High-speed operation of scanning electron microscope lenses

Abstract

The speed at which the focal properties of an iron-shrouded magnetic electron lens can be changed is fundamentally limited by Eddy currents in the magnetic material. The effects of such Eddy currents on the lens properties can be partially compensated by modifying the lens excitation to speed up the lens response. This work considers the application of excitation pre-emphasis techniques to scanning electron microscope lenses for small and large dynamic changes in excitation. Where transient material saturation effects come into play, attempts to further improve the lens response beyond that at the onset of saturation are futile. Under such circumstances, the use of a closed-loop feedback arrangement is proposed that can provide a degree of response improvement.