Prospects for Trace Analysis in the Analytical Electron-Microscope

The analytical electron microscope (AEM) uses a high energy (>100 kV) beam of electrons to generate a range of signals from a thin foil sample as shown in figure la [1,2]. Various detectors are configured in the AEM to pick up most of the generated signals (fig. lb). Microanalysis is usually performed using the characteristic x-ray signal, detected by an energy dispersive spectrometer (EDS) although occasionally the electron energy loss spectrum is also used. This paper will emphasize x-ray microanalysis only. The specific advantages that the AEM has for microanalysis are two-fold. First the instrument can be operated as a high resolution transmission electron microscope, thus permitting the analytical information to be related directly to the microstructure of the sample. Second, in the AEM most microanalysis is performed with a probe size <z 10 nm and a specimen thickness < = 100 nm. This results in an analyzed volume 10' of that commonly encountered in bulk microanalysis, for example, in the electron probe microanalyzer (EPMA). This small volume means that the spatial resolution of microanalysis is relatively good (routinely <50 nm) but generally trace analysis in the AEM is relatively difficult, because generated signal intensities are low.