Towards sub-0.5 Å electron beams

In the 4 years since the previous meeting in the SALSA series, aberration correction has progressed from a promising concept to a powerful research tool. We summarize the factors that have enabled 100-120kV scanning transmission electron microscopes to achieve sub-A resolution, and to increase the current available in an atom-sized probe by a factor of 10 and itiore. Once C, is corrected, fifth-order spherical aberration (C-5) and chromatic aberration (C-c) pose new limits on resolution. We describe a quadrupole/octupole corrector of a new design, which will correct all fifth-order aberrations while introducing less than 0.2 mm of additional C-c. Coupled to an optimized STEM column, the new corrector promises to lead to routine sub-Angstrom electron probes at 100 kV, and to sub-0.5 Angstrom probes at higher operating voltages. (C) 2003 Elsevier Science B.V. All rights reserved.