Boron carbide thin film deposition using supersonic plasma jet with substrate biasing

Hard, microcrystalline boron carbide thin films have been deposited from th e thermal dissociation of hydrogen, methane and boron trichloride in a supe rsonic plasma jet. The influence of negative and positive substrate bias on the film properties and morphology has been investigated. A continuous ion bombardment has been found to increase the film crystallinity, however, it has led to poor adhesion to the substrate. Pulsed d.c. positive biasing ha s been developed as a means to elevate the electron temperature and control the gas phase chemistry, while limiting the total current flowing in the s econdary discharge. In this case, it has been found that the deposition rat es increase with bias voltage as a function of the third power, without aff ecting the film hardness and morphology. Also, the boron-to-carbon atomic r atio of the films increases with increasing positive bias voltage, from car bon-rich to stoichiometric boron carbide. Correlations between deposition r ates and gas species line emission indicate that atomic boron is the primar y growth species. The pulsed d.c. biasing enhancement presented in this pap er constitutes a novel approach to controlling the film composition and dep osition rate. (C) 1999 Elsevier Science S.A. All rights reserved.