Test and characterization of a new post-column imaging energy filter

Spectrometers and energy filters for electron energy-loss spectroscopy and electron spectroscopic imaging in transmission electron microscopy have a long history. The first commercial post-column imaging energy filter became available already in the early 1990s. Since then this technique has become an important tool for analytic electron microscopy. Nevertheless, the imaging performance of post-column energy-filters is undoubtedly inferior compared to in-column energy filters and their principal design is clearly restricted by practical constraints we found that significant improvements with respect to optical performance and alignment flexibility are still possible compared to the presently available instrumentation. By applying calculation methods originally developed for the design of aberration correctors and spending much care on the precise design of the optical elements and the alignment strategies we could develop a new post-column imaging energy filter with a minimalistic and robust optical design but remarkable optical properties. Compared to other state-of-the-art devices in our design the aberrations determining the energy resolution are reduced by one order of magnitude for the same entrance aperture. Therefore, if desirable, the field of view can be enlarged by more than a factor of two in area while the optical aberrations stay comparable or even better. We describe the principal optical design of the CEOS Energy-Filtering and Imaging Device (CEFID), explain methods for aberration measurement and optical characterization, and finally demonstrate its optical performance by measured imaging and spectroscopy data from two prototype systems.