The Use of Electron Probe MicroAnalysis to Determine the Thickness of Thin Films in Materials Science

Abstract

Electron Probe MicroAnalysis (EPMA) was born around 1950 when Raymond Castaing, a French graduate student working under the supervision of Andre Guinier, built his first microanalyser (Castaing & Guinier, 1950; Castaing, 1951; Grillon & Philibert, 2002). The principle of EPMA is to bombard the sample surface with a focused electron beam and to collect the X-rays emitted from the sample. The X-rays are dispersed using Bragg diffraction on a mobile monochromator, which enables to get the whole X-ray spectrum from zero to more than 10 keV. The technique of dispersion is called WDS (Wavelength Dispersive X-ray Spectroscopy). The spectrum is made of continuous background (Bremstrahlung emission) and characteristic peaks, which allow elemental qualitative and quantitative analysis of the material. EPMA-WDS can be carried out in dedicated instruments called “microprobes” (usually fitted with 4 WDS spectometers) or in high current Scanning Electron Microscopes (SEM) equipped with one single WDS spectrometer. More recently, Energy Dispersive X-ray Spectroscopy has been developed and has nowadays become a cheap and widespread technique. Most SEM in materials science laboratories are equipped with an EDS spectrometer. EPMA-EDS performances are far below those of EPMA-WDS with regards to sensitivity and spectral resolution, although it can achieve very good qualitative and quantitative results in many cases. EPMA quantification of the sample composition is usually possible by using a standard material of known composition. Unfortunately there is in general no direct proportionality between the concentration of an element and the X-ray emission intensity (peak height) coming from this element. Since the early years of EPMA, many models have been proposed in the literature to correlate the concentration of an element and the associated X-ray emission intensity: ZAF (Philibert & Tixier, 1968), MSG (Packwood & Brown, 1981), PAP and XPP (Pouchou & Pichoir, 1987; Pouchou et al., 1990)... The XPP model (implemented in several microanalysis software packages) has now reached a good level of maturity and reliability, even in difficult situations involving light elements and strong bulk absorption. It is based on an accurate calculation of the (z) curve, which describes the depth-dependence of the X-ray emission intensity of a particular line in the sample. In EPMA analysis of