Wavelength-dependent ripple propagation on ion-irradiated prepatterned surfaces driven by viscous flow corroborates two-field continuum model

Glass surfaces were patterned by milling periodic trench structures with wavelengths from 150 to 750 nm in a focused ionbeam (FIB) system. Upon exposure to 30 keV Ga super(f) ion irradiation under an incidence angle of 52[degrees] with respect to the surface normal, those patterns were found to transform into "ripple"-like nanostructures. A quantitative agreement of the experimental findings with this model is obtained if the presence of a viscous flow is effective in that thin surface layer. Ex situ atomic force microscopy was employed to derive the final crest-to-valley amplitudes of the ripple structures; their values were ~50-100 nm and thus similar to the depth of the original trenches. On the pristine surface areas (which have not been patterned), ripples were also formed by ion bombardment. Their wavelength increases with ion fluence [Phi] from ~250 to ~420 nm, following a dependence [lambda] [infinity] [Phi] super(0.19).