The roles of hydrogen and fluorine in the deposition of cubic boron nitride films by DC jet plasma chemical vapor deposition in a gas mixture of Ar-N-2-BF3-H-2 were investigated. Fluorine was demonstrated to preferentially etch the hexagonal phase during deposition. Hydrogen was found necessary to produce solid boron nitride from gas phase and to balance excessive etching of fluorine. An in situ etching process by switching off hydrogen and substrate bias was found to be effective to increase phase purity and crystal quality. A critical bias voltage was observed for the formation of cubic phase and its value decreased as the H/F ratio decreased.