Ellipsometric and X-ray specular reflection studies on naturally grown overlayers on aluminium thin films

Abstract

The results of detailed investigations on the natural surface layer formed at room temperature on aluminium films exposed to air are presented. Aluminium films of high perfection, deposited onto very smooth glass substrates, have been studied over a 2 year period using ellipsometry. Soft X-ray specular reflection analysis revealed a composite surface layer structure composed by a thin ( d 2 = 0.8 nm) very compact alumina layer in contact with the aluminium substrate and by a thick ( d 3 = 3 nm) hydrated oxide layer. A new computer procedure was applied for this composite layer system, which evaluated 72 ellipsometric experimental data and achieved a best fit of the measured and calculated Ψ and Δ values. The resultant optical constants of the aluminium substrate were n = 1.09, 0.95, 0.535 and 0.370 for λ = 579 nm, 546 nm, 436 nm and 365 nm respectively, whereas k = 6.72, 6.40, 4.96 and 4.23 respectively for the same mercury lines. Among widely scattered data from the literature, these are in good agreement with results of Hass on the assumption of a similar surface layer structure, using n 2 = 1.77 (alumina) and n 3 = 1.58 (hydrated oxide). Our optical constants for aluminium were applied for evaluating ellipsometric experimental data obtained during the 2 year period. A slight systematic change in the d 2 and d 3 values of the samples was found, owing to hydration.