EPMA using low-energy (0.1-1.0 keV) X-rays-an historical perspective

Abstract

Studies with the electron-probe microanalyser using low-energy (<1 keV) X-rays are reviewed with particular reference to analysis of the ultra-light elements (atomic number Z<11). The paper begins by describing the historical development of methods for measuring low-energy X-ray spectra and goes on to show that minimum detection levels achieved for ultra-light elements are now comparable to those for heavier elements, in ideal circumstances about 100 ppm using wavelength-dispersive spectrometry and about 1000 ppm with energy-dispersive spectrometry. Regarding quantitative analysis, the correction models developed during the 1960s worked fairly well in most cases but, because they were based upon an X-ray depth distribution function of limited validity, were unsatisfactory for the ultra-light elements. Since then, advances made using empirical curve-fitting methods to derive this function have led to accuracies of a few per cent. relative, close to figures for heavier elements, any limitations being most likely associated with the accuracy of the mass absorption coefficients used rather than the models themselves. The merit of utilising low-energy radiation for surface analysis is discussed and it is shown that, by employing a low-voltage electron probe to restrict X-ray excitation to surface layers and recording low-energy X-ray emission, surface films down to about 1 nm thickness may be investigated. Finally, examples are given to show the value of studying the shape of low-energy spectra as a means of identifying the phase of microstructural features.