Silicon nitride films are prepared by helium-excited magnetron radio-frequency sputtering. Excitation energy transfer from He to N2 and existence of hydrogenation and oxidation source (O+ and OH.) in the plasmas are confirmed by optical emission spectroscopy. The structure and characteristics of the resultant films depend on the gas pressure during sputtering : Films produced at pressures above 5 Pa are etched rapidly in a buffered hydrogen fluoride solution and have low refractive indices because their structure is coarse and rich in oxygen and hydrogen or both.