ISSN 00213640, JETP Letters, 2015, Vol. 101, No. 6, pp. 394–397. © Pleiades Publishing, Inc., 2015. Original Russian Text © D.A. Zayarny, A.A. Ionin, S.I. Kudryashov, S.V. Makarov, A.A. Rudenko, S.G. Bezhanov, S.A. Uryupin, A.P. Kanavin, V.I. Emel’yanov, S.V. Alferov, S.N. Khonina, S.V. Karpeev, A.A. Kuchmizhak, O.B. Vitrik, Yu.N. Kulchin, 2015, published in Pis’ma v Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 2015, Vol. 101, No. 6, pp. 428–432.
Nanoscale Boiling during SingleShot Femtosecond Laser Ablation of Thin Gold Films¶ D. A. Zayarnya, A. A. Ionina, S. I. Kudryashova, b, *, S. V. Makarova, c, A. A. Rudenkoa, S. G. Bezhanova, b, S. A. Uryupina, b, A. P. Kanavina, b, V. I. Emel’yanovd, S. V. Alferove, S. N. Khoninae, S. V. Karpeeve, A. A. Kuchmizhakf, O. B. Vitrik f, g, and Yu. N. Kulchin f, g a
Lebedev Physical Institute, Russian Academy of Sciences, Leninskii pr. 53, Moscow, 119991 Russia *email:
[email protected] b National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe sh. 31, Moscow, 115409 Russia c ITMO University, St. Petersburg, 197101 Russia d Moscow State University, Moscow, 119899 Russia e Image Processing Systems Institute, Russian Academy of Sciences, Samara, 443001 Russia f Institute for Automation and Control Processes, Far Eastern Branch, Russian Academy of Science, Vladivostok, 690041 Russia g Far Eastern Federal University, Vladivostok, 690041 Russia Received January 26, 2015
A nanoscale chaotic relief structure appears as a result of subthreshold singleshot femtosecond laser ablation of gold films in the regimes of fabrication of microbumps and nanospikes, but only for a relatively thick film. The observed nanoablation tendency versus film thickness makes it possible to suppose the existence of a sub surface temperature maximum in thicker gold films and its absence within thinner film, which results from competing evaporative cooling and electronic heat conduction, as demonstrated by numerical simulations of the thermal dynamics. DOI: 10.1134/S0021364015060132
1. Despite numerous preceding observations of various surface mesostructures—nanocrowns, microbumps, nanojets, and nanoholes—in thin films and on surfaces of bulk materials during their single and multishot nanoscale ablation by nanosecond [1– 5] and femtosecond [6–8] laser pulses, their formation mechanisms are not well understood yet. A rather good approximation in understanding the mecha nisms of nanoscale ablation is the description of mac roscopic femtosecond (fs) laser ablation provided in a number of recent studies [9–15]. In particular, it was demonstrated that spallative femtosecond laser abla tion of thin films and bulk materials is initiated by delayed subsurface boiling of a molten layer [13, 14], rather than thermoelastic stresses, with picosecond evaporative cooling of the layer suppressing its phase explosion [15]. As a result, nanoscale spallation in thin films proceeds in two stages: (1) irreversible formation of a microbump apparently owing to high vapor pres sure in a vapor cavity between the film and its substrate [2, 10, 12] and (2) complete film liftoff during hydro dynamic expulsion of a molten nanojet [11]. ¶The article was translated by the authors.
In the present work, we report results of a compar ative study of singleshot fs laser ablation of thinner and thicker gold films, indicating similarity of their relief evolution versus fs laser fluence, with exception of a subthreshold nanoroughness on the thicker film. 2. In our studies, laser irradiation of fresh spots of samples was performed by single femtosecond, second harmonic (SH) pulses of a Yb3+doped fiber laser (Satsuma, Amplitude Systemes) [16]: SH wavelength is 515 nm, FWHM duration is 200 fs, maximum pulse energy is 4 μJ in TEM00 mode, repetition rate is 0– 2 MHz. Laser radiation was focused onto the sample surface in air by a microscope objective with numerical aperture NA = 0.25 into a circular spot with the radius R1/e ≈ 0.45 μm (Fig. 1). The sample was arranged on a threedimensional motorized translation stage with minimum step of 150 nm, moved from pulse to pulse. The resulting ablative relief was visualized using a JEOL 7001F scanning electron microscope (SEM). As samples, we used thin films of gold (80%) and pal ladium (20%) alloy with their thicknesses of h ≈ 60 and ≈200 nm, deposited in an argon atmosphere on dielec tric (silica glass) substrates by means of a magnetron sputtering system (SC7620, Quorum Technologies).
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Fig. 1. (Color online) Experimental setup for nanoscale laser ablation of material surfaces: (RA) reflection attenuator, (AC) auto correlator, (EM) pyroelectric energy meter, (WL) whitelight halogen illumination lamp, (BS) beam splitter, (CCD) CCD cam era, and (PC) laptop for control of the laser and 3D motorized translation stage.
3. Singleshot fs laser exposure of the thinner film at the peak laser fluence F ≈ 0.5 J/cm2 resulted in its partial melting and resolidification, becoming evident in the form of significantly larger recrystallized metal lic nanograins (Fig. 2a). In contrast, lowfluence fs laser modification of the thicker film at F ≈ 1.5 J/cm2 occurred as surface nanofoaming with the nanorough ness scale increasing toward the spot center (Fig. 2d). The origin of such nanoscale surface foaming becomes clear at higher laser fluences: for the thinner film at F ≈ 0.7 J/cm2 a central microbump appears, besides the film melting (Fig. 2b). Similarly, for the thicker film at F ≈ 1.8 J/cm2, such a central microbump also emerges on the ablated spot, but the nanofoam structure is now smoothened on its remolten top (Fig. 2e). Finally, at a further increase in fluence, a central nanospike (“fro zen nanojet”) appears for both the thinner (F ≈ 0.9 J/cm2) and thicker (F ≈ 2.5 J/cm2) films (Figs. 2c, 2f), with one or a few nanoparticles atop. In the latter regime, the foamlike structure on the thicker film is displaced to the crater periphery, appearing as a regu lar nanocrown with one or two circular rows of tips (Fig. 2f). The observed evolution of the surface topog raphy of the thicker film—nanofoam, microbump, nanospike—demonstrates obvious consistency not only in the peak fluence domain but, apparently, also in terms of the film temperature increasing to near critical magnitudes to initiate explosive growth of a nanojet [11, 17]. Importantly, the characteristic flu JETP LETTERS
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ence values for the appearance of the different nanofeatures in the thicker films are well scalable to the thinner film, accounting for the ratio of their thicknesses. In our opinion, these facts indicate that, for the thicker film, subsurface boiling takes place, which is, among other physical and chemical factors, the main driving process not only for microbump and nanojet formation on surfaces of thin films [2, 10, 12], but also for surface spallation in bulk materials [13, 14]. One possible reason for subsurface boiling can be the sub surface temperature maximum [5, 18–23], emerging on a subnanosecond timescale within a thick film owing to a balance of heat conduction and intense evaporative cooling, as experimentally demonstrated in [15]. Otherwise, in the case of thin film, heat con duction is fast and evaporative cooling shifts the corre sponding temperature maximum toward the unirradi ated film edge. In order to calculate temperature profiles in these films, numerical simulations were undertaken for temperatures of electrons and lattice for the condi tions of our experiments. The equation for electron temperature accounted for a heating source term cor responding to optical absorption of the incident inho mogeneous electromagnetic field in the films, elec tronic heat conduction term, and electron–lattice coupling term. The latter term was also included in the equation for lattice temperature. The corresponding
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Fig. 3. Temperature profile in the 200nm thick gold film 13 ps after single 515nm, 200fs, 10TW/cm2 pump pulse. Inset: Temperature profile in the 60nmthick gold film 11 ps after single 515nm, 200fs, 7TW/cm2 pump pulse. The displacement of each curve relative to the beginning of the x axis indicates the thickness of the vapor ized layer at the corresponding time instant.
Fig. 2. Obliqueincidence (≈45°) SEM images of 60nm and 200nm gold films ablated by single fs laser pulses at F ≈ (a) 0.5, (b) 0.7, (c) 0.9, (d) 1.5, (e) 1.8, and (f) 2.5 J/cm2. The remolten central zone is shown by the dashed circle in image (a). The scale bars are different for different images.
temperature dependences of heat capacities, elec tron–lattice coupling, and effective collision frequen cies were taken from [24–26], which were previously tested in simulations of gold film absorbance [27]. Moreover, the fivefold lower thermal conductivity of the gold alloy, as compared to that of pure gold, was also taken into account [28]. The set of temperature equations was solved until equilibration of electron and lattice temperatures at ≈7 ps, corresponding to the electron–lattice energy exchange timescale for the electron temperature of 0.6 eV. As a result, at that time, the transient lattice temperature exceeded the melting temperature of gold, Tmelt ≈ 1337 K [29]. The laserinduced film melting resulted in significant reduction of its thermal conductivity [30], accounted for in an additional equation for melt temperature. Also, on this timescale, the film was intensely cooled by surface evaporation [5, 18–23], as was accounted for by the boundary conditions [21]. The temperature profiles calculated for the 60nm and 200nm films for the 200fs laser pulse are shown in Fig. 3. In the case of the 200nm film, at the laser intensity of 10 TW/cm2 and the time instant of 13 ps, the temperature profile demonstrates a pronounced maximum at a distance of 50–60 nm from the outer surface with its magnitude
considerably exceeding the corresponding surface temperature, as well as the normal boiling temperature of gold Tboil ≈ 3150 K [29], in agreement with surface nanotopographies presented in Figs. 2a and 2d. The results of the numerical calculation for the 60nm film at the laser intensity of 7 TW/cm2 at the time instant of 11 ps are presented in the inset to Fig. 3. As seen from the inset, the temperature maximum within the thin ner film is absent owing to less intense evaporative cooling and efficient heat conduction across the film [25, 31]. 4. Therefore, in this study, pronounced effects of subsurface boiling were observed for the first time dur ing singleshot femtosecond laser ablation of sup ported gold films in the regimes of fabrication of microbumps and nanospikes, supporting the evapora tive origin of the latter surface features. Our theoretical analysis and numerical simulations demonstrate that subsurface boiling becomes possible in relatively thick films owing to subnanosecond appearance of a sub surface temperature maximum as a result of a balance between electronic heat conduction and surface evap orative cooling in the films. This work was supported by the Russian Founda tion for Basic Research (project nos. 130200971a, 130201377a, 140200460a, 140200748a, 14 3250026 mol_nr), by the Presidium of the Russian Academy of Sciences, by the ITMO PostDoctoral Fellowship Program (government subsidy no. 074 U01), and by the Ministry of Education and Science of the Russian Federation (resolution no. P218, con tract no. 02.G25.31.0116 on August 14, 2014, between Open Joint Stock Company Ship Repair Center JETP LETTERS
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