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.

Interpretation and Modeling of Laser-Induced Backside Wet Etching Procedure

“…This behavior was observed by many research groups [13][14][27][28][29][30][31][32], but correct, coherent explanation have not existed yet. We have already presented a LIBWE model in our previous papers [15,27], which gave relatively good results for LIBWE applying ArF laser (nevertheless the calculated threshold fluence was higher than the measured one), but this model did not work for KrF experiments, namely etching was not predicted in the applied fluence range (up to ≈2 J/cm 2 ). The main difference between the two laser sources is in their wavelengths and therefore in the significant difference between the absorption coefficients of the absorbing liquid (α ).…”

“…These layers can be identified as the modified layers, although these thicknesses were overestimated by the calculations. This residual melted and quickly refrozen layer might answer the question, how the modified layer is reproduced from pulse to pulse: the carbon (originate from the organic liquid molecules due to the high energy UV photons) probably can build in the upper part of the residual melted fused silica layer (by diffusion and/or due to the high pressure in the liquid [27,29,38]). …”

Laser-Induced Backside Wet Etching of Transparent Materials with Organic and Metallic Absorbers

“…The effect of H-LIBWE to the properties of the processed surface was investigated in [38,39] and gave evidence for a strong surface modification and incubation. Some studies aim at the comparison of experimental LIBWE results with (numerical) model calculations [32,40]. In [32], it was found that the interface temperature at H-LIBWE is below the melting temperature of the fused silica for the laser fluences at the etch threshold; however, in this work interface modifications were neglected.…”

“…Some studies aim at the comparison of experimental LIBWE results with (numerical) model calculations [32,40]. In [32], it was found that the interface temperature at H-LIBWE is below the melting temperature of the fused silica for the laser fluences at the etch threshold; however, in this work interface modifications were neglected. In Table 1, the most important characteristics of patterning processes for transparent materials are listed for H-and M-LIBWE in comparison to laser ablation.…”

“…For very high repetition rates, the bubble lifetime can exceed the pulse interval whereby the etching process is influenced and lower etch rates were found [30,31]. A wide range of liquids were investigated for H-LIBWE comprising mainly solutions of dyes in organic solvents [22,32], solutions of inorganic and organic absorbers in water [25,26,33,34], and liquid metals [27,35]. Only some experiments on LIBWE with ultrashort pulse lasers are known [36,37].…”