Evaluation of aberrations of immersion objective lensesin relation to electron emission microscopy

Abstract

This paper aims to evaluate the spatial resolution achievable with immersion objective lenses for electron emission microscopy. The simulations, using Simion 7 software, follow a brief reminder of objective lens properties as determined by analytical studies. In general, the aperture and chromatic aberrations should be characterised concurrently since the aperture stop causes an energy-dependent angular discrimination. This last effect adds to the longitudinal chromatic shift produced during the initial electron acceleration. The simulation is applied first to monochromatic electron beams for PEEM objective lenses described in the literature and for several other lens configurations. Then, for energy dispersed electron beams (0.2–2, 0.2–4,..., 0.2–10 eV), the image quality is deduced from density plots involving convolution weighting factors which include the change in angular acceptances. It is shown that PEEM objective lenses associated with an aperture stop of a diameter of 30 µm, may provide resolutions below 2 nm for electrons emitted within the initial energy window 0–10 eV. However for the same aperture stop and a moderate acceleration voltage of 4000 V other objective lenses might provide equally good resolutions. The acceleration bias mode for the focussing element seems especially promising since all along their paths the electron beams keep high energy and are thus less submitted to residual magnetic fields. Also the misalignment effects caused by the electrostatic field disturbances on the sample surface are reduced when a quite long acceleration length is used: the object nodal point being removed far from the emitting surface.