Deposition of boron nitride coatings by reactive RF magnetron sputtering: Correlation between boron and nitrogen contents and the flux of energetic Ar+ ions at the substrate

Thin films BNxOy have been deposited by rf Reactive Magnetron Sputtering from a BN target in Ar+ plasma (1 Pa) at different substrate bias conditions. The possibility of obtaining dense and hard BN coatings at a target-to-substrate distance larger than the mean free path of the B and N atoms has been investigated. In order to deduce the plasma potential and the ion density near the substrates, the discharge characteristics have been measured by means of an electrostatic probe of the Langmuir type properly coupled to the glow discharge for avoiding the rf interference. From the probe data both flux and energy of the Ar+ ions have been determined and correlated to the net contents of boron and nitrogen as measured by means of the N(d,p) and 11B(p,α) nuclear reactions. The results of this study show that in the investigated experimental conditions the flux of energetic Ar+ ions at the substrates is insufficient for promoting the formation of cubic BN as confirmed by XRD, FT-IR and nanohardness measurements. The reported data agree with recent literature findings and support the need of monitoring both the bombarding rate and energy of the Ar ions in rf discharges if the c-BN deposition is to be attained in a reproducible manner