High spatial resolution surface potential measurements using secondary electrons

Abstract

High spatial resolution surface potential and work function measurements have been made by observing the onset energy of secondary electron emission using a band-pass analyser (CMA) in an ultra high vacuum scanning electron microscope (UHV-SEM). Multiple secondary electron spectra can be displayed for closely spaced positions on the sample. Alternatively a feedback loop is used to maintain a constant surface potential as the SEM beam is scanned across the sample. Both methods have been shown to yield linear measures of surface potential, and of work function differences, with a sensitivity of < 40 mV and a spatial resolution < 0.4 μm in suitable circumstances. Examples are given for polycrystalline tungsten and for a cesium monolayer edge on tungsten. In the latter case it is seen that the non-uniform potential distribution above the surface (the patch field) affects the low energy electron trajectories, and consequently spatial discrimination depends on the magnitude of work-function differences. Nevertheless very low energy electrons (<1 eV) are observed from regions which have a repulsive surface field. Computer simulations of low energy electron trajectories indicate that these may escape by multiple reflection along the surface. This effect, although not completely understood, enables high spatial resolution measurements to be made.