Adsorbed layer etching of fused silica by excimer laser with nanometer depth precision

High quality etching of transparent materials for applications in micro- and nano-structuring as well as in precision engineering is still a challenge for current laser processing techniques. The recently developed LESAL technique allows precise etching of transparent materials, e.g., fused silica, at low laser fluences with small etch rates in the nanometer scale. The etch process is based on the pulsed UV-laser backside irradiation of a transparent material that is covered with an adsorbed toluene layer. This layer absorbs the laser radiation causing the etching of the solid. A constant etch rate of 1.4 nm/pulse was measured in a fluence range from 2.0 to 4.5 J/cm2 for fused silica. The etched surfaces with a depth of more than 1 μm feature a roughness of below 1 nm rms. With increasing pulse number the rising roughness is mainly caused by the growing waviness and spikes. Mask projection techniques were applied for the fabrication of surface structures in the micron range with nanometer depth precision and a low roughness.