Interpretation of ESCA Thickness Measured on Non-Uniform Oxide Layers

Abstract

The ESCA method of determining oxide layer thickness depends upon a comparison of the photoelectron intensity received from the overlayer of oxide with that from the unoxidised metal substrate underneath. As photoelectron intensity varies .nonlinearly with the depth of material, however, the method requires that the oxide be uniform in thickness. The errors caused by a non-uniform oxide layer are investigated theoretically for a flat plate. It is shown that the method always underestimates the mean thickness of irregular oxide layers. The error incurred is strongly dependent on the mean oxide thickness, becoming substantial when the layer is comparable in thickness to the inelastic mean free path of the electrons or thicker. Error calculations are given for several irregular oxide layers using a simple geometric model which assumes that the oxide consists of relatively thick islands interspersed on a thin oxide skin on the metal substrate. Results of measurements on gas atomised aluminium powder are interpreted and are shown to be consistent with the proposed model. PM/0542