Comparison of the Focal Properties of Selected Magnetic and Electrostatic Lenses and Tests of Their Theoretical Models

Abstract

The main goal of the present research is to determine whether there is a significant difference between the aberrations of magnetic and electrostatic electron lenses when they are used in the same way. Comparison of the lenses was done in an electron-optical bench, set up to resemble a scanning electron microscope (SEM). Five different lenses, two magnetic and three electrostatic, were compared. Each group included two lenses with different lens-field lengths (pole-face spacing in magnetic lenses and interior length in electrostatic lenses). The lenses were used in turn to focus the electron beam into a demagnified image, or probe. The shadowgraph method was used to determine the paraxial properties and the spherical aberration coefficients Cs' of the probes. Graphs of Cs' versus working distance (WD) were used to compare the spherical aberrations of the probes focused by the different lenses. The main difference was shown to be not the type of lens, but rather differences in the lengths of the lens fields. For the same WD, lenses of either type, with shorter lens fields, had higher values of Cs' than did lenses with longer lens fields. An exception occurs when the focal length f0 goes through a minimum, causing Cs' to be lower for a short-field lens. Theoretical models with the same physical basis for focusing as the respective lens types, but with analytical solutions for the paraxial image and lens properties, were introduced for comparison with the experimental lenses. Lowering the activation of the models brought the curves of model properties into agreement with the corresponding lens graphs. The reduction in model activation needed for agreement was greater for short-field than for long-field lenses, showing that short-field lenses had weaker paraxial fields and higher spherical aberration coefficients. The analytical curves of lens properties obtained from the reduced activation models were used to evaluate the chromatic coefficients Cc' of the probes. Curves of Cc' vs. WD show that is higher for the electrostatic lenses than for the magnetic lenses.