Ion-induced roughening and ripple formation on polycrystalline metallic films

We present a study of nanopattern formation on polycrystalline Ni surfaces upon low energy Ar ion bombardment. At low angles of ion incidence an isotropic, rough morphology develops on the surface while at grazing incidence a ripple structure parallel to the ion beam direction is formed (so-called perpendicular mode ripples). To explain this behavior we propose a model which is based on a combination of (a) surface roughening due to sputter yield variation between different crystalline grains and (b) anisotropic nonlinearity resulting from the oblique angle bombardment. By computer simulations we show that the combination of these two phenomena excellently reproduces the experimental behavior, in particular the dependence of the surface morphology on the ion incidence angle. Importantly, the formation of ripples at grazing incidence does not involve any linear instability, in strong contrast to the present model of the ripple formation process.