Minimizing electron optical aberrations.

摘要

Electron optical aberrations play important roles in high voltage electron optical instruments used for lithography (SCALPEL) and high-resolution electron microscopy (HREM). In this thesis, we developed new theoretical approaches for the aberration corrections in these two important fields. In the field of electron beam projection lithography, we studied the reduction of the field curvature in SCALPEL projection systems. By investigating the extreme aberration performance of conventional projection systems, we proposed a doublet design capable of minimizing the field curvature in the magnetic-round-lens scheme. We also demonstrated the principle of using space charge to correct the field curvature in an electron beam projection system. Several arrangements of space charge foils are found capable of reducing the field curvature, and it is feasible to make the field curvature to be the only affected aberration. In the field of electron microscopy, we studied the correction of chromatic aberration for high-resolution transmission electron microscopy with a large-gap polepiece. Designs based on the quadrupole-octopole multiplet for the C _c-correction were investigated numerically in which roles of higher order and off-axial aberrations are thoroughly studied. We obtained upper limit of power instability of electrostatic elements in such a corrector for given resolution goals and proposed an optimum design with satisfactory aberration performance. We also systematically studied the role of transverse stray field effect in high-resolution electron microscopy and pointed out the necessity of magnetic shielding in a high-resolution electron microscope with large-gap polepiece.