Aberration Correction in Electron Microscopy

Spherical and chromatic aberrations limit the resolution of conventional electron microscopes. Both defects are unavoidable in the case of static rotationally symmetric electromagnetic fields (Scherzer’s theorem). To compensate for these aberrations, multipole correctors or electron mirrors are required. The correction of the resolution-limiting aberrations is demonstrated for the hexapole corrector, the quadrupole-octopole corrector and the tetrode mirror. Electron mirrors require a magnetic beam separator, which must be free of second-order aberrations. The multipole correctors are highly symmetric telescopic systems compensating for the defects of the round lenses. The hexapole corrector has the simplest structure yet eliminates only third-order spherical aberration and coma, whereas the mirror and the quadrupole-octopole (QO) corrector are able to correct the chromatic aberration as well. The QO corrector eliminates chromatic aberration by means of crossed electric and magnetic quadrupoles and the third-order spherical aberration by octopoles. At present, aberration-corrected electron microscopes obtain a resolution limit of about 1 Angstroem.