Abstract text (incl. figure legends and references)

Spectrum imaging has developed into a very useful analytical technique. Behind the energy filter or spectrometer the EEL spectrum is recorded by a fast pixelated detector. For sample navigation and drift correction it is beneficial to simultaneously record an ADF STEM signal.

In order to capture enough signal to form a high-quality EEL spectrum a small camera length of the STEM projective has to be used. As a consequence, for many existing microscopes the inner detector angle for the available ADF detector becomes rather large leading to a small signal-to-noise ratio in the ADF images. In this situation, an additional, dedicated ADF detector inserted directly above the filter entrance aperture and adapted to its geometry can significantly improve the signal quality.

Here, we use a commercially available segmented electron detector (product of PNDetector GmbH [1]) originally developed for back-scattered electron imaging in SEM combined with a modified pre-amplifier configuration. The pneumatic insertion mechanism has been developed to be compatible with the CEOS pre-filter camera housing. The simultaneous acquisition of STEM EELS and ADF signals is fully integrated into the spectrum imaging workflows of our software CEOS Panta Rhei.

The semiconductor-type detector based on fully depleted ultra-pure silicon has a very high sensitivity up to 300 kV, a very low dark current level resulting in an excellent signal-to-noise ratio and allows for fast readout. The detector can be provided in a light insensitive version. The detector with an inner and outer diameter of Ø 5.6mm and Ø 12mm, respectively, is placed in front of the Ø 5mm entrance aperture for the CEOS Energy-Filtering and Imaging Device (CEFID [2]). While the integrated pre-amplifier provides a fixed gain, the main amplifier allows for adjustment of brightness and contrast levels. The signal from the four segments can be combined freely with a 4:1 readout using one main amplifier, while a 4:2 readout resulting in two simultaneous differential signals for orthogonal directions is possible with a second main amplifier. Additionally, the high sensitivity and discrete electron signals allow for quantitative STEM [3].

In this contribution, we exemplify different use cases and features of the detector, especially for high-resolution STEM at very small camera length.

References:
[1] Schmid et al. (2018), New Possibilities for State-of-the-Art Electron Microscopy with Fast Backscattered Electron Detectors, Microscopy and Microanalysis 24

[2] Kahl et al. (2019), Test and characterization of a new post-column imaging energy filter, Advances in Imaging and Electron Physics, Volume 212

[3] Ishikawa et al. (2014), Quantitative Annular Dark Field Electron Microscopy Using Single Electron Signals, Microscopy and Microanalysis 20

[4] CEOS GmbH has received funding from the European Union"s Horizon 2020 research and innovation program under grant agreement No. 823717 – ESTEEM3.

Figure 1: Detector mechanism with ADF STEM detector head and flange suitable for mounting to CEOS camera housing.

Figure 2: STEM image of Au particles on carbon film and corresponding histogram. The discrete electron events in the histogram allow for easily calibrate for quantitative STEM [3].