Design and testing of Omega mode imaging energy filters at 200 kV

We designed and constructed an omega filter with following optical properties. The dispersions at 100 and 200 kV are 2.1 and 1.1 μm eV^-1, respectively. Non-isochromatisity (difference of the selected energy between the centre and outer regions of an electron microscope image) is 1.3 and 2.5 eV at 100 and 200 kV, respectively, on a 25 mm diameter detector. The filter was installed and tested on a modified 200 kV high resolution TEM with an LaB₆ cathode. Zero loss high resolution lattice image of [110]Si in thick specimen regions were clearly observed. Zero loss HOLZ CBED patterns of [111]Si were taken successfully up to 6.4°at an accelerating voltage of 100 kV. Since the slit was inserted to cut 17 eV first plasmon loss of Si, the maximum observable angle limited by the non-isochromatisity is 6.8°, which agrees with the experimental 6.4°. Based on these results, a new omega filter with a non-isochromatisity of 0.9 eV on the diameter 25 mm at 200 kV was designed.