Silicon nitride films were deposited by helicon plasma enhanced radio frequency (rf) reactive sputtering. This technique was shown to produce films with low hydrogen and oxygen content at very low sputtering-gas pressure, with improved deposition rate compared with conventional rf reactive sputtering. Stoichiometric Si3N4 film with extremely low hydrogen content (<1%) and oxygen content (<1%) was obtained at the Ar/N-2 partial pressure ratio of 1.5, and total sputtering-gas pressure of less than 0.1 Pa. The influence of sputtering-gas pressure on the mechanical, chemical, optical and compositional properties of the films was investigated by stress and chemical etch rate measurements, infrared absorption spectroscopy, ultraviolet-visible spectroscopy, and Rutherford backscattering spectrometry (RBS). These studies revealed that when keeping all other deposition conditions the same, reducing the sputtering-gas pressure, can effectively reduce the film contamination of H and O, and increase the film density.