# Optimization of Laser Fixed Contact Process for the Formation of an Al-BSF Layer
## 3 Results  
We worked with an ns green laser when applying P-contact formation technology to the back of the silicon wafer through the LFC process. When the laser is used this way, a heat-affected zone (HAZ) inevitably occurs, as when the laser beam is irradiated to the metal layer on the silicon solar cell, all its energy is absorbed, apart from the reflected part. The HAZ is an area of the base material where the material does not melt when exposed to high temperatures, although its microstructure and mechanical properties are affected by both the heat generated during the laser and the sequential cooling processes. The HAZ can have undesirable effects, such as surface cracks and distortion, and ultimately causes wafer damage, degrading cell characteristics. Accordingly, it is necessary to select appropriate laser parameters to minimize the HAZ, and so, in this study, we studied ns laser process optimization so as to minimize the HAZ formed during p-contact formation. In a solar cell characterized by the LFC structure, it is essential to form a Si/Al alloy, by instantaneously melting Si and Al under the correct laser conditions. During this, if the energy of the laser is excessive, the Al layer evaporates, while if the energy is too weak, only the surface of the Al melts, and proper alloy formation will not occur. In this study, laser conditions were analyzed structurally and electrically, in terms of the frequency, power, and speed of the ns laser, when applied to LFC structure solar cell fabrication. First, we analyzed the structure of the p-contact processing region, in relation to ns laser frequency. After fixing the laser power at 7 W and a speed of 100 mm/s, the frequency was varied between 300-600 kHz. To assist our analysis of the HAZ generated during the laser process, scanning electron microscope (SEM) imagery of the affected surfaces and cross-sections were obtained, as shown in Figure 1.  
In Figure 1, it can be seen that, at 500 kHz or higher, laser damage rapidly increases. This is because, as the frequency increases, the number of processes between the laser and particles increases; the particles then become excessively heated, which transfers heat to particles that are not directly affected by the laser. This indirect thermal effect increases laser damage around the processed laser track. The SEM imagery in Figure 2 shows that the laser track width increased as the frequency increased from 300 to 600 kHz. For frequencies \(\leq\)400 kHz, the damage extended over an area <3 mm in diameter. The associated cross-sectional imagery for conditions of 300 and 400 kHz, showed that the processing had reached depths of approximately 5.2 and 9.2 mm, respectively. However, the 300 kHz image showed that the energy density for forming the Al-BSF layer was so low that only the surface had been processed, making such a frequency unsuitable for p-contact formation. Thus, a 400 kHz process was identified as the most suitable for LFC structure solar cell fabrication. In order to optimize the formation of the BSF layer in a manner in which silicon wafer thermal damage at the HAZ was \begin{table}
\begin{tabular}{c c} \hline
**Laser Parameters** & **Conditions** \\ \hline Laser type & Nanosecond green laser \\ Beam mode & Pulsed mode \\ Focal spot diameter & 3.5 mm at a laser power of 8.6 W \\ Focal spot position (\(z\)-axis) & 0 μm \\ Pulse frequency range & 10–600 kHz \\ Laser moving system & XYZ system \\ Output power range & 0.1–10 W \\ Wavelength & 532 nm \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters for the laser used in the process. minimal, the frequency was fixed at 400 kHz, and the experiment on the effect of varying laser power was allowed to proceed.