Roughness evolution of sol–gel optical coatings by ion beam sputtering

The surface roughness evolution of two silica-based sol–gel materials under 650 eV argon ion beam sputtering has been investigated. The liquid sol–gel solutions were applied on silicon substrates using the dip coating technique and thermally cured to obtain thin solid films and their thicknesses were then controlled over the samples surface using spectroscopic ellipsometry. The surface roughness of the sol–gel films was measured using both interferometric profilometry and atomic force microscopy depending on the obtained sputtering depths. We observed a significant increase of the roughness according to the sputtering depth, faster in the case of sol–gel layers than with bulk fused silica. Interestingly, the sputtering rates of the sol–gel layers were found much higher than the rate obtained on bulk fused silica. The development of micron scale holes with relatively stable interstices is supposed to rule the surface roughness evolution. AFM measurements revealed a regular submicron scale lateral structure which nanometric amplitude is amplified within sputtering.