Boron nitride (BN) films with a high cubic phase content have been synthesized. The films are deposited by reactive sputtering of a pure boron target in electron cyclotron resonance (ECR) plasmas and ion bombardment to a substrate is enhanced by rf biassing of the substrate. A cubic phase of BN is grown over a certain threshold value of ion bombardment energy. The threshold and the cubic phase content depend on the ratio of ion flux to boron atom flux to the substrate. The threshold decreases as the ratio increases; however, the cubic phase content also decreases, particularly in a high ion flux region. BN films with a high cubic phase content show surface cracking and delamination because of the high compressive stress induced by ion bombardment. An intermediate hexagonal BN layer, of a certain thickness between layer with high cubic phase content and the substrate prevents surface cracking and improves film adhesion.