Thickness measurements of thin films: comparison of techniques using characteristic X-ray line ratio techniques

Abstract

In this research, two characteristic X-ray line ratio techniques are experimentally investigated. One technique uses the ratio of the characteristic L X-ray line to the characteristic Kα X-ray line of the substrate. The other techniques uses the ration between one characteristic X-ray line of the substrate and one characteristic X-ray line of the thin film overlay. Experimental measurements were taken for both techniques by bombarding a coated sample with electrons and measuring the ratio of the areas under the characteristic X-ray line peaks. The samples used in the experiment consisted of a copper substrate coated with aluminum and a molybdenum substrate coated with aluminum. The data are presented by plotting the characteristic X-ray line ratio vs. the thin film thickness from 400 to 7000 Å for 20, 25 and 30 keV electron bombardment. On the basis of these experiments, it was found that the ratio of the characteristic X-ray lines from the substrate (L:Kα ratio) is not suitable for thin film measurements because of the complication caused by matrix enhancement effects. However, matrix enhancement effects are also partly responsible for producing a large ratio change (80%) over the range of 5000 Å of overlay coating for the ratio technique that uses an X-ray line from the substrate and an X-ray line from the thin film overlay. This ratio was independent of accelerating electron energy and inversely proportional to the thickness.