# Experimental Study on Femtosecond Laser Processing Performance of Single-Crystal Silicon Carbide
## 3 Results and Discussions
### Experimental Results
#### 3.2.2 Analysis of the Experimental Results of the Aspect Ratio and the HAZ Width of Grooves  
Figure 9 shows the effect of laser fluence on the aspect ratio and the HAZ of the groove. As shown in Figure 9a, the maximum aspect ratio of the groove is 0.15. According to Section 3.2.1, since the laser fluence has a greater impact on the groove depth than the groove width, increasing the fluence can increase the aspect ratio of the groove. It can be seen from Figure 9b that increasing the fluence can increase the HAZ of the groove, and the maximum HAZ is 5.62 \(\upmu\)m.  
Figure 8: Effect of numerical aperture on groove size, HAZ and MRR. (**a**) Depth; (**b**) width; (**c**) HAZ; (**d**) MRR. Figure 9: Influence of laser fluence on the aspect ratio and HAZ of grooves. (**a**) Aspect ratio; (**b**) HAZ. Figure 10 shows the effect of scan speed on the aspect ratio and HAZ of grooves. When the scan speed is 5 \(\upmu\)m/s, the maximum aspect ratio is 0.6 and the HAZ is 7.52 \(\upmu\)m. Figure 11 shows the effect of scan speed on the ablation morphology. It can be seen from the figure that with the increase in scan speed, ripples appear on the side wall of the groove and in the HAZ. Due to a decrease in scan speed and an increase in spot overlap rate, the injected laser energy increases, resulting in a recast layer at the edge of the groove and a valley-like overlap layer on the side wall of the groove [5,6]. Figure 12 shows the effect of multipass scanning on the aspect ratio and the HAZ of grooves. As shown in Figure 12a, as the number of scans increases, the aspect ratio of the groove increases. When the number of scans is 60, the maximum aspect ratio is 0.79. Figure 12b shows that the HAZ is basically unchanged due to the limitation of the spot diameter when the number of scans is 10, 30 and 60. Figure 10: Effect of scan speed on the aspect ratio and HAZ of grooves. (**a**) Aspect ratio; (**b**) HAZ. Figure 13 shows the effect of multipass scanning on the ablation morphology. When scanning with a single laser beam, due to the Gaussian distribution of the laser beam, the center of the groove is coarse ripple structure, and the edge is fine ripple structure. As the number of multi-pass scans increases, thermal damage caused by thermal accumulation will form at the edge of the laser spot, resulting in an increase in irregular craters on the sidewall of the groove. Figure 14 shows the effect of multipass scanning on the ablation profile. As the number of scans increases, the depth and width of the groove increase. Figure 12: Effect of multipass scanning on the aspect ratio and HAZ of grooves. (**a**) Aspect ratio; (**b**) HAZ. Figure 13: Effect of multipass scanning on femtosecond laser ablation morphology. (**a**) Morphology at \(c=1\); (**b**) side wall at \(c=1\); (**c**) HAZ at \(c=1\); (**d**) morphology at \(c=60\); (**e**) side wall at \(c=60\); (**f**) HAZ at \(c=60\). Figure 15 shows the effect of polarization angle on the aspect ratio and the HAZ of grooves. As shown in Figure 15a, the polarization angle has little effect on the aspect ratio of the groove. Due to the same material removal mechanism, laser polarization mainly affects the direction of the electric field distribution, without affecting the total energy transferred to the workpiece. Therefore, the depth of the groove is mainly affected by variables related to energy, such as laser fluence, scan speed, repetition rate, etc., but is little affected by the polarization of the stimulated light. It can be seen from Figure 15b that the HAZ slightly increases with the increase in polarization angle due to the acceleration of the photoelectric field on the super-hot electron jet. By using a z-layer feeding layer-by-layer machining method, both sides of the groove will interfere with the laser beam, so excessive feeding layers will limit the removal of the groove. For the same objective and laser parameters, the Rayleigh length is a collimation distance, and the ablation process is carried out on the focal plane.