  We have observed the formation of ripples in a number of thin metal films,
e.g. Au, Pt, Ag, Cu and Co under Ar^{+} ion beam sputtering at grazing
incidence. The structures are found to be quite stable under ambient
conditions. The results show that the ripple formation in polycrystalline
metallic films relies on the erosion-induced surface instability similar to
that in amorphous materials.
