A beam separator with small aberrations

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Abstract
Beam separators are utilized for splitting the illuminating electrons from the imaging electrons in cathode lenses or for splitting the incident beam from the reflected beam in a mirror corrector. In the latter case the beam separator must have small aberrations to enable a significant increase in resolution. For this purpose a highly-symmetric system is outlined with imaging properties similar to round lenses. A simple and systematic calculation procedure is presented which obtains the aberrations of the beam separator up to arbitrary rank. To include the effect of the fringe fields, a charge simulation method has been developed which yields a high-precision design. The aberrations of the optimized system are compared with those of equivalent standard electron-optical elements.

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