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M. Stöger-Pollach, T. Schachinger, K. Biedermann, V. Beyer:
"Valence EELS below the limit of inelastic delocalization using conical dark field EFTEM or Bessel beams";
Ultramicroscopy, 173 (2017), 24 - 30.

In this experimental work we present novel methods to increase the spatial resolution of valence electron energy
loss spectrometry (VEELS) investigations below the limit given by the inelastic delocalization. For this purpose
we analyse a layer stack consisting of silicon/silicon-oxide/silicon-nitride/silicon-oxide/silicon (SONOS) with
varying layer thickness down to the 2 nm level. Using a combination of a conical illumination and energy filtered
transmission electron microscopy we are able to identify the layers by using low energy losses. Employing Bessel
beams we demonstrate that VEELS can be performed in dark-field conditions while simultaneously the Bessel
beam increases the spatial resolution of the elastic image due to less sensitivity to the spherical aberration of the
condenser lens system. The dark-field conditions also guarantee that only electrons are collected that have
neither undergone an energy loss being due to the C ˇ erenkov effect, nor due to the excitation of transition
radiation or light guiding modes. We consequently are able to measure the optical properties of a 2.5 nm thin
oxide being sandwiched by the silicon substrate and a silicon-nitride layer.

http://dx.doi.org/10.1016/j.ultramic.2016.11.022