Recent Advances to Establish XPS as an Accurate Metrology Tool

Abstract

A brief review is given of the project to measure the thickness of SiO₂ on Si in the range 1.5 nm to 8 nm by XPS. An outline is provided for the rationale of the work and the way that progress was organised to achieve accuracy better than 1%. Key elements of the uncertainty that needed addressing were the establishment of a reference geometry to avoid forward focusing from the single crystal silicon substrate, adequate signal strength to reduce the signal uncertainty, sufficient angular acceptance in the analyser to average the remaining forward focusing structure but not so large as to cause a bias, and accurate setting of the angle of emission for the reference geometry. With these parameters controlled, XPS becomes a linear and repeatable method for determining the thickness of SiO₂ on Si. By measuring the difference in thicknesses of a series of films against one or more other methods such as ellipsometry, X-ray reflectance, neutron reflectance, etc, the relevant attenuation lengths may be determined, thus converting XPS from a precise method into an accurate and traceable method. This general procedure can be used for any material layer. Details are given that led to a final result with a standard uncertainty better than 1%.