Open Access
Add to BibSonomyAbstract
Results on the growth of highly organized, periodic microstructures on a copper substrate utilizing a nanosecond pulsed Nd:YVO4 laser at 532 nm are reported. At the laser energy fluence of ~2.5 J/cm2 (intensity of ~0.21 GW/cm2) arrays of microstructures with average periods ranging from ~40 μm to ~80 μm, depending on the distance between the consecutive laser scans, are generated. The employed technique for irradiating copper and the mechanism for formation of the highly organized microstructures are discussed.
©2011 Optical Society of America
1. Introduction
Many within the photonics community are familiar with the formation of self-assembled conical and periodic structures that can be produced on various surfaces when the energy fluence of the laser beam is near or at the damage threshold of the material. Indeed, microstructures have been observed in many materials under inert and reactive ambient gases, with laser wavelengths from UV to IR and laser pulse durations from nanosecond to femtosecond [1–22]. In some cases, such as pulsed laser deposition of thin films they appear at the bottom of craters after laser ablation processes. In other cases, they grow from the surface of the target, e.g. in surface modification treatment at various laser fluences. The latter results from the melting of a surface layer and a low vaporization rate depending on the intensity of the source and number of accumulated pulses. Unlike self-assembled conical structures that have been reported on laser machined polymer surfaces, the process by which microcones and the like are produced on a metallic substrates is a melt flow dominated process [11–13,15,17] rather than vaporization–redeposition process [9,16,19]. Often, the formation of self-assembled microstructures on metallic surfaces, such as steel, was attributed to the melt flow produced by surface tension gradients resulting from temperature non-uniformity on the surface. The productions of such features are believed to have applications as black body sources [20], and in the fields of surface wettability [21], and microbiology [22].
Recently, we reported results on the growth of highly organized, reproducible, periodic microstructure arrays on a stainless steel substrate using multi-pulsed Nd:YAG (wavelength of 1064 nm, pulse duration of 7 ns) laser irradiation in standard atmospheric environment (room temperature and normal pressure) [23]. The peculiarity of our work was on the utilized approach for scanning the laser beam over the surface.
In this paper, we show that the suggested approach is scalable to other metals. Here, we explore experimental conditions required for the formation of highly organized, periodic microstructures on a copper (Cu) target upon multi-pulsed laser irradiation at 532 nm in air. The microstructures exhibited an average separation between tips ranging from 40 to 80 μm, depending on the hatching overlap between consecutive scans. The tips of the generated structures are at the level of the original substrate.
To the authors’ knowledge, the irradiation parameters employed for processing of copper and formation of highly organized structures on this metal have not been reported before. Copper, its alloys and metals with similar metallurgical behavior (e.g., gold) are important materials for many technical applications due to their unrivalled thermal and electrical conductivity. The latest social and environmental developments lead to a much higher electrification of our everyday lives. Therefore, laser processing (structuring, welding etc.) of metals like copper and gold are key technologies for this trend. However, the high thermal conductivity (~16 times higher than that of stainless steel) together with low absorptivity of these materials at the fundamental wavelength of typical solid-state bulk, fiber and diode lasers makes their processing a challenge.
2. Experimental methods
The experiments were performed using commercially available copper foils with purity of 99.99% and thickness of 1 mm. The thermal conductivity (k) and thermal diffusivity (D) of copper are 401 W/m⋅K, and 1.1234 × 10−4 m2/s, respectively [24]. The foils were first ultrasonically cleaned with ethanol and deionized water to remove organic contamination. A Nd:YVO4 laser with a maximum average power of 10 W at λ = 532 nm, pulse length of τ = 12 ns and repetition rate of ƒ = 30 kHz was utilized for irradiation of the samples in standard atmospheric environment (room temperature and normal pressure).
The laser beam had a Gaussian intensity profile (M2 ~1.1) and was focused onto the target surface using a flat field scanning lens system, a specialized lens system in which the focal plane of the deflected laser beam is a flat surface. The flat field scanning lens systems are commonly used in laser processing applications to offset the off axis deflection of the beam through the focusing lens system.
The diameter of the focused spot between the points where the intensity has fallen to 1/e2 of the central value was ϕ ≈60 μm. This resulted to the Rayleigh range of R = 4.83 mm, given by [25]:
This is the distance from the beam waist of diameter ϕ to the position where it is √2ϕ. This large Rayleigh range results in a negligible change of the beam spot size on the target, providing a uniform ablation trace throughout the experiments, as will be discussed later.The laser beam was raster scanned over the surface of the target at a velocity of V = 10 mm/s, using a computer-controlled scanner system. The hatch distance d, i.e. the distance between adjacent raster scans, was varied for each experiment as will be described in the next section.
Two different scanning techniques were employed for the experiments, namely line scanning, shown in Fig. 1(a) , and crosshatched scanning, shown in Fig. 1(b). In the line-hatching (LH) regime the laser was scanned only in the x-direction (horizontal direction) and the hatch distance was varied between the lines. In the cross hatching (CH) regime the laser was scanned over the surface in two directions (both in x and y) to form a grid pattern, with the total number of pulses fired onto the target surface being twice as many as in the LH regime. The number of pulses fired per spot (N) in each scan is calculated by:
Using the above experimental parameters, during each scan 180 pulses per spot were fired onto the target. One can tailor N to one’s need by, for instance, increasing the number of scans.
Fig. 1 (a) Line-hatching (LH) regime for scanning the laser beam over the surface of the copper foil. (b) The cross-hatching (CH) regime where the lasers beam was scanned in both horizontal and vertical directions, to form a grid pattern.
The optical characterizations of the samples were performed using a JASCO V-670 UV/VIS/NIR Spectrophotometer equipped with an ISN-723 60 mm Diameter Integrated Sphere, and KEYENCE Digital Microscope VHX-1000.
3. Results and discussion
Reflectivity of the copper foils at 532 nm was measured to be ~43%. This value increases to > 90% above 700 nm. For the first set of experiments and in order to estimate the required energy fluence for structuring the target, the laser beam was raster scanned over the surface of the target in the LH regime (Fig. 1(a)). The hatch distance (d) was set to 80 µm, approximately 33% larger than the beam focus spot diameter of 60 µm, to ensure no overlap between the written lines. For each experiment, approximately N = 1800 pulses per spot were fired onto the target. The target was first processed at laser energy fluence of ~2 J/cm2. Surface and cross-section of the processed area are shown in Figs. 2(a) and 2(b), respectively. As it can be seen from Fig. 2(b), penetration of the beam at this fluence is negligible.
Fig. 2 (a) and (b) are the surface and cross-section of copper after irradiation using laser fluence of ~2 J/cm2. (c) and (d) are the surface and cross-section of the target after irradiation using laser fluence of ~2.5 J/cm2. In both cases the LH regime was employed, the hatch distance is 80 µm, and 1800 pulses per spot were fired onto the target. The depth of the grooves in (d) were measured to be ~77 µm from the original surface of the target. Some structures in the form of melt can be seen on the original surface of the metal and next to the grooves.
Increasing the laser energy fluence to ~2.5 J/cm2, i.e. above the damage threshold of the target and while keeping N constant, resulted in the formation of well-defined grooves. Surface and cross-section of the processed area for this energy fluence are shown in Figs. 2(c) and 2(d), respectively. The depth of the grooves were measured to be ~77 µm from the original surface of the target. Keeping the laser energy fluence at 2.5 J/cm2 but decreasing the hatch distance (d) between the lines from 70 µm to 40 µm in steps of 10 µm, resulted in the formation of some structures at the edge of the beam (Fig. 3 ). Decreasing the hatch distance did not affect the depth of the grooves. Higher levels of overlap between the lines (< 40 µm) only resulted in the substantial ablation of the material.
Fig. 3 Microscope images of the surface after laser irradiation at 2.5 J/cm2 in the LH regime. The hatch distances were varied from (a) to (d) in steps of 10 µm. The hatch distances are 40, 50, 60 and 70 µm form (a) to (d), respectively. In all cases the number of pulses fired per spot is 1800.
Formation of the grooves (even for large hatch distances) indicates that the driving force behind the material removal is the pressure of the expanding ablation products at the center of the laser beam interaction site. Here, the heat diffusion length (LT) is approximately equal to 2.3 μm, given by [26,27]:
where D is the heat diffusivity of copper (1.1234 × 10−4 m2/s) and τ is the laser beam dwell time (equal to the laser pulse length of 12 ns). The calculated value for LT is much smaller than the laser spot size on the target of ~60 μm. Hence, the lateral heat flow can be ignored and the temperature distribution of the target in z-direction (penetration direction) can be obtained using the one-dimensional heat equation [25–27].The molar mass and mass density (at 300 K) of copper are 63.5 g/mole and 8.96 g/cm3, respectively [27]. Using the calculated value for the heat diffusion length as depth, the average enthalpy per pulse for the laser spot can be estimated to be approximately 50 kJ/mol. This value is less than the enthalpy required for heating copper from room temperature, through fusion to evaporation of approximately 394 kJ/mole [27]. These lead to the following discussion.
For long laser pulses (> 1 ns), the absorption of laser radiation and ablation of the target material occur at the same time [28]. If the intensity of the beam is high enough to produce plasma then the duration of the ablation can be considered to be close to the duration of the laser pulse. In such circumstances, there is enough time for the thermal wave to propagate into the target and to create a relatively large layer of melted material. The lifetime of the melt pool is in approximately the same order of magnitude as the time between pulses (~tens of μs) [29], which will result in an interaction of the persistent melt with the pressure in the near-surface plasma/vapor layer. In the absence of definite polarisation of the laser beam and spatial modulation of the radiation intensity (in our experiments, the large Rayleigh range of R = 4.83 mm maintained a high irradiance owing to the good beam quality of the laser), melt instability in the field of ablation plume pressure results in the growth of large-scale surface structures [30].
We therefore believe that here because of the high intensity and good beam quality (Gaussian beam with high axial intensity profile) of the source, and the build up of energy by successive pulses, ablation/vaporization takes place at the center of the melt pool (which is above the ablation threshold of the material) with concomitant build-up of backpressure ejecting the melt laterally. This could be enhanced by plasma formation, originating from laser coupling with vapor plume, and the plasma shock wave impinging on the molten surface. Formation of plasma in the center of the beam results in a high thermal gradient between the spot center and the outer spot diameter. The hydrodynamic expansion of the ablated material creates an extended density profile where the laser energy is absorbed. At the periphery of the beam, melting is occurring instead. All this can be clearly seen from Figs. 2(c) and 2(d) and Fig. 3, where next to the grooves a relatively large layer of melted material containing large-scale surface structures is created.
Following the above observations and in order to grow organized structures, second set of experiments were performed. Results of the irradiation of copper in CH regime are shown in Fig. 4 . Here for all cases a laser energy fluence of ~2.5 J/cm2 is used. The number of pulses per spot fired in each direction (horizontal and vertical) is 1800. The distance between the consequent scans were increased, from 30 µm (Fig. 4(a)) to 80 µm (Fig. 4(f)) in steps of 10 µm, in both horizontal and vertical directions. It has to be noted that the sizes of the structured areas were each 1 mm2 and that the microscopic images in Fig. 4 only represent small selected areas.
Fig. 4 Microscope images of the copper surface after laser structuring in the CH regime. The laser beam scanned over the surface in both horizontal and vertical directions. In each direction 1800 pulses per spot were fired onto the target. In both directions and in each area, the hatch distance was fixed between the scanned lines. The hatch distances are 30, 40, 50, 60, 70 and 80 µm for (a), (b), (c), (d), (e) and (f), respectively.
As can be seen from Fig. 4, from the hatch distances of 40 µm onward the structures are uniform. We observed that the heights of the structures are equal to the ablated layer thickness and that the average structure period versus the hatch distance follows a linear trend. The measured values for period of the organized structures (peak to peak distances) in Fig. 4 are as follows: ~42 µm in (b); ~53 µm in (c); ~59 µm in (d), and ~67 µm in (e). For the peculiar structures presented in Fig. 4(f), the measured distance from the center of one valley to the next valley is ~81 µm. This observation suggests that formation of similar but much smaller organized structures are possible by using a smaller beam spot sizes in the focus and smaller hatch distances.
The laser-material interaction for the LH and CH regimes is identical, with the only difference being that in the CH mode the target surface is pre-heated. In the range of irradiation parameters used here (pulse duration of 12 ns and intensity in the order of 0.2 GW/cm2), the formation of the highly organized structures is induced through a spatial modulation of the pressure in the near-surface plasma layer, followed by melt outflow from pits to humps and subsequent solidification. It has to be pointed out that during the lifetime of the molten phase the structures are damped due to the viscous nature of the liquid. Upon re-solidification, the actual shape of the surface freezes. These structures can then act as precursors to the formation of microcones by altering the reflectivity of the target surface, and hence introducing a non-uniform temperature distribution on the target. It can be seen from Fig. 4 that the period of the surface structures is fairly close to the distance between the adjacent traces (hatch distance). Increasing the hatch distance results in a less efficient damping of the structures during the lifetime of the molten surface, with the consequence of increasing the average period of the structures. Increasing the hatch distance has a consequence of decreasing the total number of pulses fired onto the surface. This results in the formation of larger microstructures at the expense of the extinction of smaller ones.
4. Conclusion
In summary, microstructuring of copper using a nanosecond (12 ns) pulsed Nd:YVO4 laser at 532 nm is demonstrated. At the laser energy fluence of ~2.5 J/cm2 (intensity of ~0.21 GW/cm2), and using a unique scanning technique, arrays of highly organized microstructures with average periods ranging from ~40 μm to ~80 μm are generated. We showed that this average period depends on the hatching overlap between the consecutive laser scans. The employed technique for irradiating copper and the mechanism for formation of the microstructures were discussed. High intensity and brightness of the laser sources had considerably facilitated the uniform processing of the material. Moreover, our observations suggest that formation of similar but much smaller organized structures are possible, e.g., by reducing the beam spot size in the focus (tighter focusing) and smaller hatch distances.
We believe that our technique is practical and scalable to other metals. Depending on the material, controllable formation of such organized structures can have applications in areas ranging from energy harvesting and display technology to biocompatibility and wettability.
Acknowledgments
This work was conducted under the aegis of the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom. Amin Abdolvand is an EPSRC Career Acceleration Fellow at the University of Dundee (EP/I004173/1).
References and links
1. T. J. Bastow, “Ordering of microcraters produced by a laser on a metal surface,” Nature 222(5198), 1058–1060 (1969). [CrossRef]
2. J. E. Sipe, J. F. Young, J. S. Preston, and H. M. van Driel, “Laser-induced periodic surface structure. I. Theory,” Phys. Rev. B 27(2), 1141–1154 (1983). [CrossRef]
3. S. A. Akhmanov, V. I. Emelyanov, N. I. Koroteev, and V. N. Seminogov, “Interaction of powerful laser radiation with the surfaces of semiconductors and metals: nonlinear optical effects and nonlinear optical diagnostics,” Sov. Phys. Usp. 28(12), 1084–1124 (1985). [CrossRef]
4. R. Kelly and J. E. Rothenberg, “Laser sputtering. Part III. The mechanism of the sputtering of metals low energy densities,” Nucl. Instrum. Methods Phys. Res. B 7-8, 755–763 (1985). [CrossRef]
5. P. E. Dyer, S. D. Jenkins, and J. Sidhu, “Development and origin of conical structures on XeCl laser ablated polyimide,” Appl. Phys. Lett. 49(8), 453–455 (1986). [CrossRef]
6. D. J. Krajnovich and J. E. Vazquez, “Formation of intrinsic surface defects during 248 nm photoablation of polyimide,” J. Appl. Phys. 73(6), 3001–3008 (1993). [CrossRef]
7. S. R. Foltyn, Pulsed Laser Deposition of Thin Films, D. B. Christy and G. K. Huber, eds. (Wiley, 1994).
8. J. Heitz, J. D. Pedarnig, D. Bäuerle, and G. Petzow, “Excimer-laser ablation and micro-patterning of ceramic Si3N4,” Appl. Phys., A Mater. Sci. Process. 65(3), 259–261 (1997). [CrossRef]
9. T.-H. Her, R. J. Finlay, C. Wu, S. Deliwala, and E. Mazur, “Microstructuring of silicon with femtosecond laser pulses,” Appl. Phys. Lett. 73(12), 1673–1675 (1998). [CrossRef]
10. J. F. Silvain, H. Niino, S. Ono, S. Nakaoka, and A. Yebe, “Surface modification of elastomerrcarbon composite by Nd:YAG laser and KrF excimer laser ablation,” Appl. Surf. Sci. 141(1-2), 25–34 (1999). [CrossRef]
11. S. I. Dolgaev, S. V. Lavrishev, A. A. Lyalin, A. V. Simakin, V. V. Voronov, and G. A. Shafeev, “Formation of conical microstructures upon laser evaporation of solids,” Appl. Phys., A Mater. Sci. Process. 73(2), 177–181 (2001). [CrossRef]
12. S. I. Dolgaev, J. M. Fernandez-Pradas, J. L. Morenza, P. Serra, and G. A. Shafeev, “Growth of large microcones in steel under multipulsed Nd:YAG laser irradiation,” Appl. Phys., A Mater. Sci. Process. 83(3), 417–420 (2006). [CrossRef]
13. P. V. Kazakevich, A. V. Simakin, and G. A. Shafeev, “Formation of periodic structures by laser ablation of metals in liquids,” Appl. Surf. Sci. 252(13), 4457–4461 (2006). [CrossRef]
14. A. Bensaoula, C. Boney, R. Pillai, G. A. Shafeev, A. V. Simakin, and D. Starikov, “Arrays of 3D micro-columns generated by laser ablation of Ta and steel: modelling of a black body emitter,” Appl. Phys., A Mater. Sci. Process. 79(4-6), 973–975 (2004). [CrossRef]
15. S. I. Dolgaev, N. A. Kirichenko, A. V. Simakin, and G. A. Shafeev, “Laser-assisted growth of microstructures on spatially confined substrates,” Appl. Surf. Sci. 253(19), 7987–7991 (2007). [CrossRef]
16. N. S. Murthy, R. D. Prabhu, J. J. Martin, L. Zhou, and R. L. Headrick, “Self-assembled and etched cones on laser ablated polymer surfaces,” J. Appl. Phys. 100(2), 023538 (2006). [CrossRef]
17. R. Lloyd, A. Abdolvand, M. Schmidt, P. Crouse, D. Whitehead, Z. Liu, and L. Li, “Laser-assisted generation of self-assembled microstructures on stainless steel,” Appl. Phys., A Mater. Sci. Process. 93(1), 117–122 (2008). [CrossRef]
18. A. J. Pedraza, J. D. Fowlkes, and D. H. Lowndes, “Laser ablation and column formation in silicon under oxygen-rich atmospheres,” Appl. Phys. Lett. 77(19), 3018–30121 (2000). [CrossRef]
19. E. György, A. P. Pino, P. Serra, and J. L. Morenza, “Laser-induced growth of titanium nitride microcolumns on biased titanium targets,” J. Mater. Res. 20(01), 62–67 (2005). [CrossRef]
20. D. Starikov, C. Boney, R. Pillai, A. Bensaoula, G. A. Shafeev, and A. V. Simakin, “Spectral and surface analysis of heated micro-column arrays fabricated by laser-assisted surface modification,” Infrared Phys. Technol. 45(3), 159–167 (2004). [CrossRef]
21. B. Wu, M. Zhou, J. Li, X. Ye, G. Li, and L. Cai, “Superhydrophobic surfaces fabricated by microstructuring of stainless steel using a femtosecond laser,” Appl. Surf. Sci. 256(1), 61–66 (2009). [CrossRef]
22. J. Chen, J. P. Ulerich, E. Abelev, A. Fasasi, C. B. Arnold, and W. O. Soboyejo, “An investigation of the initial attachment and orientation of osteoblast-like cells on laser grooved Ti-6Al-4V surfaces,” Mater. Sci. Eng. C 29(4), 1442–1452 (2009). [CrossRef]
23. A. Abdolvand, R. Lloyd, M. Schmidt, D. Whitehead, Z. Liu, and L. Li, “Formation of highly organized, periodic microstructures on steel surfaces upon pulsed laser irradiation,” Appl. Phys., A Mater. Sci. Process. 95(2), 447–452 (2009). [CrossRef]
24. J. P. Holman, Heat Transfer, 9th ed. (McGraw-Hill, 2002).
25. W. M. Steen and J. Mazumder, Laser Material Processing, 4th ed. (Springer-Verlag London Limited, 2010).
26. D. Bäuerle, Laser Processing and Chemistry, 3rd ed. (Springer, 2000).
27. M. von Allmen and A. Blatter, Laser-Beam Interactions with Materials, 2nd ed. (Springer-Verlag, 1995).
28. C. Momma, B. N. Chichkov, S. Nolte, F. von Alvensleben, A. Tunnermann, H. Welling, and B. Wellegenhausen, “Short-pulse laser ablation of solid targets,” Opt. Commun. 129(1-2), 134–142 (1996). [CrossRef]
29. Z. Kántor, Zs. Geretovszky, and T. Szorenyi, “The effect of target temperature on the deterioration of metal surfaces under pulsed laser irradiation,” Appl. Surf. Sci. 154–155(1-4), 78–82 (2000). [CrossRef]
30. S. I. Anisimov and V. A. Khokhlov, Instabilities in Laser-Matter Interaction (CRC Press, Inc., 1995).
