Performance of a Monochromator for a 200 kV Analytical Electron Microscope

Abstract

We have been developing a 200 kV analytical electron microscope, which is equipped with a monochromator [1]. The target performance of the microscope is to achieve an energy resolution of 0.2 eV with a smaller than 2 nm diameter probe on a specimen plane. Though the ultimate energy resolution of 0.14 eV was obtained with our first monochromator, the shape of the beam on the specimen plane was oval [2]. The new monochromator consists of two dodecapole-type Wien-filters (Fig. 1) of 30 mm length and a slit on the symmetric plane of the two filters [2], [3]. The upper (1st) filter and the electro-static round lens at the entrance of the monochromator make an energy-dispersed focus on the slit. The lower (2nd) filter cancels the energy dispersion and makes an achromatic and stigmatic focus at the exit of the monochromator. We obtained the energy dispersion of 19.5 μm/eV on the slit experimentally, which is sufficient to obtain the energy resolution of 0.2 eV using the slit. Figure 2 shows the shape of a 200 keV electron beam on the specimen plane with the new monochromator. It shows that an almost round shaped beam or a better achromatic beam of a 4 nm (FWHM) was obtained. A smaller beam than a 2 nm diameter on the specimen plane at the achromatic condition of the monochromator will be obtained by using a higher excitation condition of the probe forming lens system. We have succeeded in obtaining an energy-dispersed beam on the slit and a stigmatic and achromatic beam at the exit of the monochromator.