Topography and roughness evolution of microstructured surfaces at laser-induced backside wet etching

The topography and roughness evolution of surfaces etched by laser-induced backside wet etching (LIBWE) is investigated in detail. The etching of sub-mum gratings with a period of 760 nm into flat surfaces by means of interfering laser beams shows a saturation of the grating depth within 20 pulses. The over-etching of already microstructured surfaces results in the change of the cross section and in reduction of the microstructure height with increasing pulse number. The decrease in height of sub-micron gratings from 125 to less than 10 nm within 15 laser pulses causes a substantial roughness reduction. The depth limitations in etching of the gratings as well as the height reduction of microstructures are the result of the influence of the surface topography to the heat flow. The more efficient heating of surface peaks in contrast to the valleys results in higher etch rates and probably causes the smooth surfaces observed in LIBWE processing. The thermal diffusion length determines the structure dimension influenced by this "smoothing" effect.