Formation Mechanism and Transverse Broadening Analysis of Ion Microbeams Generated Using a Micrometer-Scale Pinhole

Micrometer-scale pinholes provide a compact approach for generating localized ion beams, but ion scattering from the pinhole wall can produce spatially extended background that degrades implantation selectivity. To clarify the transport mechanisms underlying a SiV implantation experiment, 28Si ion transport through a 1 μm tapered pinhole was investigated using PHITS. The spatial distributions and exit angles of wall-scattered ions were evaluated, with attention to beam-energy dependence. Within the investigated parameter ranges and selected response metric, sensitivity analysis identified incident energy as the dominant parameter controlling transverse broadening. At 0.4 MeV, strong energy loss during ion–wall interactions limited forward transport, and exit angles of 10–30° corresponded to transverse displacements of approximately 170–580 μm at a 1 mm pinhole-to-sample distance. In contrast, 1–3 MeV ions retained more residual energy and produced a more continuous peripheral distribution near the beam axis. Separately, the radial flux distribution showed that the scattered-ion flux decreased rapidly within about 100 μm of the beam center and then formed a weak long-range tail. No distinct transport threshold was found at 400 μm, indicating that the >400 μm separation used experimentally is better interpreted as a conservative measure for suppressing scattering-induced background rather than a sharply defined physical threshold.