ACCURATE DIRECT RAY TRACING IN CHARGED PARTICLE OPTICS

Abstract

This thesis investigates the accuracy of direct ray tracing on finite element method(FEM) solved potential distributions for magnetic lenses. Two structured mesh refinement techniques are used, the conformal mesh and the graded mesh. The conventional approach of first-order element solutions on region block meshes is compared with the use of second-order elements and first-order finite elements with the conformal mesh and the graded mesh. The results show that reliable results can be obtained by second-order elements and mesh refinement. Two test magnetic objective lenses are used for the investigation, the Cambridge S100 magnetic objective lens and a single pole magnetic objective lens. The axial field distributions of second-order elements and first-order elements with mesh refinement are analysed in terms of their continuity. Using the structured mesh refinement, high-order continuity of the axial field distribution can be obtained with first-order FEM. Direct rays are plotted through the lenses to calculate third and fifth-order spherical aberration coefficients. Using either second-order elements or structured mesh refinements, the truncation noise variations can be kept low. For the direct ray tracing of secondary electrons, the single pole objective test example is used to demonstrate that field interpolation may limit the final accuracy, in which case the structured mesh refinement is the most efficient means for improving the accuracy.