Abstract
Calculation formulae for the residual field curvature, astigmatism and distortion aberrations are made. The resolution of a typical symmetrical magnetic doublet with dynamically compensated field aberrations is limited by the diffraction, the residual field curvature and astigmatism aberrations. From the beam trajectories, it has been clarified that distortion is generated because the crossover is shifted from the normal position in the direction of the object for the outer trajectories by the spherical aberration of the first projection lens, and that the distortion is reduced if the crossover position for each trajectory is corrected to the normal position. With this correction, the field curvature and astigmatism are not changed. For a 1.5 m optical path length from the object to the image and for 20 and 0.25 mm of main- and subfield sizes, respectively, the residual beam resolution and distortion are 33 and 1.1 nm, respectively, when the beam energy is 50 keV.
Published Version
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