# Numerical and Experimental Research on the Laser-Water Jet Scribing of Silicon
## 4 Results and Discussion
### Fabrication of Micro-Grooves with Large DTWR
During the construction of groove two, The refracted and reflected laser beam in the deposited water enhanced the unilateral ablation, which resulted in the remodulation of the groove wall and the formation of the step structure. For 132 sequence, the construction of groove one and three generated the bumpy structures in the central area. This structure could \"split\" the incident LWJ, forming a symmetric distributed pool for water deposition. The refracted and reflected laser beam in this channel suppressed the construction of groove three, as the laser energy was dissipated to enhance the ablation of groove one and groove two where the effective LWJ was absent. In contrast, a clearly V-shaped micro-groove with no step and bump structures was obtained through 213 sequence. The well morphology should be given credit for the formation of the central water channel after the construction of groove two. In specific, the LWJ could interact with the side wall of groove one and groove three directly without the disturbance from deposited water. The slag could be ejected from the groove fluently as well. Therefore, 213 sequence was the most appropriate scanning strategy on the same layer. Figure 17: Scanning strategies on the same layer, **(a)** 123 sequence, **(b)** 213 sequence, and **(c)** 132 sequence. Based on the above analyses, two scanning strategies along the depth direction was raised, and the schematic is shown in Figure 19. The corresponding groove morphologies are shown in Figure 20, and the machining parameters are listed in Table 5. The surface-to-depth sequence offered larger DTWR and lower taper. Analyses showed that under the assistance of the argon atmosphere, the larger ablation region enabled the deposited water and slag to eject fluently. The LWJ could be delivered to the deeper position of the microscope instead of being dissipated. However, the application of depth-to-surface sequence weakened the water and slag ejection seriously along depth direction. The reflection and refraction of the laser beam within the deposited water induced extra damage to the groove wall, and the enlargement of DTWR was prohibited. In summary, the excellent ejection channel of the deposited water was of great significant for the fabrication of micro-grooves. The 213 sequence on the same layer and the surface-to-depth sequence on different layers were the most suitable scanning strategies. Using the parameters in Table 6 the micro-grooves with DTWR and taper up to 19 and 0.013 were constructed, as shown in Figure 21. The morphological parameters were measured and are listed in Table 7 The wall of the groove was nearly vertical. Machining defects, such as attached debris, edge crack, and heat-affected zones were absent, which solidly supports the feasibility of LWJ. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Laser Power**} & \multirow{2}{*}{**Repetition Rate**} & **Scanning** & **Scanning** & **Scanning** & **Scanning** & **Scanning** \\  & & **Interval on the** & **Interval along** & **Layer along** & **Scanning Time** & **Speed** \\  & & **Same Layer** & **Depth** & **Depth** & **Seenning Time** & **Speed** \\ \hline
50 W & 10 kHz & 20\(\upmu\)m & 100\(\upmu\)m & 60 & 1 & 10 mm/s \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Machining parameters of micro-grooves in Figure 21. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Groove Number** & **Entrance Width/\(\upmu\)m** & **Bottom Width/\(\upmu\)m** & **Depth/\(\upmu\)m** & **DTWR** & **Taper** \\ \hline
1 & 306.4 & 161.3 & 5778.6 & 18.86 & 0.013 \\
2 & 307.2 & 164.6 & 5831.1 & 18.98 & 0.012 \\
3 & 305.6 & 151.5 & 5815.5 & 19.03 & 0.013 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Morphological parameters of micro-grooves in Figure 21.  
Figure 20: Surface and end face morphologies of micro-grooves fabricated by (**a**) depth-to-surface sequence and (**b**) surface-to-depth sequence.