CORRECTION DEVICE FOR CORRECTING THE SPHERICAL ABERRATION IN PARTICLE-OPTICAL APPARATUS

摘要

Electron-optical rotationally symmetrical lenses inevitably suffer from spherical aberration which often imposes a limit on the resolution. This lens defect cannot be eliminated by compensation by means of rotationally symmetrical fields. In order to enhance the resolution nevertheless, it has already been proposed to correct the spherical aberration by means of a correction device provided with two hexapoles (24, 26) and two rotationally symmetrical transmission lens systems (28, 30). Each transmission lens system in the known correction device consists of two lenses. According to the invention, one or both transmission lens systems can be replaced by a single lens without reducing the corrective capacity (when only one system (30) is replaced) or while reducing it only slightly (when both systems (28, 30) are replaced).