<title>Dynamic correction of aberrations in focusing and deflection systems with shaped beams</title>

Abstract

In electron beam focusing and deflection systems, certain aberrations (such as deflection field curvature, astigmatism and distortion) can be `dynamically corrected', by applying appropriate focusing and stigmation fields corrections at each point in the deflection field. Such corrections are particularly important for large-area shaped-beam systems. In this paper, the question of which aberrations can be dynamically corrected is discussed, and the principle of applying dynamic corrections using dynamic focus coils and stigmators is summarized. The computation of the fields in magnetic and electrostatic stigmators is then described, and a unified aberration theory for handling electron lenses, deflectors, dynamic focus coils and stigmators is outlined. This theory has been implemented and incorporated in an optimization program. An example of the computer-aided design and optimization of a shaped beam system with dynamic corrections is presented. The results lead to the interesting conclusion that, as well as correcting field curvature and astigmatism, dynamic correction elements can also be used to significantly reduce the hybrid distortions of shaped beam systems, thereby largely eliminating pattern distortions and stitching errors.