The Emergence of Aberration Correctors for Electron Lenses

In 1936, Otto Scherzer published his famous theoretical paper 1 on the correction of aberrations in round magnetic electron lenses. For fundamental reasons, the aberration coefficients are positive and to introduce negative corrections, the electron optical elements have to produce either charges or poles on the optic axis, or contain a mirror or be a multipole system. In 1997, three papers appeared with actual demonstrations of the correction of aberrations in imaging lenses, 2 probe forming lenses 3 and electron mirrors. 4 It took almost two thirds of a century for this seemingly straightforward development to occur and for the promise of wavelength limited electron imaging to emerge as a practical proposition. In this paper I will put this achievement into perspective, i.e., try to explain why this was so difficult, give some indication of the implications for our field and provide a sense of the contributions of two scientists, Albert Crewe and Harald Rose, whose work has immeasurably influenced the work we do now and, in my view, has even more influenced the future of this field.