Thin silicon nitride films have been deposited by a low temperature (20 degrees C) Electron Cyclotron Resonance (ECR) plasma directly on Si substrates. Varying the process pressure, gas composition and radio frequency bias power, films with different properties were obtained. Characterization by Fourier transform infra-red spectrometry reveals the presence of Si-N, Si-H and N-H bonds in the films. Refractive indexes in the range from 1.77 to 2.9 and deposition rates from 13 to 18 nm/min were determined by ellipsometry. Buffered hydrofluoric acid (BHF) etch rates from 0.7 to 509 nm/min, and KOH etch rates lower than I nm/min were obtained. Optical emission spectroscopy showed a strong correlation between the concentration of NH molecules produced in the plasma and porosity of the films. Finally, the films that presented high resistance to etching in KOH and BHF were used to fabricate suspended membranes on Si substrates. With these results, the ECR plasma produced SiNx films that have been used for fabrication of membranes in microelectromechanical systems. (c) 2008 Elsevier B. V. All rights reserved.