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Abstract
A novel combined laser pulses (CLPs) consisting of a millisecond (ms) pulse and an assisted nanosecond (ns) pulse train was proposed for drilling alumina ceramic. The processing efficiency and quality were well improved by spatially and temporally superposing the ms and ns laser beams. As a result, due to the multi-reflection of keyhole and ejection of melt, the temporally superposed CLPs could decrease the energy consumption of the drilling by an order of magnitude compared with the conventional ms pulse. On the other hand, the spatial distribution of the ns laser on the focal plane was elliptical due to the off-axis distortion of the optical system. However, since the reflection of the laser in the keyhole was non-uniform, the spatially superposed CLPs showed no dependence on the shape of the focused elliptical ns laser spot in terms of the drilling quality. The research results have an important guiding for improving the efficiency and quality of laser processing, especially for the alumina ceramic laser processing.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nowadays, alumina ceramic has been widely used in electronic substrate applications due to its outstanding mechanical and physical properties. It is essential to drill multiple holes for interconnecting vias with an ongoing trend to further miniaturization imposing a persistent demand for smaller hole diameters below 100 µm. In general, the ceramic with high hardness processed by conventional mechanical methods or short (or ultra-short) pulse is extremely laborious and time consuming [1–3], which is impossible to meet the demand of industrial drilling efficiency. In this context, the rapid micro hole laser drilling in the alumina ceramic substrate using single mode millisecond (ms) laser has been obtained with the association of the high-pressure gas jet [4,5]. However, high-pressure gas jet assisted ms laser drilling will cause the loss of laser energy and a large amount of gas consumption [6]. Besides, the method of increasing the drilling efficiency by improving the pressure of the assist gas leads to a change of the refractive index of the medium between the workpiece and the nozzle, which can cause the defocusing of laser beam [7]. At the same time, the additional surface processing is always required because of the residual spatters [8]. On the contrary, in the case of stand-off drilling at a distance without gas-assisted, the drilling efficiency of the ms pulse is rather low, and the drilling quality of the ms pulse is far from meeting the requirements.
Up to now, the ideas of developing combined laser pulses (CLPs) have been explored for improving the efficiency and quality of laser processing in a wealth of applications, such as drilling [9–11], welding [12], cutting [13], annealing [14], material damaging [15–18] and laser-induced periodic surface structures (LIPSS) [19–21]. In general, the CLPs technology is supposed by two kinds of pulses, including a main pulse and an assisted pulse, and the assisted pulse can help to significantly improve the performance of the main pulse. As reported, a typical assisted short pulse with high peak power density has been widely adopted. First, the rapid temperature rising caused by the assisted short pulse could be used to increase the absorptivity of materials [22,23], introduce the defects [24], and generate recoil pressure for stirring [12], ejecting [25] or structuring surface [26,27]. Second, the assisted short pulse with sufficient peak power density to ablate the material could improve the modified formation and removal of closures [28], or improve the surface finish [13]. Third, the assisted short pulse with high peak power density could introduce plasma shock wave that helped to eject the melt [29–31], or improve the utilisation of the laser energy through the multi-photo absorption [32]. However, the assisted short pulse used in the CLPs was always a single pulse, the delay time between the assisted short pulse and the main pulse would have a great impact on the processing efficiency, and the small number of the assisted short pulses would limit the performance improvement [22,24,25]. In this context, recently, a novel CLPs which consisted of a main pulse and an assisted pulse train has become an intense research subject [33–35]. In 2019, Qin et al. reported the laser drilling of metal by the CLPs consisting of a ms pulse with pulse duration from 1 ms to 3 ms and a nanosecond (ns) pulse train with repetition rate of 4 kHz. The results show that with the assistance of the high pressure induced by the ns pulses, melt generated by the ms pulse could be expelled within several ms with velocities of tens of m/s, which finally led to the formation of a hole [33]. In addition, Jia et al. applied a ns pulse train to assist the continue wave (CW) laser to drill the metal, the results show the assisted short pulse train could periodically eject the melt, which improved the drilling efficiency and quality [34,35].
In this paper, a novel CLPs consisting of a ms pulse and a ns pulse train was proposed for drilling the alumina ceramic. The spatial combination technology was used to superpose the ms laser and ns laser. Due to the off-axis distortion of the optical system, the spatial distribution of the ns laser on the focal plane was elliptical. The influence of the focused elliptical ns laser spot on the circularity of the hole drilled by the CLPs was analysed to reveal the underlying physical mechanism. On the other hand, the drilling results of the CLPs was compared with the conventional ms pulse, and the enhancement mechanism of the CLPs performance was systematically explained.
2. Experimental procedure
Figure 1(a) shows the schematic diagram of the CLPs experimental system. The ms laser was a gated 1070 nm CW fiber laser (Beijing Raypower Laser Technologies Co., Ltd, Corepower-1070-800-CW, China). The core diameter of the fiber was 20 µm (NA: 0.06). After collimation, the beam size of ms laser was ∼3.5 mm. The ns pulse train was generated by a 1064 nm Q-switch Nd:YAG laser (Huaray, Spruce-1064-20, China). The M2 was about 1.3. After expanding through a 5-fold beam expander, the beam size of ns laser was ∼6.0 mm. The ns laser and ms laser used in our experiment were both non-polarized. A programmable control system was used to realize the synchronous control of the ms pulse and ns pulse train. Mirrors 1, 2, and 3 were high reflectivity mirrors (R > 99.6% at∼1064 nm and ∼1070 nm). The ms laser propagated along the optical axis, while the ns laser propagated off-axis and parallel to the ms laser. The distance between the two beams was ∼10.0 mm, as shown in Fig. 1(b). Then the CLPs were focused on the sample surface by one focusing lens with a focal length of 120 mm and a diameter of 50 mm.
Fig. 1. (a) The schematic diagram of the experimental system. (b) The spots of the ms pulse and ns pulse before being focused, respectively. (c) The spatial combination part of the CPLs in the experiment.
Figure 2(a) shows the schematic spots of ms laser and ns laser on the focal plane. In the experiments, the waist positions of the two lasers were on the sample surface. The spot shape of the focused ms laser was a circle and the beam waist of the ms laser was calculated to be 50 µm. On the other hand, due to the off-axis distortion of spatial combination optical system, the spot shape of the focused ns laser was an ellipse, and the spot size of elliptical ns laser measured with the D2-method had a major and minor axes of 47 ± 4 µm and 29 ± 6 µm, respectively [36]. In the time domain, the ms laser had a repetition rate of 50 Hz and a duty cycle of 10%. The ns laser was operated at 3 kHz with a fixed pulse duration and single pulse energy of 17 ns and 4 mJ, respectively. The CLPs constituted one ms pulse and one ns pulse train with 6 short pulses, which was realized by precisely adjusting the synchronous control system, as shown in Fig. 2(b). The samples were alumina ceramic plates with a thickness of 0.5 mm (96% Al2O3), and each hole was repeatedly drilled ten times under the same operating condition. There was no high-pressure gas jet assisted during the drilling process. The morphology of the hole surface was characterized by optical microscopy (OM, Axio Lab A1, Germany). The circularity ($C$), taper angle ($\theta$) of the hole were calculated by the following formulas [37]:
Fig. 2. (a) The schematic spots of ms pulse and ns pulse after being focused. (b) The schematic oscillogram diagram of the CLPs.
Where ${r_{\min }}$ and ${r_{\max }}$ were the maximum radius and the minimum radius of a hole entrance and exit, respectively; ${D_1}$ and ${D_2}$ were the diameter of the hole upper surface and lower surface, respectively; h was the thickness of the sample.
3. Results and discussion
Normally, for the 1064 nm laser, the alumina ceramic surface has an absorptivity of less than 0.2 [38], but the absorptivity will be increased to 0.8-1.0 after the keyhole formed [39]. High peak power density is required at the beginning of the ms pulse drilling. However, after the keyhole is formed, the ms pulse with high peak power density will cause excessive ablation due to the enhanced-absorption [40]. Therefore, the hole drilled by the conventional ms pulse is much larger than the focused laser spot, and there will also be thick recast layer and obvious cracks [41], which is called changing absorptivity problem here. In order to effectively solve this problem, the CLPs with a lower peak power ms pulse and a high peak power ns pulse train was employed. In the experiment, the parameters of ns pulse remained unchanged and the peak power of ms pulse could be adjusted. For the alumina ceramic plates with a thickness of 0.5 mm, 10 pulses of the CLPs were used to drill through the sample. After systematic study finalised to find the optimal parameters of the CLPs, the peak power of ms pulse in the CLPs was set to 60 W. In order to compare the CLPs drilling results with the conventional ms pulse, 10 pulses of the ms laser were used to drill through the sample. The peak power of the ms pulse was 700 W. In addition, the pure ns laser drilling process was added as a reference for the CLPs drilling, which used 4050 pulses of the ns laser to drill through the sample. The detailed experimental results are shown in Table 1.
Table 1. The energy used for drilling
3.1 Analysis of the drilling efficiency of temporally superposed CLPs
Table 1 lists the energy used to drill through the sample by the CLPs, ms pulses and ns pulses. The drilling of ns pulses used the most energy, which was due to the energy loss caused by the plasma screening [42] and latent heat of vaporization [22,31]. On the other hand, it can be seen the energy used by the CLPs was less than 1/10 of the ms pulses, which can be explained in terms of two aspects. First, the keyhole ablated by the ns pulse train increased the absorptivity of alumina ceramic. Second, the rapid temperature rising caused by the ns pulse train increased the absorption of ms pulse and generated recoil pressure for ejecting the molten ceramic.
In order to verify that the keyhole ablated by the ns pulse train could increase the absorptivity of alumina ceramic, a ns-ms two-step drilling method was adopted. The parameters are shown in Fig. 3(a). Firstly, an initial ns pulse train with 60 pulses was used to drill a keyhole in the sample surface. After a delay time of ∼1 s, a ms pulse train with 10 pulses was used to drill through the sample. The peak power of the ms pulse was 90 W. The total energy used for drilling was 2.0 J, which was 7 times lower than that of conventional ms pulse drilling, as shown in Fig. 3(b). That is to say, the keyhole ablated by the ns pulse train could significantly increase the absorptivity of the alumina ceramic.
Fig. 3. (a) The schematic of ns-ms two-step drilling method. (b) The energy used for drilling through the sample by the conventional ms pulse, ns-ms two-step drilling method and CLPs.
In addition, it can be known from Fig. 3(b) that the CLPs used the lowest energy comparing with the conventional ms pulse and ns-ms two step drilling method. In fact, at the beginning of the CLPs drilling, the ns pulse train could continuously ablate the ceramic surface and change the surface roughness. The increased roughness and keyhole depth could increase the absorption of subsequent ms pulses [43]. Then the absorption-enhanced ms pulses could further increase the absorption of ns pulses through the preheating effect [22]. As the increased interaction between the ms pulse and ns pulse, a keyhole could be formed more quickly. On the other hand, the process of splash was mainly influenced by buoyancy, surface tension gradient and recoil pressure [44–46]. The recoil pressure generated during evaporation was the dominant driving force that clears the melt out of the molten pool. For the CLPs drilling the alumina ceramic, the drilling efficiency could be improved due to the recoil pressure generated by the high intensity laser pulse [25] or the high pressure of the shock wave induced by an igniting laser plasma [47–49]. Therefore, the temporally superposed CLPs could further decrease the energy used for drilling.
3.2 Analysis of the drilling quality of spatially superposed CLPs
Figure 4 shows the OM images of the holes drilled by the ns pulses, ms pulses and CLPs. The upper surface of the hole drilled by the ns pulses was elliptical, as shown in Fig. 4(a). The main reason was the off-axis distortion of the optical system. On the contrary, the hole drilled by the ms pulses had a better circularity than that of ns pulses, but there was obvious splatter on the hole surface, as shown in Figs. 4(b) and 4(c). In addition, for the CLPs drilling, the hole with best quality was obtained. The marginal standard error of the hole diameter was about 6 µm. Comparing with the hole drilled by the conventional ms pulse, it can be seen, the diameter of the hole was smaller, the circularity was increased, the spatter volume was decreased, and the taper angle was decreased, as shown in Figs. 4(d) and 4(e).
Fig. 4. The optical microscopy (OM) images of the hole. (a) The upper surface of the hole drilled by the ns pulses with 60 pulses. (b) The upper surface of the hole drilled by the ms pulses. (c) The lower surface of the hole drilled by the ms pulses. (d) The upper surface of the hole drilled by the CLPs. (e) The lower surface of the hole drilled by the CLPs.
As shown in Table 2, the upper surface diameter of the hole drilled by the conventional ms pulse was ∼160 µm, which was much larger than the diameter of ms laser spot. This was due to the changing absorptivity problem during the conventional ms pulse drilling. On the contrary, by using the ns pulses to ablate a keyhole, the absorptivity of the alumina ceramic to the ms pulses of the ns-ms two step drilling method or CLPs was keep around 0.8-1.0. It could help to avoid the changing absorptivity problem and reduce the energy used for drilling. Thus, the hole diameter was decreased. On the other hand, compared with the ns-ms two step drilling method, the melt generated by the ms pulse of the CLPs could be expelled within several ms with velocities of tens of m/s under the ejection of the ns pulse train [33], which could help to reduce the splatter deposited on the sample surface, the diameter of the upper surface and the taper angle of the hole.
Table 2. Drilling quality comparison between the CLPs, ns-ms two step drilling method and conventional ms pulse
In particular, it can be found that although the shape of focused ns laser spot was elliptical, the hole drilled by the CLPs still had a good circularity, just as shown in Figs. 4(a) and 4(d). In order to explain this phenomenon, the commercial software COMSOL with the module “Ray Optics Module” was used for the numerical modelling. Since the absorptivity of the alumina ceramic composed of the arithmetic average of the parallel and vertical polarized parts did not change much with the incident angle [50], the absorptivity of the alumina ceramic was set to an average of 0.15. The upper surface of the initial keyhole was an ellipse, and the major and minor axes of 50 µm and 25 µm, respectively. The keyhole depth was set to 50 µm as the drilling depth of the first pulse of CLPs, as shown in Fig. 5.
Fig. 5. The simulation results of the number of the reflection distributed on the inner wall of the keyhole. The front (a) and top (b) view of the initial keyhole (ellipsoid).
Figure 5 shows the simulation results of the number of the reflection distributed on the keyhole inner wall. As can be seen from Fig. 5(a), after the multi-reflection, most of the rays were focused on the bottom of the keyhole. Due to the lower curvature near the minor axes and shorter distance between the two endpoints of the minor axes, the rays focused on the keyhole bottom were more concentrated on the minor axes, as shown in Fig. 5(b).
Figure 6 shows the simulation results of the laser intensity distributed on the keyhole inner wall. At first, an elliptical keyhole was ablated by the off-axis propagated ns laser. In the keyhole, the highest laser density was concentrated on the minor axes on the keyhole bottom, as shown in Fig. 6(a). So, the alumina ceramic at this position would be removed with the highest ablation rate first. As the drilling progressed, more energy would be injected and focused on the minor axes position, so the eccentricity (e) would be increased constantly, as shown in the Figs. 6(a) to 6(c). Then, the keyhole would become a truncated cone and the laser intensity distributed on the keyhole would be uniform, as shown in Fig. 6(d). Finally, with the further increase of the keyhole depth irradiated by the laser with uniform intensity distribution, a through hole with a good circularity would be obtained. That is to say, the elliptical keyhole ablated by the ns pulse train will not affect the circularity of the final though hole.
Fig. 6. The top view of the simulation laser intensity distributed on the inner wall of the keyhole. The minor axes are 12.5um (a), 18um (b), 23um (c) and 25um (d), respectively. During the simulation, there were 106 rays in total and the laser intensity was normalized [51].
4. Conclusions
In conclusion, micro-hole drilling of alumina ceramic by spatially and temporally superposing the ms pulse and ns pulse train was performed. The drilling results of the CLPs were compared to the conventional ms pulse. The results show that the energy used for drilling by the CLPs could be decreased by an order of magnitude. The multiple enhancement mechanisms were the multi-reflection of the keyhole, preheating effect and ejecting of molten ceramics, etc. On the other hand, the relationship between the hole circularity and the shape of the focused elliptical ns laser spot were analysed theoretically and experimentally. The results show that the elliptical keyhole ablated by the ns pulse train would not affect the circularity of the final though hole, and better drilling quality could be achieved by the CLPs.
Results about CLPs drilling the alumina ceramic presented in this paper not only can provide a reference in the field of high-speed ceramic drilling, but also provides a high efficiency and quality processing method for laser processing applications.
Funding
Key Technologies Research and Development Program (2016YFE0202500); National Natural Science Foundation of China (61975060).
Disclosures
The authors declare no conflicts of interest.
References
1. H. Dong, X. Zhu, and K. Lu, “Morphology and composition of nickel–boron nanolayer plate on boron carbide particles,” J. Mater. Sci. 43(12), 4247–4256 (2008). [CrossRef]
2. Y. Zhang, Y. Wang, J. Zhang, Y. Liu, X. Yang, and Q. Zhang, “Micromachining features of TiC ceramic by femtosecond pulsed laser,” Ceram. Int. 41(5), 6525–6533 (2015). [CrossRef]
3. V. Khuat, Y. Ma, J. Si, T. Chen, F. Chen, and X. Hou, “Fabrication of through holes in silicon carbide using femtosecond laser irradiation and acid etching,” Appl. Surf. Sci. 289, 529–532 (2014). [CrossRef]
4. B. Adelmann and R. Hellmann, “Rapid micro hole laser drilling in ceramic substrates using single mode fiber laser,” J. Mater. Process. Technol. 221, 80–86 (2015). [CrossRef]
5. M. Mendes, V. Kancharla, C. Porneala, X. Song, M. Hannon, R. Sarrafi, J. Schoenly, D. Sercel, S. Dennigan, R. V. Gemert, J. Bickley, and J. Sercel, “Laser machining with QCW fiber lasers,” J. Laser Appl. 2014(1), 79–83 (2014). [CrossRef]
6. M. J. J. Schmidt, L. Li, and J. T. Spencer, “Removal of chlorinated rubber coatings from concrete surfaces using an RF excited CO2 laser,” J. Mater. Process. Technol. 114(2), 139–144 (2001). [CrossRef]
7. N. B. Dahotre and S. P. Harimkar, Laser fabrication and machining of materials (Springer Science & Business Media, 2007).
8. M. Schneider, L. Berthe, R. Fabbro, M. Muller, and M. Nivard, “Gas investigation for laser drilling,” J. Laser Appl. 19(3), 165–169 (2007). [CrossRef]
9. K. Walther, M. Brajdic, and E. W. Kreutz, “Enhanced processing speed in laser drilling of stainless steel by spatially and temporally superposed pulsed Nd: YAG laser radiation,” Int. J. Adv. Des. Manuf. Technol. 35(9-10), 895–899 (2008). [CrossRef]
10. Q. Wang, S. Luo, Z. Chen, H. Qi, J. Deng, and Z. Hu, “Drilling of aluminum and copper films with femtosecond double-pulse laser,” Opt. Laser Technol. 80, 116–124 (2016). [CrossRef]
11. Z. Wang, L. Jiang, X. Li, A. Wang, Z. Yao, K. Zhang, and Y. Lu, “High-throughput microchannel fabrication in fused silica by temporally shaped femtosecond laser Bessel-beam-assisted chemical etching,” Opt. Lett. 43(1), 98–101 (2018). [CrossRef]
12. S. Yan, Z. Hong, T. Watanabe, and J. G. Tang, “CW/PW dual-beam YAG laser welding of steel/aluminum alloy sheets,” Opt. Laser Eng. 48(7-8), 732–736 (2010). [CrossRef]
13. J. P. Parry, J. D. Shephard, F. C. Dear, N. Jones, N. Weston, and D. P. Hand, “Nanosecond-laser postprocessing of millisecond laser-machined zirconia (Y-TZP) Surfaces,” Int. J. Appl. Ceram. Technol. 5(3), 249–257 (2008). [CrossRef]
14. T. Seino, Y. Arai, N. Kobayashi, T. Kudo, and K. Sano, “Backside activation of power device IGBTs by microsecond-pulsed green laser annealing thermally assisted with CW diode laser,” in 18th International Conference on Advanced Thermal Processing of Semiconductors (RTP) (IEEE, 2010), pp. 140–143.
15. Y. Pan, H. Zhang, J. Chen, B. Han, Z. Shen, J. Lu, and X. Ni, “Millisecond laser machining of transparent materials assisted by nanosecond laser,” Opt. Express 23(2), 765–775 (2015). [CrossRef]
16. M. Mrohs, L. Jensen, S. Günster, T. Alig, and D. Ristau, “Dual wavelength laser-induced damage threshold measurements of alumina/silica and hafnia/silica ultraviolet antireflective coatings,” Appl. Opt. 55(1), 104–109 (2016). [CrossRef]
17. S. Guizard, S. Klimentov, A. Mouskeftaras, N. Fedorov, G. Geoffroy, and G. Vilmart, “Ultrafast Breakdown of dielectrics: Energy absorption mechanisms investigated by double pulse experiments,” Appl. Surf. Sci. 336, 206–211 (2015). [CrossRef]
18. Y. Ding, L. Yang, and M. Hong, “Enhancement of pulsed laser ablation assisted with continuous wave laser irradiation,” Sci. China: Phys., Mech. Astron. 62(3), 34211 (2019). [CrossRef]
19. S. Höhm, A. Rosenfeld, J. Krüger, and J. Bonse, “Laser-induced periodic surface structures on zinc oxide crystals upon two-colour femtosecond double-pulse irradiation,” Phys. Scr. 92(3), 034003 (2017). [CrossRef]
20. M. Hashida, T. Nishii, Y. Miyasaka, H. Sakagami, M. Shimizu, S. Inoue, and S. Sakabe, “Orientation of periodic grating structures controlled by double-pulse irradiation,” Appl. Phys. A 122(4), 484 (2016). [CrossRef]
21. W. Ge, C. Xing, V. Veiko, and Z. Li, “All-optical, self-focused laser beam array for parallel laser surface processing,” Opt. Express 27(20), 29261–29272 (2019). [CrossRef]
22. X. Lv, Y. Pan, Z. Jia, Z. Li, H. Zhang, and X. Ni, “Laser-induced damage threshold of silicon under combined millisecond and nanosecond laser irradiation,” J. Laser Appl. 121(11), 113102 (2017). [CrossRef]
23. W. Duan, X. Dong, K. Wang, Z. Fan, X. Mei, W. Wang, and J. Lv, “Effect of temporally modulated pulse on reducing recast layer in laser drilling,” Int. J. Adv. Des. Manuf. Technol. 87(5-8), 1641–1652 (2016). [CrossRef]
24. Y. Pan, X. Lv, H. Zhang, J. Chen, B. Han, Z. Shen, J. Lu, and X. Ni, “Millisecond laser machining of transparent materials assisted by a nanosecond laser with different delays,” Opt. Lett. 41(12), 2807–2810 (2016). [CrossRef]
25. C. Lehane and H. S. Kwok, “Enhanced drilling using a dual-pulse Nd:YAG laser,” Appl. Phys. A: Mater. Sci. Process. 73(1), 45–48 (2001). [CrossRef]
26. A. Temmler, O. Pütsch, J. Stollenwerk, E. Willenborg, and P. Loosen, “Optical set-up for dynamic superposition of three laser beams for structuring and polishing applications,” Opt. Express 22(2), 1387–1393 (2014). [CrossRef]
27. A. Temmler, D. Liu, S. Drinck, J. Luo, and R. Poprawe, “Experimental investigation on a new hybrid laser process for surface structuring by vapor pressure on Ti6Al4 V,” J. Mater. Process. Technol. 277, 116450 (2020). [CrossRef]
28. M. Brajdic, L. Walther, and U. Eppelt, “Analysis of laser drilled deep holes in stainless steel by superposed pulsed Nd: YAG laser radiation,” Opt. Laser Eng. 46(9), 648–655 (2008). [CrossRef]
29. J. A. Fox, “A method for improving continuous wave laser penetration of metal targets,” Appl. Phys. Lett. 26(12), 682–684 (1975). [CrossRef]
30. S. Donadello, A. G. Demir, and B. Previtali, “Probing multipulse laser ablation by means of self-mixing interferometry,” Appl. Opt. 57(25), 7232–7241 (2018). [CrossRef]
31. N. Shen, J. D. Bude, S. Ly, W. J. Keller, A. M. Rubenchik, R. Negres, and G. Guss, “Enhancement of laser material drilling using high-impulse multi-laser melt ejection,” Opt. Express 27(14), 19864–19886 (2019). [CrossRef]
32. M. Gedvilas, J. Mikšys, J. Berzinš, V. Stankevič, and G. Račiukaitis, “Multi-photon absorption enhancement by dual-wavelength double-pulse laser irradiation for efficient dicing of sapphire wafers,” Sci. Rep. 7(1), 5218 (2017). [CrossRef]
33. Y. Qin, D. J. Förster, R. Weber, T. Graf, and S. Yang, “Numerical study of the dynamics of the hole formation during drilling with combined ms and ns laser pulses,” Opt. Laser Technol. 112, 8–19 (2019). [CrossRef]
34. X. Jia, Y. Zhang, Y. Chen, H. Wang, G. Zhu, and X. Zhu, “Combined pulsed laser drilling of metal by continuous wave laser and nanosecond pulse train,” Int. J. Adv. Des. Manuf. Technol. 104(1-4), 1269–1274 (2019). [CrossRef]
35. X. Jia, Y. Chen, G. Zhu, H. Wang, K. Aleksei, and X. Zhu, “Experimental study on the optimum matching of CW-nanosecond combined pulse laser drilling,” Appl. Opt. 58(33), 9105–9111 (2019). [CrossRef]
36. V. N. Lednev, S. M. Pershin, E. D. Obraztsova, S. I. Kudryashov, and A. F. Bunkin, “Single-shot and single-spot measurement of laser ablation threshold for carbon nanotubes,” J. Phys. D: Appl. Phys. 46(5), 052002 (2013). [CrossRef]
37. J. Zhang, Y. Long, S. Liao, H. Lin, and C. Wang, “Effect of laser scanning speed on geometrical features of Nd: YAG laser machined holes in thin silicon nitride substrate,” Ceram. Int. 43(3), 2938–2942 (2017). [CrossRef]
38. Y. Yan, L. Li, K. Sezer, D. Whitehead, L. Ji, Y. Bao, and Y. Jiang, “Experimental and theoretical investigation of fibre laser crack-free cutting of thick-section alumina,” Int. J. Mach. Tool. Manu. 51(12), 859–870 (2011). [CrossRef]
39. A. N. Samant and N. B. Dahotre, “Computational predictions in single-dimensional laser machining of alumina,” Int. J. Mach. Tool. Manu. 48(12-13), 1345–1353 (2008). [CrossRef]
40. M. M. Hanon, E. Akman, B. G. Oztoprak, M. Gunes, Z. A. Taha, K. I. Hajim, E. Kacar, O. Gundogdu, and A. Demir, “Experimental and theoretical investigation of the drilling of alumina ceramic using Nd:YAG pulsed laser,” Opt. Laser Technol. 44(4), 913–922 (2012). [CrossRef]
41. Y. Yan, L. Ji, Y. Bao, and Y. Jiang, “An experimental and numerical study on laser percussion drilling of thick-section alumina,” J. Mater. Process. Technol. 212(6), 1257–1270 (2012). [CrossRef]
42. J. Yuan, L. Liang, G. Lin, X. Li, and M. Jiang, “Experimental study on the laser-matter-plume interaction and its effects on ablation characteristics during nanosecond pulsed laser scanning ablation process,” Opt. Express 27(16), 23204–23216 (2019). [CrossRef]
43. E. Ohmura and S. Noguchi, “Laser drilling simulation considering multiple reflection of laser, evaporation and melt flow,” in Materials with complex behavior, (Springer, 2010).
44. M. Von Allmen, “Laser drilling velocity in metals,” J. Appl. Phys. 47(12), 5460–5463 (1976). [CrossRef]
45. C. L. Chan and J. Mazumder, “One-dimensional steady-state model for damage by vaporization and liquid expulsion due to laser-material interaction,” J. Appl. Phys. 62(11), 4579–4586 (1987). [CrossRef]
46. P. Solana, P. Kapadia, J. Dowden, W. S. O. Rodden, S. S. Kudesia, D. P. Hand, and J. D. C. Jones, “Time dependent ablation and liquid ejection processes during the laser drilling of metals,” Opt. Commun. 191(1-2), 97–112 (2001). [CrossRef]
47. N. Krstulovic and S. Miloševic, “Drilling enhancement by nanosecond–nanosecond collinear dual-pulse laser ablation of titanium in vacuum,” Appl. Surf. Sci. 256(13), 4142–4148 (2010). [CrossRef]
48. S. T. Hendow, R. Romero, S. A. Shakir, and P. T. Guerreiro, “Percussion drilling of metals using bursts of nanosecond pulses,” Opt. Express 19(11), 10221–10231 (2011). [CrossRef]
49. X. D. Wang, A. Michalowski, D. Walter, S. Sommer, M. Kraus, J. S. Liu, and F. Dausinger, “Laser drilling of stainless steel with nanosecond double-pulse,” Opt. Laser Technol. 41(2), 148–153 (2009). [CrossRef]
50. M. Geiger, K. H. Leitz, H. Koch, and A. Otto, “A 3D transient model of keyhole and melt pool dynamics in laser beam welding applied to the joining of zinc coated sheets,” Prod. Eng. 3(2), 127–136 (2009). [CrossRef]
51. G. Zhu, X. Zhu, and C. Zhu, “Analytical approach of laser beam propagation in the hollow polygonal light pipe,” Appl. Opt. 52(23), 5619–5630 (2013). [CrossRef]
