Unbalanced magnetron sputtered Si–Al coatings: plasma conditions and film properties versus sample bias voltage

Abstract

In this paper we present the results for 100–200 nm Si–Al coatings of different stoichiometry, produced by unbalanced magnetron sputtering, deposited on glassy carbon substrates with different SixAly target source compositions varying from pure aluminium to pure silicon and for substrate bias voltage ranging from 0 to −100 V.The structure of the coatings has been analysed with cross-section transmission electron microscopy (TEM) and X-ray diffraction (XRD), while the composition and thickness were measured by Rutherford backscattering spectroscopy (RBS).Cross-sectional TEM shows columnar growth, perpendicular to the surface of the substrate. The width of aluminium and silicon peak diffraction patterns indicates that all films containing both elements are composed of separated silicon and aluminium phases. The aluminium grains are bigger than the silicon ones and the dimension of the silicon grains decreases when the silicon concentration increases. When bias voltage is applied to the substrates, a contamination of argon is observed. On the contrary, no argon contamination is observed in pure aluminium films for all grounded coatings. The maximum argon concentration appears for bias voltages around −30 V and is proportional to the plasma ion density, which depends on the target composition. At lower bias voltages the argon concentration decreases and stays nearly constant below −60 V. The presence and the concentration of argon is correlated to heating of the substrate due to the impinging ions and electrons.