A beam separator with small aberrations

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.