Gold-film coating assisted femtosecond laser fabrication of large-area, uniform periodic surface structures Pin Feng, Lan Jiang, Xin Li,* Wenlong Rong, Kaihu Zhang, and Qiang Cao Laser Micro/Nano-Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China *Corresponding author: [email protected] Received 6 November 2014; revised 14 January 2015; accepted 15 January 2015; posted 16 January 2015 (Doc. ID 226429); published 12 February 2015
A simple, repeatable approach is proposed to fabricate large-area, uniform periodic surface structures by a femtosecond laser. 20 nm gold films are coated on semiconductor surfaces on which large-area, uniform structures are fabricated. In the case study of silicon, cross-links and broken structures of laser induced periodic surface structures (LIPSSs) are significantly reduced on Au-coated silicon. The good consistency between the scanning lines facilitates the formation of large-area, uniform LIPSSs. The diffusion of hot electrons in the Au films increases the interfacial carrier densities, which significantly enhances interfacial electron–phonon coupling. High and uniform electron density suppresses the influence of defects on the silicon and further makes the coupling field more uniform and thus reduces the impact of laser energy fluctuations, which homogenizes and stabilizes large-area LIPSSs. © 2015 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (320.2250) Femtosecond phenomena; (220.4241) Nanostructure fabrication. http://dx.doi.org/10.1364/AO.54.001314
1. Introduction
In recent years, femtosecond laser induced periodic surface structures (LIPSSs, also called ripples) have been widely accepted as a natural phenomenon. People mainly pay attention to study LIPSS formation through adjusting laser parameters (pulse number [1,2], laser fluence [3,4], polarization [5], or double pulse delay [6]) or changing processing environments (liquid [7,8], temperature [9]). The materialdependent formation mechanisms of LIPSSs, such as excitation of surface plasmon polaritons (SPPs) [10–12], effect of grating-assisted surface plasmonlaser coupling [13], self-organization [14], and second harmonic generation [15], have been proposed. However, fabrication of large-area LIPSSs with uniformity and repeatability remains the research hotspot and challenge [16]. Recently, a lot of 1559-128X/15/061314-06$15.00/0 © 2015 Optical Society of America 1314
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experiments and simulations have been conducted to fabricate large-area, uniform LIPSSs. For example, Oktem et al. demonstrated a simple, low-cost method based on exploiting naturally occurring feedback mechanisms for the creation of laser-induced metaloxide nanostructures with unprecedented uniformity [17]. And de la Cruz et al. pointed out that uniform, large-area LIPSSs could be created on Cr film by a high repetition rate femtosecond laser [18]. Moreover, a variety of methods, such as laser-induced chemical etching [19], electron-beam lithography [20], and laser interference lithography [21], have been established to process large-area, uniform LIPSSs on various materials. Nevertheless, those methods are either complicated or costly. Therefore, a new efficient method is desperately needed to fabricate large-area, uniform LIPSSs. In this paper, a simple, efficient, and repeatable approach, that is, plating 20 nm gold films on silicon surfaces, is proposed to fabricate large-area, uniform LIPSSs by radiation of a femtosecond laser.
Cross-links and broken structures are significantly reduced, and good consistency between the scanning lines is found in the laser induced surface structures on Au-coated silicon. The proposed approach might have potential applications in absorption enhancement of material surfaces, high surface-enhanced Raman scattering, and generation of hydrophobic surfaces. 2. Experiment Setup
A regenerative Ti:sapphire laser system (800 nm, 35 fs, 1 kHz) is used in the experiment to fabricate large-area, uniform periodic surface structures in air at ambient temperature and pressure. The p-polarized laser radiation is focused perpendicular to the substrate using an achromatic doublet (f 100 mm). The diameter of the Gaussian beam is measured as ∼20 μm, which is determined using a method by Liu [22]. The initial pulse duration of ∼35 fs is stretched to ∼50 fs (measured by an autocorrelator) after passing a series of lenses. The sample is mounted on a computer-controlled, six-axis translation stage (M-840.5DG, PI, Inc.) with a
positioning accuracy of 1 μm in the x and y directions and 0.5 μm in the z direction. The laser fluence is accurately adjusted by the combination of an achromatic half-wave plate and a linear polarizer. The gold film is coated on the silicon substrate (10 mm × 10 mm × 0.5 mm) by a magnetron sputtering method, and the thickness can be accurately controlled by the magnetron sputtering time. The surface roughness of the Au-coated silicon and silicon is less than 1 nm (Ra < 1 nm). The surface morphologies after laser processing are imaged by scanning electron microscopy (SEM) and atomic force microscopy (AFM). 3. Results and Discussion
As shown in Fig. 1, the morphologies of the scanning lines are carefully studied on Au-coated silicon [Figs. 1(a1), 1(a2), and 1(a3)] and on silicon [Figs. 1(b1), 1(b2), and 1(b3)]. The periods of LIPSSs are ∼600 nm, and the orientations are perpendicular to the laser polarization. The fluence is varied between 0.18 and 0.27 J∕cm2 . The scanning speed (v) is 100 μm∕s, and the effective number of pulses per spot is ∼200.
Fig. 1. SEM images and AFM profiles of the surface morphology written on samples. (a1), (a2), and (a3) are written on Au-coated silicon, and (b1), (b2), and (b3) are written on silicon. The laser fluences are 0.27, 0.23, and 0.18 J∕cm2 , as labeled on the images. (c) and (d) illustrate AFM profiles of the surface morphology on Au-coated silicon and silicon at 0.27 J∕cm2 , respectively. The scanning speed is 100 μm∕s, and the effective number of pulses per spot is ∼200. The polarization direction is labeled in (a1). 20 February 2015 / Vol. 54, No. 6 / APPLIED OPTICS
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For example, when the laser fluence is 0.23 J∕cm2 , it is found that well-defined LIPSSs on silicon only exist in part of the region, and some of the adjacent LIPSSs are joined up, which is defined as cross-links [Fig. (b2)]. And some of the LIPSS are broken, which refers to some holes or bifurcations that interrupt the continuity of LIPSS. Those defects account for almost half of the whole LIPSS covered area. For comparison, there are no cross-links on Au-coated silicon [Fig. (a2)]. Furthermore, the boundary between the LIPSS covered area and the unprocessed area on Au-coated silicon is clear, whereas, on silicon, the edge of scanning line is wavy, which is unfavorable to uniform, large-area LIPSS processing. Relatively uniform periodic structures can be processed on silicon as the laser fluence changes between 0.18 and 0.27 J∕cm2 . The cross-links on silicon are also more than those on Au-coated silicon. The experiments with each material are repeated at least five times in our experiments. Moreover, AFM is used in order to gain a deep insight into the information about the spatial characteristics. As shown in Figs. 1(c) and 1(d), when the laser fluence is 0.27 J∕cm2 , compared with silicon, the LIPSSs on Au-coated silicon have clear peaks, valleys, and stable periods. In contrast, the LIPSSs on silicon fluctuate severely, which is mainly due to the existence of cross-links and broken structures. Accordingly, it is much easier to form large-area, uniform nanostructures on Au-coated silicon than on silicon. In order to accurately analyze the formation process on Au-coated silicon and silicon, the evolution of the surface morphology is observed after laser irradiation. Figure 2 shows the SEM images of the irradiated areas on Au-coated silicon and silicon. The laser pulse number N increases from 1 to 300, which is controlled externally by a fast mechanical shutter synchronized with the laser repetition rate. At N 1, the Au layer is ablated, leading to exposure of the substrate. As shown in Fig. 3 and Table 1, the concentration of Au is 8.4%, which is analyzed by auger electron spectroscopy (AES). The inset image shows the atomic concentration as a function of sputter depth. The concentration of Si increases sharply, and the Au is close to zero with increasing sputter depth. For comparison, a damage spot is generated by a single laser pulse on the silicon surface. The optical properties of the irradiated area on silicon (e.g., absorption coefficient and surface reflectivity) may have been changed [23,24]. When the pulse number increases to 10, the irradiated region on Au-coated silicon contains almost no gold, while a number of uniform tiny LIPSSs appear in the substrate, as shown in Fig. 2(b). Instead, there is still no obvious change on silicon [Fig. 2(h)]. At N 20, the LIPSSs are formed at the center of the irradiated area on Au-coated silicon. However, on silicon, the LIPSS can be obtained at the upper side of the irradiated area. With the increasing of the pulse number from 50 to 300, the LIPSSs on Au-coated silicon keep uniform; only the ablated area becomes larger. As a 1316
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Fig. 2. SEM images of the structures on Au-coated silicon and silicon induced by different laser pulse numbers. The laser fluence is 0.27 J∕cm2 . The polarization direction is labeled in (a).
comparison, LIPSSs with defects would be formed on silicon. Especially, at N 300, the quality of the LIPSSs on silicon seriously decline. Most of the adjacent LIPSS are joined up and a few of them are broken, indicating the quality degradation of the periodic structure. Besides, the thickness of Au film (d) is also an important factor for processing. Hence, the Au films with thicknesses of 10–30 nm have been investigated. Since the 20 nm film thickness is just larger than the optical penetration depth (∼13 nm) [25,26],
Fig. 3. AES analysis of atomic concentration (Au, Si, C, O) on the surface of Au-coated silicon.
Table 1. Atomic Concentration (Au, Si, C, O) on the Surface of Au-coated Silicon with Increasing Pulse Number
Atomic Concentration %
Au Si C O
N1
N5
N 10
8.4 13.8 46.6 30.3
5.5 14.0 60.0 19.0
0 18.1 58.8 22.8
it can avoid the negative effect of a nonuniform laser coupling field caused by direct excitation of silicon and weaken the effects of thermal damage, namely melting and vaporization, which will occur with increasing film thickness [27,28]. The initialization of LIPSSs both on silicon and Au-coated silicon has been widely accepted to be explained by the interference between the incident laser and surface plasmons (SPs), and then the mechanism of grating assisted SP-laser coupling dominates the evolution of LIPSSs [11]. For silicon, under irradiation by a Gaussian beam, the laser energy fluctuations and inhomogeneous distribution along the radial direction of the irradiated area might cause nonuniformity of the LIPSS period [29,30], which would be further enlarged by grating assisted SP-laser coupling. As shown in Fig. 2(j), the period of LIPSS at the edge of the irradiated area (∼550 nm) is smaller than at the middle part (∼640 nm). Besides, the intrinsic defects in silicon might lead to the structures initiating from multiple seed locations concurrently and independently [17]. Therefore, silicon is much more sensitive to the changes in intrinsic defects and irradiation properties. In this paper, as a benefit of gold, it is much easier to fabricate large-area, uniform LIPSSs on a gold-coated material surface. The possible mechanism of gold-film coating assisted laser fabrication is discussed as
Fig. 4. SEM image of femtosecond laser scanning treated surfaces of Au-coated silicon. The scanning speed is 100 μm/s, and the scanning interval D is 8 μm. The laser fluence is 0.27 J∕cm2 .
follows. In the femtosecond time domain, the distribution of electron temperature at the interface between Au film and silicon layer becomes uniform owing to the high thermal conductivity of gold film [31,32]. The diffusion of hot electrons in the Au films increases the interfacial carrier densities and enhances the interfacial electron–phonon coupling [32,33]. High and uniform electron density on the surface of Aucoated silicon makes the coupling field between laser radiation and surface plasmons enhanced and more uniform. It may further suppress the influence of localized ionization caused by the defects on silicon, which makes the periodic structures with fewer crosslinks much easier to process. Moreover, the excited electron density (ne ) is closely related to the LIPSS period [11,34]. The electron density of gold film is much higher than the excited electron density of silicon so that it can reduce the impact of inhomogeneous deposition of laser energy on silicon. Therefore, plating gold on silicon is beneficial to the formation of an initial uniform LIPSS, which will play an important role for the formation of final uniform LIPSS [35]. After 5 to 10 pulses, the gold layer disappears. When the initial LIPSSs are formed, grating assisted SP–laser coupling will play an important role for the formation and evolution of the latter ones [11]. The effect of grating assisted SP–laser coupling generated by the initial LIPSS effectively influences the interference mechanism of the incident laser with SPs. For this study, plating gold on silicon can make the initial LIPSS more uniform so that the structures formed by grating coupling evolve toward a more uniform direction. Since the coated Au film effectively suppresses the disadvantages of defects and energy fluctuations on the uniformity of structures, multiple pulses can further enhance the above structures by an incubation effect. As a consequence, uniform and stable surface periodic structures would be formed owing to the SPP and the subsequent gratingassisted SP–laser coupling [11]. The image of the scanning area induced by femtosecond lasers on Au-coated silicon is shown in Fig. 4 (F 0.27 J∕cm2 , v 100 μm∕s). The interval is 8 μm so that two adjacent scanning lines can be slightly overlapped with each other in order to form large-area, uniform periodic structures. The newly developed LIPSSs are oriented well to the old ones so that the LIPSSs induced by two separate laser scannings can be coherently linked. It is concluded that better coherence between the scanning lines can be obtained on an Au-coated silicon surface. The cross-links and broken structures have been greatly reduced between the lines. Therefore, large-area uniform periodic surface structures can be much easier produced on an Au-coated silicon surface by a simple scanning technique with appropriate irradiation conditions. 4. Conclusion
Large-area, uniform periodic surface structures have been fabricated by a femtosecond laser on Au-coated 20 February 2015 / Vol. 54, No. 6 / APPLIED OPTICS
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silicon. Compared with silicon, the structures formed on Au-coated silicon have stable periods, much fewer cross-links, and better coherence between the scanning lines. Ultimately, a large area with uniform periodic structures is obtained on Au-coated silicon. The diffusion of hot electrons in the Au films increases carrier densities and enhances the interfacial electron–phonon coupling. High and uniform electron density suppresses the influence of localized ionization caused by the defects on silicon and further makes the coupling field more uniform, which reduces the impact of laser energy fluctuations due to the high nonlinearity of the energy absorption. Little fluctuations of absorbed energy would result in great variation of LIPSS structuring. In addition, a large-area, uniform periodic structure formed by plating gold on a material surface would exhibit great application potential, which opens a new road in nanoscience and technology. This research is supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 51025521 and 51105037) and the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20111101120010). References 1. J. Bonse and J. Krüger, “Pulse number dependence of laserinduced periodic surface structures for femtosecond laser irradiation of silicon,” J. Appl. Phys. 108, 034903 (2010). 2. J. Vincenc Oboňa, J. Z. P. Skolski, G. R. B. E. Römer, and A. J. Huis in `t Veld, “Pulse-analysis-pulse investigation of femtosecond laser-induced periodic surface structures on silicon in air,” Opt. Express 22, 9254–9261 (2014). 3. Y. Li, F. Liu, Y. F. Li, L. Chai, Q. R. Xing, M. L. Hu, and C. Y. Wang, “Experimental study on GaP surface damage threshold induced by a high repetition rate femtosecond laser,” Appl. Opt. 50, 1958–1962 (2011). 4. F. Liang, R. Vallée, and S. L. Chin, “Pulse fluence dependent nanograting inscription on the surface of fused silica,” Appl. Phys. Lett. 100, 251105 (2012). 5. P. J. Liu, L. Jiang, J. Hu, W. N. Han, and Y. F. Lu, “Direct writing anisotropy on crystalline silicon surface by linearly polarized femtosecond laser,” Opt. Lett. 38, 1969–1971 (2013). 6. Y. P. Yuan, L. Jiang, X. Li, C. Wang, L. Yuan, L. T. Qu, and Y. F. Lu, “Adjustments of dielectrics craters and their surfaces by ultrafast laser pulse train based on localized electron dynamics control,” Appl. Opt. 52, 4035–4041 (2013). 7. M. Y. Shen, J. E. Carey, C. H. Crouch, M. Kandyla, H. A. Stone, and E. Mazur, “High-density regular arrays of nanometerscale rods formed on silicon surfaces via femtosecond laser irradiation in water,” Nano Lett. 8, 2087–2091 (2008). 8. M. Ulmeanu, F. Jipa, C. Radu, M. Enculescu, and M. Zamfirescu, “Large scale microstructuring on silicon surface in air and liquid by femtosecond laser pulses,” Appl. Surf. Sci. 258, 9314–9317 (2012). 9. G. L. Deng, G. Y. Feng, K. Liu, and S. H. Zhou, “Temperature dependence of laser-induced micro/nanostructures for femtosecond laser irradiation of silicon,” Appl. Opt. 53, 3004–3009 (2014). 10. S. Höhm, A. Rosenfeld, J. Krüger, and J. Bonse, “Area dependence of femtosecond laser-induced periodic surface structures for varying band gap materials after double pulse excitation,” Appl. Surf. Sci. 278, 7–12 (2013). 11. M. Huang, F. L. Zhao, Y. Cheng, N. S. Xu, and Z. Z. Xu, “Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incident laser,” ACS Nano 3, 4062–4070 (2009). 1318
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12. A. Y. Vorobyev and C. L. Guo, “Femtosecond laser-induced periodic surface structure formation on tungsten,” J. Appl. Phys. 104, 063523 (2008). 13. M. Huang, Y. Cheng, F. L. Zhao, and Z. Z. Xu, “The significant role of plasmonic effects in femtosecond laser-induced grating fabrication on the nanoscale,” Ann. Phys. (Berlin) 525, 74–86 (2013). 14. J. Reif, F. Costache, M. Henyk, and S. V. Pandelov, “Ripples revisited: non-classical morphology at the bottom of femtosecond laser ablation craters in transparent dielectrics,” Appl. Surf. Sci. 197-198, 891–895 (2002). 15. X. Du, H. Zhang, C. Cheng, S. F. Zhou, F. T. Zhang, Y. Z. Yu, G. P. Dong, and J. R. Qiu, “Space-selective precipitation of ZnO crystals in glass by using high repetition rate femtosecond laser irradiation,” Opt. Express 22, 17908–17914 (2014). 16. L. Hong, Rusli, X. C. Wang, H. Y. Zheng, H. Wang, and H. Y. Yu, “Femtosecond laser fabrication of large-area periodic surface ripple structure on Si substrate,” Appl. Surf. Sci. 297, 134–138 (2014). 17. B. Oktem, I. Pavlov, S. Ilday, H. Kalaycıoğlu, A. Rybak, S. Yavas, M. Erdoğan, and F. Ö. Ilday, “Nonlinear laser lithography for indefinitely large-area nanostructuring with femtosecond pulses,” Nat. Photonics 7, 897–901 (2013). 18. A. Ruiz de la Cruz, R. Lahoz, J. Siegel, G. F. de la Fuente, and J. Solis, “High speed inscription of uniform, large-area laserinduced periodic surface structures in Cr films using a high repetition rate fs laser,” Opt. Lett. 39, 2491–2494 (2014). 19. A. Pan, B. Gao, T. Chen, J. H. Si, C. X. Li, F. Chen, and X. Hou, “Fabrication of concave spherical microlenses on silicon by femtosecond laser irradiation and mixed acid etching,” Opt. Express 22, 15245–15250 (2014). 20. T. Ishikawa, S. Kohmoto, and K. Asakawa, “Site control of selforganized InAs dots on GaAs substrates by in situ electronbeam lithography and molecular-beam epitaxy,” Appl. Phys. Lett. 73, 1712–1714 (1998). 21. K. V. Sreekanth, J. K. Chua, and V. M. Murukeshan, “Interferometric lithography for nanoscale feature patterning: a comparative analysis between laser interference, evanescent wave interference, and surface plasmon interference,” Appl. Opt. 49, 6710–6717 (2010). 22. J. M. Liu, “Simple technique for measurements of pulsed Gaussian-beam spot sizes,” Opt. Lett. 7, 196–198 (1982). 23. J. Bonse, K. W. Brzezinka, and A. J. Meixner, “Modifying singlecrystalline silicon by femtosecond laser pulses: an analysis by micro Raman spectroscopy, scanning laser microscopy and atomic force microscopy,” Appl. Surf. Sci. 221, 215–230 (2004). 24. J. Bonse, S. Baudach, J. Krüger, W. Kautek, and M. Lenzner, “Femtosecond laser ablation of silicon-modification thresholds and morphology,” Appl. Phys. A 74, 19–25 (2002). 25. M. Nicoul, U. Shymanovich, A. Tarasevitch, D. V. D. Linde, and K. Sokolowski-Tinten, “Picosecond acoustic response of a laser-heated gold-film studied with time-resolved x-ray diffraction,” Appl. Phys. Lett. 98, 191902 (2011). 26. J. Hohlfeld, J. G. Müller, S.-S. Wellershoff, and E. Matthias, “Time-resolved thermoreflectivity of thin gold films and its dependence on film thickness,” Appl. Phys. B 64, 387–390 (1997). 27. M. D. Qian, Y. Dai, X. N. Yan, and G. H. Ma, “Formation of nanostructures by an intense femtosecond pulse laser irradiating Au film on sapphire substrate,” Appl. Phys. A 114, 1031–1037 (2014). 28. J. Krüger, D. Dufft, R. Koter, and A. Hertwig, “Femtosecond laser-induced damage of gold films,” Appl. Surf. Sci. 253, 7815–7819 (2007). 29. S. K. Das, H. Messaoudi, A. Debroy, E. McGlynn, and R. Grunwald, “Multiphoton excitation of surface plasmon polaritons and scaling of nanoripple formation in large bandgap materials,” Opt. Mater. Express 3, 1705–1715 (2013). 30. G. D. Tsibidis, M. Barberoglou, P. A. Loukakos, E. Stratakis, and C. Fotakis, “Dynamics of ripple formation on silicon surfaces by ultrashort laser pulses in subablation conditions,” Phys. Rev. B 86, 115316 (2012). 31. D. S. Ivanov and L. V. Zhigilei, “Combined atomisticcontinuum modeling of short-pulse laser melting and disintegration,” Phys. Rev. B 68, 064114 (2003).
32. J. B. Lee and S. H. Lee, “Thermal boundary resistance effect on non-equilibrium energy transport in metal–dielectric thin films heated by femtosecond pulse lasers,” Mater. Trans. 52, 1492–1499 (2011). 33. Y. Wang, X. Ruan, and A. K. Roy, “Two-temperature nonequilibrium molecular dynamics simulation of thermal transport across metal–nonmetal interfaces,” Phys. Rev. B 85, 205311 (2012).
34. Y. Shimotsuma, P. G. Kazansky, J. R. Qiu, and K. Hirao, “Selforganized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. L 91, 247405 (2003). 35. X. Jia, T. Q. Jia, N. N. Peng, D. H. Feng, S. A. Zhang, and Z. R. Sun, “Dynamics of femtosecond laser-induced periodic surface structures on silicon by high spatial and temporal resolution imaging,” J. Appl. Phys. 115, 143102 (2014).
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A simple, repeatable approach is proposed to fabricate large-area, uniform periodic surface structures by a femtosecond laser. 20 nm gold films are coated on semiconductor surfaces on which large-area, uniform structures are fabricated. In the case study of silicon, cross-links and broken structures of laser induced periodic surface structures (LIPSSs) are significantly reduced on Au-coated silicon. The good consistency between the scanning lines facilitates the formation of large-area, uniform LIPSSs. The diffusion of hot electrons in the Au films increases the interfacial carrier densities, which significantly enhances interfacial electron–phonon coupling. High and uniform electron density suppresses the influence of defects on the silicon and further makes the coupling field more uniform and thus reduces the impact of laser energy fluctuations, which homogenizes and stabilizes large-area LIPSSs. © 2015 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (320.2250) Femtosecond phenomena; (220.4241) Nanostructure fabrication. http://dx.doi.org/10.1364/AO.54.001314
1. Introduction
In recent years, femtosecond laser induced periodic surface structures (LIPSSs, also called ripples) have been widely accepted as a natural phenomenon. People mainly pay attention to study LIPSS formation through adjusting laser parameters (pulse number [1,2], laser fluence [3,4], polarization [5], or double pulse delay [6]) or changing processing environments (liquid [7,8], temperature [9]). The materialdependent formation mechanisms of LIPSSs, such as excitation of surface plasmon polaritons (SPPs) [10–12], effect of grating-assisted surface plasmonlaser coupling [13], self-organization [14], and second harmonic generation [15], have been proposed. However, fabrication of large-area LIPSSs with uniformity and repeatability remains the research hotspot and challenge [16]. Recently, a lot of 1559-128X/15/061314-06$15.00/0 © 2015 Optical Society of America 1314
APPLIED OPTICS / Vol. 54, No. 6 / 20 February 2015
experiments and simulations have been conducted to fabricate large-area, uniform LIPSSs. For example, Oktem et al. demonstrated a simple, low-cost method based on exploiting naturally occurring feedback mechanisms for the creation of laser-induced metaloxide nanostructures with unprecedented uniformity [17]. And de la Cruz et al. pointed out that uniform, large-area LIPSSs could be created on Cr film by a high repetition rate femtosecond laser [18]. Moreover, a variety of methods, such as laser-induced chemical etching [19], electron-beam lithography [20], and laser interference lithography [21], have been established to process large-area, uniform LIPSSs on various materials. Nevertheless, those methods are either complicated or costly. Therefore, a new efficient method is desperately needed to fabricate large-area, uniform LIPSSs. In this paper, a simple, efficient, and repeatable approach, that is, plating 20 nm gold films on silicon surfaces, is proposed to fabricate large-area, uniform LIPSSs by radiation of a femtosecond laser.
Cross-links and broken structures are significantly reduced, and good consistency between the scanning lines is found in the laser induced surface structures on Au-coated silicon. The proposed approach might have potential applications in absorption enhancement of material surfaces, high surface-enhanced Raman scattering, and generation of hydrophobic surfaces. 2. Experiment Setup
A regenerative Ti:sapphire laser system (800 nm, 35 fs, 1 kHz) is used in the experiment to fabricate large-area, uniform periodic surface structures in air at ambient temperature and pressure. The p-polarized laser radiation is focused perpendicular to the substrate using an achromatic doublet (f 100 mm). The diameter of the Gaussian beam is measured as ∼20 μm, which is determined using a method by Liu [22]. The initial pulse duration of ∼35 fs is stretched to ∼50 fs (measured by an autocorrelator) after passing a series of lenses. The sample is mounted on a computer-controlled, six-axis translation stage (M-840.5DG, PI, Inc.) with a
positioning accuracy of 1 μm in the x and y directions and 0.5 μm in the z direction. The laser fluence is accurately adjusted by the combination of an achromatic half-wave plate and a linear polarizer. The gold film is coated on the silicon substrate (10 mm × 10 mm × 0.5 mm) by a magnetron sputtering method, and the thickness can be accurately controlled by the magnetron sputtering time. The surface roughness of the Au-coated silicon and silicon is less than 1 nm (Ra < 1 nm). The surface morphologies after laser processing are imaged by scanning electron microscopy (SEM) and atomic force microscopy (AFM). 3. Results and Discussion
As shown in Fig. 1, the morphologies of the scanning lines are carefully studied on Au-coated silicon [Figs. 1(a1), 1(a2), and 1(a3)] and on silicon [Figs. 1(b1), 1(b2), and 1(b3)]. The periods of LIPSSs are ∼600 nm, and the orientations are perpendicular to the laser polarization. The fluence is varied between 0.18 and 0.27 J∕cm2 . The scanning speed (v) is 100 μm∕s, and the effective number of pulses per spot is ∼200.
Fig. 1. SEM images and AFM profiles of the surface morphology written on samples. (a1), (a2), and (a3) are written on Au-coated silicon, and (b1), (b2), and (b3) are written on silicon. The laser fluences are 0.27, 0.23, and 0.18 J∕cm2 , as labeled on the images. (c) and (d) illustrate AFM profiles of the surface morphology on Au-coated silicon and silicon at 0.27 J∕cm2 , respectively. The scanning speed is 100 μm∕s, and the effective number of pulses per spot is ∼200. The polarization direction is labeled in (a1). 20 February 2015 / Vol. 54, No. 6 / APPLIED OPTICS
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For example, when the laser fluence is 0.23 J∕cm2 , it is found that well-defined LIPSSs on silicon only exist in part of the region, and some of the adjacent LIPSSs are joined up, which is defined as cross-links [Fig. (b2)]. And some of the LIPSS are broken, which refers to some holes or bifurcations that interrupt the continuity of LIPSS. Those defects account for almost half of the whole LIPSS covered area. For comparison, there are no cross-links on Au-coated silicon [Fig. (a2)]. Furthermore, the boundary between the LIPSS covered area and the unprocessed area on Au-coated silicon is clear, whereas, on silicon, the edge of scanning line is wavy, which is unfavorable to uniform, large-area LIPSS processing. Relatively uniform periodic structures can be processed on silicon as the laser fluence changes between 0.18 and 0.27 J∕cm2 . The cross-links on silicon are also more than those on Au-coated silicon. The experiments with each material are repeated at least five times in our experiments. Moreover, AFM is used in order to gain a deep insight into the information about the spatial characteristics. As shown in Figs. 1(c) and 1(d), when the laser fluence is 0.27 J∕cm2 , compared with silicon, the LIPSSs on Au-coated silicon have clear peaks, valleys, and stable periods. In contrast, the LIPSSs on silicon fluctuate severely, which is mainly due to the existence of cross-links and broken structures. Accordingly, it is much easier to form large-area, uniform nanostructures on Au-coated silicon than on silicon. In order to accurately analyze the formation process on Au-coated silicon and silicon, the evolution of the surface morphology is observed after laser irradiation. Figure 2 shows the SEM images of the irradiated areas on Au-coated silicon and silicon. The laser pulse number N increases from 1 to 300, which is controlled externally by a fast mechanical shutter synchronized with the laser repetition rate. At N 1, the Au layer is ablated, leading to exposure of the substrate. As shown in Fig. 3 and Table 1, the concentration of Au is 8.4%, which is analyzed by auger electron spectroscopy (AES). The inset image shows the atomic concentration as a function of sputter depth. The concentration of Si increases sharply, and the Au is close to zero with increasing sputter depth. For comparison, a damage spot is generated by a single laser pulse on the silicon surface. The optical properties of the irradiated area on silicon (e.g., absorption coefficient and surface reflectivity) may have been changed [23,24]. When the pulse number increases to 10, the irradiated region on Au-coated silicon contains almost no gold, while a number of uniform tiny LIPSSs appear in the substrate, as shown in Fig. 2(b). Instead, there is still no obvious change on silicon [Fig. 2(h)]. At N 20, the LIPSSs are formed at the center of the irradiated area on Au-coated silicon. However, on silicon, the LIPSS can be obtained at the upper side of the irradiated area. With the increasing of the pulse number from 50 to 300, the LIPSSs on Au-coated silicon keep uniform; only the ablated area becomes larger. As a 1316
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Fig. 2. SEM images of the structures on Au-coated silicon and silicon induced by different laser pulse numbers. The laser fluence is 0.27 J∕cm2 . The polarization direction is labeled in (a).
comparison, LIPSSs with defects would be formed on silicon. Especially, at N 300, the quality of the LIPSSs on silicon seriously decline. Most of the adjacent LIPSS are joined up and a few of them are broken, indicating the quality degradation of the periodic structure. Besides, the thickness of Au film (d) is also an important factor for processing. Hence, the Au films with thicknesses of 10–30 nm have been investigated. Since the 20 nm film thickness is just larger than the optical penetration depth (∼13 nm) [25,26],
Fig. 3. AES analysis of atomic concentration (Au, Si, C, O) on the surface of Au-coated silicon.
Table 1. Atomic Concentration (Au, Si, C, O) on the Surface of Au-coated Silicon with Increasing Pulse Number
Atomic Concentration %
Au Si C O
N1
N5
N 10
8.4 13.8 46.6 30.3
5.5 14.0 60.0 19.0
0 18.1 58.8 22.8
it can avoid the negative effect of a nonuniform laser coupling field caused by direct excitation of silicon and weaken the effects of thermal damage, namely melting and vaporization, which will occur with increasing film thickness [27,28]. The initialization of LIPSSs both on silicon and Au-coated silicon has been widely accepted to be explained by the interference between the incident laser and surface plasmons (SPs), and then the mechanism of grating assisted SP-laser coupling dominates the evolution of LIPSSs [11]. For silicon, under irradiation by a Gaussian beam, the laser energy fluctuations and inhomogeneous distribution along the radial direction of the irradiated area might cause nonuniformity of the LIPSS period [29,30], which would be further enlarged by grating assisted SP-laser coupling. As shown in Fig. 2(j), the period of LIPSS at the edge of the irradiated area (∼550 nm) is smaller than at the middle part (∼640 nm). Besides, the intrinsic defects in silicon might lead to the structures initiating from multiple seed locations concurrently and independently [17]. Therefore, silicon is much more sensitive to the changes in intrinsic defects and irradiation properties. In this paper, as a benefit of gold, it is much easier to fabricate large-area, uniform LIPSSs on a gold-coated material surface. The possible mechanism of gold-film coating assisted laser fabrication is discussed as
Fig. 4. SEM image of femtosecond laser scanning treated surfaces of Au-coated silicon. The scanning speed is 100 μm/s, and the scanning interval D is 8 μm. The laser fluence is 0.27 J∕cm2 .
follows. In the femtosecond time domain, the distribution of electron temperature at the interface between Au film and silicon layer becomes uniform owing to the high thermal conductivity of gold film [31,32]. The diffusion of hot electrons in the Au films increases the interfacial carrier densities and enhances the interfacial electron–phonon coupling [32,33]. High and uniform electron density on the surface of Aucoated silicon makes the coupling field between laser radiation and surface plasmons enhanced and more uniform. It may further suppress the influence of localized ionization caused by the defects on silicon, which makes the periodic structures with fewer crosslinks much easier to process. Moreover, the excited electron density (ne ) is closely related to the LIPSS period [11,34]. The electron density of gold film is much higher than the excited electron density of silicon so that it can reduce the impact of inhomogeneous deposition of laser energy on silicon. Therefore, plating gold on silicon is beneficial to the formation of an initial uniform LIPSS, which will play an important role for the formation of final uniform LIPSS [35]. After 5 to 10 pulses, the gold layer disappears. When the initial LIPSSs are formed, grating assisted SP–laser coupling will play an important role for the formation and evolution of the latter ones [11]. The effect of grating assisted SP–laser coupling generated by the initial LIPSS effectively influences the interference mechanism of the incident laser with SPs. For this study, plating gold on silicon can make the initial LIPSS more uniform so that the structures formed by grating coupling evolve toward a more uniform direction. Since the coated Au film effectively suppresses the disadvantages of defects and energy fluctuations on the uniformity of structures, multiple pulses can further enhance the above structures by an incubation effect. As a consequence, uniform and stable surface periodic structures would be formed owing to the SPP and the subsequent gratingassisted SP–laser coupling [11]. The image of the scanning area induced by femtosecond lasers on Au-coated silicon is shown in Fig. 4 (F 0.27 J∕cm2 , v 100 μm∕s). The interval is 8 μm so that two adjacent scanning lines can be slightly overlapped with each other in order to form large-area, uniform periodic structures. The newly developed LIPSSs are oriented well to the old ones so that the LIPSSs induced by two separate laser scannings can be coherently linked. It is concluded that better coherence between the scanning lines can be obtained on an Au-coated silicon surface. The cross-links and broken structures have been greatly reduced between the lines. Therefore, large-area uniform periodic surface structures can be much easier produced on an Au-coated silicon surface by a simple scanning technique with appropriate irradiation conditions. 4. Conclusion
Large-area, uniform periodic surface structures have been fabricated by a femtosecond laser on Au-coated 20 February 2015 / Vol. 54, No. 6 / APPLIED OPTICS
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silicon. Compared with silicon, the structures formed on Au-coated silicon have stable periods, much fewer cross-links, and better coherence between the scanning lines. Ultimately, a large area with uniform periodic structures is obtained on Au-coated silicon. The diffusion of hot electrons in the Au films increases carrier densities and enhances the interfacial electron–phonon coupling. High and uniform electron density suppresses the influence of localized ionization caused by the defects on silicon and further makes the coupling field more uniform, which reduces the impact of laser energy fluctuations due to the high nonlinearity of the energy absorption. Little fluctuations of absorbed energy would result in great variation of LIPSS structuring. In addition, a large-area, uniform periodic structure formed by plating gold on a material surface would exhibit great application potential, which opens a new road in nanoscience and technology. This research is supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 51025521 and 51105037) and the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20111101120010). References 1. J. Bonse and J. Krüger, “Pulse number dependence of laserinduced periodic surface structures for femtosecond laser irradiation of silicon,” J. Appl. Phys. 108, 034903 (2010). 2. J. Vincenc Oboňa, J. Z. P. Skolski, G. R. B. E. Römer, and A. J. Huis in `t Veld, “Pulse-analysis-pulse investigation of femtosecond laser-induced periodic surface structures on silicon in air,” Opt. Express 22, 9254–9261 (2014). 3. Y. Li, F. Liu, Y. F. Li, L. Chai, Q. R. Xing, M. L. Hu, and C. Y. Wang, “Experimental study on GaP surface damage threshold induced by a high repetition rate femtosecond laser,” Appl. Opt. 50, 1958–1962 (2011). 4. F. Liang, R. Vallée, and S. L. Chin, “Pulse fluence dependent nanograting inscription on the surface of fused silica,” Appl. Phys. Lett. 100, 251105 (2012). 5. P. J. Liu, L. Jiang, J. Hu, W. N. Han, and Y. F. Lu, “Direct writing anisotropy on crystalline silicon surface by linearly polarized femtosecond laser,” Opt. Lett. 38, 1969–1971 (2013). 6. Y. P. Yuan, L. Jiang, X. Li, C. Wang, L. Yuan, L. T. Qu, and Y. F. Lu, “Adjustments of dielectrics craters and their surfaces by ultrafast laser pulse train based on localized electron dynamics control,” Appl. Opt. 52, 4035–4041 (2013). 7. M. Y. Shen, J. E. Carey, C. H. Crouch, M. Kandyla, H. A. Stone, and E. Mazur, “High-density regular arrays of nanometerscale rods formed on silicon surfaces via femtosecond laser irradiation in water,” Nano Lett. 8, 2087–2091 (2008). 8. M. Ulmeanu, F. Jipa, C. Radu, M. Enculescu, and M. Zamfirescu, “Large scale microstructuring on silicon surface in air and liquid by femtosecond laser pulses,” Appl. Surf. Sci. 258, 9314–9317 (2012). 9. G. L. Deng, G. Y. Feng, K. Liu, and S. H. Zhou, “Temperature dependence of laser-induced micro/nanostructures for femtosecond laser irradiation of silicon,” Appl. Opt. 53, 3004–3009 (2014). 10. S. Höhm, A. Rosenfeld, J. Krüger, and J. Bonse, “Area dependence of femtosecond laser-induced periodic surface structures for varying band gap materials after double pulse excitation,” Appl. Surf. Sci. 278, 7–12 (2013). 11. M. Huang, F. L. Zhao, Y. Cheng, N. S. Xu, and Z. Z. Xu, “Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incident laser,” ACS Nano 3, 4062–4070 (2009). 1318
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