Cubic boron nitride (c‐BN) thin films have been deposited by unbalanced rf (13.56 MHz) magnetron sputtering of a hexagonal boron nitride target in a pure argon discharge. Deposition parameters have been 300 W rf target power, 8×10−4 mbar argon pressure, 3.5 cm target substrate distance, and 800 K substrate temperature. Under these conditions the ion current density is 2.25 mA/cm2 and the growth rate is ∼1.1 Å/s. By applying a rf substrate bias the ion plating energy is varied from plasma potential of 37 eV up to 127 eV. The films have been characterized by infrared (IR) and Auger electron spectroscopy (AES), x‐ray diffraction (XRD), x‐ray reflectivity, elastic recoil detection (ERD), Rutherford backscattering (RBS), nuclear resonance analysis (NRA), and stress measurements. The subplantation model proposed by Lifshitz and Robertson can be applied to the c‐BN formation. An energy of about 85±5 eV is found where the stress (25 GPa, 200 nm film thickness) and the c‐BN content (≳90%) have a maximum. The grain size of the crystalline c‐BN phase was estimated to be in the range of 5 nm. Below an energy of 67±5 eV no c‐BN could be detected. An excellent adhesion has been obtained by a special interface treatment. © 1996 American Institute of Physics.