Ion Beam Nanopatterning Beyond Conventional Geometries

Abstract

This thesis explores the evolution of nanopatterns on solid substrates under various unconventional ion beam configurations in contrast to the conventional ones. These configurations include intermittent sputtering sequences to study surface relaxation effects, azimuthally swinging geometry at variable speeds to study surface morphological transitions and azimuthal swinging to study coarsening phenomena and hierarchical surface formation. Additionally, the potential for increased pattern order using these techniques is investigated. Conventional ion beam irradiation methods have been compared with these unconventional approaches. The primary objectives were to understand the mechanisms behind nanopattern formation and to explore methods for improving pattern quality and increasing structural ordering. The intermittent sputtering method involves irradiating the surface intermittently with Ar' ions at oblique incidence, allowing it to relax within intervals of varying durations. Nanoripples dominate the irradiated surfaces, accompanied by hierarchical triangular structures at lower energies. We find that ripple ordering peaks at intermediate sputtering intervals, and the area of triangular structures varies with these intervals. Our findings align with numerical simulations, suggesting that intermittent sputtering holds promise for tailoring ion-beam nanostructured surfaces. In the next study, we explored the morphological changes on silicon surfaces induced by 500 eV Ar™ jon sputtering at a 67° i technique. By varying azimuthal angles and rotation speeds, we identified four distinct idence angle, using a novel substrate swinging regimes of nanostructure morphology, including ripples with and without triangular structures, smooth surfaces, and disordered ripple topographies. Notably, ripple wave vector orientations exhibited sudden shifts at certain azimuthal angles. These findings enhance our understanding of surface evolution under unconventional ion sputtering, potentially advancing theoretical models and offering insights into practical applications of surface patterns. In our follow-up study, we investigated how Si(100) surfaces evolved under 500 eV Ar* irradiation at a polar angle of 67°, employing a novel technique of azimuthal sample swinging (20° (= A¢). We found that the swinging sample exhibited prolonged wavelength coarsening, while ripple anticoarsening was observed for the static sample at high fluences. This unconventional geometry directly influenced both linear and nonlinear growth regimes, leading to reduced asymmetries in surface structures. Our findings high light the role of near-surface mass redistribution events, sputtered atom re-deposition effects, and differential exposure conditions in the swinging geometry. This study contributes to a better understanding of the spatio-temporal pattern formation benefits of unconventional azimuthal swinging geometry in ion beam sputtering, with potential applications in various fields. A further work explores the impact of asymmetric azimuthal swinging angles (A¢) on self-organized nanostructures under low-energy Ar* ion beam irradiation. This configuration reveals rotated triangular structures and nanoripples. These patterns challenge traditional explanations based on curvature-dependent instability terms. A parameter space for Ar' ion bombardment with swinging substrates inducing rotated nanostructures on Si surfaces is identified. In summary, the experimentally observed results presented in this thesis are very useful. They contribute significantly to the fundamental understanding of pattern formation and ion-solid interaction. Furthermore, they facilitate the investigation i of pattern formation under unconventional techniques aimed at enhancing surface ordering and reducing surface defect density.