# Numerical and Experimental Research on the Laser-Water Jet Scribing of Silicon
## 4 Results and Discussion
### Fabrication of Micro-Grooves with Large DTWR  
Based on the analysis in Sections 4.1 and 4.2, single-row scribing experiments were put forward to clarify the influence of laser power, pulse energy, and scanning speed on the groove depth and width. Every parameter group was repeated five times and each groove was scanned once. The morphological results were measured and are listed in Figures 14 and 15. With fixed pulse energy and scanning speed, the increasing laser power indicated deeper groove depth. Similarly, with fixed laser power and scanning speed, the increasing laser pulse energy also resulted in deeper groove depth. When the laser power  
Figure 13: Schematic of single-spot LWJ machining. and pulse energy were fixed, the decreasing scanning speed led to deeper groove depth as well. To explain these rules, quantitative analyses concerning the effective pulse overlap were carried out and are listed in Table 3. The decreasing distance between neighboring laser spots equaled to more effective pulse overlap, as illustrated in Figure 16. Furthermore, the higher laser pulse energy and more effective pulse overlap contributed to the higher incident laser fluence in the unit area. Taking the analyses in Sections 4.1 and 4.2 into consideration, the higher incident laser fluence led to a stronger thermal effect in the machining region, which was capable of promoting ablation volume [20; 21]. Therefore, the groove depth increased correspondingly. Figure 14: The variation of groove depth under pulse energy of (**a**) 0.45 mJ, (**b**) 0.65 mJ, (**c**) 0.85 mJ, and (**d**) 1.05 mJ. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Group**} & \multicolumn{2}{c}{**Laser Pulse**} & \multirow{2}{*}{**Laser Power/W**} & \multirow{2}{*}{**Scanning Speed/mm\(\cdot\)s\({}^{-1}\)**} & \multicolumn{2}{c}{**Distance between Neighboring Laser**} \\  & & **Energy/mJ** & & **Spot/\(\mu\)m** \\ \hline \multirow{2}{*}{1} & 1.1 & 0.45 & 5 & 10 & 0.9 \\ \cline{2-5}  & 1.2 & 0.45 & 17 & 10 & 0.26 \\ \cline{2-5}  & 2.1 & 0.45 & 5 & 10 & 0.9 \\ \cline{2-5}  & 2.2 & 1.05 & 5 & 10 & 2.1 \\ \cline{2-5}  & 3.1 & 0.45 & 5 & 2 & 0.9 \\ \cline{2-5}  & 3.2 & 0.45 & 5 & 10 & 4.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Quantitative analyses of effective pulse overlap. Nevertheless, the groove width was not sensitive to these parameters. The reason is that during single-row LWJ scribing, the laser power was trapped within the waterjet. The increase in laser pulse energy and power may not alter the working state of LWJ. Referring to Figure 13, the limited refraction and reflection of the laser beam within the deposited water in the machining region led to the slightly broadening of groove width. The increase in scanning speed could suppress the water deposition to some extent as it provided a better drainage channel under the assisted argon atmosphere, but it still hardly influenced groove width [22]. Figure 16: Comparison of high and low effective pulse overlap in the unit area. Figure 15: The variation of groove width under pulse energy of (**a**) 0.45 mJ, (**b**) 0.65 mJ, (**c**) 0.85 mJ, and (**d**) 1.05 mJ. According to the single-row experimental results, higher laser pulse energy and relatively small scanning speed were fundamental factors for fabricating deeper grooves. On this basis, multi-row scanning strategies were investigated from two progressive aspects. First of all, three scanning strategies on the same layer were put forward, the schematic of which are illustrated in Figure 17. The groove morphologies are shown in Figure 18, and the corresponding machining parameters are listed in Table 4. An obvious step structure could be discovered on the groove wall fabricated by 123 sequence, which could be attributed to the asymmetric scanning trace. Specifically, the construction of groove one provided a unilateral channel for water ejection.