We investigated the influence of process parameters in electron cyclotron resonance plasma-enhanced chemical-vapor deposition (ECR-PECVD) of silicon nitride on the intrinsic stress of thin SiNx films and on their composition, to obtain SiNx films suitable for micromechanical applications. The silane-to-nitrogen gas flow ratio R, along with the addition of helium to the gas mixture, was found to be a critical parameter for the tuning of intrinsic stress in ECR-PECVD SiNx films, from compressive to tensile stress within a large window of R from 0.3 to 0.7, with a maximum related to the largest Si–N bond density in the film.