Backside laser etching of fused silica using liquid gallium

Laser-induced backside wet etching (LIBWE) that is regularly performed with hydrocarbon solutions is demonstrated with the liquid metal gallium as a new class of absorbers for the first time. Well-contoured square etch pits in fused silica with smooth bottoms and well-defined edges were achieved already with the first pulse from a 248 nm excimer laser. The etching is characterized by a threshold fluence of 1.3 J/cm2 and a straight proportional etch rate growth with the fluence up to 8.2 J/cm2. In addition, the etch depth increases linearly for onward pulsed laser irradiation and gives evidence for an only marginal incubation effect. The high fluences necessary for etching originate from the high reflection losses as well as the high thermal conductivity of the metallic absorber. The suggested etch mechanism comprises the heating of the fused silica up to or beyond the fused silica melting point by the laser heated gallium and the removing of the softened or molten fraction of the material by mechanical forces from shock waves, bubbles, high pressures, or stress fields.