Aberration Correction and Electron Holography Hannes Lichte, Dorin Geiger, Martin Linck and Michael Lehmann* Triebenberg Laboratory, Institute of Structure Physics, Technische Universitaet Dresden, Germany, www.triebenberg.de * Institut für Optik und Atomare Physik, Technische Universitaet Berlin, Berlin, Germany, www.ioap.tu-berlin.de/menue/arbeitsgruppen/ag_lehmann Otto Scherzer has shown that aberrations cannot be avoided with the usual round electron lenses [1]. However, he also has shown how electron microscopy can make the best out of it. In his famous paper [2] he outlined the wave optical aspects in a TEM and derived the transfer functions for both Phase Contrast and Amplitude Contrast. He showed that aberrated imaging of atoms – considered as phase objects – needs optimizing phase contrast by means of the so-called Scherzer-focus. At the end, after improving the performance of electron sources and lenses, atomic structures were visible, however, averaged about the point-spread function (“delocalization”) with a diameter of about five times Scherzer resolution. Despite the remaining restrictions, TEM contributed indispensably to the progress in materials and life sciences. Amongst others, since 1936 Scherzer struggled for an aberration corrector to overcome the resolution limit with the help of non-round electron lenses. His scholars Harald Rose [3] and Maximilian Haider [4] finally solved the involved problems and built the aberration corrector hence brought TEM to real atomic resolution; a lateral resolution of 0.05nm is within reach. With these correctors, the point-spread function provoked by the residual aberrations is finally smaller than the atoms; therefore, position and intensity of atoms can reliably be interpreted. Additionally, like in any optical system, opening up the imaging aperture improves not only lateral resolution but also enhances the signal strength by increasing the collection efficiency for the diffracted electron waves. Therefore, the perceptibility of weak structures such as light atoms is considerably improved. Nevertheless, already from the papers of Scherzer, it became also evident that there is an additional problem: the object modulates both amplitude and phase of the electron waves. Therefore, the question remains What do we see in the intensity image? With correction of aberrations, these questions get even more severe in that phase modulations are completely invisible in an intensity image recorded at perfect correction. A solution is offered by electron holography. Dennis Gabor proposed electron holography as a lens-less imaging method in order to by-pass the limitations presented by the aberrations [5]. In fact, lens-less electron holography is not yet successful because of the comparably poor coherence provided by electron beams. Therefore lenses are needed to take image plane holograms at highest lateral resolution; fortunately, the effects of lens aberrations in the reconstructed wave can be corrected a-posteriori; a review is found in [6]. From such a hologram, the complete object exit wave can uniquely be determined in an amplitude image and a phase image, and hence all information needed for a complete interpretation in terms of the object is available. Then, phase images allow revealing details of the atomic structure, such as • difference of atomic numbers of different constituents • number of atoms in an atomic column • interatomic electric potentials • potentials across interfaces Examples are given in [7]. Microsc Microanal 15(Suppl 2), 2009 Copyright 2009 Microscopy Society of America doi: 10.1017/S1431927609096160 1460 https://doi.org/10.1017/S1431927609096160 Published online by Cambridge University Press