Wet etching of fused silica: a multiplex study

Abstract

The machining process of transparent materials using the laser induced backside wet etching (LIBWE) procedure was studied. In the course of this, experimental investigations and numerical calculations were carried out. Fused silica plates were irradiated by an ArF excimer laser, using a naphthalene–methyl methacrylate solution as an absorbing liquid (concentration 0.85 mol dm−3, absorption coefficient at 193 nm 52 200 cm−1). The etch rate dependence on the applied laser fluence (varied from 110 to 860 mJ cm−2) was derived from the etch depths, measured using an atomic force microscope (AFM). The etch rate was found to be 4.7–49.5 nm/pulse, depending on the laser fluence. The surface morphology of the etched edges was also investigated by AFM. A fast photographic arrangement was used for time resolved observation of bubble development in the liquid absorbent, which is an important phenomenon of LIBWE. The internal pressure of the expanding bubbles was calculated using recorded snapshots. It was found to be 22–120 MPa 17.2 ns after the excimer pulse peak. The one-dimensional heat flow equation, including the melting of the treated fused silica layer and the vaporization of the absorbing solution, was solved using the finite difference method. The surface temperature of the fused silica was found to be a maximal 17.2 ns after the excimer pulse peak. Based on our results, we present a possible interpretation of the LIBWE procedure of fused silica.