Superior imaging resolution in scanning helium-ion microscopy: A look at beam-sample interactions

Scanning helium-ion microscopy (SHIM) offers high-resolution imaging at the subnanometer scale. We employ Monte Carlo simulations to show the characteristics of the beam-sample interaction involved in SHIM, and compare it with those of scanning electron microscopy and scanning gallium-ion microscopy. We discuss the electronic and nuclear stopping powers and demonstrate that helium ions in matter possess a distinctive "electronic loss phase" in which inelastic collisions strongly dominate. This allows for large penetration depth, localized secondary electron emission, and high signal-to-noise ratio. We investigate the energy dependence of the interaction volume and show that for SHIM, spatial resolution can be further increased with energies beyond the customary 30 keV level. This paper provides a better understanding of the physics of SHIM and its conditions for high performance.