Symmetric Magnetic Doublet Optics with Dynamically Compensated Field Aberration for Reducing Image Projection System

Abstract

Aberrations calculated from the electron trajectories are compared with those estimated from the third-order aberration coefficients. Field curvature and radial and azimuthal distortions are consistent with each other for moderate radial value R. In order to optimize lens positions, a method is adopted which makes the lens condition under which the principal ray trajectory from the largest objective radius R o crosses the optical axis at its normal position equal to that under which the trajectories diverging from the same Ro focus on the image. For the 50 cm object-to-image distance optics, residual aberrations are calculated through the trajectory calculation after the corrections of lens current, crossover and beam position. When the main and subfield sizes are 20 mm and 0.25 mm square, the residual field curvature, astigmatism, and radial and azimuthal distortions are 13.1 nm, 6.4 nm, 19 nm, and 8.35 nm, respectively, when the beam semiangle is 0.1 mrad, and the beam energy is 100 keV.