The deposition of amorphous silicon nitride (a-SiNx:H) films at high deposition rates (similar to 3 nm/s) and at low substrate temperatures (< 150 degrees C) has been studied using the expanding thermal plasma technique operated on an Ar-NH3-SiH4 reactant mixture. To increase the atomic density of the films by ion bombardment, low frequency (If, 400 kHz) and radio-frequency (rf, 13.6 MHz) substrate biasing has been employed during deposition such that the ions are accelerated towards the substrate up to energies of similar to 250 eV From spectroscopic ellipsometry and Rutherford backscattering measurements, it is demonstrated that the film density increases with increasing substrate bias even under these high deposition rate conditions. An increase in film atomic density from 7.6x 10(22) cm(-3) to 8.8 x 10(22) cm(-3) has been observed for rf biasing when going from almost zero substrate bias to a bias voltage of -250 V It is shown that this increased film density reduces the oxygen content in the a-SiNx:H caused by post-deposition oxygen and/or moisture permeation by more than 50%. (c) 2005 Elsevier B.V. All rights reserved.