A new corrector of spherical aberration (Cs) for a dedicated scanning transmission electron microscope (STEM) is described and its results are presented. The corrector uses strong octupoles and increases Cc by only 0.2 mm relative to the uncorrected microscope. Its overall stability is greatly improved compared to our previous design. It has achieved a point-to-point resolution of 1.23 Å in high-angle annular dark field images at 100 kV. It has also increased the current available in a 1.3 Å-sized probe by about a factor of ten compared to existing STEMs. Its operation is greatly assisted by newly developed autotuning software which measures all the aberration coefficients up to fifth order in less than one minute. We conclude by discussing the present limits of aberration-corrected STEM, and likely future developments.

Keywords:high resolution electron microscopy, scanning transmission electron microscopy, spherical aberration, aberration correction, Ronchigram analysis