Electron optics of skewed micro-Einzel lenses

Abstract

Micro-Einzel lenses always suffer from chromatic and spherical aberration, even when the electron beam is exactly on the optical axis of the lens. When the inclination of the electron beam with respect to the lens axis increases, additional effects such as coma, astigmatism, and defocus start to dominate. An example of inclined electron beams in micro-Einzel lenses can be found in multi-electron-beam systems with a single source: the performance of a micro-Einzel lens array in front of a single Schottky electron source in a high brightness, high resolution multi-electron-beam scanning electron microscope is limited by its field aberrations. A model is presented to analyze the performance of inclined electron beams in micro-Einzel lenses. A first solution to improve this performance is to introduce micro-Einzel lenses of which the apertures are aligned with the center of the electron beam by shifting them perpendicular to the system optical axis, resulting in an array of skewed micro-Einzel lenses with reduced field aberrations. The model is used to prove the principles of this concept. A second solution is to fully compensate astigmatism and defocus by introducing elliptical lens holes with a diameter increasing with the off-axis distance. The presented solutions can be used to control the field aberrations of the multi-electron-beam system.