Applied Surface Science

Applied Surface Science 273 (2013) 101 106 Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Ultrafast laser parallel microprocessing using high uniformity binary Dammann grating generated beam array Zheng Kuang, Walter Perrie, Dun Liu, Stuart P. Edwardson, Yao Jiang, Eamonn Fearon, Ken G. Watkins, Geoff Dearden Laser Group, Centre for Material and Structures, School of Engineering, University of Liverpool, Brownlow Street, Liverpool L69 3GQ, United Kingdom a r t i c l e i n f o Article history: Received 11 November 2012 Received in revised form 25 January 2013 Accepted 28 January 2013 Available online 4 February 2013 Keywords: Ultrafast laser Special Light Modulator Dammann grating a b s t r a c t Ultrafast laser parallel processing using diffractive multi-beam patterns generated by a spatial light modulator (SLM) has demonstrated a great increase in processing throughput and efficiency. Applications ranging from surface thin film patterning to internal 3D refractive index modification have been recently reported with the parallel processing technology. Periodic and symmetrical geometry design (e.g. N M beam array) of the multi-beam pattern must be avoided to guarantee the required high uniformity in these applications, which, however, limited the processing flexibility. In this paper, Dammann gratings are used to create diffractive 1 5 and 5 5 beam arrays for the parallel processing. The 0-th order, observed slightly stronger than the other higher orders, can be adjusted by superimposing a Fresnel zone lens (FZL) and tuning the degree of defocusing at the processing plane. The uniformity (presented by the variation of the machined hole diameter) is measured to be <4% after the adjustment. Additionally, a parallel surface patterning of indium tin oxide (ITO) thin film with periodic array structures was demonstrated using the Dammann grating generated beam array without requiring the complicated geometry separation and the time-consuming positioning. 2013 Elsevier B.V. All rights reserved. 1. Introduction Parallel processing using diffractive multi-beam patterns generated by a spatial light modulator (SLM) is a novel method to greatly increase the processing throughput and efficiency of ultrafast laser material processing [1 4]. Applications ranging from surface thin film patterning of transparent conducting oxides (TCOs) [5] to internal 3D refractive index modification of poly(methyl methacrylate) [6,7] have been recently reported with the parallel processing technology. In order to reach the required high uniformity in these applications, periodic and symmetrical geometry design (e.g. N M beam array) of the diffractive multi-beam pattern must be avoided. This is due to the fact that periodic and symmetrical designs can significantly increase the probability of overlap of unwanted diffraction peaks, called ghosts, and degrade the reconstruction accuracy when using lenses and gratings (LG) algorithm [8]. This, however, limited the processing flexibility. With this limitation in mind, binary Dammann gratings [9 11] which can create uniformed beam arrays were applied for the parallel processing. According to grating equation, sin m = m/, (where m is the diffractive order and m is m order s diffractive angle), a Corresponding author. Tel.: +44 1517944851. E-mail addresses: kz518@msn.com, kuang@liv.ac.uk (Z. Kuang). 2D binary grating can generate m m diffractive beam array. The energy directed into each of the diffracted orders is dictated by the shape and nature of the surface profile within a single grating period, called a grating unit cell [11]. Dammann et al. [9] firstly developed a straightforward method of calculating the unit cell structure for binary gratings with uniformed energy distribution. These gratings are named Damman gratings. In this paper, Two Dammann gratings (1 5 and 5 5), provided by Holoeye Photonics, were used for the parallel processing of Ti64. The 0-th order, observed slightly larger than the other 24 higher orders, can be adjusted by superimposing a Fresnel zone lens (FZL) and tuning the degree of defocusing at processing plane. The uniformity (presented by the variation of the machined hole diameter) is measured to be <4% after the adjustment. Additionally, a parallel surface patterning of indium tin oxide (ITO) thin film with periodic array structures was demonstrated using the Dammann grating generated beam array, without requiring the complicated geometry separation and the time-consuming positioning. 2. Experimental The ultrafast laser system used for this research is a custom made Nd: VAN seeded regenerative amplifier laser system (High-Q IC-355-800ps, Photonic Solutions). Schematic of the experimental setup is shown in Fig. 1. The laser output (t p = 10 ps, = 1064 nm, 0169-4332/$ see front matter 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.195

102 Z. Kuang et al. / Applied Surface Science 273 (2013) 101 106 Fig. 1. Experimental setup. f = 10 khz) passes through a half wave plate used for adjusting the linear polarization direction, a beam expander (M 3), and after reflection on mirrors 1, 2 and 3, illuminates a reflective phase only SLM, A Hamamatsu X10468-03 liquid crystal on silicon (LCoS) device with 800 600 pixels and dielectric coated for 1064 nm wavelength (reflectivity 95%), oriented at <10 degree angle of incidence. A flipping mirror, placed after lens 1, can reflect the beam to a charge-coupled device (CCD) camera-based laser profiler (Spiricon) to observe the reconstructed multi-beam patterns when it is flipped into beam line. A 4f-optical system is formed from A to D to remove the unwanted 0-th order beam [4]. The beam then enters a scanning galvanometer with f 0 = 100 mm flat field f-theta lens (Nutfield) producing an agile focusing system. The light utilization efficiency of the system (i.e. the ratio of the energy output after the laser head and the output after the scanning galvanometer) is measured to be >90%. Substrates are mounted on a precision 5-axis (x, y, z, p, q) motion control system (Aerotech) allowing accurate positioning of the substrate surface at the laser focus. The spectral bandwidth, < 0.3 nm, is relatively narrow and important in eliminating chromatic dispersion of the SLM [4,12]. 3. Results and discussions 3.1. Uniformity measurement of the beam arrays created by Dammann grating Fig. 2 demonstrates the Dammann gratings used in this research. As shown in grating unit cells, the periodic binary structure has two gray levels, G1 and G2. Fig. 3 shows the uniformity of the holes machined by the Damman grating generated beam arrays (1 5 and 5 5 patterns) on a polished Ti64 sample versus the gray level difference between G1 and G2. The uniformity was quantitatively presented by the variation of the hole diameter, V. The variation, V i, was calculated by: V i = ( i /a i ) 100%, where a i and i are arithmetic mean and standard deviation of the hole diameter, respectively. When calculating a i and i, the number of the included measured hole diameters depended on the applied diffractive pattern (i.e. 5 holes for the 1 5 pattern and 25 holes for the 5 5 pattern). Each diffractive pattern has been tested five times and the subscript, i, stands for the number of the test. The final value of V was the arithmetic mean of each test: V = V i /5. G1 = 0 remained unchanged, while G2 was changing from 50 to 200 (Since the grating is an 8-bit computer generated hologram, there are totally 2 8 gray levels i.e. 0 255.). As shown, both 1 5 and 5 5 patterns reached the highest uniformity when the gray level difference was around 125, equivalent to a phase depth of. This shows the experimental results in accordance with theoretical prediction, because a phase depth of was assumed when calculating the Dammann gratings. Fig. 2. 1 5 (upper) and 5 5 (bottom) Dammann grating. Fig. 4(a) shows the laser profiler (Spiricon) observed 1 5 and 5 5 arrays, while Fig. 4(b) shows the holes machined on a polished Ti64 sample by 1 5 and 5 5 arrays (the total input pulse energy, measured in front of the scanning galvanometer, was approximately 0.2 mj). As demonstrated, 0-th order is always slightly stronger than the other diffracted orders, which decreases the uniformity of the whole pattern. Fig. 5(a) and (b) show the uniformity comparison between the pattern including and excluding the 0-th order machined hole. In this case, the gray level difference was focussed on a smaller range (100 150), because both patterns reached the highest uniformity when the gray level difference was around 125 (see Fig. 3). As shown, the uniformity was significantly degraded due to the slightly stronger 0-th order. Fig. 3. Variation of the hole size machined by 1 5 (diamonds) and 5 5 (squares) diffractive patterns versus gray level difference between G1 and G2.

Z. Kuang et al. / Applied Surface Science 273 (2013) 101 106 103 Fig. 4. (a) 1 5 (left) and 5 5 (right) arrays observed by laser profiler (Spiricon) when G1 = 0 and G2 = 125. (b) Blind holes machined on a polished Ti64 sample by applying 1 5 (left) and 5 5 (right) Dammann grating (G1 = 0, G2 = 125). 3.2. Adjustment of 0-th order by superimposing a Fresnel zone lens (FZL) As demonstrated in Section 3.1, the uniformity of the whole pattern was degraded by the slightly stronger 0-th order. Accordingly, if the 0-th order energy can be adjusted to be comparable to the diffracted orders at the processing plane, the pattern uniformity will be improved. This can be achieved by adding a phase hologram of Fresnel zone lens (FZL) to the Dammann gratings (DG). By superimposing a phase hologram of FZL, as shown in Fig. 6, the focal plane of the diffracted orders can be adjusted by changing the focal length of FZL, f FZL, while 0-th order beam remains unchanged. This creates a focal plane separation, d, between 0-th order and diffracted orders, as shown in Fig. 7. d > 0 when the added FZL works as a positive lens, and d < 0 when it works as a negative lens. When the processed sample is placed at the focal plane of the diffracted orders, the degree of defocusing 0-th order can be adjusted by d. This technique has been previously reported by us to remove the unwanted 0-th order [12]. As shown in Fig. 8(a), the hole size variation including the 0-th order decreased to V 4% when d > 3 mm. Fig. 8(b) demonstrates the holes machined on a polished Ti64 sample by 1 5 and 5 5 arrays (the total input pulse energy, measured in front of the scanning galvanometer, was approximately 0.2 mj) with the DG superimposed by a positive FZL, giving d = 3 mm. The pattern uniformity is significantly improved compared to Fig. 4(b).

104 Z. Kuang et al. / Applied Surface Science 273 (2013) 101 106 Fig. 5. (a) Variation of the hole size machined by 1 5 diffractive patterns include 0-th order (diamonds) and exclude 0-th order (squares) versus gray level difference between G1 and G2. (b) Variation of the hole size machined by 5 5 diffractive patterns include 0-th order (diamonds) and exclude 0-th order (squares) versus gray level difference between G1 and G2. Fig. 6. Dammann grating superimposed by a phase hologram of FZL. 3.3. Parallel surface patterning of indium tin oxide (ITO) using high uniformity Dammann grating (DG) generated beam array A parallel ultrashort pulse laser surface processing of ITO on glass was demonstrated by us previously with diffractive multibeam patterns [5]. The periodic and symmetrical geometry design was avoided to guarantee the high uniformity when using lenses and gratings (LG) algorithm [5,8]. To generate a beam array structure, the pattern was separated into several asymmetric parts playing one after another with accurate synchronization of scanning motions, which however significantly complicated the processing. Since DG generated beam arrays demonstrated high Fig. 7. Schematic showing the way to separate focal plane of diffracted orders and 0-th order. The added FZL can work as either positive lens (upper) or negative lens (lower) to obtain the separation: d = f 0 d 2 = f 0 (f FZLf 0 f 0d 1)/(f FZL + f 0 d 1) [12], where f FZL and f 0 are the focal length of FZL and f-theta lens respectively, d 1 is the distance between FZL and f-theta lens, d 2 is the distance between f-theta lens and sample plane. d > 0 when the added FZL works as a positive lens, and d < 0 when it works as a negative lens.

Z. Kuang et al. / Applied Surface Science 273 (2013) 101 106 105 Fig. 8. (a) Variation of the hole size machined by 1 5 (diamonds) and 5 5 (squares) diffractive patterns versus d. (b) Blind holes machined on a polished Ti64 sample by applying 1 5 (left) and 5 5 (right) Dammann grating superimposed a FZL giving d = 3 mm. uniformity (V < 4%), as shown in Section 3.2, they were applied in the present research to generate beam array structures directly on an ITO coated glass sample. The sample was generated from an ITO precursor solution prepared by SAFC Hitech. The precursor was evenly coated on a glass slide by a spin-coater and then annealed by a furnace to create the ITO coating (thickness 50 nm) with high transparency and conductivity. As shown in Fig. 9, the concentric circle arrays were generated by scanning a DG (superimposed by an FZL to give d = 3 mm) generated 5 5 beam array at a speed of v = 50 mm/s, while the LLEC pattern below (containing 32 diffractive spots) was generated by applying a computer generated hologram (CGH) calculated by the LG algorithm [13,14]. The total input pulse energy was approximately 0.2 mj. The 5 5 concentric circle pattern, which covered about S 5 5 0.25 mm 2 area on the ITO sample, was completed within t 5 5 < 10 ms (i.e. the patterning speed was 4 s/cm 2 ). This pattern can be repeated using the scanning galvanometer hence increasing the processing area. The LLEC pattern was completed by only t LLEC 1 ms allowing 10pulses (per spot) going through to achieve the thin film removal. The ITO coating was successfully removed by thermal-free ablation without any damages to the glass substrate, as shown in the inserted scanning electron microscope images. The machined structures were reasonably uniform (V 5 5 3.7% and V LLEC 8.9%). The ghosts in the LG algorithm generated LLEC pattern were below the ablation threshold of the ITO coating and can be neglected, because periodic and symmetrical geometry designs were avoided. However, the uniformity of the LLEC pattern is lower than the DG generated 5 5 beam array pattern. This is due to the fact that the degeneration caused by overlap of unwanted diffraction peaks may still degrade the uniformity of the desired peaks when using LG algorithm.

106 Z. Kuang et al. / Applied Surface Science 273 (2013) 101 106 Fig. 9. Parallel surface processing of ITO coated glass (left column: reconstruction of diffractive patterns; middle column: optical micrograph of the processed ITO sample; right column: scanning electron microscope (SEM) images showing the removal of ITO thin film they demonstrated elliptical structures, because the sample was tilted in the SEM). 4. Conclusion Uniform diffractive beam arrays (1 5 and 5 5) were created by Dammann grating (DG) for parallel processing using an ultrashort pulse laser (t p = 10 ps, = 1064 nm). The 0-th order, observed slightly stronger than the other higher orders, was adjusted by superimposing a Fresnel zone lens (FZL) and tuning the degree of defocusing at the processing plane. The uniformity is measured to be V < 4% after the adjustment. Additionally, a parallel surface patterning of indium tin oxide (ITO) thin film with periodic array structures was demonstrated using the Dammann grating generated beam array without requiring the complicated geometry separation and the time-consuming positioning. Acknowledgements The authors gratefully acknowledge the support of the Technology Strategy Board (through project PARALASE), the Northern Way programme and SAFC Hitech (Dr. Andrew Kingsley, for his help in making the ITO precursor solutions). References [1] Y. Hayasaki, T. Sugimoto, A. Takita, N. Nishida, Variable holographic femtosecond laser processing by use of a spatial light modulator, Applied Physics Letters 87 (2005) 031101. [2] S. Hasegawa, Y. Hayasaki, N. Nishida, Holographic femtosecond laser processing with multiplexed phase Fresnel lenses, Optics Letters 31 (2006) 1705 1707. [3] Z. Kuang, W. Perrie, J. Leach, M. Sharp, S.P. Edwardson, M. Padgett, G. Dearden, K.G. Watkins, High throughput diffractive multi-beam femtosecond laser processing using a spatial light modulator, Applied Surface Science 255 (5) (2008) 2284 2289. [4] Z. Kuang, D. Liu, W. Perrie, S. Edwardson, M. Sharp, E. Fearon, G. Dearden, K.G. Watkins, Fast parallel diffractive multi-beam femtosecond laser surface micro-structuring, Applied Surface Science 255 (13 14) (2009) 6582 6588. [5] Z. Kuang, W. Perrie, D. Liu, P. Fitzsimons, S.P. Edwardson, E. Fearon, G. Dearden, K.G. Watkins, Ultrashort pulse laser patterning of indium tin oxide thin films on glass by uniform diffractive beam patterns, Applied Surface Science 258 (2012) 7601 7606. [6] D. Liu, Z. Kuang, W. Perrie, J. Cheng, S. Shang, S.P. Edwardson, E. Fearon, G. Dearden, K.G. Watkins, High-speed uniform parallel 3D refractive index microstructuring of poly(methyl methacrylate) for volume phase gratings, Applied Physics B 101 (4) (2010) 817 823. [7] D. Liu, W. Perrie, Z. Kuang, P.J. Scully, A. Baum, S. Liang, S.P. Edwardson, E. Fearon, G. Dearden, K.G. Watkins, Multi-beam second harmonic generation in beta barium borate with a spatial light modulator and application to internal structuring in poly(methyl methacrylate), Applied Physics B 107 (3) (2012) 795 801. [8] J.E. Curtis, C.H.J. Schmitz, J.P. Spatz, Symmetry dependence of holograms for optical trapping, Optics Letter 30 (2005) 2086 2088. [9] H. Dammann, K. Gortler, High-efficiency in-line multiple imaging by means of multiple phase holograms, Optics Communications 3 (5) (1971) 312 315. [10] U. Krackhardt, N. Streibl, Design of dammann-gratings for array generation, Optics Communications 74 (1-2) (1989) 31 36. [11] D.C. O Shea, T.J. Suleski, A.D. Kathman, D.W. Prather, Diffractive Optics: Design, Fabrication, and Test, 2003, pp. 83 113 (Chapter 5). ISBN: 9780819451712. [12] Z. Kuang, W. Perrie, D. Liu, S. Edwardson, J. Cheng, G. Dearden, K.G. Watkins, Diffractive multi-beam surface micro-processing using 10ps laser pulses, Applied Surface Science 255 (22) (2009) 9040 9044. [13] J. Liesener, M. Reicherter, T. Haist, H.J. Tiziani, Multi-functional optical tweezers using computer-generated hologram, Optics Communications 185 (2000) 77 82. [14] 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, Applied Optics 45 (2006) 897 903.