We report the fabrication of planar sub-micron gratings in silicon with a period of 720 nm using a modified Michelson interferometer and femtosecond laser radiation. The gratings consist of alternated stripes of laser ablated and unmodified material. Ablated stripes are bordered by parallel ridges which protrude above the unmodified material. In the regions where ridges are formed, the laser radiation intensity is not sufficient to cause ablation. Nevertheless, melting and a significant temperature increase are expected, and ridges may be formed due to expansion of silicon during resolidification or silicon oxidation. These conclusions are consistent with the evolution of the stripes morphology as a function of the distance from the center of the grating. (C) 2013 Elsevier Ltd. All rights reserved

Conde, O.

Oliveira, J. C.

Oliveira, Vitor

Polushkin, N. I.

Serra, R.

Vilar, Rui

Optics & Laser Technology

Repositório Científico do Instituto Politécnico de Lisboa

  Title: Sub-micron structuring of silicon using femtosecond laser interferometry Author(s):  Oliveira, V. [1,3]; Vilar, R. [2,3]; Serra, R. [4]; Oliveira, J. C. [4]; Polushkin, N. I. [2,3]; Conde, O. [3,5]  Source: Optics and Laser Technology Volume: 54 Pages: 428-431 DOI: 10.1016/j.optlastec.2013.06.031 Published: Dec 30 2013 Document Type: Article Language: English Abstract:  We report the fabrication of planar sub-micron gratings in silicon with a period of 720 nm using a modified Michelson interferometer and femtosecond laser radiation. The gratings consist of alternated stripes of laser ablated and unmodified material. Ablated stripes are bordered by parallel ridges which protrude above the unmodified material. In the regions where ridges are formed, the laser radiation intensity is not sufficient to cause ablation. Nevertheless, melting and a significant temperature increase are expected, and ridges may be formed due to expansion of silicon during resolidification or silicon oxidation. These conclusions are consistent with the evolution of the stripes morphology as a function of the distance from the center of the grating. (C) 2013 Elsevier Ltd. All rights reserved.  Author Keywords: Silicon patterning; Femtosecond laser; Michelson interferometer  KeyWords Plus: Fabricaton; Ablation; Pulses; Si; Interference; Surfaces; Gratings; Lithography; Solids; Arrays   Reprint Address: Oliveira, V (reprint author) - Inst Super Engn Lisboa, Ave Rovisco Pais 1, P-1049001 Lisbon, Portugal.  Addresses: [1] Inst Super Engn Lisboa, P-1959007 Lisbon, Portugal [2] Inst Super Tecn, P-1049001 Lisbon, Portugal [3] ICEMS, P-1049001 Lisbon, Portugal [4] Univ Coimbra, CEMUC, Dept Engn Mecan, P-3030788 Coimbra, Portugal [5] Univ Lisbon, Fac Ciencias, Dept Fis, P-1749016 Lisbon, Portugal  E-mail Addresses: voliveira@adf.isel.pt  Funding: Funding Agency Grant Number Portuguese Foundation for Science and Technology (FCT)   PTDC/FIS/121588/2010  SSFRH/BPD 17382712010   Publisher: Elsevier SCI LTD Publisher Address: The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, Oxon, England  ISSN: 0030-3992  Citation: OLIVEIRA, V.; VILAR, R.; SERRA, R.; OLIVEIRA, J. C.; POLUSHKIN, N. I.; CONDE, O. - Sub-micron structuring of silicon using femtosecond laser interferometry. Optics and Laser Technology. ISSN 0030-3992. Vol. 54 (2013), p. 428-431.  

Sub-micron structuring of silicon using femtosecond laser interferometry

We report the fabrication of planar sub-micron gratings in silicon with a period of 720 nm using a modified Michelson interferometer and femtosecond laser radiation. The gratings consist of alternated stripes of laser ablated and unmodified material. Ablated stripes are bordered by parallel ridges which protrude above the unmodified material. In the regions where ridges are formed, the laser radiation intensity is not sufficient to cause ablation. Nevertheless, melting and a significant temperature increase are expected, and ridges may be formed due to expansion of silicon during resolidification or silicon oxidation. These conclusions are consistent with the evolution of the stripes morphology as a function of the distance from the center of the grating

Vilar, R.

Oliveira, J.C.

Polushkin, N.I.

Optics & Laser Technology 54 (2013) 428–431Contents lists available at SciVerse ScienceDirectOptics & Laser Technology0030-39http://dn CorrPais noE-mjournal homepage: www.elsevier.com/locate/optlastecSub-micron structuring of silicon using femtosecondlaser interferometryV. Oliveira a,c,n, R. Vilar b,c, R. Serra d, J.C. Oliveira d, N.I. Polushkin b,c, O. Conde c,ea Instituto Superior de Engenharia de Lisboa, Avenida Conselheiro Emídio Navarro no 1, 1959-007 Lisboa, Portugalb Instituto Superior Técnico, Avenida Rovisco Pais no 1, 1049-001 Lisboa, Portugalc ICEMS - Instituto de Ciência e Engenharia de Materiais e Superfícies, Avenida Rovisco Pais no 1, 1049-001 Lisboa, Portugald CEMUC - Centro de Engenharia Mecânica da Universidade de Coimbra, Departamento de Engenharia Mecânica, Universidade de Coimbra,Rua Luís Reis Santos, 3030-788 Coimbra, Portugale Faculdade de Ciências da Universidade de Lisboa, Departamento de Física, Campo Grande Ed. C8, 1749-016 Lisboa, Portugala r t i c l e i n f oArticle history:Received 16 May 2013Received in revised form27 June 2013Accepted 28 June 2013Available online 24 July 2013Keywords:Silicon patterningFemtosecond laserMichelson interferometer92/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.optlastec.2013.06.031espondence to: Instituto Superior de Engenha1, 1049-001 Lisboa, Portugal. Tel.: +351 218 3ail address: voliveira@adf.isel.pt (V. Oliveira).a b s t r a c tWe report the fabrication of planar sub-micron gratings in silicon with a period of 720 nm using amodiﬁed Michelson interferometer and femtosecond laser radiation. The gratings consist of alternatedstripes of laser ablated and unmodiﬁed material. Ablated stripes are bordered by parallel ridges whichprotrude above the unmodiﬁed material. In the regions where ridges are formed, the laser radiationintensity is not sufﬁcient to cause ablation. Nevertheless, melting and a signiﬁcant temperature increaseare expected, and ridges may be formed due to expansion of silicon during resolidiﬁcation or siliconoxidation. These conclusions are consistent with the evolution of the stripes morphology as a function ofthe distance from the center of the grating.& 2013 Elsevier Ltd. All rights reserved.1. IntroductionSurface patterning using pulsed lasers and holographic opticalset-ups proved to be a powerful and versatile tool for micro-machining and patterning [1–6]. The principle of holographicsurface patterning is simple: when two or more pulses of coherentradiation overlap in time and space, an interference pattern isgenerated that can be used to imprint periodic structures on amaterial surface. Femtosecond lasers are particularly attractive forthis purpose, as compared to continuous-wave and long-pulseduration lasers [7,8] because, due to their extremely short pulseduration, a very high peak power is achieved. As a result, non-linear effects and a plasma-mediated ablation mechanism thatminimizes collateral thermal effects are favored [7–9]. Takingadvantage of this, high quality periodic micro and nanostructureswere successfully fabricated using femtosecond laser radiation andinterferometric techniques in bulk transparent dielectric materials[4,10], metals [5,11] and thin ﬁlms [3,12]. Due to the importance ofsilicon in the microelectronic and solar cell industries, Si surfacetexturing using femtosecond laser radiation has been extensivelyreported in the literature. The large majority of the studies on Sitexturing deal with the formation of self-organized arrays ofmicrocones [13,14] or sub-wavelength laser-induced periodic sur-face structures [15,16] created by irradiating silicon with multiplell rights reserved.ria de Lisboa, Avenida Rovisco17 135; fax: +351218317138.femtosecond laser pulses of near-IR radiation, and aimed at con-trolling the material wettability [17,18], optical properties [19], orﬁeld emission efﬁciency [20]. In contrast, only a few studies reportthe formation of periodic patterns in silicon using femtosecondlaser interferometry [21,22]. Tan et al. [21] successfully fabricatedplanar gratings with periods between 1.6 and 4.0 μm in bulksilicon, using a Mach–Zender interferometer and radiation from aTi-Sapphire laser (400 nm wavelength, 200 fs pulse duration). Themain drawback of this Mach–Zender interferometer is the mini-mum achievable gratings period which depends on the ratio D/f,where D is the focusing lens diameter and f its focal length. As aresult, in order to achieve sub-micron structures with commonnear-infrared lasers (λ∼1 μm), lenses with a D/f ratio greater than1 must be used. In this paper, we describe the fabrication of sub-micron planar gratings on silicon using a modiﬁed Michelsoninterferometer and a single femtosecond laser pulse. The mainadvantages of this setup with respect to those reported previouslyare the simplicity of the interferometer design and alignment, thepossibility of sub-micron structuring using ordinary focusing lenses,and of the procedure used to monitor time coincidence. The workpresented shows that femtosecond laser interferometry may beused to create versatile nanoscaled structures such as siliconnanowires for sensing applications [23].2. Materials and methodsAll experiments were performed in air on 500 μm thick Si (100)wafers. Processing was carried out with a commercial Yb:KYWV. Oliveira et al. / Optics & Laser Technology 54 (2013) 428–431 429chirped-pulse-regenerative ampliﬁcation laser system (AmplitudeSystèmes, s-Pulse HP) providing linearly polarized laser pulseswith 560 fs duration at a central wavelength λ¼1030 nm. Thepatterns were produced using a beam delivery system based on aMichelson interferometer, similar to the one previously usedto create sub-micron patterns in thin-ﬁlm magnetic alloys [24](Fig. 1(a)). In this optical set-up, the laser beam is ﬁrstly split intwo by a non-polarizing beam splitter. Beams 1 and 2 are reﬂectedby mirrors M1a and M2a, respectively and, after crossing the beamsplitter, directed to the focusing lenses FL1 and FL2 by mirrors M1band M2b, respectively. Finally, the two beams are directed to thesame area on the sample's surface by mirrors M1c and M2c. Thefringe spacing is given by d¼ λ=½2 sin ðθ=2Þ, where λ denotes theradiation wavelength and θ the angle between the interferingbeams. The angle θ depends on the distance between the mirrorsM1c and M2c, D, and between the sample and the line deﬁned bythe centers of the two mirrors, L. It results from simple trigono-metry that θ=2¼ tan 1ðD=2LÞ. The fringes spacing can be con-trolled by changing either D or L. In the present study, D¼40 mmand L¼18 mm, leading to a fringe spacing d∼700 nm. In order tooverlap the two beams in time, the optical path length of beam2 can be controlled by a precise translation of mirror M2a. Todetect time coincidence, the sample is substituted by a common3M post-it green paper, and the paper two-photon ﬂuorescencemonitored by a video camera. Because the two-photon absorptionis proportional to the square of the light intensity, the lightintensity emitted by the paper increases when time coincidenceoccurs. This is demonstrated in Fig. 1(b) where images of the lightemitted by the paper before and after time coincidence aredepicted. After processing, the surfaces were observed by scanningelectron microscopy (SEM) and atomic force microscopy (AFM).SEM was performed using a JEOL 7001 ﬁeld-emission scanningelectron microscope (FE-SEM). AFM measurements were carriedout in air and at room temperature with a Brunker Innova systemin tapping mode using a Si probe with 8 nm tip radius.Fig. 2. (a) Top view FE-SEM image of the central region of a grating created insilicon using single laser pulse; (b) its magniﬁed view tilted by 301.3. Results and discussionThe conditions for gratings production were optimized byvarying the laser pulse energy. Good quality and approximatelycircular gratings with about 125 μm diameter were produced bythe interference of two 100 μJ laser pulses. The gratings consist ofparallel stripes with a constant period of about 720 nm, in goodagreement with the predicted 700 nm. Figs. 2 and 3 depict SEMand AFM images of the central region of a grating. The dark stripesFig. 1. (a) Optical scheme for direct laser interference patterning. The notations used are abeam 1 and 2, FL 1(2) are focusing lenses. Mirror M2a was mounted on a translation st(b) Images and intensity proﬁle (inset) of the light emitted by a common green paperbeams are coincident in time.in the SEM image correspond to higher radiation intensity regions,where silicon was ablated, while the bright stripes correspond toregions of lower radiation intensity, where no signiﬁcant mod-iﬁcations occurred. Interestingly, the ablated stripes present 80 nmwide longitudinal parallel ridges at each side, separated by∼340 nm. The AFM topographic proﬁle of Fig. 3(b) shows thatthese ridges typically protrude 3–4 nm above the original surfacelevel, and the ablated regions are 1–2 nm deep.We calculated the radiation intensity distribution at the targetsurface, I. Assuming that the spatial radiation intensity distributionof the two interfering beams is the same, I can be expressed asI¼ 2I0½1þ cos ð2πx=dÞ; ð1Þwhere d is the fringe period and I0 the interfering beams intensitydistribution. Assuming that the interfering beams are Gaussians follows: BS is beam splitter, M1a ((b) and (c)) and M2a ((b) and (c)) are mirrors forage for ﬁnding overlapping in time by adjusting a time delay between the beams.when (1) the interfering beams are not coincident in time; and (2) the interferingFig. 3. (a) AFM image of the central region of a grating and (b) the correspondingcross-sectional topographic proﬁle.Fig. 4. Calculated radiation intensity proﬁle in the vicinity of a stripe in the centerof the grating and corresponding FE-SEM image and AFM topographic proﬁle.Fig. 5. SEM images and AFM proﬁles of the same stripe as a function of thedistance r from the grating center, and corresponding intensity proﬁle calculatedusing Eq. (1): (a) r¼0; (b) r¼10 μm; (c) r¼20 μm; (d) r¼30 μm; and (e) r¼60 μm.V. Oliveira et al. / Optics & Laser Technology 54 (2013) 428–431430and square laser pulses, I0 is given byI0 ¼2Eτπw2e2r2=w2 ð2Þwhere E is the pulse energy, τ is the pulse duration, r is the radialdistance, and w is the grating radius. The radiation intensitydistribution calculated using Eq. (1), taking E¼100 μJ, w¼62.5 μm,τ¼560 fs and r¼0 is depicted in Fig. 4 together with the FE-SEMimage of a stripe obtained in the central region of the grating usingthese parameters. The ablated region III corresponds to radiationintensities in the range of 1.0–1.21013 W/cm2. The protrudingridges (region II) correspond to radiation intensities in the range of0.6–1.01013 W/cm2. Finally, the unmodiﬁed material (region I)correspond to radiation intensities below 0.61013 W/cm2. Basedon the above considerations, the grating proﬁle is explained asfollows. In region III, silicon is ablated because the radiation intensityis higher than the ablation threshold. On the contrary, in region IIthe radiation intensity is below the ablation threshold and ablationdo not occur. Nevertheless, melting and a signiﬁcant temperatureincrease are expected. As a result, the protruding ridges may beformed due to melting followed by expansion of silicon duringresolidiﬁcation [25,26], or, since the laser treatment was performedin air, due to silicon oxidation favored by the temperature increase inthese regions [27,28]. Finally, in region I, the radiation intensity isinsufﬁcient to cause signiﬁcant visible changes in the target material.The proposed explanation is consistent with the evolution ofthe pattern proﬁle from the center of the grating to its periphery.Fig. 5 shows FE-SEM images and AFM topographic proﬁles of thesame stripe at different distances from the grating center, and thecorresponding radiation intensity distribution calculated usingEq. (1). In the central region of the gratings where the radiationintensity is maximum, the three distinct regions previously men-tioned are observed (Fig. 5a and b), but in the periphery of thegrating only two regions are observed because the radiationintensity was not sufﬁcient to cause ablation (Fig. 5c and d).Finally, at the border of the grating, the radiation intensity is toosmall to cause any signiﬁcant changes in the material.4. ConclusionsIn summary, planar gratings with a period of about 720 nmnanometers were produced in silicon by single-pulse laser treat-ment using a modiﬁed Michelson interferometer and femtosecondlaser radiation. The gratings consist of alternated stripes of ablatedand unaffected material. The ablated stripes are surrounded byparallel ridges separated of about 340 nm which protrude 3–4 nmabove the initial surface. The laser radiation intensity in theregions of ablated stripes is high enough to cause ablation. Onthe contrary, in the regions where ridges are formed melting and asigniﬁcant temperature increase is expected. As a consequence,ridges may be formed due to silicon expansion during resolidiﬁca-tion or oxidation.AcknowledgmentsWork was supported by the Portuguese Foundation for Scienceand Technology (FCT) through the Project PTDC/FIS/121588/2010and Grant SSFRH/BPD 17382712010 (R.S.).References[1] Rezek B, Nebel CE, Stutzmann M. Interference laser crystallization of micro-crystalline silicon using asymmetric beam intensities. Journal of Non-Crystalline Solids 2000;266:650–3.[2] Kaganovskii Y, Vladomirsky H, Rosenbluh M. Periodic lines and holes pro-duced in thin Au ﬁlms by pulsed laser irradiation. Journal of Applied Physics2006;100:044317.[3] Kawamura K, Sarukura N, Hirano M, Hosono H. Holographic encoding of ﬁne-pitched micrograting structures in amorphous SiO2 thin ﬁlms on silicon by asingle femtosecond laser pulse. Applied Physics Letters 2001;78:1038–40.V. 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Sub-micron structuring of silicon using femtosecond laser interferometry

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