# A Quest for the Mechanism of Ultrahigh Resolution SEM Imaging

## TL;DR

This paper uses simulations to show how secondary electrons contribute to high-resolution imaging in scanning electron microscopy.

## Contribution

A new simulation framework clarifies the role of SE2 electrons in achieving sub-nanometer resolution in SEM.

## Key findings

- SE1 and SE2 electrons have similar emission characteristics except for spatial spreading.
- Morphology-modulated SE2 electrons near edges enhance sub-nanometer resolution.
- Excluding SE3 electrons improves resolution in ultrahigh-resolution SEM imaging.

## Abstract

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.

Using Monte Carlo simulations with explicit definitions of SE1 and SE2 align with a cascade secondary electron production model, this work shows that the 3D nanostructure modulated SE2 emission enhances other than to reduce topographical contrast via a “miniature Faraday cup” effect. This explains the mechanism of sub‐nanometer resolution SEM imaging.