A new omega filter has successfully been installed in 200kV TEMs(see FIG. 1). Energy filtering TEM (EFTEM) is extremely useful to enhance contrast of biological specimens, to observe defects in thick semiconductor tips, to obtain a good compositional map in grain boundaries, to give structure image in the thick specimen regions and to obtain quantitative data by removing plasmon loss signals. In-column filters such as omega filter has advantages over postcolumn filters from points of cancellation of aberrations, to obtain iso-chromatic images and a freedom of selecting variety of magnifications.

Here, we discuss conditions of designing an omega filter. In usually, curved axis systems, which are common to energy analysis equipment, has large second order aberrations. Those aberrations are compensated in analyzers with single sector magnet with multi stage sextupole correctors. However, the adjustment of those correctors is difficult and residual aberrations destroy quality of the filtered image.