An electrostatic aberration corrector for improved Low-Voltage SEM imaging

An electrostatic aberration corrector (AC), based on a quadrupole-octupole design, for a low voltage scanning electron microscope (LV-SEM) has been developed, integrated and tested in a modified commercial SEM for improving image quality. After quantitative assessment and adjustment of the chromatic aberration and qualitive adjustment of the spherical aberration, LV-SEM image resolution and contrast improved by almost a factor three. Some unavoidable electromagnetic interference (EMI) accounts for the difference between the experimentally demonstrated AC-SEM edge resolution of 3.0 nm at a beam energy of 1000 eV and the corresponding theoretical probe size of 2.2 nm. After cancelling the chromatic and spherical aberrations of the objective lens of a scanning electron microscope (SEM) the reachable image resolution is limited by spot blur due to EMI, higher order aberrations and, more fundamentally, by the interaction volume of the focused electron beam in a sample and beam-induced alterations to the sample. Furthermore, the practical performance of the purely electrostatic aberration corrector integrated into an AC-SEM is demonstrated on typical material and life science samples at a beam energy of 500 and 1000 eV. Whereas electro-magnetic aberration correctors struggle with re-alignment iterations after a beam energy change due to remanent magnetic fields, a purely electrostatic corrector is swiftly adjusted by proportional scaling of electrode voltages. In principle, an electrostatic corrector can also be applied to low-voltage ion microscopy. Summarising, an easy-to-use purely electrostatic corrector has been developed which, after proper integration into a state-of-the-art SEM, is capable of delivering the ultimate low-voltage SEM images.