Journal
ULTRAMICROSCOPY
Volume 173, Issue -, Pages 24-30Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.ultramic.2016.11.022
Keywords
Coulomb interaction; Inelastic delocalization; VEELS; Bessel beams
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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 terenkov 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.
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