In situ infrared ellipsometry study of plasma processing of metallic surfaces

Abstract

Aluminium and stainless-steel surfaces were processed using H 2 plasma in an Integrated Distributed Electron Cyclotron Resonance reactor. H 2 plasma treatment was monitored by in situ infrared (IR) ellipsometry in the range 2800–3050 cm −1 and correlated with ex situ AES depth profiling measurements. The removal of hydrocarbon surface contamination was first demonstrated. Real-time monitoring was also performed during treatment. The evolution of the ellipsometrical angle Δ at fixed wavelength, out of any vibration range, reveals a reduction of the native oxide layer. On Al, Δ shifts are attributed to the reduction of the external hydroxylated and porous oxide phase, the dense Al 2O 3 passivating layer remaining unchanged. In contrast, stainless-steel oxide is found less stable than Al 2O 3 and its reduction is almost complete (thickness estimated to be 1.1 ± 0.2 nm) using the same plasma conditions. As a consequence, the adhesion of SiO 2 thin films to stainless-steel substrate is found to be enhanced after H 2 plasma treatment. The enhancement is attributed to an increase in the substrate surface energy due simultaneously to cleaning and to native oxide reduction, which leads to stronger interactions with SiO 2. In conclusion, IR ellipsometry appears a very promising in situ technique to probe plasma processing of surfaces.