Principles of Dual-Beam Low-Energy Electron Microscopy

A novel low-energy electron microscope (LEEM) aimed at improving the throughput and extending the applications to nonconductive substrates has been developed. A dual-beam approach, where two beams illuminate the field of view, is used to mitigate the charging effects when the LEEM is used to image semiconductor substrates with insulating or composite (insulator, semiconductor, metal) surfaces. The negative charging effect, created by a partially absorbed mirror beam, is compensated by the positive charging effect of either a higher-energy secondary electron beam with an electron yield exceeding 1, or a photon beam. Several implementations of a dual-beam LEEM are presented. Novel electron-optical components and concepts enabling this approach are discussed in detail, including an immersion magnetic objective lens, dual-beam gun, matched immersion magnetic gun lens, tunable prism array, and semitransparent electron mirror. We have experimentally demonstrated the dual-beam approach by imaging a variety of semiconductor device wafers without deleterious charging effects.