Transverse chromatic aberration in a symmetric magnetic doublet with dynamically compensated field aberrations

Abstract

Radial and azimuthal transverse chromatic aberrations, which are calculated from the electron trajectories, are much larger than those calculated from the axial magnetic field distribution and its first derivative. The generation mechanism for such large transverse chromatic aberrations is assumed to be that the electron trajectories initiated at the same object position with different beam energies travel through different paths. This assumption is roughly confirmed by studying the crossover position deviation and the magnetic vector potentials as a function of radius along the Z axis and their curvature. When the object side and image side lenses are shifted in the crossover direction by 32 and 8 mm, respectively, the azimuthal and radial transverse chromatic aberrations are 1.9 and 1.3 nm/eV for a field size of 20×0.25 mm2 and a beam energy of 100 keV. The radial and azimuthal distortions in the subfield are much smaller than 10 nm, when the radial subfield size is 250 μm and the target rotation is corrected.