The invention generally includes a new technique for making cubic boron nitride films with low contamination from other forms of boron nitride such as hexagonal and amorphous boron nitride. Films including either hexagonal or amorphous boron nitride are etched in a gas atmosphere including a halogen and/or hydrocarbon radical, preferably a methyl radical (CH.sub.3 ). Such atmospheres may be a plasma etching atmosphere also including hydrogen and hydrogen atoms. The etching technique is successful in removing hexagonal or amorphous boron nitride and leaving cubic boron nitride, or in converting hexagonal or amorphous boron nitride into cubic boron nitride, thus increasing the concentration of cubic boron nitride in the film. Interestingly, little or no etching of hexagonal or amorphous boron nitride occurs using only hydrogen or hydrogen atoms.