Mechanisms and control of bending of monocrystalline and polycrystalline films by gallium ion irradiation

Abstract

The ion beam-induced bending of thin metal films represents a potent technique for fabricating three-dimensional structures using focused ion beam technology. This technique exhibits significant potential in the realm of micro and nano-scale structure fabrication for applications in various novel devices, such as plasmonic detectors. However, achieving greater control is imperative to ensure the reproducible fabrication of intricate structures. Here, we investigate mechanisms of bending and their dependence on conditions using molecular dynamics (MD) simulations for both monocrystalline and polycrystalline gold films. Our findings reveal the distinct differences in the bending behaviors between monocrystalline and polycrystalline films, as well as the significant dependence of bending on irradiation conditions that can be utilized for control purposes. The results show that mass transports are associated with defect formation and migrations, as well as migration of grain boundaries in the polycrystalline case, which play key roles. We observe a slow initial stage of film bending by following the commencement of irradiation, which is attributed to non-uniform stress fields from non-uniform defect generation; however, mass transport becomes increasingly important at higher doses. There is a strong interplay between this mass transport and grain boundaries, leading to differences in the behavior of monocrystalline and polycrystalline films.