Use of in-situ spectroscopic ellipsometry to study the behavior of metallic surfaces in different solutions

Visible spectroscopic ellipsometry is applied to monitor in-situ the behaviour of metal and metal oxides in various aqueous solutions. Ellipsometry measures the change in polarisation state upon reflection on a sample and is widely used for the determination of the optical properties of surfaces and thin films. The technique has the advantage that no reference measurements are needed. The use of in-situ ellipsometry in the fields of electrochemistry and corrosion is illustrated by means of three cases. These show the possibilities to obtain film thickness, film refractive index, and the surface roughness of the metal. The first case is related to oxide films on aluminium. For the native oxide (several nm thick) on the metal the roughening of the substrate as well as the changes in oxide film thickness can be observed independently. On thicker oxides (> 100 nm), it is possible to independently determine in-situ the refractive index and thickness of the oxide, as well as the interface roughness. This was shown in a study of the effect of aggressive solution son aluminium/aluminium oxide surface. The second case concerns the electrochemical polishing process of copper. A good coincidence is achieved between the interface roughness and layer thickness from ellipsometry and the expected surface structure for the different electrochemical conditions. A last example shows the possibility of ellipsometry to study the copper corrosion in an aggressive solution. For this case, the thickness and the refractive index of the corrosion film can only be obtained in that part of the spectrum where the oxide is transparent. The degree of corrosion protection was characterised by monitoring the protective film thickness.