METHOD OF OPERATING A PARTICLE-OPTICAL APPARATUS

摘要

Electron-optical, rotationally-symmetrical lenses inevitably exhibit spherical and chromatic aberrations which usually determine the limit of the resolution. Such lens aberrations cannot be eliminated by compensation by means of rotationally-symmetrical fields. In order to enhance the resolution nevertheless, it has already been proposed to reduce said lens aberrations by means of a Wien-type corrector. Such a configuration must satisfy very severe requirements as regards manufacturing precision, mechanical and electrical stability and alignment of the various elements relative to one another. Consequently, it is extremely difficult to perform readjustement of the electron-optical apparatus by means of such a corrector in the case of changing circumstances. According to the invention there is provided a combination of a correction unit and a doublet to be corrected. By constructing the objective to be corrected as a doublet (5) and by independently controlling the two lenses (6 and 8) of the doublet (5), the setting of the correction unit (28) can remain unchanged, it being possible to vary the free object distance and the electron voltage nevertheless during operation of the electron-optical apparatus.