On the Role of Supercritical Water in Laser-Induced Backside Wet Etching of Glass

The features and mechanisms of microcrater formation in optical silicate glass by laser-induced backside wet etching (LIBWE) are determined in a wide range of energy densities (Φ) from 4 to 103 J/cm2 for laser pulses of 5 ns length and 1 kHz repetition rate. The existence of two different mechanisms of laserinduced microcrater formation is revealed: (i) chemical etching in supercritical water (SCW), and (ii) cavitation. At Φ > 102 J/cm2 irregular craters of 1–20 μm in depth with rough walls and distinct cracks around microcrater are formed testifying that in such mode (“hard”) laser induced cavitation plays a dominant role in glass removal. At Φ < J/cm2 neat glass craters with smooth walls are formed, their size and shape are easily reproducible, cracks are not formed, and the processing area is limited to the laser spot area. In this mode (“soft mode with active cavitation”), a microcirculation of water is stimulated by cavitation without causing undesirable shock breakage. The latter is achieved thanks to the fast removal of glass etching products by microcirculation, and the inflow of “fresh” etchant (SCW) to the glass surface in the vicinity of the formed microcraters. Such mode is optimal for highly controlled laser microstructuring of glass and other optically transparent materials.