Growth mechanism of cubic boron nitride thin films by ion beam assist sputter deposition

Cubic boron nitride (c-BN) thin films were successfully grown on Si(100) substrates by dual ion beam sputter deposition and the growth mechanism was studied with angle resolved in situ x-ray photoelectron spectroscopy (XPS) analysis. Boron was sputter deposited by 1 keV argon ions from a boron target and simultaneously bombarded with low energy nitrogen and argon ions mixture. Through Fourier transform infrared absorption spectroscopy (FTIR) and in situ XPS experiment, the optimum conditions for the c-BN growth such as the substrate temperature, the assist ion current density and the ion energy were determined to be 460 °C, 60 μA/cm² and 350 eV, respectively. Angle resolved in situ XPS analysis showed that the 1.2±0.2 nm surface layer of the c-BN film is always in the hexagonal boron nitride (h-BN) phase, which clearly shows that c-BN phase grows by the transformation from the initially formed h-BN phase by low energy ion bombardment. Cross-sectional high resolution transmission electron microscopy (TEM) images show that the BN thin films on Si has a sequential layered structure which consists of an initial amorphous BN (a-BN) layer (∼4 nm), a transitional h-BN layer (∼5 nm), and a main c-BN layer. The transformation of the h-BN to the c-BN phase and the unusual sequential layered structure were discussed in view of the compressive stress model.