Evaluation of in-column energy filters for analytical electron microscopes

Abstract

Performance of three electron energy filters: B-type omega filter (dispersion D=1.2 μm/eV at 200 kV), twin column filter ( D=5.1 μm/eV) and S-filter ( D=9.5 μm/eV ) are compared. Comparison was made for the non-isochromaticity at various total magnifications and beam illumination conditions in imaging and diffraction modes and for the energy resolution in electron energy loss spectroscopy. Although non-isochromaticity is considered to be a property of the imaging filter, it depends on the size and energy spread of the incident beam in practice. The incident beam size and energy width are changed to investigate how the non-isochromaticity and energy resolution changes by those parameters. When the energy width is 1 eV (without using monochromator), the large dispersion filter (S-filter) has a merit under low magnification and large illumination angle. The non-isochromaticity approaches up to the energy width of the incident beam when the magnification becomes high (100,000 times) or the beam illumination angle becomes small (1 mrad). Under the diffraction condition, the B-type omega filter has a difficulty in obtaining 4 eV non-isochromaticity when the diffracted beam with the half angle of 60 mrad is used. However, the other two filters can give smaller non-isochromaticity because of its larger dispersion. The elongation of the beam on the slit plane occurs due to the energy spread of the incident beam. The energy resolution of the filter is determined by the incident beam energy width when it is larger than 0.1 eV for all filters.