Simulation of a hollow cone-shaped probe in aberration-corrected STEM for high-resolution tomography

In a simulation study, we found that focal depth extension using a hollow cone-shaped probe with an annular aperture is useful for three-dimension (3D) tomography of aberration-corrected scanning transmission electron microscopy (STEM). Our calculations showed that, for 200 kV STEM, a sub-angstrom sized probe could extend the focal depth from a few to more than several tens nm. We also examined the influence of obstructing bridges, including actual fabricated annular apertures, on focused probe intensity distribution. We found that, to avoid any distortion of probe intensity, the width of the bridges should be narrow. Quantitative evaluation showed that the ratio of obstructing area of the bridges to the area of the annular slit should be less than 0.11.