ELSEVIER Diamond and Related Materials 5 (1996) 548-551 DIAMOND AND RELATED MATERIALS Knock-on subplantation-induced formation of nanocrystalline c-BN with r.f. magnetron sputtering and r.f. argon ion plating S. Ulrich a j. Schwan a W. Donner b H. Ehrhardt a a Technische Physik, Universitiit Kaiserslautern, Erwin SchrOdinger Strafle, D-67663 Kaiserslautern, Germany b Institutffir Materialwissenschaften der Bergischen Universitdt Wuppertal, Mfingstener Str. 10, D-42285 Wuppertal, Germany Abstract 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% sp3) and X-ray reflectivity (approximately 100% sp3). Furthermore, the films were characterized by stress measurements and atomic force microscopy (AFM). The film-forming particles (flux ~on) are mainly sputtered neutral boron and nitrogen atoms, and the plating particles (flux ~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 q~i/~,= 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 UV-assisted deposition and post-annealing, are discussed. Keywords: Boron nitride; Thin films; Magnetron sputtering; Subplantation 1. Introduction Cubic boron nitride (c-BN) is an extremely interesting material, since it belongs to the class of superhard materials with a Vickers hardness in the range 6000-7000 kp cm -2 [1,2]. Only diamond and the hypothetical fl- C3N4 have a higher hardness, but in contrast with diamond c-BN is chemically stable against oxygen and ferrous materials. Furthermore, it may be possible to perform n- and p-type doping of this material. For these reasons, the interest in c-BN is increasing, leading to different deposition methods for c-BN formation [3-8]. We deposited c-BN by unbalanced r.f. magnetron sput- tering from a hexagonal boron nitride (h-BN) target with r.f. argon ion plating in a pure argon atmosphere [9]. The advantage of magnetron sputtering is that this technique is widely established in industry as a high rate deposition method and allows the deposition of large areas. 2. Experimental details Boron nitride thin films were prepared by unbalanced r.f. magnetron sputtering with r.f. argon ion plating. In Elsevier Science S.A. SSDI 0925-9635 (95) 00464-5 principle, the deposition method is shown in Fig. 1. Boron and nitrogen atoms were sputtered from a crystal- line h-BN target by argon ions. The argon pressure was adjusted to 8 x 10 -4 mbar and caused nearly no inelastic collisions of the sputtered atoms in the plasma. The sputter flux yields the film-forming particle flux q~n = ~bB + ~N. All films were deposited on heated Si(100) substrates with an r.f. target power of 300 W, which generated an ion current density q~i of 2.25 mA cm 2 on the substrate. When the target power was increased, the electron temperature Te remained almost constant (approximately 7.1 eV) and the plasma density n. was enhanced up to 5.6 × 10 l° cm -3 at 300 W r.f. power, as measured by double probe plasma diagnostics. As a consequence, the ion current density ~i increased (Fig. 2): q~i = ~Ar + °cnex/-~" The ion energy distribution is given by the electron temperature T~ at a target- substrate distance of 6 cm, i.e. the full width at half- maximum is AEFwHM = kT~/2. When the substrate-target distance was decreased, the influence of the r.f. amplitude on the ion energy distribution increased and produced a shoulder in the distribution at higher energies; this is shown in Fig. 3 for the working distance of 3.5 cm. Without external substrate bias, the maximum of the ion energy distribution was determined at 37 eV.