We have studied the electron cyclotron resonance plasma chemical vapor deposition (ECR PCVD) of silicon nitride films onto unheated substrates to evaluate their potential for use in micromechanical devices. We demonstrate that this low temperature process (<120 degrees C) is compatible with lift-off patterning of both thin and thick films. We measured the deposition rate, stress and buffered HF (BHF) etch rate for different gas flows of silane, nitrogen and argon. The deposition rate is rate controlled by the silane flow and we obtained rates sufficiently high (0.7-1.4 mu m/h) to obtain films for practical use in micromechanics (0.5-2 mu m) in about 1 h. We measured stresses ranging from 1000 MPa compressive to 60 MPa tensile, and we can repeatedly obtain films with stress of less than 10 MPa. These films are ideal for both low-stress and stress-compensated device designs. We observed BHF (NH4F:HF 6:1 at 20 degrees C) etch rates from 5 nm/min to 150 nm/min, which are five times faster than those for films deposited by CVD. ECR films are ideal for fast etched sacrificial layers at 150 nm/min or as slowly etched masking layers at 5 nm/min. We found that stress and the BHF etch rate were strongly correlated : low etch rates are observed for highly compressively stressed films and high etch rates for lower stress. This correlation has a major impact on the design of micromechanical devices and on their fabrication processes using ECR PCVD.