Formation of nanoripples on amorphous alumina thin films during low-energy ion-beam sputtering: Experiments and simulations

The formation of nanopatterns induced by low-energy (0.5--1.5 keV) ${\mathrm{Xe}}^{+}$ ion-beam sputtering of amorphous alumina thin films is investigated by atomic force microscopy and grazing incidence small-angle x-ray scattering. The observed dependence of the surface morphology on ion incidence angle, temperature, ion energy, and fluence is compared with the predictions of linear and nonlinear continuum theoretical models. The results show that ion-induced mass redistribution stabilizes the surface at near-normal and very grazing incidence angles, while curvature-dependent erosion governs the formation of periodic nanoripples in the range of incidence angles between ${50}^{\ensuremath{\circ}}$ and ${65}^{\ensuremath{\circ}}$. Surface-confined ion-induced viscous flow is shown to be the dominant relaxation mechanism during erosion. Moreover, pattern evolution with ion fluence (pattern ordering and asymmetry of the ripple profile, in particular) suggests that nonlinear effects that are ignored by the Sigmund's collision cascade theory of sputtering contribute strongly to the observed dynamics of ripple formation.