Development state of the CEOS ground-potential monochromator

BIO Web of Conferences 129, 05001 (2024) EMC 2024 https://doi.org/10.1051/bioconf/202412905001 Development state of the CEOS ground-potential monochromator Felix Börrnert1, Stephan Uhlemann1, Heiko Müller1, Volker Gerheim1, Klaus Hessenauer1, Holger Schulz1, Maximilian Haider1 1CEOS GmbH, Heidelberg, Germany Electron energy loss spectroscopy (EELS) is a long established analytical method to investigate the chemistry as well as the electronic and optical properties of materials. There are several kinds of EELS, namely regards the energy of the primary electrons and if the geometry is in transmission or reflection. Each method provides distinct information about the sample. In particular, EELS often is integrated into an electron microscope adding spatial information to the spectroscopic signal with about the spatial resolution of the hosting microscope. In this respect, EELS in transmission at primary electron energies in the order of several 10 keV and above gained much popularity in the last decades due to the success of (scanning) transmission electron microscopy ((S)TEM). It evolved into a widespread characterisation method being able to deliver spectroscopic information at the atomic level. The energy resolving power in modern instruments is often sufficient for just identifying the elemental composition of a material by absorption edge onset fingerprinting, and in many cases the respective bonding states can be investigated by looking at the edge fine structure. Nevertheless, a higher energy resolution would help to find very weak signals and make the fine structure less ambiguous. More importantly, a better energy resolution gives access to very small energy loss signals that otherwise would be shadowed by the fringe of the primary electron beam signal. There, most interesting information about the physical properties of the investigated specimen can be gained. In spectroscopy using electrons, the energy distribution of the primary electron beam ∆ E₀ is a major limiting factor for the energy resolving power of the spectrometer. Reducing ∆ E₀ by means of a monochromator can enhance the energy resolving power of the system accordingly. Additionally, in (S)TEM, ∆ E₀ in conjunction with the chromatic aberration of the imaging system is one of the limiting factors for the maximum spatial information transfer, especially at lower energies. Therefore, a monochromator can also help to enhance the spatial resolving power of (S)TEM instruments. There are different types of electron monochromators; crossed electric and magnetic fields, called Wien filter or trochoidal monochromator, and electric or magnetic sector fields (prisms) combined in various geometries (α- or Ωshaped, hemispheres, or using electron mirrors), leaving out more exotic schemes like radio frequency cavities for pulsed electron beams. To date, four © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (https://creativecommons.org/licenses/by/4.0/). BIO Web of Conferences 129, 05001 (2024) EMC 2024 https://doi.org/10.1051/bioconf/202412905001 different types of electron monochromators for (S)TEM instruments have been commercialised. First, the simple single-stage Wien filter and second, the more advanced double-stage Wien filter, both employing crossed electric and magnetic multipole fields and a straight optical axis. Third, the purelyelectro-static Ω-monochromator using inhomogeneous sector fields bending the optical axis into an Ω-shape and fourth, the purely magneto-static groundpotential monochromator combining magnetic sector fields in such a way that the optical axis follows an α-shaped trajectory. For the first three cases the monochromator is added before the accelerator at low electron energies or at high potential. The fourth implementation is different in this respect. Here, the monochromator is added after the accelerator at high electron energies or at ground potential. The latter implementation ties in with earlier concepts from electron spectroscopy, where the energy-filtered source and the energy analyser are at the same potential. In that case, only the interaction with the specimen causes energy differences and disturbing effects from instabilities of the accelerator can be avoided. Moreover, having the monochromator, the spectrometer and the specimen at ground potential is very beneficial from a technological point of view since additional high-tension feed-through and energy transfer are not necessary. Recently, with a ground-potential monochromator implemented in a dedicated STEM, an unprecedented energy-resolution has been demonstrated and employed for phonon and aloof-beam plasmon spectroscopy. All four presently used monochromator implementations have in common, that the optical design are orthogonal systems with single-section symmetry, that is, the optical axis and the dispersive trajectories are all situated in one section. The optical axis can be curved in one section but not in two sections as long as a perfectly manufactured system without tolerances is considered. In this contribution, we present the development state of our new design of a ground-potential monochromator based on magnetic prisms in a threedimensional arrangement. This design abandons the energy selection at the central symmetry plane and the optical axis is not only curved in one section but in two sections. This allows for a highly symmetric and compact design with a variable energy window but no mechanically adjustable energyfiltering slits. Fixed blocking blades in combination with optical deflectors reside at distinct dispersive planes making the system robust and flexible. The practical design fits primary electron energies from 30 keV up to 300 keV and provides an energy resolving power of better than 2·10−7 with respect to the primary electron energy. Also, the resulting beam cross-over after the monochromator is free of residual spatial or angular dispersion. Very importantly, the new monochromator is designed to be potentially retrofittable to existing microscopes, adding about 33 cm to the total height of the microscope column. 2 BIO Web of Conferences 129, 05001 (2024) EMC 2024 https://doi.org/10.1051/bioconf/202412905001 Graphic: Keywords: EELS, monochromator, TEM, STEM 3  (...truncated)