A post-column imaging energy filter compatible with multiple detectors

Abstract text (incl. figure legends and references)

Recent developments of detector technologies and acquisition schemes cause a demand for modular post-column energy filters with good performance for EFTEM and EELS experiments. Owing to decades of research in academia and at Gatan company for post-column filters and at Zeiss and JEOL companies for in-column filters these techniques are well established [1,2]. At CEOS we re-investigated the optical design and technology and identified some incremental improvements by using design methods, manufacturing technologies and alignment strategies originally developed for aberration correctors. Complementary to established plug-and-play solutions which primarily allow for a predefined combination of microscope / spectrometer / detector, we provide an open platform with utmost freedom in the selection of components, regarding both hardware and software.

The optics design of the CEOS Energy-Filtering and Imaging Device (CEFID) is based on a highly optimized sector magnet combined with two 12-pole elements. For all other focusing elements we use pure quadrupoles supplemented by some deflectors and separate two- and three-fold stigmators. This approach already allows for a 12mm entrance aperture with a peak-to-peak non-isochromaticity below 1eV at 200kV (Fig. 1) and a root-mean-square geometric distortion below 0.35% in imaging mode. In spectroscopy mode dispersions from 1meV/channel up to 1eV/channel are possible even at 80kV. The use of well-designed quadrupole elements made from high-quality soft iron helps to largely avoid remanence effects and improves the reproducibility of alignments. The filter design introduces a clear separation between pre- and post-slit alignments. All second- and higher-order spectrum aberrations can be measured and tuned at the slit plane by using identical methods for EFTEM and EELS modes [3]. This simplifies operation and enables to switch between imaging and spectroscopy modes with different dispersions or even different detectors with no or only little re-tuning. With an entrance aperture of 5 mm an energy resolution in the sub-20meV range should be possible and a drift of less than 1eV over 12 hours at 300 kV has been achieved.

Multiple pixelated detectors can be added at different distance from the final cross-over, e.g. large-field of view scintillator-coupled or direct-detection CMOS cameras or frame- or event-based hybrid-pixel detectors with a large pixel size (Fig. 2). Our Python-based software targets for open interfaces and flexibility for user-scripting also for unusual experimental setups. EFTEM and EELS including STEM spectrum imaging are readily possible [4].

References:
[1] Reimer, L. (1995) Energy-filtering transmission electron microscopy. Springer, Berlin.
[2] Egerton, R. F. (2011) Electron energy-loss spectroscopy in the electron microscope. Springer, Berlin.
[3] F. Kahl et al. (2019) AIEP 212 including Proceedings CPO-10.
[4] A. Ruiz-Caridad et al. (2022) Micron 160, Volume 160.

<Fig1>

Figure1. Non-isochromaticity measured at 200kV for the full Ø12mm entrance aperture and the inner Ø8mm area with increased post-filter magnification.

<Fig2>

Figure 2. CAD Model with a selection of currently possible pre-filter and post-filter detectors. Typically only two post-filter detectors are installed on the same system.