Reducing the impurity incorporation from residual gas by ion bombardment during high vacuum magnetron sputtering

The influence of ion energy on the hydrogen incorporation has been investigated for alumina thin films, deposited by reactive magnetron sputtering in an Ar/O2/H2O environment. Ar+ with an average kinetic energy of ~5 eV was determined to be the dominating species in the plasma. The films were analyzed with x-ray diffraction, x-ray photoelectron spectroscopy, and elastic recoil detection analysis, demonstrating evidence for amorphous films with stoichiometric O/Al ratio. As the substrate bias potential was increased from –15 V (floating potential) to –100 V, the hydrogen content decreased by ~70%, from 9.1 to 2.8 at.%. Based on ab initio calculations, these results may be understood by thermodynamic principles, where a supply of energy enables surface diffusion, H2 formation, and desorption [Rosén et al., J. Phys.: Condens. Matter 17, L137 (2005)]. These findings are of importance for the understanding of the correlation between ion energy and film composition and also show a pathway to reduce impurity incorporation during film growth in a high vacuum ambient.