## 4 Discussion
Following several pulses, we propose a hypothesis that the accumulated metal residuum layer and the ablation-induced glass defects cause an increase in the laser absorption rate of the quartz glass, as shown in Figure 9c[25]. Moreover, inverse  
Figure 9: Schematic illustration of laser-induced plasma ablation (LIPA) processing: (**a**) the laser beam heats the metal target through the quartz plate; (**b**) target begins to boil and plasma occurs; (**c**) the quartz plate is etched by the plasma; (**d**) the saturation of plasma-induced ablation; (**e**) the close-up of the etching hole on the quartz rear surface. bremsstrahlung absorption gives rise to a change in the optical properties of the quartz, increasing the absorption rate as well [21]. Then, the laser ablation effect on the glass sample includes both plasma thermal ablation and direct laser-quartz interaction. This can fully explain Figure 6, showing that, when the pulse number was more than 70, the etching rate increased dramatically. In Figure 9d, during ablation processing, the height of the ablation gap between the quartz and the metal surface increased and eVentually exceeded the distance of the plasma ejection range. Therefore, less and less plasma vapor can approach the top surface and ablate the quartz, and the ablation efficiency tends to saturate. In this case, the contribution of plasma-induced ablation can be omitted. Only the direct laser ablation effect continues, with absorption enhancement of the metal residuum. This is why the ablation depth increased slowly with more than 150 laser pulses in Figure 6, and the ablation still occurred eVen without the metal target in Figure 8. However, the residuum does not distribute uniformly on the surface (see Figure 9e), but mainly on the ablation gap edge, and glass edge chipping occurs due to the nonuniform heat conduction (similar to Figure 5f). To validate our hypothesis of absorption enhancement resulting from the metal residuum, the absorption rates of the resulting quartz sample after the LIPA at 4 J/cm\({}^{2}\) fluence were measured using a UV spectrometer (UH-4150, HITACHI Co., Ltd., Marunouchi, Tokyo, Japan). As shown in Figure 10, the absorption ecofficiencies at 248 nm were measured to be 2.5% and 3.2% after exposure to 10 and 50 laser pulses, respectively. Nevertheless, the absorption rate of the bare quartz plate before exposure was measured to be less than 0.01%. The absorption rate increased by at least two orders of magnitude. After analyzing the LIPA processing mechanism, we attempted to implement a microscale channel for biomedical application using the optimized parameters. As shown in Figure 11, the LIPA scanning process of a straight-line pattern on fused quartz glass was launched. Considering the practical microchannel dimension requirements and the manufacturing efficiency, the processing parameters were set as follows: laser energy density, 4 J/cm\({}^{2}\); translation velocity of the stage, 50 \(\upmu\)m/s; laser pulse frequency, 10 Hz. After processing, the metal residuum and molten quartz dust were removed by HF corrosion in the supersonic cleaner. We can observe from Figure 11 the good surface quality of the glass microchannel bottom without edge cracking in the entire structure. The depth was 28 \(\upmu\)m, and the roughness on the bottom surface was approximately 150 nm. The channel can be qualified for biomedical microfluidic applications including cell culture or DNA analysis [5; 6; 7]. It should be noted that, during ablation, the neighboring ablation positions overlapped partly; thus, the depth after the scan was slightly more than that in the single localized hole measured before. Figure 10: Absorption spectrum of the quartz glass sample.