Solid-state wetting and spreading of tin (Sn) on aluminium interfaces

Abstract

The deposition of tin (Sn) layers on polycrystalline aluminium surfaces exhibits pronounced islanding. The detection of a continuous Sn coverage (“wetting layer”) between the islands indicates a Stranski–Krastanov growth mode. If the Al–Sn-bilayer system is covered by an Al-capping layer the surface morphology of this overlayer is significantly different depending on whether the surface of the underlying Al–Sn-bilayer was oxidized or not. The present study investigates the islanding mechanism and the dependence of the morphology of the Al-overlayer on the oxidation state of the underlying surface by combined SEM, atomic force microscopy and scanning Auger microscopic (SAM) investigations. The islanding mechanism as well as the overlayer morphology are related to the wetting of the Al-surface by Sn in the absence of oxygen. This was proved by in situ sputter cleaning of the sample surface in the SAM chamber. For the Al–Sn-bilayer it was possible to completely remove the wetting layer by the sputter cleaning process. A clean, oxygen free Al-surface was recovered at room temperature by Sn after several minutes as observed by spatially resolved SAM measurements. Sn emerges from the Sn islands and spreads over the Al-surface uniformly. In the case of an Al–Sn–Al sandwich structure Sn permeated to the surface of the topmost Al-layer when the Al-overlayer was deposited on an unoxidized Al–Sn-bilayer. For Al-deposition on an oxidized Al–Sn-bilayer no Sn transport to the overlayer surface was detected at all. These results indicate that metallic Sn is wetting every type of interface of polycrystalline Al (grain boundaries as well as the solid/vacuum interface) and also acts as a surfactant which changes the morphology of the Al-overlayer. Oxygen acts as a potent inhibitor of Sn wetting and spreading.