Monochromators and Imaging Energy Filters

The ultimate goal of high-resolution analytical electron microscopy is the acquisition of detailed information about the atomic structure, the chemical composition, and the local electronic states of real objects whose structure deviates from ideal crystalline periodicity. To obtain detailed information on the interatomic bonding, an energy resolution of about 0.1 eV is necessary. The presently available electron microscopes do not fulfill this requirement because electron sources with a maximum energy spread of 0.1 eV at a sufficiently high current do not yet exist for conventional transmission electron microscopes. The energy width of field emitters lies in the range between 0.3 and 0.8 eV depending on the current. Hence, to enable electron spectroscopy with an energy resolution of 0.1 eV, we must employ a monochromator which filters out the electrons which deviate more than ±0.05 eV from the most probable energy. A feasible monochromator reduces the energy spread of the beam without affecting the spectral brightness and the effective size of the source. To preserve the emission characteristic of the source and to prevent a loss of lateral coherence, the dispersion must vanish on the far side of the monochromator. Moreover, the monochromator should be as compact as possible to avoid an unduly lengthening of the column. These conditions cannot be satisfied satisfactorily by Wien filters. In order that the monochromator does not affect the size and the radiation characteristic of the effective source, the second-order aberrations and the dispersion must vanish behind the monochromator. Therefore, the energy selection must be performed within the monochroma-tor at a position where the dispersion is at its maximum. Different versions of such dispersion-free energy filters have been proposed [75,90]. Because the monochromators are placed at high tension, electrostatic designs are most appropriate [77, 78].