3.1. Comparison of drilling efficiency between NSP and LP
Figure 3a shows an SEM image and depth profile of a crater on a Metglss foil formed using NSP at 20 mJ. The surface morphology of the foils after laser irradiation closely resembles that of similar amorphous alloy foils (Metglas® 2605S2) shown in results of an earlier study . The average depth of the craters formed by single pulse was 3 μm. The height of the debris surrounding the craters was 3 μm. To obtain a hole through the foil, about 10 pulses were necessary (Fig. 3b). The height of the debris around the hole was increased to 15 mm. No increase in the drilling efficiency was observed when laser irradiation was done using NSP at 266 nm wavelength.
Significant different results were obtained when laser irradiation was conducted using LPs. A portrayed in Fig. 4a, holes through the foil were formed by an LP at 20 mJ. In addition, through-holes were formed when the laser intensity was reduced to 1 mJ (Fig. 4b). The decrease in the diameter of holes formed at 1 mJ is attributable to the Gaussian-like intensity distribution of the laser pulse: The intensity of the central portion of the laser beam is higher than that at the peripheral portion. The debris height around the holes was 3 µm at 1 mJ and 4–5 µm at 20 mJ, indicating that the height of the debris formed around a through hole by an LP is lower than that formed by tens of NSPs.
To investigate the drilling depth of an LP, laser irradiation of stacked foils (30 layer) was conducted because thicker foil was unavailable. The focal point was set at the top foil. After irradiation of LPs with various amounts of energy, the number of foils for which each LP formed a through hole was counted. The average value and the standard deviation of the drilling depth for each amount of laser energy was obtained as presented in Fig. 5, which shows that the drilling depth of an LP at 20 mJ is ca. 480 µm (ca. 19–20 layers of foil), indicating that the drilling depth of an LP is ca. 160 times greater than that of an NSP at 20 mJ. Additionally, when the drilling efficiencies (drilling depth per unit of laser energy) of an NSP at 20 mJ and of an LP at 1 mJ are compared, the drilling efficiency of an LP at 1 mJ (ca. 75 µm/mJ) is ca. 500 times higher than that of an NSP at 20 mJ (ca. 0.15 µm/mJ). Figure 5 also shows that, although the drilling depth increased with increasing laser energy, the drilling efficiency of an LP becomes lower with energy greater than 15 mJ. We confirmed that this phenomenon is not attributable to the deviation of the focal condition because no significant difference of the average drilling depth was observed at 30 mJ when the focal point was set at the bottom layer.
3.2. Mechanism of LP drilling
It is remarkable that the LP drilling efficiency is considerably higher than that of an NSP. We investigated what factors of LP irradiation bring this higher drilling efficiency.
As such a factor, the influence of plasma shielding was considered first because the appearance of a plasma plume observed during NPS irradiation and that observed during LP irradiation are vastly different (Fig. 6). When laser ablation of a metal target is done, plasma generated by the former part of a laser pulse is well-known to absorb and reflects the latter part of the pulse, resulting in decreased ablation efficiency [11–13].
To investigate the influence of plasma shielding in the present case, optical emission spectra of laser-generated plasma plume were observed. As portrayed in Fig. 7, the optical emission intensity of plasma generated by an NSP at 20 mJ is significantly higher than that of plasma generated by anLP at 20 mJ. The higher optical emission intensity of plasma generated by an NSP will be attributable to the higher peak energy of a NSP than that of an LP. Consequently, plasma shielding will occur more efficiently for an NSP than for an LP, suggesting that the drilling efficiency of a NSP becomes lower than that of an LP. Additionally, the fact that the optical emission intensity of plasma generated by an LP increases with increasing laser intensity, as observed in Fig. 7, suggests that lowering of the drilling efficiency in the higher laser energy region of LP irradiation observed in Fig. 6 is attributable to the plasma shielding, indicating that the plasma shielding effect becomes prominent at higher energy, even when an LP is used. In other words, the use of lower energy is expected to be a key condition that enables us to carry out efficient drilling.
It can be proposed that the second important factor enhancing the drilling efficiency of LP irradiation is the characteristic temporal laser pulse profile of an LP. As presented in Fig. 1, an LP comprises many sub-pulses, meaning that laser energy is distributed on these sub-pulses. This feature contributes to suppression of the peak laser energy. Additionally, these sub-pulses might irradiate the same spot of the foil repeatedly , leading to promotion of drilling. To investigate such a feature of the sub-pulse irradiation, high-speed camera observations were made of the laser ablation process caused by an LP. As shown in Figs. 8a and 8b, the optical emissions of plasma were observed several times during an LP irradiation. Additionally, the ejection of molten matter was observed after the end of sub-pulses, as indicated by the circles in Figs. 8c and 8d. These results suggest that repeated irradiation of sub-pulses in an LP promotes the drilling.
We also applied low-energy LP drilling for various metal foils to investigate the versatility of this method. As shown in Fig. 9, in most cases, a through hole was formed by the irradiation of an LP at 5 mJ, indicating this method as versatile. For Ag and Cu foils, some LP shots were unable to form a through hole.Figure 9 also shows that the hole diameter depends on the metal species. These results suggest that some physical properties of the materials influence the drilling efficiency. To investigate such influences of materials properties on the drilling efficiency, we conducted the similar experiment to that to obtain the data shown in Fig. 5 in which the drilling depth was determined by counting holes formed on the stacked foils. Fig. 10 shows the relation between some physical properties (melting point and thermal conductivity of metals) and the average drilling depth for a LP at 5 mJ. As shown in Fig. 10, the drilling depth seems to correlate with the melting point except for Cu.
This finding suggests that the drilling of an LP would proceed via melting of materials. This assumption is consistent with the fact that the amount of plasma emission generated by an LP is small, and is consistent with the morphology of the debris observed circumjacent to the holes. Fig. 10 also shows that the thermal conductivity is also an important factor. The lower drilling efficiency found for Ag and Cu can be explained based on their higherthermal conductivities than those of other metals. The temperatures of laser-irradiated portions of Ag and Cu foils are expected to become lower due to the faster heat dispersion. Results of these analyses suggest that the balance of the plasma shielding effect and heat dispersion leads to the high drilling efficiency of an LP. Additionally, when the data for the amorphous alloy foil (Metglas) are plotted on Fig. 10, it is suggested that the thermal properties of this material (relatively lower melting point, relatively lower thermal conductivity) would enable us to carry out efficient drilling. In other words, the low-energy LP drilling is particularly suitable for materials with such thermal properties.