UD

ZKL

a

ARRAA

KUSD

1

aimfin[nabTcdah

wlm

0h

Applied Surface Science 273 (2013) 101– 106

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j our nal ho me p age: www.elsev ier .com/ loc ate /apsusc

ltrafast laser parallel microprocessing using high uniformity binaryammann grating generated beam array

heng Kuang ∗, Walter Perrie, Dun Liu, Stuart P. Edwardson, Yao Jiang, Eamonn Fearon,en G. Watkins, Geoff Dearden

aser Group, Centre for Material and Structures, School of Engineering, University of Liverpool, Brownlow Street, Liverpool L69 3GQ, United Kingdom

r t i c l e i n f o

rticle history:eceived 11 November 2012eceived in revised form 25 January 2013ccepted 28 January 2013vailable online 4 February 2013

eywords:ltrafast laser

a b s t r a c t

Ultrafast laser parallel processing using diffractive multi-beam patterns generated by a spatial light mod-ulator (SLM) has demonstrated a great increase in processing throughput and efficiency. Applicationsranging from surface thin film patterning to internal 3D refractive index modification have been recentlyreported with the parallel processing technology. Periodic and symmetrical geometry design (e.g. N × Mbeam array) of the multi-beam pattern must be avoided to guarantee the required high uniformity inthese applications, which, however, limited the processing flexibility. In this paper, Dammann gratingsare used to create diffractive 1 × 5 and 5 × 5 beam arrays for the parallel processing. The 0-th order,

pecial Light Modulatorammann grating

observed slightly stronger than the other higher orders, can be adjusted by superimposing a Fresnel zonelens (FZL) and tuning the degree of defocusing at the processing plane. The uniformity (presented bythe variation of the machined hole diameter) is measured to be <4% after the adjustment. Additionally, aparallel surface patterning of indium tin oxide (ITO) thin film with periodic array structures was demon-strated using the Dammann grating generated beam array without requiring the complicated geometryseparation and the time-consuming positioning.

. Introduction

Parallel processing using diffractive multi-beam patterns gener-ted by a spatial light modulator (SLM) is a novel method to greatlyncrease the processing throughput and efficiency of ultrafast laser

aterial processing [1–4]. Applications ranging from surface thinlm patterning of transparent conducting oxides (TCOs) [5] to inter-al 3D refractive index modification of poly(methyl methacrylate)6,7] have been recently reported with the parallel processing tech-ology. In order to reach the required high uniformity in thesepplications, periodic and symmetrical geometry design (e.g. N × Meam array) of the diffractive multi-beam pattern must be avoided.his is due to the fact that periodic and symmetrical designsan significantly increase the probability of overlap of unwantediffraction peaks, called ‘ghosts’, and degrade the reconstructionccuracy when using lenses and gratings (LG) algorithm [8]. This,owever, limited the processing flexibility.

With this limitation in mind, binary Dammann gratings [9–11]

hich can create uniformed beam arrays were applied for the paral-

el processing. According to grating equation, sin�m = m�/�, (where 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).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.01.195

© 2013 Elsevier B.V. All rights reserved.

2D binary grating can generate m × m diffractive beam array. Theenergy directed into each of the diffracted orders is dictated bythe shape and nature of the surface profile within a single gratingperiod, called a grating unit cell [11]. Dammann et al. [9] firstlydeveloped a straightforward method of calculating the unit cellstructure for binary gratings with uniformed energy distribution.These gratings are named Damman gratings.

In this paper, Two Dammann gratings (1 × 5 and 5 × 5), providedby Holoeye Photonics, were used for the parallel processing of Ti64.The 0-th order, observed slightly larger than the other 24 higherorders, can be adjusted by superimposing a Fresnel zone lens (FZL)and tuning the degree of defocusing at processing plane. The uni-formity (presented by the variation of the machined hole diameter)is measured to be <4% after the adjustment. Additionally, a par-allel surface patterning of indium tin oxide (ITO) thin film withperiodic array structures was demonstrated using the Dammanngrating generated beam array, without requiring the complicatedgeometry separation and the time-consuming positioning.

2. Experimental

The ultrafast laser system used for this research is a custommade Nd: VAN seeded regenerative amplifier laser system (High-QIC-355-800ps, Photonic Solutions). Schematic of the experimentalsetup is shown in Fig. 1. The laser output (tp = 10 ps, � = 1064 nm,

102 Z. Kuang et al. / Applied Surface Science 273 (2013) 101– 106

ftaodwibfiwAelttm(pte

3

3D

AtmadpwmWhhdsmwc0uaaw

Fig. 2. 1 × 5 (upper) and 5 × 5 (bottom) Dammann grating.

Fig. 1. Experimental setup.

= 10 kHz) passes through a half wave plate used for adjustinghe linear polarization direction, a beam expander (M ≈ ×3), andfter reflection on mirrors 1, 2 and 3, illuminates a reflective phasenly SLM, A Hamamatsu X10468-03 liquid crystal on silicon (LCoS)evice with 800 × 600 pixels and dielectric coated for 1064 nmavelength (reflectivity � ≈ 95%), oriented at <10 degree angle of

ncidence. A flipping mirror, placed after lens 1, can reflect theeam to a charge-coupled device (CCD) camera-based laser pro-ler (Spiricon) to observe the reconstructed multi-beam patternshen it is flipped into beam line. A 4f-optical system is formed from

to D to remove the unwanted 0-th order beam [4]. The beam thennters a scanning galvanometer with f0 = 100 mm flat field f-thetaens (Nutfield) producing an agile focusing system. The light utiliza-ion efficiency of the system (i.e. the ratio of the energy output afterhe laser head and the output after the scanning galvanometer) is

easured to be >90%. Substrates are mounted on a precision 5-axisx, y, z, p, q) motion control system (Aerotech) allowing accurateositioning of the substrate surface at the laser focus. The spec-ral bandwidth, �� < 0.3 nm, is relatively narrow and important inliminating chromatic dispersion of the SLM [4,12].

. Results and discussions

.1. Uniformity measurement of the beam arrays created byammann grating

Fig. 2 demonstrates the Dammann gratings used in this research.s shown in grating unit cells, the periodic binary structure has

wo gray levels, G1 and G2. Fig. 3 shows the uniformity of the holesachined by the Damman grating generated beam arrays (1 × 5

nd 5 × 5 patterns) on a polished Ti64 sample versus the gray levelifference between G1 and G2. The uniformity was quantitativelyresented by the variation of the hole diameter, V. The variation, Vi,as calculated by: Vi = (�i/ai) × 100%, where ai and �i are arithmeticean and standard deviation of the hole diameter, respectively.hen calculating ai and �i, the number of the included measured

ole diameters depended on the applied diffractive pattern (i.e. 5oles for the 1 × 5 pattern and 25 holes for the 5 × 5 pattern). Eachiffractive pattern has been tested five times and the subscript, i,tands for the number of the test. The final value of V was the arith-etic mean of each test: V = �Vi/5. G1 = 0 remained unchanged,hile G2 was changing from 50 to 200 (Since the grating is an 8-bit

omputer generated hologram, there are totally 28 gray levels i.e.–255.). As shown, both 1 × 5 and 5 × 5 patterns reached the highestniformity when the gray level difference was around 125, equiv-

lent to a phase depth of . This shows the experimental results inccordance with theoretical prediction, because a phase depth of as assumed when calculating the Dammann gratings.

Fig. 4(a) shows the laser profiler (Spiricon) observed 1 × 5and 5 × 5 arrays, while Fig. 4(b) shows the holes machined on apolished Ti64 sample by 1 × 5 and 5 × 5 arrays (the total inputpulse energy, measured in front of the scanning galvanometer,was approximately 0.2 mJ). As demonstrated, 0-th order is alwaysslightly stronger than the other diffracted orders, which decreasesthe uniformity of the whole pattern. Fig. 5(a) and (b) show theuniformity comparison between the pattern including and exclud-ing the 0-th order machined hole. In this case, the gray leveldifference was focussed on a smaller range (100–150), becauseboth patterns reached the highest uniformity when the gray leveldifference was around 125 (see Fig. 3). As shown, the unifor-mity was significantly degraded due to the slightly stronger 0-thorder.

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 applying1

3l

tidwo

tiu

× 5 (left) and 5 × 5 (right) Dammann grating (G1 = 0, G2 = 125).

.2. Adjustment of 0-th order by superimposing a Fresnel zoneens (FZL)

As demonstrated in Section 3.1, the uniformity of the whole pat-ern was degraded by the slightly stronger 0-th order. Accordingly,f the 0-th order energy can be adjusted to be comparable to theiffracted orders at the processing plane, the pattern uniformityill be improved. This can be achieved by adding a phase hologram

f Fresnel zone lens (FZL) to the Dammann gratings (DG).

By superimposing a phase hologram of FZL, as shown in Fig. 6,

he focal plane of the diffracted orders can be adjusted by chang-ng the focal length of FZL, fFZL, while 0-th order beam remainsnchanged. This creates a focal plane separation, �d, between 0-th

order and diffracted orders, as shown in Fig. 7. �d > 0 when theadded FZL works as a positive lens, and �d < 0 when it works asa negative lens. When the processed sample is placed at the focalplane of the diffracted orders, the degree of defocusing 0-th ordercan be adjusted by �d. This technique has been previously reportedby us to remove the unwanted 0-th order [12].

As shown in Fig. 8(a), the hole size variation including the 0-thorder decreased to V ≈ 4% when |�d| > 3 mm. Fig. 8(b) demon-strates the holes machined on a polished Ti64 sample by 1 × 5

and 5 × 5 arrays (the total input pulse energy, measured in frontof the scanning galvanometer, was approximately 0.2 mJ) with theDG superimposed by a positive FZL, giving �d = 3 mm. The patternuniformity 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 differencebetween 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 leveldifference between G1 and G2.

Fig. 6. Dammann grating superimpos

ed by a phase hologram of FZL.

3.3. Parallel surface patterning of indium tin oxide (ITO) usinghigh uniformity Dammann grating (DG) generated beam array

A parallel ultrashort pulse laser surface processing of ITO onglass was demonstrated by us previously with diffractive multi-beam patterns [5]. The periodic and symmetrical geometry designwas avoided to guarantee the high uniformity when using lensesand gratings (LG) algorithm [5,8]. To generate a beam arraystructure, the pattern was separated into several asymmetric

parts playing one after another with accurate synchronization ofscanning motions, which however significantly complicated theprocessing. Since DG generated beam arrays demonstrated high

Fig. 7. Schematic showing the way to separate focal plane of diffracted orders and0-th order. The added FZL can work as either positive lens (upper) or negative lens(lower) to obtain the separation: |�d| = |f0 − d2| = |f0 − (fFZLf0 − f0d1)/(fFZL + f0 − d1)|[12], where fFZL and f0 are the focal length of FZL and f-theta lens respectively, d1 isthe distance between FZL and f-theta lens, d2 is the distance between f-theta lensand sample plane. �d > 0 when the added FZL works as a positive lens, and �d < 0when it works as a negative lens.

Z. Kuang et al. / Applied Surface Science 273 (2013) 101– 106 105

F s) difa �d = 3

utaIebt

bepaLmaw

ig. 8. (a) Variation of the hole size machined by 1 × 5 (diamonds) and 5 × 5 (squarepplying 1 × 5 (left) and 5 × 5 (right) Dammann grating superimposed a FZL giving

niformity (V < 4%), as shown in Section 3.2, they were applied inhe present research to generate beam array structures directly onn ITO coated glass sample. The sample was generated from anTO precursor solution prepared by SAFC Hitech. The precursor wasvenly coated on a glass slide by a spin-coater and then annealedy a furnace to create the ITO coating (thickness ≈ 50 nm) with highransparency and conductivity.

As shown in Fig. 9, the concentric circle arrays were generatedy scanning a DG (superimposed by an FZL to give �d = 3 mm) gen-rated 5 × 5 beam array at a speed of v = 50 mm/s, while the ‘LLEC’attern below (containing 32 diffractive spots) was generated bypplying a computer generated hologram (CGH) calculated by the

G algorithm [13,14]. The total input pulse energy was approxi-ately 0.2 mJ. The 5 × 5 concentric circle pattern, which covered

bout S5 × 5 ∼ 0.25 mm2 area on the ITO sample, was completedithin t5 × 5 < 10 ms (i.e. the patterning speed was ≈4 s/cm2). This

fractive patterns versus �d. (b) Blind holes machined on a polished Ti64 sample by mm.

pattern can be repeated using the scanning galvanometer henceincreasing the processing area. The ‘LLEC’ pattern was completedby only tLLEC ≈ 1 ms allowing 10pulses (per spot) going through toachieve the thin film removal. The ITO coating was successfullyremoved by ‘thermal-free’ ablation without any damages to theglass substrate, as shown in the inserted scanning electron micro-scope images.

The machined structures were reasonably uniform (V5 × 5 ≈ 3.7%and VLLEC ≈ 8.9%). The ‘ghosts’ in the LG algorithm generated ‘LLEC’pattern were below the ablation threshold of the ITO coating andcan be neglected, because periodic and symmetrical geometrydesigns were avoided. However, the uniformity of the ‘LLEC’ pattern

is lower than the DG generated 5 × 5 beam array pattern. This is dueto the fact that the degeneration caused by overlap of unwanteddiffraction peaks may still degrade the uniformity of the desiredpeaks when using LG algorithm.

106 Z. Kuang et al. / Applied Surface Science 273 (2013) 101– 106

F of diffr ITO tht

4

bsssdtpses

A

opm

R

[

[

[

[

ig. 9. Parallel surface processing of ITO coated glass (left column: reconstruction

ight column: scanning electron microscope (SEM) images showing the removal ofhe SEM).

. Conclusion

Uniform diffractive beam arrays (1 × 5 and 5 × 5) were createdy Dammann grating (DG) for parallel processing using an ultra-hort pulse laser (tp = 10 ps, � = 1064 nm). The 0-th order, observedlightly stronger than the other higher orders, was adjusted byuperimposing a Fresnel zone lens (FZL) and tuning the degree ofefocusing at the processing plane. The uniformity is measuredo be V < 4% after the adjustment. Additionally, a parallel surfaceatterning of indium tin oxide (ITO) thin film with periodic arraytructures was demonstrated using the Dammann grating gen-rated beam array without requiring the complicated geometryeparation and the time-consuming positioning.

cknowledgements

The authors gratefully acknowledge the support of the Technol-gy Strategy Board (through project PARALASE), the Northern Wayrogramme and SAFC Hitech (Dr. Andrew Kingsley, for his help inaking the ITO precursor solutions).

eferences

[1] Y. Hayasaki, T. Sugimoto, A. Takita, N. Nishida, Variable holographic femtosec-ond 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 processingwith 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. Dear-den, K.G. Watkins, High throughput diffractive multi-beam femtosecond laser

[

ractive patterns; middle column: optical micrograph of the processed ITO sample;in film – they demonstrated elliptical structures, because the sample was tilted in

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. Dear-den, K.G. Watkins, Fast parallel diffractive multi-beam femtosecond lasersurface 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 onglass 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 micro-structuring of poly(methyl methacrylate) for volume phase gratings, AppliedPhysics 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 inbeta barium borate with a spatial light modulator and application to internalstructuring 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 foroptical trapping, Optics Letter 30 (2005) 2086–2088.

[9] H. Dammann, K. Gortler, High-efficiency in-line multiple imaging bymeans 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 twee-

zers 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 opticaltweezers control, Applied Optics 45 (2006) 897–903.