Optical investigations of the effect of temperature and plasma conditions on the growth of sp3-bonded BN thin films

Boron nitride (BN) films with high cubic (sp3) content using various conditions were deposited on c-Si substrates by rf magnetron sputtering. Fourier Transform IR Spectroscopic Ellipsometry (FTIRSE) has been employed for the investigation of the effect of the substrate temperature (Ts) and gas partial pressure on the films bonding structure. A two-layer optical model was applied for the analysis of the FTIRSE spectra leading to the determination of the individual layer thickness and optical properties, as a function of the deposition parameters in order to get better insights on the phase evolution and microstructure of the BN films. As a result, the critical experimental conditions required to sustain c-BN growth were determined. It was found that at higher Ts, the formation of c-BN is favored and followed by a significant reduction of the corresponding layer thickness as a combined effect of the reduction of the B atoms sticking coefficient, the preferential etching of the h-BN regions and the existence of thermal spikes in the growing BN films. Furthermore, the investigation of the effect of plasma parameters on BN phase evolution and c-BN optical properties showed that the addition of N atoms in the plasma promotes the growth of c-BN. These results have been supported and justified by the study of the mechanical behavior of the BN films by additional depth sensing nanoindentation measurements.