Analytical Multilayer Model Revisited for High Atomic Numbers at Low Voltage

Electron probe micro-analysis (EPMA), originally developed to determine the composition of the bulk samples at the micron scale, has become a well established technique to determine the compositions and the thickness of thin multilayer deposited on a substrate [1-4]. This technique can be used to determine the film thickness in a range of a few micrometers to a monolayer. By varying the accelerating voltage, and thus the excitation depths, the different layers can be analyzed. Typical accelerating voltages of 5– 40 kV have excitation depths from 0.2-10 μm. The thin-multilayer method by EPMA is based on the comparison of the ratios of x-ray intensities (k-ratio) emitted by the elements of the various layers to those emitted from bulk standards under same experimental conditions. To convert the measured k-ratio from elements of the layers in thicknesses and compositions, a multilayer model requires an accurate description of the x-ray depth distribution ((ρz)) from which the emitted x-ray intensities are calculated. It is worth noting that for an element corresponds a specific (ρz) distribution which varies with accelerating voltages and with the thicknesses and compositions of the layers.