Micro-X-ray fluorescence combined with SEM/EDX

Mathias PROCOP
IfG – Institute for Scientific Instruments, Berlin, Germany

X-ray fluorescence spectroscopy (XRF) belongs to the analytical methods having the lowest minimum detection limits. Knowing this researchers attempted a soon as EDX spectrometers became commercially available to combine electron and photon excited X-ray spectroscopy in the scanning electron microscope (SEM). The SEM itself was used as the X-ray tube, but its low power in the µW range resulted in long spectrum acquisition times. Recent developments of small power (30 to 50 W) micro-focus X-ray tubes and focussing polycapillary X-ray optics enable a much more efficient excitation of X-ray fluorescence and, moreover, focussing the primary X-rays down to about 50 microns. X-ray tube and optics form a micro-focus X-ray source which has been commercially available as attachment to any SEM since a few years.  Fluorescence spectra are measured with the existing EDX spectrometer at count rates comparable with electron beam excitation. The advantage to have XRF as a second analytical method with the SEM is not only the lower detection limit, but also the extension of the measurable spectrum range up to 40 keV and the detection of elements or objects buried a few microns beneath the specimen surface.

The presentation explains the operation of the micro-focus X-ray source, similarities and differences in spectrum processing and quantification in comparison with electron beam excitation and gives finally a few examples of application.