EVALUATION OF TAKE-OFF-ANGLE-DEPENDENT XPS FOR DETERMINING THE THICKNESS OF PASSIVATION LAYERS ON ALUMINUM AND SILICON

Investigating the effect of a cleaner (used to remove lubrication oil residues) on the surface composition of rolled aluminium foils, we applied angle-dependent XPS to determine the thickness of the passivation layer on these foils. We found the simple uniform overlayer model, which has frequently been applied to translate XPS intensity ratios into a value for the overlayer thickness, to be inapplicable on these aluminium foils. However, data obtained from an Si(100) single-crystal surface were in rather good agreement with the model. SEM images from the aluminium foils show that their surface is rough on the (sub)micron scale. We believe this roughness to be the reason for the inapplicability of the uniform overlayer model. Simulations of XPS intensity ratios for model rough surfaces support this point of view. Both experiment and simulations make it clear that one should not base XPS thickness determinations on one measurement only, because in that case the applicability of the uniform overlayer model cannot be checked.