## 3 Results  
In this experiment, 210 \(\upmu\)m Si wafers were used as samples. Monocrystalline silicon is known to be very hard but brittle [23], prone to potential damage during the micromachining process. The processing parameters, such as machining speed and number of pattern repetitions, were therefore chosen very carefully and within a limited range. The optimum values were found to be 7 mm/s and between 15 and 20 repetitions (see Figure 3 for reference). There is however a potential to further increase the scanning speed, which will translate to better efficiency, but lower ablation rate. A desired crater depth should be considered according to specific applications. To analyze the micromachining results, two different approaches were required. The samples, that were cut through during the process, were analyzed by scanning electron microscopy technique (Figure 4). The samples scribed with sub-picosecond laser were analyzed using confocal microscopy. Figure 3: The results of a study performed to optimize the machining parameters of 210 \(\upmu\)m silicon wafers. The samples were scribed with 4 different values of scanning speed (0.7–13 mm/s), in 4 variants of pattern repetitions (6–15x). The ablation depth of the craters was then measured for each case. Laser beam parameters: 650 fs, 343 nm, 200 kHz, 27.5 \(\upmu\)J.  
Figure 2: The experimental setup. A laser confocal microscope imaging allowed higher contrast and measurement resolution. The 3D images were created to make crater profile and depth measurements (Figure 5). Figure 4: SEM images of the Si wafers post sub-picosecond micromachining. Laser beam parameters: 650 fs, 343 nm, 200 kHz, 27.5 \(\upmu\)J; processing parameters: 7 mm/s, 20 pattern repetitions. High quality laser micromachining of monocrystalline silicon is difficult to achieve with pulses in the range of pico or nanoseconds [24; 25; 26]. The technique is well established for laser pulses of 5 to 400 fs duration [24; 27]. Our research shows that applying the pulses of 650 fs, i.e., sub-picosecond range, gave high quality results in both cutting and scribing of the Si wafers. Figure 5: Confocal microscopy images and crater profile of the Si wafers post sub-picosecond micromachining. Laser beam parameters: 650 fs, 343 nm, 200 kHz, 27.5 uJ; processing parameters: 7 mm/s, 15 pattern repetitions. For ultrashort pulse durations, the fundamental physical processes like energy deposition, melting, and ablation are separated in time. Because the electron-lattice heating and melting occurs roughly on a picosecond timescale [28], the laser-matter interactions in sub-picosecond regime leave the area surrounding the micromachining target unaffected. SEM images show clean surface with lack of micro-cracks or debris. The confocal microscope images indicate a minimal range of a so called heat affected zone. The crater profile shown in Figure 4 has a clear, conical shape, 60-\(\upmu\)m depth, and is 55-\(\upmu\)m wide (Table 2). In semiconductors and dielectrics irradiated with high laser intensities, the loss of crystalline order is possible within less than one picosecond. The state of the material after irradiation depends strongly on the type of material and on the laser properties such as intensity and wavelength [29]. The results obtained within our research show that both, the laser beam and processing parameters, are in the optimum range for micromachining of silicon. The fact that the surface surrounding the target area remains unchanged might indicate that the crystalline order of the Si wafers is not affected. However, due to some material changes shown in the confocal microscope images, it should be subjected to further investigation. However, it can be stated that it enables a safe solar cell micromachining, e.g., edge isolation processes. Figure 6 shows a large scale image of the Si wafers surface hatching that demonstrate the applicability of this process for solar cell micromachining, e.g., for micro-structuration.