Performance of the SALVE III corrector for EFTEM applications

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The Sub‐Angstroem‐Low‐Voltage‐Electron‐Microscope (SALVE) corrector was designed and built by the CEOS GmbH for the SALVE III project [1], a joined project of the group of Prof. Dr. Ute Kaiser at the University of Ulm (Germany), FEI company in Eindhoven (Netherlands) and CEOS GmbH (Germany). This C c ‐C s ‐corrector is a dedicated low‐voltage corrector based on the so‐called Rose‐Kuhn‐Design [2], operated in a cubed FEI Titan Themis TEM for acceleration voltages from 20kV to 80kV. For all these high tensions it can be aligned such that it provides uniform phase contrast transfer for all image features up to an aperture angle of θ max = 50mrad and at the same time for a considerable field of view. To achieve such an excellent performance the corrector allows correcting all axial aberrations of fourth order and for certain unround axial aberrations of fifth order. Furthermore, C 5 can be adjusted to its optimum positive value for bright atom contrast. All axial aberrations up to third order and all off‐axial aberrations up to third order depending linearly on the distance from the axis can be adjusted. For all residual axial aberrations of fourth and fifth order, the lower order aberrations of same respective multiplicity can be adjusted for optimal compensation. This means that the integrated squared deviation from the ideal phase shift (±π/2 in case of C 1 , C 3 , C 5 and zero for all other multiplicities) over the entire aperture is minimized for each azimuthal multiplicity separately [3]. The predicted performance of the corrector has already been demonstrated for the five acceleration voltages 20kV, 30kV, 40kV, 60kV and 80kV [4]. Figure 1 (a) shows that even within the largest field of view reasonable with the mounted Ceta 4k camera the wave aberration hardly changes up to a scattering angle of 50mrad. Since the corrector will also be used together with a post column energy filter, we currently investigate both, theoretically and experimentally, how the optical performance of an EFTEM image is affected by the corrector. After correction of the linear chromatic aberration C c , an energy window of at least 20eV can be transferred by the corrector with a negligible focus change even at a beam energy of 20keV, see figure 1 (b). However, there are many more potentially harmful types of chromatic aberrations to consider for an EFTEM image of a given finite width of the energy window: Axial chromatic aberrations (depending on scattering angle and energy loss) can deteriorate the quality of the axial PCTF. For large EFTEM windows also the chromatic spherical aberration has to be taken into account. Off‐axial chromatic aberrations (depending on scattering angle, distance from the axis and energy loss) can effectively decrease the field of view, because they affect the quality of the transfer function in the outer image parts. Chromatic distortions (depending on distance from the axis and energy loss) can deteriorate the information limit in the outer regions of an EFTEM image. Residual dispersions of higher order in energy change could affect the information limit of all regions of the EFTEM image. In this work we will analyze in detail how the residual higher order chromatic aberrations, the remaining chromatic distortions and the residual dispersions of the SALVE corrector affect the EFTEM performance. [5]