Method of investigating and correcting aberrations in a charged-particle lens system

摘要

A method of investigating aberrations in a charged-particle lens system, which lens system has an object space comprising an object plane and an image space comprising an image plane, whereby an object placed on said object plane can be imaged by the lens system onto said image plane, the lens further having an entrance pupil, which method comprises the following steps:- Selecting a fixed pivot point on said object plane;- Directing a charged-particle beam through said pivot point, entrance pupil and lens system and onto said image plane, said beam having a relatively small cross-sectional area relative to the area of the entrance pupil;- Changing the orientation of said beam through said pivot point, so as to trace out an entrance figure on said entrance pupil and a corresponding image figure on said image plane;- Registering said image figure;- Repeating this procedure at a series of different focus settings of the lens system, thus acquiring a set of registered image figures at different focus settings;- Analyzing said set so as to derive lens aberrations therefrom.;This analyzing step will generally comprise performing a mathematical fit of said set to a collection of theoretical image figures predicted using a mathematical model. Such a model may describe wavefront deformation by the lens system in terms of localized alterations in phase and phase gradient per point on a wavefront, using a two-dimensional function. This two-dimensional function may be expanded as a Taylor Series, whose coefficients yield information on the magnitude of the various lens aberrations being investigated. Said series of different focus settings preferably includes points at or near best focus, and may extend through best focus to include under-focus and over-focus points.