Applied Surface Science

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$com/locate/apsusc High throughput diffractive multi-beam femtosecond laser processing using a spatial light modulator Zheng Kuang a, *, Walter Perrie a, Jonathan Leach b, Martin Sharp a, Stuart P.$ Edwardson a, Miles Padgett b, Geoff Dearden a, Ken G.

Edwardson a, Miles Padgett b, Geoff Dearden a, Ken G.

1 Applied Surface Science 255 (2008) Contents lists available at ScienceDirect Applied Surface Science journal homepage: High throughput diffractive multi-beam femtosecond laser processing using a spatial light modulator Zheng Kuang a, *, Walter Perrie a, Jonathan Leach b, Martin Sharp a, Stuart P. Edwardson a, Miles Padgett b, Geoff Dearden a, Ken G. Watkins a a Laser Group, Department of Engineering, University of Liverpool Brownlow Street, Liverpool L69 3GQ, UK b Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK ARTICLE INFO ABSTRACT Article history: Received 23 May 2008 Received in revised form 10 July 2008 Accepted 10 July 2008 Available online 25 July 2008 PACS: Jv Cf Wk High throughput femtosecond laser processing is demonstrated by creating multiple beams using a spatial light modulator (SLM). The diffractive multi-beam patterns are modulated in real time by computer generated holograms (CGHs), which can be calculated by appropriate algorithms. An interactive LabVIEW program is adopted to generate the relevant CGHs. Optical efficiency at this stage is shown to be 50% into first order beams and real time processing has been carried out at 50 Hz refresh rate. Results obtained demonstrate high precision surface micro-structuring on silicon and Ti6Al4V with throughput gain >1 order of magnitude. ß 2008 Elsevier B.V. All rights reserved. Keywords: Femtosecond laser Spatial light modulator (SLM) Computer generated holograms (CGH) 1. Introduction Laser surface micro-structuring of materials (e.g. metals and semi-conductors) with ultrashort laser pulses can demonstrate high precision with little thermal damage due to the ultrafast timescale on which energy is coupled to the electronic system [1 3]. In metals, this reduces heat diffusion during the temporal pulse length to the nanometer scale, comparable to the optical skin depth, provided the fluence F is kept in the low regime, where F < 1Jcm 2 [4 6]. Consequently, the pulse energies used at low fluence for micro- and nano-structuring are often <10 mj while high gain regenerative amplifier systems running at n 1 khz repetition rate provide mj level output pulse energies. The need to highly attenuate the output beam can have a severe limit on potential throughput. This restriction may be one of the reasons why industrial uptake of khz femtosecond systems has been limited. * Corresponding author. Tel.: address: z.kuang@liv.ac.uk (Z. Kuang). Spatial light modulators (SLM) are a dynamic diffractive optical element which can modulate the phase of an incoming wavefront. By applying a computer generated hologram (CGH) to the SLM, one can transform the incident beam intensity distribution (e.g. Gaussian) into that desired for micro-manipulation, for example, in multi-trapping of micro-particles in optical tweezers [7 12], or material micro-processing first demonstrated recently by Hayasaki et al. [13 16]. The ability to address these devices in real time and synchronize with the scanning method adds an additional flexibility for surface micro-structuring. Here, we demonstrate parallel processing of a surface with arbitrary multiply diffracted spots through the application of CGHs. 2. Experimental The experimental setup is shown in Fig. 1. The output from a femtosecond laser system (Clarke-MXR CPA2010, with 160 fs pulsewidth, 775 nm central wavelength, 1 mj pulse energy and 1 khz repetition rate) traverses a half wave-plate and glan laser polariser and directed to a reflective SLM liquid crystal on silicon /$ see front matter ß 2008 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 Z. Kuang et al. / Applied Surface Science 255 (2008) Fig. 1. Experiment setup. Fig. 2. (a) The interface of the LabVIEW program generated by Leach et al. [7] and (b) a CGH calculated by the program (left) and its computational reconstruction (right).

2286 Z. Kuang et al. / Applied Surface Science 255 (2008) 2284 2289 Fig. 2. (Continued ). (LCoS) device with 1024 768 pixels (Holoeye LC-2500), oriented at 458 angle of incidence.

$The diffracted spot pattern (zero and higher order) enter the 10 mm aperture of a scanning galvo system and flat field lens with focal length f = 100 mm.$

3 2286 Z. Kuang et al. / Applied Surface Science 255 (2008) Fig. 2. (Continued ). (LCoS) device with pixels (Holoeye LC-2500), oriented at 458 angle of incidence. A beam expanding telescope, with magnification M 2 helps to reduce the average intensity on the SLM. The diffracted spot pattern (zero and higher order) enter the 10 mm aperture of a scanning galvo system and flat field lens with focal length f = 100 mm. The proximity of the SLM to the input aperture of the galvo ensures that diffracted beams are not clipped on the galvo mirrors and the flat field of the f-theta lens produces a near perfect focusing system. Substrates are mounted on a precision 4-axis (x, y, z, u) motion control system (Aerotech) allowing accurate positioning of the substrate surface at the laser focus. Appropriate holograms required to create arbitrary multibeam patterns at the substrate surface are generated via an interactive LabVIEW program using a number of algorithms to calculate the CGHs [7]. Fig. 2(a) shows the interface of the LabVIEW program, and Fig. 2(b) is a CGH calculated by the program and its computational reconstruction. 3. Results and discussions Fig. 3(a) shows optical micrographs of surface structuring on silicon with an LLEC pattern comprising 32 blind holes while Fig. 3(b) demonstrates a random spot pattern comprising 30 holes. Each hole pattern here was micro-machined simultaneously on the silicon wafer by creating the spot patterns in LabVIEW then applying the calculated CGHs to create the desired geometries. The Fig. 3. Optical images of two different multi-beam patterns. (Exposure time was 1 s, i.e. 1 k pulses.) (a) LLEC pattern comprising 32 micro-sized holes and (b) random spots pattern comprising 30 micro-sized holes.

4 Z. Kuang et al. / Applied Surface Science 255 (2008) incident pulse energy on the SLM was E P 300 mj. All diffractive spots have similar dimensions indicating accurate calculation of the CGHs using the Grating and Lenses algorithm [7,11]. The large hole above is generated by the residual zero order reflection highlighting the loss of precision with excessive ultrashort pulse energies. In order to evaluate the pulse energy of each of the diffracted spots, we compared the size of hole produced to a calibrated series of drilled holes using a single beam of varying pulse energy. When using the SLM, the diameter of each of the 30 diffractive spots was measured to be 20 mm, which indicated that each of the spots had a pulse energy of approximately 5 mj. The total input pulse energy was evaluated by measuring the power (to be 300 mj) before the aperture of the scanning galvo. Consequently, the diffraction efficiency of the SLM is approximately 50%, which is fairly typical of commercially available units, especially when used slightly away from their recommended operating wavelength (in this case nm) Graphs of measured hole size and eccentricity versus distance from the zero order hole in both LLEC and random spots patterns are shown in Fig. 4(a) and (b). The uniformity of the diffracted beams is shown by the measurements of ablated spot diameters using the Wyko NT1100 optical surface profiler: F 1 = mm ( LLEC pattern) and F 2 = mm (random spots pattern). Additionally, there is a modest increase in eccentricity with distance, which is probably the result of the finite bandwidth of the laser source [16].All diffractive optics suffer from the same limitation when used with a broadband source, namely for a given design of CGH, the shift of the focused spot away from the zero order is proportional to the wavelength (l) of the spectral component. Accordingly, the spot is elongated by an amount equal to its fractional spectral bandwidth multiplied by the number of equivalent grating line-pairs in the CGH design. For example, our 160 fs source has a fractional spectral bandwidth of approximately 0.7% (l = 775 nm, Dl = 5 nm). A spot shifted away from the zero order using a CGH with 25 line-pairs per mm would result in an eccentricity of the diffraction limited focal spot of approximately 2:1, in agreement with the observed results. Laser source bandwidth would therefore appear to set an upper limit on the useful field of view of the system. To avoid large variations in the required intensity distributions between spots, it is advantageous to avoid patterns with a high degree of symmetry, for example, straight lines, squares, or parallel geometries [17]. While static holograms are useful and can be combined with the galvo system to demonstrate parallel processing with fixed spot geometry, processing by real time control the CGHs is demonstrated in Fig. 5(a) and (b). Fig. 5(a) shows a pattern comprising 121 holes completed by real time playing of 15 stored CGHs at 20 Hz refresh rate, and Fig. 5(b) demonstrates the formation of the pattern which was completed within 0.75 s. The incident pulse energy on the SLM was E P 300 mj. Fig. 4. Graphs of hole size and eccentricity vs. the distance from each diffracted spot to the zero order hole in both LLEC and random spots patterns. (a) Holes size (area) vs. distance away from zeroth order hole and (b) eccentricity a/b vs. distance away from zeroth order hole.

2288 Z. Kuang et al. / Applied Surface Science 255 (2008) 2284 2289 Fig. 5.

5 2288 Z. Kuang et al. / Applied Surface Science 255 (2008) Fig. 5. (a) Pattern completed by real time playing 15 series of CGHs at 20 Hz refresh rate (50 ms duration per CGH) and (b) the formation of the pattern. By combining real time control of the CGHs with scanning, diffractive multi-beam processing has significant potential to produce complex surface micro-machining patterns [18]. Fig. 6(a) and (b) illustrates this on a polished Ti6Al4V substrate where 6 micro-channels, a f were generated by applying the appropriate CGHs at 50 Hz refresh rate while simultaneously scanning the diffracted spots at a speed of 1 mm/s. The resulting micro-channels a f are 40 mm wide and 10 mm deep. The large channel above, again is due to the zero order. Ti6Al4V, which is an metallic alloy widely used in aerospace and bioscience, has a relatively low ablation threshold (0.1 J cm 2 ) [19] hence the advantages of diffractive multi-beam processing are demonstrated as extensive attenuation was largely avoided. 4. Conclusions The results obtained demonstrate that precision diffractive multi-beam ultrafast laser micro-structuring using an SLM is a novel method to increase processing throughput. Pulse energies up to 300 mj at 1 khz repetition rate have been diffracted into >30 spots with 50% efficiency. This constitutes a throughput gain >1 order of magnitude for precision micro-structuring tasks requiring micro-joule level pulse energies. Future research will focus on full

6 Z. Kuang et al. / Applied Surface Science 255 (2008) Zheng Kuang would like to thank the scholarship of Overseas Research Students Award Scheme (ORSAS) funded by the UK Government which financially supports his PhD study at the University of Liverpool. References Fig. 6. Micro-channels obtained by combining CGHs real time control with galvo scanning. (a) Whole pattern view (5 objective); 6 channels a f were generated by the diffractive beams while the wider channel 0 above was formed by zero order beam and (b) 3D larger zoom view by Wyko NT1100 optical surface profiler, where 10 mm depth (10 times over scan) micro-channels are illustrated. (For interpretation of the references to color in this artwork, the reader is referred to the web version of the article.) synchronizing action of hologram application with scanning techniques hence producing more complex 2 and 3D structures. The zero order reflected light creates unwanted surface features but can be removed by modifying the optical setup to include a telescope with unity magnification and blocking this component at the Fourier plane. Acknowledgements The authors gratefully acknowledge the support of the North West Development Agency (NWDA) in the UK, and the author [1] D. Du, X. Liu, G. Korn, J. Squier, G. Mourou, Laser-induced breakdown by impact ionization in SiO 2 with pulse widths from 7 ns to 150 fs, Appl. Phys. Lett. 64 (1994) [2] X. Liu, D. Du, G. Mourou, Laser ablation and micromachining with ultrashort laser pulses, IEEE J. Quant. Electron. 33 (1997) [3] B.N. Chichkov, C. Momma, S. Nolte, F. von Alsvenleben, A. Tunnermann, Femtosecond, picosecond and nanosecond laser ablation of solids, Appl. Phys. A 63 (1996) [4] R.Le. Harzic, D. Breitlung, M. Weikert, S. Sommer, C. Fohl, S. Valette, C. Donnet, E. Audouard, F. Dausinger, Pulse width and energy influence on laser micromachining of metals in a range of 100 fs to 5 ps, Appl. Surf. Sci. 249 (2005) [5] W. Perrie, M. Gill, G. Robinson, P. Fox, W. O Neill, Femtosecond laser microstructuring of aluminium under helium, Appl. Surf. Sci. 230 (2004) [6] A. Luft, U. Franz, A. Emsemann, J. Kasper, A study of thermal and mechanical effects on materials induced by pulsed laser drilling, Appl. Phys. A 63 (1996) [7] J. Leach, K. Wulff, G. Sinclair, P. Jordan, J. Courtial, L. Thomson, G. Gibson, K. Karunwi, J. Cooper, Z.J. Laczik, M. Padgett, Interactive approach to optical tweezers control, Appl. Opt. 45 (2006) [8] J. Leach, G. Sinclair, P. Jordan, J. Courtial, M.J. Padgett, J. Cooper, Z.J. Laczik, 3D manipulation of particles into crystal structures using holographic optical tweezers, Opt. Express 12 (2004) [9] G. Sinclair, J. Leach, P. Jordan, G. Gibson, E. Yao, Z. Laczik, M. Padgett, J. Courtial, Interactive application in holographic optical tweezers of a multi-plane Gerchberg Saxton algorithm for three-dimensional light shaping, Opt. Express 12 (2004) [10] G. Whyte, G. Gibson, J. Leach, M. Padgett, An optical trapped microhand for manipulating micron-sized objects, Opt. Express 14 (2006) [11] J. Liesener, M. Reicherter, T. Haist, H.J. Tiziani, Multi-functional optical tweezers using computer-generated holograms, Opt. Commun. 185 (2000) [12] M. Reicherter, S. Zwick, T. Haist, C. Kohler, H. Tiziani, W. Osten, Fast digital hologram generation and adaptive force measurement in liquid-crystal-displaybased holographic tweezers, Appl. Opt. 45 (2006) [13] Y. Hayasaki, T. Sugimoto, A. Takita, N. Nishida, Variable holographic femtosecond laser processing by use of a spatial light modulator, Appl. Phys. Lett. 87 (2005) [14] S. Hasegawa, Y. Hayasaki, N. Nishida, Holographic femtosecond laser processing with multiplexed phase Fresnel lenses, Opt. Lett. 31 (2006) [15] H. Takahashi, S. Hasegawa, Y. Hayasaki, Holographic femtosecond laser processing using optimal-rotation-angle method with compensation of spatial frequency response of liquid crystal spatial light modulator, Appl. Opt. 46 (2007) [16] Y. Kuroiwa, N. Takeshima, Y. Narita, S. Tanaka, K. Hirao, Arbitrary micropatterning method in femtosecond laser microprocessing using diffractive optical elements, Opt. Express 12 (2004) [17] J.E. Curtis, C.H.J. Schmitz, J.P. Spatz, Symmetry dependence of holograms for optical trapping, Opt. Lett. 30 (2005) [18] B. Bhushan, Nanotribology and nanomechnanics of MEMS/NEMS and BioMEM/ BioNEMS materials and devices, Micoelectron. Eng. 84 (2007) [19] P. Mannion, J. Magee, E. Coyne, G.M. O Connor, Ablation thresholds in ultrafast laser micro-machining of common metals in air, Proc. SPIE 4876 (2003).

FEMTOSECOND LASER INTERNAL STRUCTURING OF MATERIALS USING A SPATIAL LIGHT MODULATOR Paper (P152)

FEMTOSECOND LASER INTERNAL STRUCTURING OF MATERIALS USING A SPATIAL LIGHT MODULATOR Paper (P152) Dun Liu 1, Zheng Kuang 1, Walter Perrie 1 *, Patricia. J. Scully 2, Alexandra Baum 2, Shijie Liang 2, Anca