The nano-probe profile of the hf-2000 field-emission TEM

Abstract

The probe profile is as important as the probe size for nano-meter area X-ray analysis. Assuming the electron distribution of the probe as the Gaussian, the probe size d is usually defined as the full width half maximum (FWHM) of the probe distribution, which is suitable for SEM/STEM resolution. However, for the X-ray analysis of thin specimens, the full width tenth maximum (FWTM) which contains 90 % of the incident electrons, and thus 90 % of the × rays generated, is more suitable. For a Gaussian probe FWTM = 1.82 × FWHM. Recent electron microscopes with higher than 200 kV acceleration voltage have enough image resolution to observe objects of a few nm such as precipitates at a grain boundary or small particles on a catalysis surface. They are so small that it have been difficult to aquire statistically sufficient amount of x-ray signals with thermionic TEMs. To obtain larger probe current, larger condenser apertures are used and the condenser lens excitation is stronger so that the demagnified electron source image is formed on the specimen.