Progress in aberration-corrected scanning transmission electron microscopy

A new corrector of spherical aberration (C-s) for a dedicated scanning tran smission electron microscope (STEM) is described and its results are presen ted. The corrector uses strong octupoles and increases C-c by only 0.2 mm r elative to the uncorrected microscope. Its overall stability is greatly imp roved compared to our previous design. It has achieved a point-to-point res olution of 1.23 Angstrom in high-angle annular dark field images at 100 kV. It has also increased the current available in a 1.3 Angstrom -sized probe by about a factor of ten compared to existing STEMs. Its operation is grea tly assisted by newly developed autotuning software which measures all the aberration coefficients up to fifth order in less than one minute. We concl ude by discussing the present limits of aberration-corrected STEM, and like ly future developments.