Evaluation of take-off-angle dependent XPS for determining the thickness of passivation layers on aluminium and silicon

Angle-dependent XPS was applied to det. the surface compn. and thickness of the passivation layer on rolled aluminum foils. The simple uniform overlayer model, which has frequently been applied to translate XPS intensity ratios into a value for the overlayer thickness, was found to be inapplicable on these aluminum foils. However, data obtained from an Si(100) single-crystal surface were in rather good agreement with the model. SEM images from the aluminum foils show that their surface is rough on the (sub)micron scale. This roughness is believed 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 expt. and simulations make it clear that one should not base XPS thickness detns. on one measurement only, because in that case the applicability of the uniform overlayer model cannot be checked. [on SciFinder (R)]