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SixOyNz ,S i 3N4 and SiO2 films deposited on silicon in a low frequency PECVD reactor (187.5 kHz) from a SiH4 ,N 2O and/or NH3 gas mixture are always in compressive mechanical stress state after deposition because of ion bombardment of the growing film during deposition. The stress value is, as expected, a decreasing function of oxygen content. It is demonstrated that by reducing drastically the deposition temperature down to 200 8C, stoechiometric silicon nitride films and silicon oxynitride films (0 � O=Si � 1:8) with a low tensile stress can be obtained after high temperature annealing (800 8C). Rutherford backscattering experiments (RBS), elastic recoil detection analyses (ERDA) and infrared spectrometry measurements show that this stress variation is related to the initial composition, to hydrogen desorption from Si‐H, N‐H and O‐H bonds and to cross-linking. By controlling the initial hydrogen concentration with the deposition temperature, the stress value after annealing can be tuned precisely. This simple process was applied to the fabrication of flat dielectric membranes with a thickness in the range of 90‐400 nm by KOH bulk micromachining. # 2002 Elsevier Science B.V. All rights reserved.