Ion irradiation induced roughening and smoothing of metal films on dielectric substrates

This study investigates the morphology evolution of ultrathin Pt films on dielectric substrates produced by heavy ion bombardment. The films roughen and form nanopatterns which coarsen in a certain dose range. The microstructure evolution is characterized using AFM, SEM and RBS. The results are explained using a novel irradiation induced dewetting mechanism involving local melting and viscous flow. The temperature-independent dewetting process scales with energy deposition density (J/cm2). Fissures in the initial films are identified as nuclei of dry patches for the dewetting process. Growth kinetics of the dry patches is followed using in situ TEM. Due to line tension in the curved contact lines during local melting, irradiation induced dewetting occurs much slower than dewetting under homogeneous heating. At high irradiation doses, nanopattern disappear to form nanoparticles with decreased surface roughness. The particles embed into the substrates, leading to the smoothing of the surface. The embedding kinetics is studied using cross-sectional TEM. An analytical model based on capillary driving force and viscous flow of the substrate is built to quantitatively explain the experimental results. The irradiation-induced viscosity of SiO₂ is extracted to be ∼0.9 × 1023 Pa ion cm−2, consistent with previous measurements using stress relaxation.