For films which are thinner than 500-1000 nm, the film thickness can be measured simultaneously with the chemical composition. In this case, the electron beam will penetrate through the film into the substrate. The method is even possible with twofold or threefold film systems (see adjacent Figure). The prerequisite is that each chemical element only occurs in one of the layers or in the substrate, in order for the element radiation to be unambiguously assigned to one of the layers or the substrate.
The adjacent graphic below shows an example of a magneto-optical storage disk made from polycarbonate with a Gd-Tb-Fe functional layer at the base, which is furnished with an Al protective layer. With the aid of EPMA thin-film analysis, the thickness of both films and the stoichiometry of the concealed magneto-optical layer can be simultaneously determined position-dependent point-by-point over the radius of the disk. A strong gradient in the film thickness of the GdTbFe film from 70 to 20 nm can be observed. At the same time, the (Gd+Tb)/Fe ratio varies significantly over the radius of the disk. The measurement duration for this analysis was approx. 45 min.
The thin-film analysis is so sensitive that, under favorable conditions, even monolayers of a material lying below some 10-100 nm of another material can be detected and measured. In a low-e glass coating, for example, it was possible to determine the thickness of a 0.3 nm-thick NiCr film below 30 nm of several metal oxide layers and to measure the stoichiometry from Ni to Cr.
More detailed explanations and further examples can be found in Vakuum in Forschung und Praxis, Vol. 31, No. 3 (June 2019), pp. 26-36.