New Measurements of the Surface Ionization at Low Energies

Abstract

In electron probe microanalysis (EPMA), the surface ionization φ(0) describes the increase of ionizations in a thin layer of a given element located at the sample surface due to electrons backscattered from the substrate. Accurate knowledge of this parameter, for incident electron energies from ionization threshold up to 40 keV, is of interest for both theoretical and practical uses. On the one hand, φ(0) appears in most of the analytical parameterizations of the depth distribution of ionizations or φ(ρz) function used in quantitative EPMA procedures. While for bulk specimens the evaluated concentrations depend on this parameter especially when x-ray absorption is important, for thin films and multilayers, EPMA results are largely sensitive to the accuracy of the adopted φ(0) values. On the other hand, the surface ionization is also used in quantitative Auger electron spectroscopy (AES), where it is commonly referred to as the Auger backscattering factor (r), and it describes the increase of the Auger current due to backscattered electrons. Last but not least, the surface ionization is an excellent test to check the reliability of electron transport calculations. Indeed, φ(0) is not only sensitive to the cross sections adopted to describe elastic scattering and slowing down of electrons but also to the model adopted for inner-shell ionization.Measurements of the surface ionization have been performed by several groups over the past years. In our previous works [1,2,3,4], we have measured the surface ionization for bulk samples and thinfilm and multilayer samples deposited on a wide variety of substrates. In spite of the experimental uncertainties (of about 5%) and the limited set of incident electron energies for which φ(0) was measured, the data obtained made it possible to asses the reliability of φ(0) calculations at moderately high energies and to guide the development of new analytical formulas. However, it is still difficult to asses the reliability of φ(0) calculations at very low energies, especially close to the ionization threshold and the need for improved experimental measurements at low energies remains open.In this communication, new measurements of the surface ionization are presented. We have determined φ(0) by using the tracer method, i.e. by measuring the characteristic x-ray intensity emitted from an ultra-thin tracer layer deposited on a bulk substrate and, according to Castaing's definition, normalizing it to the intensity emitted from an equivalent, self-supporting tracer layer. Although conceptually simple, the determination of φ(0) faces numerous difficulties, from sample preparation to the measurement itself. Indeed, the tracer layer should be as thin as possible to minimize multiple-scattering effects (finite-thickness effect), especially at low accelerating voltages, but should be thick enough to yield acceptable signal-to-noise ratios so as to minimize statistical uncertainties and systematic errors in spectral background subtraction. In this work, we have used tracer layers with thic...