On the role of electron-ion recombination in low vacuum scanning electron microscopy

Here we demonstrate the effects of electron-ion recombination on imaging signals utilized in low vacuum scanning electron microscopes (SEMs). The presented results show that, under normal operating conditions, recombination of ionized gas molecules with secondary electrons (SEs) suppresses a significant fraction of emitted electrons. If the ion flux (and hence the spatial dependence of the SE-ion recombination rate) is laterally inhomogeneous across the imaged region of a specimen, contrast in SE images can be influenced and in some cases (under conditions of high detector field strength and long ionic mean free path) dominated by variations in the recombination rate. Consequently, SE images of features such as topographic asperities can exhibit edge-darkening, leading to inversion of some topographic contrast. Recognition of the extent and nature of electron-ion recombination is required for a correct understanding of processes occurring in variable pressure SEMs and, subsequently, for models of image formation.