The reduction of the threshold fluence at laser-induced backside wet etching due to a liquid-mediated photochemical mechanism

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The determination of materials erosion thresholds is of key importance for laser micromachining processes themselves and their applications. At laser-induced backside wet etching (LIBWE) of fused silica with organic liquid toluene employing an excimer laser (λ = 248 nm, τp = 25 ns), the threshold fluence is reduced from 1.10 J cm−2 to 0.22 J cm−2 with an increasing number of pulses from a single pulse to an almost infinite quantity, respectively. A similar effect is observed with an increasing laser spot size. Here, the threshold fluence decreases from 0.52 to 0.30 J cm−2 with the growing size of the laser spot from 400 to 4225 500 µm2. As the origin of the incubation effect, a laser-induced liquid-mediated process of surface modification of the fused silica sample is discussed. A first incubation model for LIBWE is developed and applied to the experimental data that comprise an enhancement of the solid–liquid interface absorbance by a layer of decomposition products from photochemical fragmentation of the liquid.