Boron nitride (BN) thin films were deposited by unbalanced magnetron sputtering. An asymmetric bipolar pulsed de power supply was used to bias the substrate with a 218 kHz signal. BN was deposited at different negative bias voltages and films with over 80% of the zinc-blende cubic phase, as measured by Fourier transform infrared (FTIR) spectroscopy, were obtained. For a given set of deposition conditions a window of maximum negative bias voltage was defined inside which the cubic phase is stabilized. This negative bias voltage was used to derive the momentum imparted to the growing film per arriving boron atom, and it is shown that a threshold value for the formation of the cubic phase (cBN) is consistent with previous results using ion beam assisted deposition. Several films were deposited on aluminum nitride and diamond thin films previously deposited on crystalline silicon in order to study the effect of the substrate on the stabilization of the cBN phase. A twofold reduction in the full width at half maximum (FWHM) of the 1060 cm(-1) FTIR peak is obtained for films deposited on diamond, which is indicative of better ordering of the B-N cubic bonding. At high negative bias voltages the FWHM increases toward the value for BN films deposited onto crystalline silicon which is expected since the greater energetic bombardment produces excessive damage to the film.