Nucleation retardation of cubic boron nitride films caused by the addition of oxygen in argon‑nitrogen sputtering gas

Abstract

Cross-sectional transmission electron microscopy (TEM) was employed to study the effect of the additive oxygen gas on the microstructure of boron nitride (BN) films especially focusing on the intervening turbostratic BN (tBN) layer's behavior. Films in this study were deposited on diamond coated Si substrates, which were biased with negative voltages, using an unbalanced magnetron sputtering method with a boron carbide target. Various amounts of oxygen were added into argon‑nitrogen mixed sputtering gas. TEM showed that the tBN intervening layer was also observed to form prior to cubic BN (cBN) layer formation for all films, irrespective of oxygen addition. The tBN layer thickness increased with increasing oxygen content. This thickness increase implied that the addition of oxygen delayed cBN phase nucleation. On the other hand, the alignment of the hexagonal BN planes in the tBN layer, estimated from FTIR spectra, was observed to be enhanced with increasing oxygen content. Such alignment enhancement is considered as a result of oxygen adsorption on the tBN layer surface, which suppresses breaking sp2 bonds caused by Ar ion bombardments. The suppression of the sp2 bond breakage, consequently, causes the difficulty of forming sp3 bonds and delays the cBN phase nucleation.