layer was characterized by nanoindentation and stress measurements based on wafer curvature method. On the possibility to extract mechanical properties from deflection measurements, to the best of our knowledge, literature seems to lack of analytical solutions of …