A Quest for the Mechanism of Ultrahigh Resolution SEM Imaging

The long-standing view in scanning electron microscopy (SEM) holds that SE1 electrons determine image resolution, whereas SE2 electrons contribute mainly to the background. However, the quantitative relationship between SE1/SE2 emission and spatial resolution has never been rigorously established due to the lack of explicit definitions for SE1 and SE2 for a Monte Carlo simulation. In this study, we develop a comprehensive simulation framework with unambiguous SE1/SE2 definitions to investigate secondary electron emission from typical solid materials by a 0.1-30 keV primary beam. The results reveal that SE1 and SE2 exhibit no intrinsic difference in emission characteristics except for the spatial spreading, and the local 3D morphology of the surface shall modulate SE2 spatial emission. Simulations for Au nanoparticles on a carbon substrate illustrate that the morphology-modulated SE2 electrons near particle edges contribute significantly to sub-nanometer resolution (∼0.8 nm); the exclusion of SE3 by the in-lens detector also plays a role in resolution enhancement. This work overturns the conventional consideration that SE2 degrades resolution, and provides a unified physical explanation for the mechanism of ultrahigh-resolution SEM imaging.