Effect of liquid environment on laser-induced backside wet etching of fused silica

Abstract

In laser-induced backside wet etching (LIBWE), the liquid absorbent indirectly heats the transparent material, causing explosive phase change and cavitation. Accordingly, the hydrodynamics of the absorbing liquid, including the size of the liquid chamber, is strongly related to the ablation process. Because the hydrodynamics is dependent on the elastic deformation of the sample, the sample thickness also affects the performance of LIBWE. In this work, experimental analyses were performed to elucidate the hydrodynamics in LIBWE and the effect on the etch rate by varying the liquid chamber size and sample thickness. A KrF excimer laser was used to ablate fused silica samples in toluene and the etch rate was quantified using a scanning profilometer. Laser flash shadowgraphy and photodeflection probing techniques were employed for in situ measurement of the laser-induced hydrodynamics and displacement of the sample, respectively, with a time resolution of approximately nanoseconds. To directly observe the effect of increased liquid pressure on LIBWE, the liquid pressure on the etching spot was locally increased by an external shock wave and the etch results are examined. This study confirms that the photomechanical effects from the laser-induced bubble plays a key role in the LIBWE process, revealing a linear relationship between the etch rate the applied recoil momentum. However, the dependence of the etch rate on the chamber size and sample thickness could not be explained by the change in recoil momentum, i.e., by the bubble pressure. Instead, transient deformation of the sample by the pressure impact was estimated to be mainly responsible for the dependence.