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Abstract
Sapphire is a kind of ultrahard transparent material with good chemical resistance. These great properties also make sapphire functional device fabrication a big challenge. We propose a novel dual-beam laser induced plasma assisted ablation (LIPAA) for high-quality sapphire microprocessing. One laser beam is focused on a sacrificial target for nano-particle generation by LIPAA to assist the sapphire ablation by the other laser beam. The new technology can reduce the ablation threshold of sapphire and the roughness of the fabricated structures. The laser fluence for particle generation is optimized. Furthermore, we demonstrate a sapphire Dammann grating and an OAM generator fabricated by this method. This method can be expanded to arbitrary transparent material precision machining for various applications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Sapphire is a single crystal form of α-alumina, which is transparent and ultrahard. Due to its unique chemical, mechanical, electrical, optical, and biomedical properties, it has wide applications in electronics, optics, defense, and medical science [1,2]. The hardness and brittleness of sapphire make the machining processes, including cutting, patterning, grinding, lapping, and polishing, very difficult and costly [3,4]. Sapphire also has good resistance to corrosion, which can only be etched at some extreme conditions. The commercial techniques, including machining by diamond-based tools and chemical mechanical polishing, are applied to process the bulk sapphire. However, it is hard to fabricate functional sapphire devices by these methods.
Laser processing is an advanced micro-processing method, which is widely used for metal, semiconductor, and dielectric material patterning [5,6]. Due to the low light energy absorption by sapphire, two approaches are often applied for the laser sapphire processing. The first approach is directly ablating sapphire by specific lasers, including Q-switched and excimer ultraviolet (UV) lasers [7–9] as well as ultrafast picosecond (ps)/femtosecond (fs) lasers [10–15]. By high peak pulse energy and light absorption in the ultraviolet regime, the UV laser pulses can directly ablate the sapphire. The roughness can achieve around 100 nm [8]. Nonlinear absorption appears at extremely high laser intensity by using ps/fs lasers. It can remove the material with low thermal diffusion. The roughness of the pattern fabricated by fs laser on sapphire can achieve 69 nm [16]. The roughness can be further reduced by wet etching [17]. However, the fabrication speed is limited due to the requirement of well-controlled laser fluence around the ablation threshold and the complex acid wet etching [18]. The other approach is to ablate the sapphire assisted by other materials, including laser induced plasma assisted ablation (LIPAA) [19,20] and laser-induced backside wet etching (LIBWE) [21]. Through these methods, the laser energy can be absorbed by other materials, such as metals and solutions. The energy is then transferred to sapphire for its processing. However, the roughness of the patterns fabricated by conventional LIPAA and LIBWE is more than hundreds of nanometers. The plasma and bubbles in liquid make the process difficult to be controlled. A new approach is desired to directly fabricate structures on sapphire with high ablation rate and good quality.
Inspired by the double-pulse laser processing [22,23] and LIPAA [24–29], we propose a novel dual-beam LIPAA technique to control nano-particle generation to assist the sapphire laser processing. A laser beam is focused on a copper plate. The copper plasma is generated and nano-particles are deposited on the rear surface of sapphire. The other laser beam is focused on the rear surface of sapphire, which can be absorbed by the nano-particles efficiently to assist the sapphire ablation. By the dual-beam LIPAA, the laser ablation rate increases significantly and the roughness of the ablated pattern is reduced greatly. Laser fluence is optimized for the dual-beam LIPAA process. Furthermore, it is applied to fabricate diffractive optical elements.
2. Design and mechanisms
Figure 1 shows the mechanism of the dual-beam LIPAA. Laser beam 1 transmits through sapphire, and its energy is absorbed by copper to ablate copper target to generate a plasma and nano-particles as shown in Fig. 1(a). In our experiment, copper is chosen as a sacrificial target due to its high laser energy absorption. If the gap between copper and sapphire is small enough (typically < 150µm [19,30]), laser-induced plasma can directly interact with the sample surface for the ablation. When the gap distance increases, ions and electrons in plasma are recombined to become nano-particles during the expansion [31,32], which cannot directly ablate the sapphire. These particles are deposited on the rear surface of sapphire. Then, laser beam 2 energy is absorbed by the particles to ablate the sapphire as shown in Fig. 1(b). The time delay between laser beam 2 and laser beam 1 was discussed for the LIPAA with a very small gap before [19]. In our case, the ions and electrons in plasma are recombined to nanoparticles that the time delay becomes insensitive. It can make the patterning process more stable for various designs. Figure 1(c) indicates the pattern generated by the dual-beam LIPAA.
Fig. 1. Mechanism of dual-beam LIPAA of sapphire. (a) LIPAA by Laser beam 1; (b) nano-particle absorption of Laser beam 2 energy; (c) sapphire and particle removal.
A dual-beam LIPAA setup is designed as shown in Fig. 2. The laser beam generated by a femtosecond laser (Coherent Libra) passes through a half waveplate (HWP) for polarization manipulation and is splitted into two beams by a polarizing beamsplitter (PBS). The laser wavelength, repetition rate, and pulse duration are 800 nm, 1000 Hz, and 50 fs, respectively. The laser beam 1 passes through a set of lenses (flens1 = 100 mm; flens2=-40 mm; the distance between two lenses is 64 mm) as a divergence beam, while the laser beam 2 is a parallel beam. Two beams are combined together by the other PBS. The laser power difference between two beams can be tuned by rotating the HWP in the system. The combined beam is reflected in an objective (10X, NA = 0.4). Then, the laser beams 1 and 2 are focused on the copper surface and rear surface of sapphire with a diameter of around 10 and 3 µm, respectively. The sapphire and copper are separated by a spacer as shown in the inset of Fig. 2 and placed on a precision 6-axis nanopositioning stage. The thickness of spacer is 0.55 mm. The pattern can be fabricated by controlling the laser and stage movement by computer numerical control (CNC). The morphology of the pattern is measured by a Tencor Alpha-Step 500 Surface Profiler.
3. Results and discussion
3.1 Dual-beam LIPAA
To demonstrate the ablation enhancement of the dual-beam LIPAA, the same patterns are fabricated by single laser beam 1 ablation of copper (L1), the single laser beam 2 ablation of sapphire (L2), and both laser beams 1 & 2 ablations of sapphire and copper simultaneously (L1 & 2, one pass of scanning by dual-beam LIPAA). The fluence of laser beam 1 is 24.2 J/cm2, which can generate a lot of nano-particles while the sapphire cannot be directly ablated. The fluence of laser beam 2 is 8.5 J/cm2, which is around the threshold laser fluence of the sapphire ablation. The scanning speed is 1 mm/s and the scanning period is 1 µm.
Figures 3(a) & (b) show the microscopic images and surface profiles of the grooves fabricated by 8 different sequences of laser ablation. The scanning passes and sequences of laser ablation are shown in the labels. As the microscopic images of single laser beam 2 ablation (L2 and L2*2) without LIPAA, the color of the pattern is dark, which is due to high roughness of the ablated surface. It is found that some structures are generated above the sapphire surface. It is due to the phase change of the sapphire from the crystallized phase (α-Al2O3) to the amorphous phase. The single beam LIPAA of sapphire by laser beam 1 (L1) can be ignored due to weak ablation. Although the power of laser beam 1 can ablate the copper for particle generation, the laser fluence is not high enough to ablate the surface due to defocusing at the sapphire rear surface at the gap distance of 0.55 mm. When the gap distance is lower than 0.15 mm, the sapphire can be directly ablated by LIPAA. The particle distribution becomes more uniform by increasing the substrate-to-target distance. However, the number of nano-particles reduces at a higher gap distance. Hence, the gap distance is optimized at 0.55 mm, which has good uniformity and high particle density at the same time. By the dual-beam LIPAA (L1 & 2), a groove with a flat bottom at the average depth of 98.9 nm is fabricated. If using a single-beam LIPAA to prepare a layer of metal particles before the laser beam 2 ablation (L1 + L2), the sapphire can also be removed. However, the ablation rate of L2 ablation following single-beam LIPAA can only achieve 75.1% ablation rate of dual-beam LIPAA as shown in Fig. 3(c). Hence, dual-beam LIPAA can not only reduce the ablation threshold but also increase the ablation rate.
Fig. 3. (a) & (b) Microscopic images and surface profiles of the sapphire ablated by dual beams on 8 different sequences. (c) Ablation rates at different ablation sequences. L1: one pass of laser beam 1 scanning; L2: one pass of laser beam 2 scanning; L1 & 2: one pass of both laser beams 1 and 2 scanning.
For the second time scanning (L1 & 2 + L2, L1 & 2 + L1 + L2, and L1 & 2*2), the contribution of the LIPAA is not so obvious. There are two main reasons. Firstly, the surface of sapphire is not so smooth anymore, which can absorb one part of laser energy for further ablation. The ablation depth by the second time scanning is about 33% higher than that by the first-time scanning. Secondly, the diffusion of nano-particles affects the ablation rate. A control experiment is done for L1 & 2 + L2 and L1 & 2*2. The distance of the two patterns fabricated by L1 & 2 + L2 and L1 & 2*2 is far enough (1 mm) to minimize the diffused nano-particle influence. It is found that the ablation rate can be increased by 47.5% by dual-beam LIPAA (the details are included in Appendix).
3.2 Laser fluence optimization
To further discover the contribution of the LIPAA, different fluence of laser beam 1 (FLB1) is applied at the same laser fluence of Laser beam 2 (8.5 J/cm2) for the patterning. A single-pass scanning is chosen to avoid the absorption at a rough sapphire surface. Figure 4(a) shows the microscopic images of the fabricated patterns at different fluences after ultrasound cleaning in DI water. Without the assistance of laser beam 2, the laser beam 1 at the threshold fluence of directly sapphire ablation that no groove patterns can be observed. When the fluence of laser beam 1 is increased above the 20.8 J/cm2, the number of nano-particles adhered on the sapphire surface raised rapidly. At 84.3 J/cm2 fluence of laser beam 1, a lot of nano-particles are accumulated in and around the groove, which cannot be cleaned by ultrasound machine in DI water.
Fig. 4. (a) Microscopic images of the ablated patterns by dual-beam LIPAA at different fluences of laser beam 1. The fluence of laser beam 2 is fixed at 8.5 J/cm2. (b) and (c) are the ablation rates and roughness of pattern at different fluences of laser beam 1, respectively.
Figure 4(b) indicates the ablation rates at different laser fluence. When the FLB1 is below 20.8 J/cm2, the ablation rate increase is approximately linear by increasing FLB1. The ablation rate reaches the maximum at the fluence of laser beam 1 around 20.8 and 24.2 J/cm2. Then, the depth of pattern reduces at higher FLB1. It is caused by the superfluous particles generated in LIPAA which cannot be fully ablated by laser beam 2. The particles can be accumulated on the sapphire surface as a film which obstructs the laser energy absorption.
Beyond the ablation rate, the roughness is also affected by PLB1 as shown in Fig. 3(c). At low PLB1, the higher depth corresponds to the higher roughness. When the fluence of laser beam 1 is above 24.2 J/cm2, the roughness is high even at lower ablation depth, which is affected by the accumulated particles. Hence, 20.8 J/cm2 is an optimized laser fluence of double-focusing particle-assisted laser sapphire ablation in our system. At this fluence, the near-maximum ablation rate can be achieved which low roughness (Ra = 18.1 nm). Meanwhile, there are few particles adhered to the sapphire.
If reducing the gap distance between the copper and sapphire, the laser fluence can be reduced. However, the plasma can directly ablate the sapphire or doped into the crystal. The process becomes more complex and fabrication quality is difficult to be guaranteed. To reduce the cost in industrial production, the laser beam 1 can be changed to other type lasers at the same gap distance.
3.3 Diffractive optical element fabrication
Due to the advantages of low roughness by the dual-beam LIPAA, it can be applied for the optical component fabrication. Diffractive optical element (DOE) is a unique optical device. Two kinds of DOEs are designed and fabricated. One is a Dammann grating, which can split one beam into multiple beams at the same light intensity. We design a one-dimensional grating to generate 7 equal intensity beams as shown in Fig. 5(b). The π phase-shift points are designed at 0, 23, 43, and 53 µm at a period of 100 µm. Figure 5(a) shows a microscopic image of the grating fabricated by the dual-beam LIPAA in transmission mode. It shows the good transmission and low absorption. Illuminated by a 532 nm laser beam, its diffraction pattern is shown in Fig. 5(c). Seven diffraction orders array in a line is generated with similar intensities. The power of the 0th order is a bit higher than other orders, which means a part of light is not modulated by fabricated DOE. The theatrical and experiment measured modulation efficiency are summarized in Table 1. Only 14.3% light power of dual-beam LIPAA fabricated Dammann grating is unmodulated, which is much lower than 58.5% in the conventional single laser beam fabricated sample. The diffractive efficiency (DE) of the whole double-focusing method fabricated grating can achieve 73.3%.
Fig. 5. (a) & (e) Microscopic images of a Dammann Grating and an OAM generator fabricated by dual-beam LIPAA. (b) & (f) theoretically designed diffraction pattern of the Dammann Grating and OAM generator. (c) & (d) measured diffraction patterns of the Dammann Grating fabricated by dual-beam LIPAA and conventional single-beam fs laser ablation, respectively. (g) & (h) measured diffraction patterns of the OAM generator fabricated by dual-beam LIPAA and single-beam fs laser ablation, respectively.
Table 1. Modulation efficiency of the fabricated sapphire DOEs.
The other application is an orbital angular momentum (OAM) generator. We design a generator for both + 1 and -1 OAM beams. By symmetric design, the phase profile of DOE can be simplified as a 0 and π phase distribution for the + 1 and -1 OAM beams deflected in the opposite directions. The design is shown in Fig. 5(f), which includes two vortex beams with equal intensity and opposite OAM. Figure 5(g) shows the measured diffraction pattern of the OAM generator, which has a weak unmodulated 0th diffraction order. By analyzing the light distribution at -1, 0, 1 diffraction orders, 92.3% power is transferred to l=±1 OAM beams. For a 0 and 1 amplitude modulated OAM generators, less than 50% power can be modified to OAM beams. It is also much better than the sample fabricated by conventional single laser beam fabrication. The high quality and low roughness pattern fabricated by the dual-beam LIPAA method can both increase the diffraction efficiency and improve the purity of the OAM beam.
4. Conclusions
The dual-beam low energy LIPAA method has advantages in microprocessing with high ablation rate, high quality, and low roughness. The absorption of the nano-particles deposited on the sapphire surface is optimized. It is applied for high performance sapphire DOE fabrication, which has high resistance to corrosion and abrasion. Compared with other reported low roughness sapphire microprocessing methods, our technology can avoid fabrication complexity and using chemical reactions, which is environment-friendly. This method can be applied in sapphire processing as well as other transparent material manufacturing
Appendix
The sapphire patterns fabricated by L1 & 2*2 and L1 & 2 + L2 are placed with 1 mm distance to avoid nano-particles influence. The ablation rate (unit: µm3/pulse) are listed as below.
Hence, in the second pass scanning, the ablation rate can increase ∼40% compared to the case by single focusing laser ablation. By the dual-beam LIPAA, the ablation rate can be further increased by 47.5%.
Funding
RIE2020 Advanced Manufacturing and Engineering (AME) Individual Research Grant (IRG) (Grant No. A1883c0010); Fund of National Engineering Research Center for Optoelectronic Crystalline Materials.
Disclosures
The authors declare no conflicts of interest.
References
1. E. R. Dobrovinskaya, L. A. Lytvynov, and V. Pishchik, Sapphire: Material, Manufacturing, Applications (Springer Science & Business Media, 2009).
2. V. Kurlov, “Sapphire: Properties, Growth and Applications,” in Encyclopedia of Materials: Science and Technology (2001), pp. 8259–8265.
3. P. Pawar, R. Ballav, and A. Kumar, “Machining processes of sapphire: An overview,” Int. J. Mod. Manuf. Technol. 9(1), 47–72 (2017).
4. Z. C. Li, Z. J. Pei, and P. D. Funkenbusch, “Machining processes for sapphire wafers: A literature review,” Proc. Inst. Mech. Eng., Part B 225(7), 975–989 (2011). [CrossRef]
5. M. Berndt and M. Danner, “Laser Processing of Sapphire: Application Know-How Matters,” Laser Tech. J. 13(5), 42–45 (2016). [CrossRef]
6. M. Malinauskas, A. Žukauskas, S. Hasegawa, Y. Hayasaki, V. Mizeikis, R. Buividas, and S. Juodkazis, “Ultrafast laser processing of materials: From science to industry,” Light: Sci. Appl. 5(8), e16133 (2016). [CrossRef]
7. R. Moser, N. Ojha, M. Kunzer, and U. T. Schwarz, “Sub-surface channels in sapphire made by ultraviolet picosecond laser irradiation and selective etching,” Opt. Express 19(24), 24738 (2011). [CrossRef]
8. X. Wei, X. Z. Xie, W. Hu, and J. F. Huang, “Polishing sapphire substrates by 355 nm ultraviolet laser,” Int. J. Opt. 2012, 1–7 (2012). [CrossRef]
9. X. Wei, X. Guo, and X. Xie, “Influence of the condition parameters on UV pulsed laser polishing of sapphire wafer,” Opt. InfoBase Conf. Pap.TuJ2_4, 1–4 (2009).
10. A. Puišys and D. Paipulas, “Integration of fresnel zone plates in the bulk of sapphire crystal by femtosecond laser pulses,” J. Laser Micro/Nanoeng. 10(1), 96–100 (2015). [CrossRef]
11. M. Wang, J. T. Lin, Y. X. Xu, Z. W. Fang, L. L. Qiao, Z. M. Liu, W. Fang, and Y. Cheng, “Fabrication of high-Q microresonators in dielectric materials using a femtosecond laser: Principle and applications,” Opt. Commun. 395, 249–260 (2017). [CrossRef]
12. T. Kudrius, G. Šlekys, and S. Juodkazis, “Surface-texturing of sapphire by femtosecond laser pulses for photonic applications,” J. Phys. D: Appl. Phys. 43(14), 145501 (2010). [CrossRef]
13. K. Yin, J. Duan, X. Sun, C. Wang, and Z. Luo, “Formation of superwetting surface with line-patterned nanostructure on sapphire induced by femtosecond laser,” Appl. Phys. A: Mater. Sci. Process. 119(1), 69–74 (2015). [CrossRef]
14. S. Butkus, D. Paipulas, R. Sirutkaitis, E. Gaižauskas, and V. Sirutkaitis, “Rapid cutting and drilling of transparent materials via femtosecond laser filamentation,” J. Laser Micro/Nanoeng. 9(3), 213–220 (2014). [CrossRef]
15. L. Ji, T. Yan, and R. Ma, “Ionization behavior and dynamics of picosecond laser filamentation in sapphire,” Opto-Electron. Adv. 2(8), 19000301–19000307 (2019). [CrossRef]
16. Q. K. Li, Y. H. Yu, L. Wang, X. W. Cao, X. Q. Liu, Y. L. Sun, Q. D. Chen, J. A. Duan, and H. B. Sun, “Sapphire-Based Fresnel Zone Plate Fabricated by Femtosecond Laser Direct Writing and Wet Etching,” IEEE Photonics Technol. Lett. 28(12), 1290–1293 (2016). [CrossRef]
17. Q. K. Li, Q. D. Chen, L. G. Niu, Y. H. Yu, L. Wang, Y. L. Sun, and H. B. Sun, “Sapphire-Based Dammann Gratings for UV Beam Splitting,” IEEE Photonics J. 8(6), 1–8 (2016). [CrossRef]
18. X.-Q. Liu, B.-F. Bai, Q.-D. Chen, and H.-B. Sun, “Etching-assisted femtosecond laser modification of hard materials,” Opto-Electronic Adv. 2(9), 19002101–19002114 (2019). [CrossRef]
19. Y. Hanada, K. Sugioka, K. Obata, H. Takase, H. Takai, I. Miyamoto, and K. Midorikawa, “Micromachining of transparent materials by Laser-induced plasma-assisted Ablation (LIPAA),” Proc. SPIE 5662, 526–531 (2004). [CrossRef]
20. X. Lu, F. Jiang, T. Lei, R. Zhou, C. Zhang, G. Zheng, Q. Wen, and Z. Chen, “Laser-Induced-Plasma-Assisted Ablation and Metallization on C-Plane single crystal sapphire (C-Al2O3),” Micromachines 8(10), 300 (2017). [CrossRef]
21. X. Ding, T. Sato, Y. Kawaguchi, and H. Niino, “Laser-induced backside wet etching of sapphire,” Jpn. J. Appl. Phys. 42(Part 2, No. 2B), L176–L178 (2003). [CrossRef]
22. F. Fraggelakis, G. Mincuzzi, J. Lopez, I. Manek-Hönninger, and R. Kling, “Controlling 2D laser nano structuring over large area with double femtosecond pulses,” Appl. Surf. Sci. 470, 677–686 (2019). [CrossRef]
23. R. Zhou, S. Lin, Y. Ding, H. Yang, K. O. Y. Keng, and M. Hong, “Enhancement of laser ablation via interacting spatial double-pulse effect,” Opto-Electron. Adv. 1(8), 18001401–18001406 (2018). [CrossRef]
24. R. Böhme and K. Zimmer, “Low roughness laser etching of fused silica using an adsorbed layer,” Appl. Surf. Sci. 239(1), 109–116 (2004). [CrossRef]
25. M. H. Hong, K. Sugioka, D. J. Wu, K. J. Chew, Y. F. Lu, K. Midorikawa, and T. C. Chong, “Laser-induced plasma-assisted ablation and its applications,” Proc. SPIE 4830, 408 (2003). [CrossRef]
26. M. Hong, K. Sugioka, D. J. Wu, L. L. Wong, Y. Lu, K. Midorikawa, and T. C. Chong, “Laser-induced plasma-assisted ablaiton for glass microfabrication,” Proc. SPIE 4595, 138–146 (2001). [CrossRef]
27. M. H. Hong, Q. Xie, B. C. Lim, K. Sugioka, K. Midorikawa, K. D. Ye, L. P. Shi, and T. C. Chong, “Low resistivity glass metallization by laser induced plasma-assisted ablation,” Proc. SPIE 5662, 532–537 (2004). [CrossRef]
28. M. Hong, K. Sugioka, Y. Lu, K. Midorikawa, and T. C. Chong, “Optical diagnostics in laser-induced plasma-assisted ablation of fused quartz,” Proc. SPIE 4088, 359–362 (2000). [CrossRef]
29. K. Sugioka, K. Midorikawa, H. Yamaoka, Y. Gomi, M. Otsuki, M. H. Hong, D. J. Wu, L. L. Wong, and T. C. Chong, “Glass microprocessing by laser-induced plasma-assisted ablation: fundamental to industrial applications,” Proc. SPIE 5506, 1–10 (2004). [CrossRef]
30. Y. Hanada, K. Sugioka, I. Miyamoto, and K. Midorikawa, “Double-pulse irradiation by laser-induced plasma-assisted ablation (LIPAA) and mechanisms study,” Appl. Surf. Sci. 248(1-4), 276–280 (2005). [CrossRef]
31. R. Zhou, F. Shen, J. Cui, Y. Zhang, H. Yan, and J. Carlos, “Electrophoretic Deposition of Graphene Oxide on Laser-Ablated Copper Mesh for Enhanced Oil/Water Separation,” Coatings 9(3), 157 (2019). [CrossRef]
32. R. Zhou, T. Huang, L. Chen, S. Chen, S. Lin, and Y. Zhuo, “Electroless Deposition of Confined Copper Layers Based on Selective Activation by Pulsed Laser Irradiation,” J. Laser Micro/Nanoeng. 12(2), 169–175 (2017). [CrossRef]
