# Optimization of Laser Fixed Contact Process for the Formation of an Al-BSF Layer
## 3 Results
SEM imagery of the surfaces and cross-sections after the application of different laser powers (after fixing the laser speed at 100 mm/s and the frequency to 400 kHz), can be seen in Figure 3. It can be seen that 4 W was not suitable for the LFC process, as the energy density was too low for p-contact formation, and the process was not performed smoothly. The process depths of approximately 8.31 and 8.33 \(\upmu\)m were achieved when using  
Figure 1: SEM imagery of wafer surfaces and cross-sections after ns laser processing, using various laser frequencies: (**a**) 300 kHz; (**b**) 400 kHz; (**c**) 500 kHz; (**d**) 600 kHz. Figure 2: Graph of laser track width vs. laser frequency. conditions of 5 and 6 W, respectively, creating respective HAZ diameters of approximately 8.69 and 14.25 um, respectively, which were suitable conditions for p-contact formation. At 7 W, the solidification phenomenon--which occurs due to excess heating and melting--was seen to occur during the process. It was also apparent at this power that the dimensional precision of the processed portion was reduced, and that the process had become difficult to control. While the depth was approximately 8.5 um under the power of 7 W, it was simply not suitable due to the excessive melting and the size of the HAZ created during p-contact formation. Based on these findings, laser powers of 5 and 6 W were used in the next experimental phase of the optimization process, which involved changing the laser speed. Figures 4 and 5 include SEM imagery of wafers processed at different laser speeds, with laser powers of 5 and 6 W, while Figure 6 shows the laser track widths using the same 5 and 6 W laser powers. In these experiments (using 5 and 6 W laser power), the line widths were different from the expected values at these laser speeds. The laser track width was expected to decrease as the laser speed increased, however, it increased at a laser speed \(\geq\)500 mm/s. This was related to the formation of large molten layers caused by the excess energy near the laser processing area. That is, when the laser speed was low, local heat propagation lasted longer, and thus the sample surface temperature increased, owing to the higher laser beam residence time. The heat-affected region increased, and a phase change from the solid-state was seen, reflecting the increased depth of the heat-affected region during the cooling period. At lower speeds, the laser track width increased accordingly, as the laser speed decreased. At higher laser speeds, sample heat propagation was short-lived, which resulted in less sample surface damage and a decreased laser track width. The case where experimental outcomes differed from the expected values saw the total laser line width the same as the previous condition, but molten solids remained on the surface, due to the thermal effect. Through this result, it was confirmed that when the process was conducted at laser speeds \(\geq\)500 mm/s, the process failed, because the energy density applied per unit area was too low due to the combination of high speed and low power density. These outcomes revealed that the suitable structural characteristics for p-contact formation--which occurred when the melting and thermal damage such as HAZ were minimized--were observed under conditions of 5 W laser power, a speed of 100 mm/s,  
Figure 3: SEM imagery of wafer surfaces and cross-sections according to varied laser power, under conditions of 100 mm/s laser speed, and a frequency of 400 kHz: (**a**) 4 W; (**b**) 5 W; (**c**) 6 W; (**d**) 7 W.  
and a frequency of 400 kHz. To ensure that conditions for the BSF layer were optimized, layers were structurally analyzed using SEM imaging for each laser speed, at the 5 W power setting. Figure 7 shows the SEM imagery of BSF layers prepared using laser speeds from 10-500 mm/s.