The, influence of substrate temperature and ion bombardment on the formation of cubic boron nitride (c-BN) films has been examined with bias-assisted magnetron sputtering of h-BN for argon and,en as the working gases. It is shown throng a two-step deposition approach that higher nitrogen substrate temperatures are required only to establish an appropriate h-BN base layer which is mandatory as a nucleation base for c-BN. For nitrogen as the working gas the energy of the bombarding ions can also be reduced after c-BN-nucleation. Hence, the conditions for the formation of c-BN films are more restrictive for the initial growth stage than for the later c-BN growth once the nucleation into c-BN has been established. Corresponding results for argon and nitrogen display characteristic differences which lead to the conclusion that the establishment of full BN surface stoichiometry is a key condition for c-BN growth.