Optimization of electron lens shape giving minimum spherical aberration coefficient

A novel computer-aided optimizing procedure has been developed to obtain the geometry of the magnetic electron lens that give the minimum spherical aberration coefficient Cs. The minimization of Cs is done by a simplex method in which Cs is regarded as a function of up to 11 parameters characterizing the lens geometry. In this process, Cs and other optical properties of lenses are evaluated using a high-speed FEM (finite-element method) with automatic mesh generation. The optimization procedure was applied to the design of an objective lens for a 400 kV transmission electron microscope.<>