Boron nitride films were produced on silicon substrates by r.f. magnetron sputtering with pure argon as working gas. The cubic BN (c-BN) phase was achieved through an r.f. substrate bias which set up a controlled ion bombardment on the growing surface. The dependence of film structure on the deposition time was studied at fixed growth parameters by means of Fourier transform infrared spectroscopy, Auger electron spectroscopy, low energy electron loss spectroscopy, atomic force microscopy, and ellipsometry. The results show for the first time that the nucleation into c-BN takes place through a phase transformation process of the upper part of an h-BN base layer formed in the initial growth stage. Special features correlating with this transformation are presented, particularly a shrinking of the film's volume and a transient extraordinarily high roughness of the growing surface. The results indicate that c-BN nucleation is not directly governed by the instantaneous ion bombardment of the growing film. (c) 2006 Elsevier B.V All rights reserved.