Low-voltage transmission electron microscopy (LV-TEM)

Electron microscopy of beam-sensitive materials requires a rigorous reduction of the beam energy to avoid electron irradiation by knock-on damage. However, at the same time the strongly increased chromatic aberration reduces the resolution of conventional C_S-corrected TEMs dramatically and prevents (atomic) low voltage imaging. The SALVE (Sub-Ångström Low Voltage Electron microscopy) project initiated by Ulm University targeted for the development (together with the companies FEI/ThermoFisher and CEOS) of a C_C/C_S corrected TEM that still delivers atomic resolution in the voltage range from 20 to 80 kV. The SALVE TEM corrects not only axial aberrations up to the 5th order but also off-axial aberrations to the 3rd order. This  allows for very large field of views of more than 2000 data points in a single image. Moreover, the correction of chromatic aberrations allows imaging even at 20 kV exploiting the full beam intensity offered by the electron source, contrary to the other approaches that combine low voltage imaging with a monochromated source. The SALVE TEM allows an information transfer of more than 55 mrad at all voltages and achives sub-Å resolution even at 40 kV [1].

High resolution TEM imaging at very low voltages also requires enhancements of the imaging theory to fully understand and describe the experimental images. Non-linear effects that are increased at lower voltages and new dampening effects such as image spread have to be considered together with geometric and chromatic dampening envelopes. Also electron dose, camera response (MTF and DQE) have to characterized exactly and included into the image simulation [2,3].