| 摘要 |
Particle-optical rotationally-symmetrical lenses inevitably suffer from chromatic aberration. This lens defect determines the limit of the resolution of known particle-optical apparatus at a comparatively low acceleration voltage (from 0.5 kV to 5 kV). This lens defect cannot be eliminated by means of rotationally-symmetrical fields. In order to enhance the resolution nevertheless, it has already been proposed to mitigate said lens defect by means of a corrector of the Wien type. This known configuration consists of a number of electrical and magnetic multipoles. In order to adapt the corrective capacity of the corrector (28) to the strength of the objective (8) to be corrected and to achieve less severe requirements in respect of mechanical tolerances, according to the invention the corrector (28) is excited in such a manner that the trajectory of the electrons constitutes a sinusoid of a length which does not deviate more than 10 % from a full sine period, upstream and downstream from the correction device there being arranged an n-pole, where n = 4, 6, ... (27, 29), at a distance which is less than 1/4 of the length of the corrector.
|
