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nanostructuring of titanium under stationary and non-stationary femtosecond laser irradiation
Applied Surface Science 255 (2009) 7556–7560
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface micro/nanostructuring of titanium under stationary and non-stationary femtosecond laser irradiation V. Oliveira a,*, S. Ausset b, R. Vilar b a b
Instituto Superior Engenharia Lisboa, Rua Conselheiro Emı´dio Navarro no. 1, 1959-007 Lisboa, Portugal Dep. Engenharia Materiais, Instituto Superior Te´cnico, Av. Rovisco Pais no. 1, 1049-001 Lisboa, Portugal
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 February 2009 Received in revised form 24 March 2009 Accepted 8 April 2009 Available online 16 April 2009
In this paper the surface topography of titanium samples irradiated by femtosecond laser pulses is described. When the fluence is about 0.5 J/cm2 periodic ripples with a period of about 700 nm are formed. For fluences between 0.5 and 2 J/cm2, a microcolumnar surface texture develops in the center of the irradiated spots and ripples are formed in the periphery of the spots. When experiments are performed with a non-stationary sample, the microcolumns exhibit ripples similar to those observed when the radiation fluence is about 0.5 J/cm2 and in the outer regions of the irradiated areas for fluences between 0.5 and 2 J/cm2. Since the energy distribution in the transverse cross-section of the laser beam is Gaussian, we conclude that the ripples form when the microcolumns are subjected to fluences near the melting threshold of the material at the trailing edge of the moving laser beam. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Titanium Femtosecond Laser Nanostructuring
1. Introduction Materials irradiated with femtosecond laser pulses often exhibit surface textures characterized by a pattern of sharp microcolumns [1–14]. The morphology of these microcolumns depends on the laser processing parameters, including the number of incident laser pulses, radiation fluence, wavelength, pulse duration, and the ambient atmosphere composition and pressure. Over the last decade, most of the work carried out in this field has been performed in silicon [1–10], due to the importance of this material in the microelectronics industry. Besides the scientific interest of these studies, laser microstructuring of silicon also yield considerable properties improvement, such as increased optical absorption coefficient [4–6], low-threshold field-emission [7] and improved wettability [8,9]. The formation of micrometer-sized columns on titanium surfaces irradiated with Ti:Sapphire femtosecond laser pulses has been recently reported by Tsukamoto et al. [13], Nayak et al. [12] and Vorobyev and Guo [14]. This microtexture may yield interesting properties for example for biomedical applications, since surface texturing of titanium implants enhance osseointegration and the strength of the bone/implant interface [15]. Tsukamoto et al., Nayak et al., and Vorobyev and Guo used radiation with the same wavelength and the same fluence range, but the microcolumns observed by Nayak et al. exhibit a 700 nm
* Corresponding author. E-mail address: [email protected] (V. Oliveira). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.04.027
wavelength ripple pattern overlapped to the columns surface, which was not observed by Tsukamoto et al. and Vorobyev and Guo. Since the first authors carried out their experiments with a stationary sample whereas the other authors used a nonstationary one, it seems that the laser processing method may affect the surface texture of the material. The aim of the present work was to investigate the conditions of formation of the ripple pattern. The results obtained show that the ripple pattern forms for a limited fluence range near the melting threshold. As a result, they form when the sample is stationary for fluences in this range or when the sample is non-stationary and the microcolumns subjected to fluences near the melting threshold of the material at the trailing edge of the moving laser beam. 2. Experimental A commercial Yb:KYW chirped-pulse-regenerative amplification laser system is used to provide linear polarized lasers pulses with a duration of about 500 fs at a central wavelength of 1030 nm. Laser processing was performed in air on polished grade 2 titanium samples, a material typically used in low load bearing medical devices. The laser beam was perpendicular to the sample surface and focused using a 100 mm focal-length lens. The radiation fluence was calculated on the basis of the spot size and average pulse energy. The samples were mounted on a computer controlled XY stage and experiments were carried out by translating the sample relatively to the stationary laser beam with scanning velocities (v) varying between 0 and 500 mm/s or with a stationary sample (v = 0). In the latter experiments the pulse repetition rate
V. Oliveira et al. / Applied Surface Science 255 (2009) 7556–7560
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Fig. 1. SEM micrographs of titanium surfaces irradiated with 1 J/cm2 and: (a) 1; (b) 5; (c) 10; (d) 20; (e) 50; (f) 75; (g) 100; (h) 150; (i) 200; (j) 500; (k) 750 and (l) 1000 laser pulses.
was varied between 1 and 50 Hz whereas in non-stationary experiments, the pulse repetition rate was constant (50 Hz). After the laser treatment the surface topography was characterized by field emission scanning electron microscopy (SEM). 3. Results Under stationary irradiation, a microcolumnar surface texture forms in titanium targets for fluences between 0.5 and 2 J/cm2. The evolution of the surface topography treated with an average fluence of 1 J/cm2 with increasing number of laser pulses is depicted in Fig. 1. The first five pulses lead to the formation of surface pits randomly distributed (Fig. 1a and b). Between 5 and 20 pulses (Fig. 1c and d), these defects evolve to small depressions and hillocks, and after 50 pulses (Fig. 1e) the surface presents a wavy topography with an average wavelength much larger than the incident radiation wavelength. With increasing number of pulses the waves transform into microcolumns, whose tips protrude well above the initial surface level (Fig. 1f and g). the microcolumns
reach their maximum height after about 200 pulses (Fig. 1h) and do not change significantly for increasing number of pulses (Fig. 1i–l). A detailed inspection of the outer region of the irradiated areas revealed ripples with a spatial periodicity of about 700 nm, irrespectively of the number of pulses (Fig. 2). Similar ripples are observed all over the irradiated areas when the fluence exceeds the melting threshold of the material and is lower than the microcolumns formation threshold (0.5 J/cm2). Fig. 3 depicts SEM micrographs of titanium surfaces processed with a fluence of 1 J/cm2 in scanning regime. The Fig. 3a and b corresponds to a sample processed at a scanning speed of 10 mm/s, while Fig. 3c and d concern a sample processed at 50 mm/s. These scanning speeds were chosen because they are equivalent to a total input energy per unit area of the sample surface similar to that obtained when processing is carried out in single spots with 500 and 100 laser pulses. In general, a surface texture consisting of microcolumns similar to those generated in stationary irradiation is observed: at 50 mm/s, the columns are tall and protrude above the sample surface level, while for 10 mm/s the columns are
Fig. 2. SEM micrographs of ripples formed in the periphery of the irradiated regions after: (a) 50 and (b) 100 pulses.
V. Oliveira et al. / Applied Surface Science 255 (2009) 7556–7560
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Fig. 3. SEM micrographs of titanium surfaces irradiated with 1 J/cm2 and scanning speeds of (a), (b), (e) and (f) 10 mm/s; (c) and (d) 50 mm/s.
broader and their tips remain below the initial surface. Nevertheless, the columns formed at 10 mm/s present a peculiar characteristic which is not observed at 50 mm/s or in stationary irradiation: the surface is covered with ripples similar to those observed in the outer regions of the irradiated areas (Figs. 2 and 3e). Furthermore, this characteristic is not observed on the microcolumns formed immediately before the end of the scan (Fig. 3f). 4. Discussion Periodic ripples similar to those observed in Fig. 2 have been previously observed in samples of silicon [16,17], titanium [12,13] and other metals [18] processed with femtosecond laser radiation at fluences just above the melting threshold of the material. The most consensual explanation for their formation is that they originate from an inhomogeneous energy distribution due to the interference of the incident laser beam with scattered radiation, for example, by microscopic asperities of the surface, propagating parallel to the surface [19,20]. The ripples formed by this mechanism are oriented perpendicularly to the electric field vector of the incident laser beam, and their period is approximately equal to the laser radiation wavelength [19,20]. In the present
Fig. 4. Plot of Eq. (1).
V. Oliveira et al. / Applied Surface Science 255 (2009) 7556–7560
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Fig. 5. SEM micrographs of microcolumns and ripples formed under different experimental conditions. Line 1 was scanned with 10 mm/s at a fluence of 1 J/cm2 and line 2 with 200 mm/s and 0.5 J/cm2. (a) General overview; (b) microcolumns in track 1; (c) periodic ripples in track 2; and (d) microcolumns in the intersection of track 1 and 2.
work, the ripples period is about 700 nm width is appreciably less than the laser radiation wavelength, 1030 nm. This discrepancy may be explained by an increase of the real part of the material refraction index due to the development of nanostructures during the first laser pulses [18]. Under stationary irradiation ripples are formed when the radiation fluence is lower than the threshold for columns formation and higher than the melting threshold. Since the laser beam spatial profile is Gaussian, this suggests that the ripples formed in the outer regions of the irradiated areas for fluences higher than the threshold for columns formation are consequence of the lower intensity of the laser beam in these regions. The fact that under non-stationary irradiation columns formed immediately before the end of a scan do not have surface ripples (Fig. 3f) suggest that the ripples are created after the formation of the microcolumns and not simultaneously with the column as previously suggested [12]. In fact, each point of the surface is subjected to varying laser fluences. For example, under irradiation along the x-direction with a Gaussian laser beam the laser fluence F at any point (x, y) can be expressed as: 2
F ¼ 2F 0 e2½ðxx0 vtÞ
þðyy0 Þ2 =r2
(1)
where the beam radius r is the distance from the beam axis where the optical intensity drops to 1/e2 of the value on the beam axis, F0 is the average laser fluence, v is the scanning speed, t the time, and x0 and y0 are the coordinates of the beam center at time t = 0. The previous expression can be used for a pulsed laser beam as long as the movement of the beam and the temporal variation of the laser fluence during the pulse duration can be neglected. Eq. (1) is plotted in Fig. 4 for a point P located at x = 1.5r and x0 = y0 = y = 0, r = 50 mm, F0 = 1 J/cm2, and using two scanning speeds, 10 and 50 mm/s, respectively. Since P is originally outside the irradiated area, the laser fluence at this point (FP) is initially zero. As the axis of the laser beam approaches P, FP increases and reaches the threshold fluence for microcolumns formation (0.5 J/cm2) after
0.7 s for 50 mm/s, and 3.3 s for 10 mm/s. At this point a wavy topography start to form which evolve to microcolumns with increasing time. When the laser beam axis coincides with P, FP reaches its maximum value, 2 J/cm2. On the contrary, as the beam is moving away from P, FP decreases and fall again below 0.5 J/cm2, after 2.8 s for 10 mm/s and 11.7 s for 50 mm/s. At this point, the microcolumns development is over but FP is still high enough to generate ripples. As a result, ripples will begin to form on the microcolumns surface. For 50 mm/s, FP falls quickly to 0 and the number of applied pulses during this period is not enough to allow ripples to develop. On the contrary, for 10 mm/s the number of applied pulses is five times higher and ripples form. To confirm the proposed mechanism of ripple formation on microcolumns the following experiment was performed. First, a linear scan was performed with a scanning speed of 10 mm/s at a fluence of 1 J/cm2 (track 1 in Fig. 5a). The microcolumns formed in the treated area present an overlapped ripple pattern perpendicular to the scanning direction (Fig. 5b). The sample was then rotated 908 and a second linear scan was made with a lower laser fluence (0.5 J/cm2) and higher scanning speed (200 mm/s) intersecting the first track (track 2 in Fig. 5a). In this second treated track, ripples are formed but not microcolumns (Fig. 5c). Since the sample was rotated 908 between the first and the second track, the ripples in the second track are perpendicular to those in the first track (Fig. 5b and c). In the region where the tracks intersect (Fig. 5d), ripples are overlapped to the microcolumns similarly to what was observed in track 1 (Fig. 5b). However, the ripples are now parallel and not perpendicular to track 1. This shows that in this region the ripples observed on the microcolumns were formed during the second linear scan and after the formation of the microcolumns. 5. Conclusions In conclusion, microcolumns are formed when titanium surfaces are irradiated with femtosecond laser pulses in stationary
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and non-stationary regimes. In contrast with the microcolumns formed in stationary irradiation, the microcolumns formed in nonstationary irradiation present a nanotexture consisting of periodic ripples with a period of about 700 nm. These ripples were also observed in the periphery of the irradiated spots whenever the radiation fluence is lower than the threshold fluence for microcolumns formation and larger than the melting threshold. Since the energy distribution in the transverse cross-section of the laser beam is Gaussian, we conclude that the ripples form when the microcolumns are subjected to fluences near the melting threshold of the material at the trailing edge of the moving laser beam. References [1] T.H. Her, R.J. Finlay, C. Wu, S. Deliwala, E. Mazur, Microstructuring of silicon with femtosecond laser pulses, Appl. Phys. Lett. 73 (1998) 1673. [2] M.Y. Shen, C.H. Crouch, J.E. Carey, E. Mazur, Femtosecond laser-induced formation of submicrometer spikes on silicon in water, Appl. Phys. Lett. 85 (2004) 5694. [3] E. Skantzakis, V. Zorba, D.G. Papazoglou, I. Zergioti, C. Fotakis, Ultraviolet laser microstructuring of silicon and the effect of laser pulse duration on the surface morphology, Appl. Surf. Sci. 252 (2006) 4462. [4] C. Wu, C.H. Crouch, L. Zhao, J.E. Carey, R. Younkin, J.A. Levinson, E. Mazur, R.M. Farrell, P. Gothoskar, A. Karger, Near-unity below-band-gap absorption by microstructured silicon, Appl. Phys. Lett. 78 (2001) 1850. [5] C.H. Crouch, J.E. Carey, M. Shen, E. Mazur, F.Y. Genin, Infrared absorption by sulfurdoped silicon formed by femtosecond laser irradiation, Appl. Phys. A 79 (2004) 1635. [6] R. Younkin, J.E. Carey, E. Mazur, J.A. Levinson, C.M. Friend, Infrared absorption by conical silicon microstructures made in a variety of background gases using femtosecond-laser pulses, J. Appl. Phys. 93 (2003) 2626. [7] V. Zorba, P. Tzanetakis, C. Fotakis, E. Spanakis, E. Stratakis, D.G. Papazoglou, I. Zergioti, Silicon electron emitters fabricated by ultraviolet laser pulses, Appl. Phys. Lett. 88 (2006) 081103.
[8] V. Zorba, L. Persano, D. Pisignano, A. Athanassiou, E. Stratakis, R. Cingolani, P. Tzanetakis, C. Fotakis, Making silicon hydrophobic: wettability control by twolengthscale simultaneous patterning with femtosecond laser irradiation, Nanotechnol. 17 (2006) 3234. [9] T. Baldacchini, J.E. Carey, M. Zhou, E. Mazur, Superhydrophobic surfaces prepared by microstructuring of silicon using a femtosecond laser, Langmuir 22 (2006) 4917. [10] J.T. Zhu, G. Yin, M. Zhao, D.Y. Chen, L. Zhao, Evolution of silicon surface microstructures by picosecond and femtosecond laser irradiations, Appl. Surf. Sci. 245 (2005) 102. [11] B.K. Nayak, M.C. Gupta, K.W. Kolasinski, Spontaneous formation of nanospiked microstructures in germanium by femtosecond laser irradiation, Nanotechnology 18 (2007) 195302. [12] B.K. Nayak, M.C. Gupta, K.W. Kolasinski, Formation of nano-textured conical microstructures in titanium metal surface by femtosecond laser irradiation, Appl. Phys. A 90 (2008) 399. [13] M. Tsukamoto, K. Asuka, H. Nakano, M. Hashida, M. Katto, N. Abe, M. Fujita, Periodic microstructures produced by femtosecond laser irradiation on titanium plate, Vacuum 80 (2006) 1346. [14] A.Y. Vorobyev, C. Guo, Femtosecond laser structuring of titanium implants, Appl. Surf. Sci. 253 (2007) 7272. [15] O. Zinger, G. Zhao, Z. Schwartz, J. Simpson, M. Wieland, D. Landolt, B. Boyan, Differential regulation of osteoblasts by substrate microstructural features, Biomater. 26 (2005) 1837. [16] B. Tan, K. Venkatakrishnan, A femtosecond laser-induced periodical surface structure on crystalline silicon, J. Micromech. Microeng. 16 (2006) 1080. [17] R. Le Harzic, H. Schuck, D. Sauer, T. Anhut, I. Riemann, K. Konig, Sub-100 nm nanostructuring of silicon by ultrashort laser pulses, Optics Express 13 (2005) 6651. [18] A.Y. Vorobyev, V.S. Makin, C.L. Guo, Periodic ordering of random surface nanostructures induced by femtosecond laser pulses on metals, J. Appl. Phys. 101 (2007) 034903. [19] J.E. Sipe, J.F. Young, J.S. Preston, H.M. Vandriel, Laser-induced periodic surface structure. I. Theory, Phys. Rev. B 27 (1983) 1141. [20] J.F. Young, J.S. Preston, H.M. Vandriel, J.E. Sipe, Laser-induced periodic surface structure. II. Experiments on Ge, Si, Al, and brass, Phys. Rev. B 27 (1983) 1155.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface micro/nanostructuring of titanium under stationary and non-stationary femtosecond laser irradiation V. Oliveira a,*, S. Ausset b, R. Vilar b a b
Instituto Superior Engenharia Lisboa, Rua Conselheiro Emı´dio Navarro no. 1, 1959-007 Lisboa, Portugal Dep. Engenharia Materiais, Instituto Superior Te´cnico, Av. Rovisco Pais no. 1, 1049-001 Lisboa, Portugal
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 February 2009 Received in revised form 24 March 2009 Accepted 8 April 2009 Available online 16 April 2009
In this paper the surface topography of titanium samples irradiated by femtosecond laser pulses is described. When the fluence is about 0.5 J/cm2 periodic ripples with a period of about 700 nm are formed. For fluences between 0.5 and 2 J/cm2, a microcolumnar surface texture develops in the center of the irradiated spots and ripples are formed in the periphery of the spots. When experiments are performed with a non-stationary sample, the microcolumns exhibit ripples similar to those observed when the radiation fluence is about 0.5 J/cm2 and in the outer regions of the irradiated areas for fluences between 0.5 and 2 J/cm2. Since the energy distribution in the transverse cross-section of the laser beam is Gaussian, we conclude that the ripples form when the microcolumns are subjected to fluences near the melting threshold of the material at the trailing edge of the moving laser beam. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Titanium Femtosecond Laser Nanostructuring
1. Introduction Materials irradiated with femtosecond laser pulses often exhibit surface textures characterized by a pattern of sharp microcolumns [1–14]. The morphology of these microcolumns depends on the laser processing parameters, including the number of incident laser pulses, radiation fluence, wavelength, pulse duration, and the ambient atmosphere composition and pressure. Over the last decade, most of the work carried out in this field has been performed in silicon [1–10], due to the importance of this material in the microelectronics industry. Besides the scientific interest of these studies, laser microstructuring of silicon also yield considerable properties improvement, such as increased optical absorption coefficient [4–6], low-threshold field-emission [7] and improved wettability [8,9]. The formation of micrometer-sized columns on titanium surfaces irradiated with Ti:Sapphire femtosecond laser pulses has been recently reported by Tsukamoto et al. [13], Nayak et al. [12] and Vorobyev and Guo [14]. This microtexture may yield interesting properties for example for biomedical applications, since surface texturing of titanium implants enhance osseointegration and the strength of the bone/implant interface [15]. Tsukamoto et al., Nayak et al., and Vorobyev and Guo used radiation with the same wavelength and the same fluence range, but the microcolumns observed by Nayak et al. exhibit a 700 nm
* Corresponding author. E-mail address: [email protected] (V. Oliveira). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.04.027
wavelength ripple pattern overlapped to the columns surface, which was not observed by Tsukamoto et al. and Vorobyev and Guo. Since the first authors carried out their experiments with a stationary sample whereas the other authors used a nonstationary one, it seems that the laser processing method may affect the surface texture of the material. The aim of the present work was to investigate the conditions of formation of the ripple pattern. The results obtained show that the ripple pattern forms for a limited fluence range near the melting threshold. As a result, they form when the sample is stationary for fluences in this range or when the sample is non-stationary and the microcolumns subjected to fluences near the melting threshold of the material at the trailing edge of the moving laser beam. 2. Experimental A commercial Yb:KYW chirped-pulse-regenerative amplification laser system is used to provide linear polarized lasers pulses with a duration of about 500 fs at a central wavelength of 1030 nm. Laser processing was performed in air on polished grade 2 titanium samples, a material typically used in low load bearing medical devices. The laser beam was perpendicular to the sample surface and focused using a 100 mm focal-length lens. The radiation fluence was calculated on the basis of the spot size and average pulse energy. The samples were mounted on a computer controlled XY stage and experiments were carried out by translating the sample relatively to the stationary laser beam with scanning velocities (v) varying between 0 and 500 mm/s or with a stationary sample (v = 0). In the latter experiments the pulse repetition rate
V. Oliveira et al. / Applied Surface Science 255 (2009) 7556–7560
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Fig. 1. SEM micrographs of titanium surfaces irradiated with 1 J/cm2 and: (a) 1; (b) 5; (c) 10; (d) 20; (e) 50; (f) 75; (g) 100; (h) 150; (i) 200; (j) 500; (k) 750 and (l) 1000 laser pulses.
was varied between 1 and 50 Hz whereas in non-stationary experiments, the pulse repetition rate was constant (50 Hz). After the laser treatment the surface topography was characterized by field emission scanning electron microscopy (SEM). 3. Results Under stationary irradiation, a microcolumnar surface texture forms in titanium targets for fluences between 0.5 and 2 J/cm2. The evolution of the surface topography treated with an average fluence of 1 J/cm2 with increasing number of laser pulses is depicted in Fig. 1. The first five pulses lead to the formation of surface pits randomly distributed (Fig. 1a and b). Between 5 and 20 pulses (Fig. 1c and d), these defects evolve to small depressions and hillocks, and after 50 pulses (Fig. 1e) the surface presents a wavy topography with an average wavelength much larger than the incident radiation wavelength. With increasing number of pulses the waves transform into microcolumns, whose tips protrude well above the initial surface level (Fig. 1f and g). the microcolumns
reach their maximum height after about 200 pulses (Fig. 1h) and do not change significantly for increasing number of pulses (Fig. 1i–l). A detailed inspection of the outer region of the irradiated areas revealed ripples with a spatial periodicity of about 700 nm, irrespectively of the number of pulses (Fig. 2). Similar ripples are observed all over the irradiated areas when the fluence exceeds the melting threshold of the material and is lower than the microcolumns formation threshold (0.5 J/cm2). Fig. 3 depicts SEM micrographs of titanium surfaces processed with a fluence of 1 J/cm2 in scanning regime. The Fig. 3a and b corresponds to a sample processed at a scanning speed of 10 mm/s, while Fig. 3c and d concern a sample processed at 50 mm/s. These scanning speeds were chosen because they are equivalent to a total input energy per unit area of the sample surface similar to that obtained when processing is carried out in single spots with 500 and 100 laser pulses. In general, a surface texture consisting of microcolumns similar to those generated in stationary irradiation is observed: at 50 mm/s, the columns are tall and protrude above the sample surface level, while for 10 mm/s the columns are
Fig. 2. SEM micrographs of ripples formed in the periphery of the irradiated regions after: (a) 50 and (b) 100 pulses.
V. Oliveira et al. / Applied Surface Science 255 (2009) 7556–7560
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Fig. 3. SEM micrographs of titanium surfaces irradiated with 1 J/cm2 and scanning speeds of (a), (b), (e) and (f) 10 mm/s; (c) and (d) 50 mm/s.
broader and their tips remain below the initial surface. Nevertheless, the columns formed at 10 mm/s present a peculiar characteristic which is not observed at 50 mm/s or in stationary irradiation: the surface is covered with ripples similar to those observed in the outer regions of the irradiated areas (Figs. 2 and 3e). Furthermore, this characteristic is not observed on the microcolumns formed immediately before the end of the scan (Fig. 3f). 4. Discussion Periodic ripples similar to those observed in Fig. 2 have been previously observed in samples of silicon [16,17], titanium [12,13] and other metals [18] processed with femtosecond laser radiation at fluences just above the melting threshold of the material. The most consensual explanation for their formation is that they originate from an inhomogeneous energy distribution due to the interference of the incident laser beam with scattered radiation, for example, by microscopic asperities of the surface, propagating parallel to the surface [19,20]. The ripples formed by this mechanism are oriented perpendicularly to the electric field vector of the incident laser beam, and their period is approximately equal to the laser radiation wavelength [19,20]. In the present
Fig. 4. Plot of Eq. (1).
V. Oliveira et al. / Applied Surface Science 255 (2009) 7556–7560
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Fig. 5. SEM micrographs of microcolumns and ripples formed under different experimental conditions. Line 1 was scanned with 10 mm/s at a fluence of 1 J/cm2 and line 2 with 200 mm/s and 0.5 J/cm2. (a) General overview; (b) microcolumns in track 1; (c) periodic ripples in track 2; and (d) microcolumns in the intersection of track 1 and 2.
work, the ripples period is about 700 nm width is appreciably less than the laser radiation wavelength, 1030 nm. This discrepancy may be explained by an increase of the real part of the material refraction index due to the development of nanostructures during the first laser pulses [18]. Under stationary irradiation ripples are formed when the radiation fluence is lower than the threshold for columns formation and higher than the melting threshold. Since the laser beam spatial profile is Gaussian, this suggests that the ripples formed in the outer regions of the irradiated areas for fluences higher than the threshold for columns formation are consequence of the lower intensity of the laser beam in these regions. The fact that under non-stationary irradiation columns formed immediately before the end of a scan do not have surface ripples (Fig. 3f) suggest that the ripples are created after the formation of the microcolumns and not simultaneously with the column as previously suggested [12]. In fact, each point of the surface is subjected to varying laser fluences. For example, under irradiation along the x-direction with a Gaussian laser beam the laser fluence F at any point (x, y) can be expressed as: 2
F ¼ 2F 0 e2½ðxx0 vtÞ
þðyy0 Þ2 =r2
(1)
where the beam radius r is the distance from the beam axis where the optical intensity drops to 1/e2 of the value on the beam axis, F0 is the average laser fluence, v is the scanning speed, t the time, and x0 and y0 are the coordinates of the beam center at time t = 0. The previous expression can be used for a pulsed laser beam as long as the movement of the beam and the temporal variation of the laser fluence during the pulse duration can be neglected. Eq. (1) is plotted in Fig. 4 for a point P located at x = 1.5r and x0 = y0 = y = 0, r = 50 mm, F0 = 1 J/cm2, and using two scanning speeds, 10 and 50 mm/s, respectively. Since P is originally outside the irradiated area, the laser fluence at this point (FP) is initially zero. As the axis of the laser beam approaches P, FP increases and reaches the threshold fluence for microcolumns formation (0.5 J/cm2) after
0.7 s for 50 mm/s, and 3.3 s for 10 mm/s. At this point a wavy topography start to form which evolve to microcolumns with increasing time. When the laser beam axis coincides with P, FP reaches its maximum value, 2 J/cm2. On the contrary, as the beam is moving away from P, FP decreases and fall again below 0.5 J/cm2, after 2.8 s for 10 mm/s and 11.7 s for 50 mm/s. At this point, the microcolumns development is over but FP is still high enough to generate ripples. As a result, ripples will begin to form on the microcolumns surface. For 50 mm/s, FP falls quickly to 0 and the number of applied pulses during this period is not enough to allow ripples to develop. On the contrary, for 10 mm/s the number of applied pulses is five times higher and ripples form. To confirm the proposed mechanism of ripple formation on microcolumns the following experiment was performed. First, a linear scan was performed with a scanning speed of 10 mm/s at a fluence of 1 J/cm2 (track 1 in Fig. 5a). The microcolumns formed in the treated area present an overlapped ripple pattern perpendicular to the scanning direction (Fig. 5b). The sample was then rotated 908 and a second linear scan was made with a lower laser fluence (0.5 J/cm2) and higher scanning speed (200 mm/s) intersecting the first track (track 2 in Fig. 5a). In this second treated track, ripples are formed but not microcolumns (Fig. 5c). Since the sample was rotated 908 between the first and the second track, the ripples in the second track are perpendicular to those in the first track (Fig. 5b and c). In the region where the tracks intersect (Fig. 5d), ripples are overlapped to the microcolumns similarly to what was observed in track 1 (Fig. 5b). However, the ripples are now parallel and not perpendicular to track 1. This shows that in this region the ripples observed on the microcolumns were formed during the second linear scan and after the formation of the microcolumns. 5. Conclusions In conclusion, microcolumns are formed when titanium surfaces are irradiated with femtosecond laser pulses in stationary
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