Correction device for correcting the lens defects in particle-optical apparatus

摘要

Electron-optical rotationally symmetrical lenses inevitably suffer from spherical and chromatic aberration which often impose a limit on the resolution. These lens defects 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 respective system (46) of four quadrupoles (48, 50, 52, 54), without the correction capability being reduced or only hardly so. If the two central quadrupoles (50, 52) of the quadrupole system (46) forming part of the correction device (22) are also arranged to produce electric quadrupole fields, the chromatic aberration of the lens to be corrected can also be corrected.