Enhancing the Etch Rate at Backside Etching of Fused Silica

Abstract

The laser-induced backside wet etching (LIBWE) technique allows straightforward etching of transparent materials such as fused silica with nanosecond UV lasers at low laser fluences. Hydrocarbon liquids are regularly exploited for LIBWE but have only moderate absorption coefficients and the etching process is affected from incubation. The estimation of the interface temperature from a simple model shows that high absorption coefficients improve the utilization of the laser energy and can result in higher temperatures exceeding the melting point. Therefore the backside etching of fused silica by means of liquid gallium using nanosecond UV laser radiation (λ = 248 nm, 25 ns pulses, 10 Hz) is investigated. A threshold fluence of about 1.5 J/cm² was estimated and etch rates close to 1 µm/pulse were measured. The square etch pits feature welldefined edges and a smooth bottom. The etch rate decreases at smaller spot size due to the thermal losses at the boundaries. The etch process involves the laser heating of gallium near the interface, the melting of the fused silica in a near surface region, and the etching of the molten layer by thermo-mechanical processes involved in laser etching.