Knock-on subplantation-induced formation of nanocrystalline c-BN with rf magnetron sputtering and rf argon ion plating

Boron nitride thin films were deposited by unbalanced r.f. magnetron sputtering from a hexagonal boron nitride (h-BN) target with r.f. argon ion plating at an argon gas pressure of 8 x 10(-4) mbar. The content of cubic boron nitride (c-BN) is very high, as determined by infrared absorption spectroscopy (IR, 84% c-BN), factor analysis of Auger electron spectroscopy (AES, approximately 100% sp(3)), and X-ray reflectivity (approximately 100% sp(3)). Furthermore, the films were characterized by stress measurements and atomic force microscopy (AFM). The film-forming particles (flux Phi(n)) are mainly sputtered neutral boron and nitrogen atoms, and the plating particles (flux Phi(i)) are argon ions. The current density is about 2.25 mA cm(-2), as determined from energy and mass analysis. The energy dependence due to r.f. substrate bias shows a maximum in the c-BN content at 137 eV at an arrival ratio Phi(i)/Phi(n)=13 where AFM investigations show a minimum of the area roughness of 0.2 nm. Increasing the arrival ratio (to 20 and 66), the optimal c-BN formation is shifted to lower energies (87 eV and 62 eV respectively) which is in agreement with the subplantation model. Stress reduction experiments, such as W-assisted deposition and post-annealing, are discussed.