Theoretical and experimental investigations of resolution and detection limits in energy-filtering electron microscopy

Abstract

The energy-filtering transmission electron microscope (EFTEM) offers the possibility of obtaining two-dimensional elemental maps using characteristic inner-shell loss electrons. The characteristic signal lies on a background stemming from outer-shell ionizations. To subtract this unspecific background one acquires several images at energy losses below the characteristic edge to extrapolate the background. We used small uranium clusters to compare different background models and to investigate the detection and resolution limits. We found that the jump-ratio method yields the best results for the background correction, followed by the least-squares fit estimation for quantitative analyses. Particles with sizes down to 2.3 nm were detectable with a signal-to-noise ratio greater than five. To obtain the resolution limit, the technique of Young's fringes and the cross-correlation function were used. For our experiments this resulted in a resolution limit down to 2.4 nm. Theoretical calculations of the detection limit confirm these results.