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Guided self-organization of nanodroplets induced by nanosecond IR laser radiation of molybdenum films on sapphire
Optics and Lasers in Engineering 113 (2019) 55–61
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Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Guided self-organization of nanodroplets induced by nanosecond IR laser radiation of molybdenum films on sapphire Igor Zagoranskiy a,∗, Pierre Lorenz a, Martin Ehrhardt a,b, Klaus Zimmer a a b
Leibniz Institute of Surface Engineering (IOM), Permoserstraße 15, Leipzig 04318, Germany Advanced Launching Co-innovation Center, Nanjing University of Science and Technology, #200 XiaoLingWei, 210094 Nanjing, Jiangsu, People’s Republic of China
a r t i c l e
i n f o
Keywords: Nanosecond laser Self-organization Directed self-organization Ripples Droplet formation Nanostructuring
a b s t r a c t The fabrication of sub-micron and nanopatterns at large surface areas by lasers is still a challenge, but the use of laser-induced, self-organized processes provides a promising approach. Here, the guided formation of nanodroplets ordered in two directions during irradiation of 15 nm thick molybdenum (Mo) film on Al2 O3 (1–102) substrate with a fibre laser having a wavelength of 1064 nm and selectable pulse duration from 5 to 600 ns is demonstrated. The two-step process comprises laser-induced ripple formation and remelting of the ripples for nanodroplet generation. The ripples with period of ∼1.1 μm are oriented perpendicularly to the laser polarization. The ripples are fabricated by irradiation with 46 laser pulses with a duration between 25 to 100 ns and fluences of 0.3 to 1.3 J/cm2 , respectively. After a subsequent laser irradiation with 10 pulses at fluences above 1 J/cm2 and a pulse duration of 100 ns, two kinds of Mo droplets are observed: (i) large, periodically ordered droplets with a diameter of ∼360 nm and a distance along and orthogonal to the laser-generated ripples of about 1 μm and 1.1 μm, respectively, in addition to (ii) small, less-ordered droplets with a size of about 120 nm localized in the former ripple valleys.
1. Introduction Micro- and nanopattern fabrication on technical surfaces is challenging because traditional lithographic approaches need precise surfaces and can hardly be applied to non-planar substrates. Nanopattern fabrication by self-organization methods is suitable for defect-tolerating applications and must be preferred for low-cost fabrication. Self-organization processes can result in random, partially, or nearly completely ordered nanostructures. However, various applications require at least partially ordered nanostructures that cannot be achieved easily by selforganizing processes. Consequently, various approaches for guiding selforganization and self-assembly processes have been shown. Here, we focus on laser-irradiation guided and laser-induced nanopattern formation by the remelting of thin metal films on wide bandgap dielectric material, in particular on sapphire. Laser-induced nanopattern formation at the surfaces of dielectriclike fused silica, sapphire, polymers is interesting for optical [1–5], wetting [6,7], and tribological applications [8,9]. In particular, nanopatterned metal films could be applied for plasmonics, e.g., optical filters [10–14]. Sapphire is a crystalline, wide-band gap material (Eg ∼10 eV) with superb mechanical and chemical stability and an optical transparency in the wavelength range from 0.2 to 5 μm [15]. Due to these
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properties, sapphire has various applications for semiconductor fabrication and mechanical protection, as well as in the optical industry. The formation of randomly distributed nanoparticles attached to surfaces by the irradiating of extremely thin metal films on thermally insulating substrates with nanosecond-pulsed laser has been demonstrated by several groups [16–18]. At low-laser fluences, the mechanism of metal formation comprises melting of the film, dewetting of the molten metal film due to hydrodynamic instabilities, and resolidification after redistribution of the molten metal, developing nanodroplet distributions [16,19]. The formation of random or specific organized droplet distributions in different thin metal films like Co, Cu, Ag, Fe, Ni, Pt, Zn, Ti, V, and Mn (layer thickness 1–25 nm) on fused silica applying Nd: YAG laser (pulse duration 9 ns, wavelength 266 nm) was studied by Favazza, et al. [19]. Laser-induced droplet formation by thin metal film dewetting can be described by combining heat equation, phase transitions, and Navier–Stokes equation, which describe laser heating, melting, and mass transport in liquids, respectively [20]. Such metal nanodroplet distributions can be utilized for nanopattern fabrication in bulk material through laser-enhanced pattern transfer of such attached nanodroplets into the substrate, e.g., fused silica, as reported by Lorenz, et al. [21]. Laser-induced periodic surface structures (LIPSS) had been first observed in the 1960s by irradiating crystalline materials with a pulsed ruby laser [22]; this phenomenon was observed later for different ma-
Corresponding author. E-mail address: [email protected] (I. Zagoranskiy).
https://doi.org/10.1016/j.optlaseng.2018.10.005 Received 18 May 2018; Received in revised form 20 August 2018; Accepted 7 October 2018 0143-8166/© 2018 Elsevier Ltd. All rights reserved.
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terials exposed to nanosecond-pulsed lasers [23,24]. Currently, the formation of LIPSS — this term is now related to ripples and other types of laser-induced, partially ordered nanopatterns — is often observed at ultrashort, pulsed laser irradiation of surfaces [25–29]. The LIPSS can be classified into LSFL (low-spatial frequency LIPSS) and HSFL (highspatial frequency LIPSS). The wavelength Λ of LSFL has a period close to 𝜆/n where 𝜆 and n are laser wavelength and refraction index, respectively. The LSFL can be classified into two types. In type 1 LSFL, the periodic structures are regularly oriented perpendicularly to the laser beam polarization. In the case of type 2, LSFL are oriented parallel to the laser beam polarization [28]. The formation of type 1 LSFL is normally considered to be related to the excitation of surface plasmon-polaritons (SPP) [28]. Due to multipulse irradiation, the formation processes of LSFL can be enhanced through feedback. HSFL that feature a period shorter than half of the laser wavelength [28] are primarily observed close to the modification threshold for high number of applied laser pulses. The mechanism of HSFL formation is still under discussion and may include a hydrodynamic approach [30–33]. LIPSS are found by irradiation of solids in gases and in liquids, whereas the period is influenced by the surrounding material— for instance by the refractive index. A higher refractive index of the confining material (e.g., water) may result in shorter LIPSS periods in comparison to those formed in air [34]. LIPSS in non-metallic films, including diamond-like carbon and ZnO [35], have been observed for ultrashort laser pulses irradiation, too; here, the strong excitation of the material may provide conditions for the temporary formation of SPP in different materials. In relation to the current work, Scorticati, et al. [36] and Lopez, et al. [37] reported ps- and fs-laser induced ripples generation in thin molybdenum layers. LIPSS formation was studied for bulk materials by Bonse, et al. [38] using different pulse durations, ranging from 150 fs to 7 ns. It was found that LIPSS appear close to the melting point. The guided formation of droplet distributions providing at least a partial order has been shown already [39–43]. However, the final achieved periodically distributed droplets can been fabricated only by laser irradiation, if very precise lithographic metal patterns [39,40], e.g., metal strips [41] as well as metal rings on dielectric surface [42,43] have been used. This limits the applicability, as challenging lithographic or high-resolving processes has to be applied. The driving force for this periodic forming process during temporal laser melting is surface tension, which is supported by the Plateau–Rayleigh instability. Also, the laser ablation with interfering laser beams, which can be achieved by phase mask projection in order to produce periodic metal film patterns, has been utilized to guide the droplet position. In this way, nanodot arrays and grids can be induced on different metal-covered substrates [44,45]. Here, we report on the guided self-organization by utilizing only one laser source for the fast fabrication of ordered metal nanodroplets on dielectric substrates. The LIPSS are induced firstly in the form of ripples and remolded in the next step to periodic ordered droplets. Both the ripples formation in molybdenum films by ns-laser irradiation, as well as the utilization of the formed molybdenum ripples for ordered nanodroplet fabrication, demonstrate the abilities of ns lasers irradiation for guiding the self-organizing process of nanopattern fabrication with low cost equipment and fabrication allowing high processing speed.
sample is washed in deionized water and dried in a nitrogen stream. The cleaned sample is coated with 15 nm molybdenum (Mo) film. Magnetron sputtering was applied with argon pressure of 10−3 mbar, plasma power of 150 W, resulting in a deposition rate ∼0.5 nm/min. The analysis of the layer thickness distribution that was performed by optical transmission measurement with a CW laser of about 100 μm spot size, showed that the variation of the metal layer thickness is less than 1%.
2.2. Laser treatment The experimental set-up for laser irradiation is schematically presented in Fig. 1a). A fibre laser (PyroFlex 25, EOLITE) is incorporated into a workstation (INNOLAS Systems). The fibre laser emits at a wavelength of 1064 nm, features a TEM00 spatial mode, and has a beam quality M2 of less than 1.3. The pulse duration Δtp can be varied from 1 to 600 ns with a precision of 1 ns. The repetition rate is fixed at f = 10 kHz. The sample is mounted with a spacing of 10 mm to the stage in order to reduce reflections from the stage surface. The XY and Z stages allow the sample positioning in relation to the scanner head with 1 μm precision. X and Y motions of the laser spot during the laser irradiation are accomplished by the scanner head. The laser beam is focussed with an F-Theta lens (focal length of 160 mm) that is attached to the scanner. A particular Gaussian radius can be chosen by varying the distance between scanner head and the sample surface; the size needs to be evaluated for each defocusing position. Liu-method is applied in order to determine the Gaussian radius in a 1 mm defocusing step [47]. From this dataset, the waist position and Gaussian radius in focus plane can be found. In the focus plane the Gaussian radius is calculated to be 𝜔 = (12.7 ± 0.5) μm. The laser irradiation is performed in two steps: (i) Mo ripple formation and (ii) transferring of the rippled pattern into Mo droplets. Different laser spot sizes that were realized by defocusing are used for the two irradiation steps. For ripple formation, a spot with a radius of 𝜔 = (115 ± 6) μm is applied so that a wider patterning (broader rippled area) can be achieved. For this spot size, the fluence ranges for ripple formation were determined for all chosen pulse durations. It is most suitable to choose such laser fluence that ripples appear near the centre of the Gaussian energy density distribution in order to realize a reliable fabrication of rippled lines, to maximize the ripples lines width, and to avoid locally strong variation of the ripple pattern near FWHM (Full Width at Half Maximum) of the laser spot. This condition can be achieved by selecting appropriate laser pulse energy. A suitable scanning speed is found to be 0.05 m/s and can be converted in terms of number of pulses, N = 46 ± 2. Lines of 6 mm length with ripples appear orthogonal to the line scan direction are fabricated. Step two is aimed to reorganize the ripples pattern into periodic distributed droplets. Selected laser spots were irradiated in a step-andrepeat process (dots in Fig. 1b) and d). Further, the number of pulses is set to N = 10, and a Gaussian beam radius 𝜔 of (37 ± 2) μm is applied in order to achieve higher laser fluences. The laser-treated samples are analyzed by optical and scanning electron microscopy (SEM) and the sample surface is covered by 10 nm gold for the SEM measurements. From the SEM images, the size and the distribution of the formed particles are evaluated by image processing.
2. Experimental set-up 2.1. Sample preparation
3. Results
Double-sided polished Al2 O3 (1–102) wafer (diameter 76.2 mm) is used. The surface roughness is measured to be 0.25 nm RMS (root mean square value). First, the sample is cleaned by RCA 1 solution (ammoniac solution 30%, hydrogen peroxide, and water in 1:1:5 combinations). Further cleaning is realized by RCA 2 solution (hydrochloric acid, hydrogen peroxide, and water in 1:1:5 combinations) [46]. Each treatment is applied for 30 minutes in an ultra-sonic bath at 333 K. Finally, the
The laser-based fabrication is performed in two separate steps: ripples fabrication and remelting of the formed ripples to produced ordered droplets patterns. The separation in a sequence of two distinct steps allows full control on the laser processing parameters for ripple and droplet fabrication. 56
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Fig. 1. Sketch of the experimental set-up, (a) and the procedure of laser irradiation, (b). Two steps are applied for the whole laser irradiation procedure: (i) scanning with a defocused spot (larger Gaussian radius) for inducing of ripples in form of extensive long lines of Mo and (ii) forming of Mo ripples pattern into droplets by fast laser melting at higher laser fluences (applying smaller Gaussian radius). c) Adjustment of Gaussian beam radiuses for two step processing. d) SEM image of pattern on sample. Both processes are visible: Ripple formation in some fluence range along a line treatment and forming process in the form of dots.
3.1. Ripple formation at laser irradiation
3.2. Remelting of rippled Mo film
At first, the appearance of ripples in the molybdenum film is studied regarding dependence on the pulse duration and the laser fluence. Ripples are found for pulse durations Δtp from 25 to 100 ns. The lower and upper limits of the laser fluence for ripples appearance can be determined from their distance y from the laser spot centre (from laser scan center to the rippled lines) of the Gaussian distribution of the laser intensity. The lower fluence limit is defined as the transitions from a smooth to a capillary wave-like surface. The upper fluence limit is defined as the transition from a capillary wave-like surface to hole-like patterns. A typical SEM image of the irradiated area is shown in Fig. 2a), where ripples appear. The calculated local fluence of the Gaussian laser intensity distribution is presented on the right side. In addition, the determined thresholds of ripples appearance in dependence on pulse duration, including the inaccuracy of determination, are shown schematically. The measurement uncertainties of the laser pulse energy and the Gaussian beam radius have been considered for the estimation of the overall inaccuracy. SEM images show that the ripples appear orthogonally to the polarization direction of laser. The period of the ripples is calculated from SEM images to be Λ = (1086 ± 30) nm. The SEM image of Fig. 2b) shows a reference ripple structure that is utilized for further laser irradiation to form droplets. In Fig. 2c), measured limits of the laser fluence for the appearance of ripples are shown. In general, the fluence for ripple formation increases with the increasing of the laser pulse duration.
Thereafter, additional laser pulses with a pulse duration of 100 ns are applied to the rippled line pattern (Fig. 2b) in order to produce periodical distribution of droplets. In Fig. 3, different results of the remelting processes are presented. For calculating, the droplet size distribution image processing of SEM photographs is carried out in subsequent steps. Applying image-processing software, the particle size distribution is calculated. The circularity of particles was limited to the range of 0.8 to 1.0 in order to detect only droplets and to exclude strip-like structures. Data of droplet size distribution is statistically analyzed. The occurrence of particles was calculated with a data interval width of 25 nm. Fig. 3a shows incomplete separation of droplets at a fluence of F = (1.1 ±0.1) J/cm2 . In particular, metal bridges between droplets and non-circular shaped particles can be observed. With a slight increase of the laser fluence to F = (1.2 ± 0.1) J/cm2 , a complete separation of droplets can be found (Fig. 3b). They have primarily a circular shape. Additionally, deep holes can be found in the sapphire substrate at fluence F = (1.7 ± 0.1)J/cm2 (see Fig. 3c). Below the SEM images, the related droplet size distributions are shown, wherein two peaks can be clearly distinguished. The total number of droplets in the intervals representing a particular droplet size range was fitted with Gaussian curves in order to find mean values of droplet size distribution. According to the SEM image in Fig. 3b) that was taken at a fluence F = (1.2 ± 0.1) J/cm2 , a periodicity in lateral distribution of the droplet is observed. For droplets with a mean size of 0.38 ± 0.09 μm, two different periods can be found: along and orthogonally to the previous formed
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Fig. 2. (a) and (b) SEM image of the irradiated 15 nm Mo/Al2 O3 : structured with N = 46 ± 2, 𝜔 = 115 μm and Δtp = 100 ns. Lower and upper limits of fluence for ripples are shown schematically. The y scale serves to represent distance from laser spot centre. At b), the reference structure is presented that will be applied for further forming procedures (applied fluence in the range of (1.1–1.2 ± 0.1) J/cm2 ). c) Laser fluencies found for ripples formation: upper and lower limits are presented in dependence on pulse duration.
Fig. 3. (a), (b) and (c) SEM images of the irradiated 15 nm Mo/Al2 O3 : Additional N = 10 pulses with 𝜔 = 37 μm, Δtp = 100 ns and a) F = (1.1 ± 0.1) J/cm2 , b) F = (1.2 ± 0.1) J/cm2 and c) F = (1.7 ± 0.1) J/cm2 were applied to reference ripples pattern (F = (1.1–1.2 ± 0.1) J/cm2 , N = 46) presented at Fig. 2b). SEM image graphs present droplet size distribution dependent upon applied fluence. Gaussian fits were applied. Here, the tolerance of peak values corresponds to the radius of the Gaussian droplet distribution.
ripples. Droplet periods of (0.965 ± 0.118) μm and (1.095 ± 0.069) μm were measured, respectively. The droplet period orthogonal to the ripples is similar to the period of original ripple structures. In the SEM image, it can also be observed that small droplets appear between big droplets along previous ripples hills.
of the sample. The surface plasmon-polariton (SPP) can be induced by laser irradiation on interface between thin the molybdenum layer and air (Fig. 4(1)). The SPP interacts with the laser light and cause interference effects [25–28]. Finally, the periodic modulated laser irradiation results in a periodical temperature profile (Fig. 4(2)). Localised melting, in addition to temperature profile induced alterations of the surface tension and viscosity may occur [48]. The mass movement occurs obviously in liquid state and follows the gradient of temperature (Marangoni-effect) [49]. Therefore, the mass movement of molten metal is periodically predefined (Fig. 4(3)). The local thinning of the metal film finally results in rupture of the film and in dewetting of the molten molybdenum from the sapphire. Multipulse laser irradiation allows the enhancement of metal ripple formation. After several pulses, the surface tension of molten metal contributes in the material separation from rippled surface to the strips and further from strips (Fig. 4(4)) to droplets (Fig. 4(5)).
4. Discussion 4.1. LIPSS formation Formally, the formation process of ripples can be presented as a stepwise process, as shown in Fig. 4. A short representation of the process is shown here. More detailed information will be given in the following two parts. The molybdenum layer surface has a roughness of 0.25 nm RMS. Laser interaction with surface material is affected by the roughness, which results in enhanced scattering of laser light in the plane 58
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Fig. 4. Schematic illustration of the formation process. (1) The SPP is induced to interface air/molybdenum (period of ripples close to excitation wavelength). SPP interferes with scattered light, which results in a periodic temperature modulation. At moment (2), the modulated temperature field is above the melting point of molybdenum. Coloring in shades of red serves to express higher temperature. The low thermal conductivity of Al2 O3 (metal layer is too thin and negligible) allows the ripple formation in some certain pulse durations. A final product is a rippled surface (3), strips (4). or a periodic droplet distribution (5). Three different types of droplets sizes can be formed: Type S1, S2, and B.
4.2. Rippling of Mo films, upper and lower limits In the experiment, it was found that ripples could be generated in an interval of fluences that depends on the pulse duration (Fig. 3c). Considering the thermal diffusion length LTD as √ a length measure for the energy dissipation, that is given by 𝐿TD = 2 𝐷𝑇 ℎ 𝑡 with t for a characteristic time (e.g., duration of laser pulse) characteristic lengths can be calculated for material specific thermal diffusivity Dth . with the DTh ∼0.8 ∗ 10−6 m2 /s for Al2 O3 and 24.6 ∗ 10−6 m2 /s for Mo at ∼2000 K reported by Touloukian, et al. [50]. Significant contributions of the metal film to the heat dissipation can be neglected due to the low thickness of the metal film in relation to the heated thickness of the substrate. With the observed ripples period Λ of ∼1.1 μm and considering of the symmetry of the heat dissipation from the hot sections of the laser-induced temperature distribution, the maximal pulse duration below which thermal equalization is prevented can be calculated with (LTD /2)2 /(4 ∗ DTh ). The calculated value of ∼125 ns is slightly higher than experimentally observed 100 ns but explains well the reason for the upper limit of the pulse length. Thermal simulation of the temperatures for melting and evaporation for the experimental studies parameter range according to the simulations in Lorenz, et al. [48] was performed. The results show that the absolute difference fluence thresholds for evaporation and melting is reduced with decreasing pulse duration (see Fig. 5). Also in the experiment, the upper and lower limits of ripple appearance come closer at decreased pulse duration. The lower limit of ripples appearance may be related, in addition to heating effects from the laser plume or the dynamics of the heating and melting processes that reduce significantly with shorter pulse durations. In particular, the lifetime of the molten molybdenum might be too short. Longer liquid lifetimes can be achieved with higher fluencies as the cooling time down to the melting point is longer, but the threshold for evaporation / laser ablation should not be exceeded. Therefore, we conclude that the dynamics of the heating, including the related phase transitions, control the short pulse limit of ripples appearance. In general, as expected, the same tendency as in the experiments of the need for lower fluence required for heating to a significant temperature (Tm ) has been found. The achieved ripples orientation is not influenced by scanning direction or sample orientation and orthogonal to the laser polarization direction. The period of ripples Λ is (1086 ± 30)nm and close to the ex-
Fig. 5. Laser fluence thresholds for evaporation and melting of 15 nm Mo on Al2 O3 dependent upon pulse duration are estimated by FEM simulation, based on [48]. The grey curve represents the absolute difference between two values.
citation wavelength (1064 nm) and to the wavelength of the SPP at the molybdenum air interface that has been calculated according to [51] to be 1052 nm. The found ripples belong to first type of LSFL. Other types of ripples were not observed. 4.3. Remelting of rippled Mo film The formed ripples [period Λ ∼ 1086 nm, line width of the formed broad stripes (line patterns of the ripples peaks) ∼190 nm and of the slim stripes (ripples valleys) ∼65 nm] were performed by a second step of multipulse irradiation. This forming process is probably a stepwise process comprising melting, mass transport in the liquid, and resolidification from each step. The experimental results (see Fig. 3) show a transfer from strips to droplets where two different kinds of droplets have been found (see Figs. 3 and 4): Type B: big droplets are localized at the former position of the broad stripes of the ripples. Type S belongs to small droplets. The small droplets can be also be distinguished in two kinds of droplets: Type S1 — small droplets are localized at the former position of the slim stripes (in the middle between two broad stripes) and Type S2 — small droplets are localized between two big droplets (Type B) at the former position of a broad stripe. 59
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References
The fitted droplet size distribution (see Fig. 3b) feature two maxima at 140 nm (Type S) and 380 nm (Type B), respectively. The period of type B droplets that are orthogonally organized to the ripples is, with ∼1.1 μm, similar to the ripples period. The formation of the periodic ordered droplets along the ripples can be explained by the PlateauRayleigh instability. The Plateau-Rayleigh instability describes the separation of a liquid cylinder into droplets due to inherent instabilities. The period of formed droplets can be estimated by equation ΛRP = 9.02∗ R0 , where the R0 is radius of a liquid cylinder, here representing the molten film metal. For simplicity, the interaction with the sapphire surface is not taken into account. Neglecting evaporation of molybdenum, the volume of the film volume should be constant within redistribution of the film during melting. Therefore, the estimated thickness of the wide Mo stripes is Δz = 86 nm. With the radius of an equivalent cylinder that is given with R0 = ((d ∗ Δz)/𝜋)0.5 , the droplet period of ΛRP ∼700 nm can be found. This estimated value is near the experimental found period of ∼644 nm. The SEM image in Fig. 2a) can be also helpful for understanding the process of type S2 droplets formation. During laser irradiation of rippled metal film, the droplets start to separate from each other. This effect is supported by Plateau-Rayleigh instability and was already simulated for nickel strips in [41]. However, filaments between unseparated droplets can be observed (Fig. 3a). Further irradiation of such bridgecoupled droplets is caused by filament separation from the droplets and subsequent remelting of the filaments to type S2 droplets. In [41], it is mentioned that the size of these droplets and period of breaking up are dependent on filament length and width. The type S1 droplets are formed on the former position of the slim stripes that have an average width of at least a factor of 3 smaller than the broad lines. Furthermore. the SEM images imply that the slim stripes have a lower thickness compared to the broad lines (see Fig. 2b). The distinct, smaller average cross-section of the slim stripes results in droplets of type S1 that can be explained by Plateau-Rayleigh instability.
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Periodical structures induced by femtosecond laser on metals in air and liquid environments. Appl Surf Sci 2013;278:347–51. [30] Sedao X, Shugaev MV, Wu C, Douillard T, Esnouf C, Maurice C, Reynaud S, Pigeon F, Garrelie F, Zhigilei LV, Colombier JP. Growth twinning and generation of high-frequency surface nanostructures in ultrafast laser-induced transient melting and resolidification. ACS Nano 2016;10:6995–7007. [31] Li XF, Zhang CY, Li H, Dai QF, Lan S, Tie SL. Formation of 100-nm periodic structures on a titanium surface by exploiting the oxidation and third harmonic generation induced by femtosecond laser pulses. Opt Express 2014;22:28086–99. [32] Volkov SN, Kaplan AE, Miyazaki K. Evanescent field at nanocorrugated dielectric surface. Appl Phys Lett 2009;94:041104. [33] Martsinovskiǐ GA, Shandybina GD, Smirnov DS, Zabotnov SV, Golovan LA, Timoshenko VY, Kashkarov PK. Ultrashort excitations of surface polaritons and waveguide modes in semiconductors. 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5. Conclusion All-laser fabrication of periodical nanodroplet distributions by guided self-organization processes without utilization high resolution laser techniques has been shown. The process comprises two steps, with each particular process parameters: first the generation of ripples and second the remelting of the ripples stripes to ordered droplets. Hydrodynamic effects provide the main driving forces for the ripples and the droplet formation, due to the needed movement of molten material. Ripples can be generated only in a limited pulse length range from 25 to 100 ns. The long pulse limit is determined by partial vanishing of the temperature modulation, due to heat dissipation in the substrate. Hence, the thermal diffusion length in substrate material must be smaller than the ripple period that is defined by the interference of the incidence and the surface scattered waves. The short pulse limit is probably given by the reduction of the molten time of the metal film, due to the shorter pulse duration. Further laser irradiation of rippled metal film leads to formation of periodic ordered droplets. Different mechanism determine the period of the droplets in the two orthogonal directions. Hence, the distance can be selected by different process parameters, in particular by selecting the wavelength that determine the droplet period perpendicular to the ripple direction and the film thickness that determine the droplet distance along the ripple direction.
Acknowledgement We appreciate the support by the Deutsche Forschungsgemeinschaft (DFG) under LO 1986/2-1. 60
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[34] Albu C, Dinescu A, Filipescu M, Ulmeanu M, Zamfirescu M. Periodical structures induced by femtosecond laser on metals in air and liquid environments. Appl Surf Sci 2013;278:347–51. [35] Miyaji G, Miyazaki K. Origin of periodicity in nanostructuring on thin film surfaces ablated with femtosecond laser pulses. Opt Express 2008;16:16265–71. [36] Scorticati D, Skolski JZP, Römer GRBE, Veld AJHit, Workum M, Theelen M, Zeman M. Thin film surface processing by ultrashort laser pulses (USLP). In: SPIE Photonics Europe. SPIE; 2012. p. 8. [37] Camacho-Lopez S, Olea MAO, Evans R, Castillo G, Herrera MAM, Esparza A, Bauelos Muet JG, Processing of metallic thin films using Nd:YAG laser pulses, DOI 10.5772/35994(2012). [38] Bonse J, Krüger J. Pulse number dependence of laser-induced periodic surface structures for femtosecond laser irradiation of silicon. J Appl Phys 2010;108:034903. [39] Roberts NA, Fowlkes JD, Mahady K, Afkhami S, Kondic L, Rack PD. Directed assembly of one- and two-dimensional nanoparticle arrays from pulsed laser induced dewetting of square waveforms. ACS Appl MaterInterfaces 2013;5:4450–6. [40] Wu Y, Dong N, Fu S, Fowlkes JD, Kondic L, Vincenti MA, de Ceglia D, Rack PD. Directed liquid phase assembly of highly ordered metallic nanoparticle arrays. ACS Appl Mater Interfaces 2014;6:5835–43. [41] Fowlkes JD, Roberts NA, Wu Y, Diez JA, González AG, Hartnett C, Mahady K, Afkhami S, Kondic L, Rack PD. Hierarchical nanoparticle ensembles synthesized by liquid phase directed self-assembly. Nano Lett 2014;14:774–82. [42] Nguyen TD, Fuentes-Cabrera M, Fowlkes JD, Diez JA, González AG, Kondic L, Rack PD. Competition between collapse and breakup in nanometer-sized thin rings using molecular dynamics and continuum modeling. Langmuir 2012;28:13960–7.
[43] González AG, Diez JA, Kondic L. Stability of a liquid ring on a substrate. J Fluid Mech 2013;718:246–79. [44] Mäder M, Höche T, Gerlach JW, Böhme R, Rauschenbach B. Nanostructures by diffraction mask projection laser ablation. Physica Status Solidi (b) 2010;247:1372–83. [45] Mäder M, Höche T, Gerlach JW, Böhme R, Zimmer K, Rauschenbach B. Large area metal dot matrices made by diffraction mask projection laser ablation. Physica Status Solidi (RRL) Rapid Res Lett 2008;2:34–6. [46] Kern W, Puotinen D. Cleaning solutions based on hydrogen peroxide for use in silicon semiconductor technology. RCA Rev 1970;31:187. [47] Liu JM. Simple technique for measurements of pulsed Gaussian-beam spot sizes. Opt Lett 1982;7:196–8. [48] Lorenz P, Zagoranskiy I, Ehrhardt M, Bayer L, Zimmer K. Nanostructuring of sapphire using time-modulated nanosecond laser pulses. In: Proc. SPIE, 10092; 2017 100921P-100921P-100914. [49] Marangoni C. On the expansion of a droplet of a liquid floating on the surface of another liquid. Italy: Tipographia dei fratelli Fusi, Pavia; 1869. [50] Touloukian YS, Buyco EH. Thermophysical properties of matter, vol. 10. NY, Washington: IFU/PLENUM; 1970. [51] Thibault JYD, Jörg K, Jörn B. Properties of surface plasmon polaritons on lossy materials: lifetimes, periods and excitation conditions. J Opt 2016;18:115007.
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Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Guided self-organization of nanodroplets induced by nanosecond IR laser radiation of molybdenum films on sapphire Igor Zagoranskiy a,∗, Pierre Lorenz a, Martin Ehrhardt a,b, Klaus Zimmer a a b
Leibniz Institute of Surface Engineering (IOM), Permoserstraße 15, Leipzig 04318, Germany Advanced Launching Co-innovation Center, Nanjing University of Science and Technology, #200 XiaoLingWei, 210094 Nanjing, Jiangsu, People’s Republic of China
a r t i c l e
i n f o
Keywords: Nanosecond laser Self-organization Directed self-organization Ripples Droplet formation Nanostructuring
a b s t r a c t The fabrication of sub-micron and nanopatterns at large surface areas by lasers is still a challenge, but the use of laser-induced, self-organized processes provides a promising approach. Here, the guided formation of nanodroplets ordered in two directions during irradiation of 15 nm thick molybdenum (Mo) film on Al2 O3 (1–102) substrate with a fibre laser having a wavelength of 1064 nm and selectable pulse duration from 5 to 600 ns is demonstrated. The two-step process comprises laser-induced ripple formation and remelting of the ripples for nanodroplet generation. The ripples with period of ∼1.1 μm are oriented perpendicularly to the laser polarization. The ripples are fabricated by irradiation with 46 laser pulses with a duration between 25 to 100 ns and fluences of 0.3 to 1.3 J/cm2 , respectively. After a subsequent laser irradiation with 10 pulses at fluences above 1 J/cm2 and a pulse duration of 100 ns, two kinds of Mo droplets are observed: (i) large, periodically ordered droplets with a diameter of ∼360 nm and a distance along and orthogonal to the laser-generated ripples of about 1 μm and 1.1 μm, respectively, in addition to (ii) small, less-ordered droplets with a size of about 120 nm localized in the former ripple valleys.
1. Introduction Micro- and nanopattern fabrication on technical surfaces is challenging because traditional lithographic approaches need precise surfaces and can hardly be applied to non-planar substrates. Nanopattern fabrication by self-organization methods is suitable for defect-tolerating applications and must be preferred for low-cost fabrication. Self-organization processes can result in random, partially, or nearly completely ordered nanostructures. However, various applications require at least partially ordered nanostructures that cannot be achieved easily by selforganizing processes. Consequently, various approaches for guiding selforganization and self-assembly processes have been shown. Here, we focus on laser-irradiation guided and laser-induced nanopattern formation by the remelting of thin metal films on wide bandgap dielectric material, in particular on sapphire. Laser-induced nanopattern formation at the surfaces of dielectriclike fused silica, sapphire, polymers is interesting for optical [1–5], wetting [6,7], and tribological applications [8,9]. In particular, nanopatterned metal films could be applied for plasmonics, e.g., optical filters [10–14]. Sapphire is a crystalline, wide-band gap material (Eg ∼10 eV) with superb mechanical and chemical stability and an optical transparency in the wavelength range from 0.2 to 5 μm [15]. Due to these
∗
properties, sapphire has various applications for semiconductor fabrication and mechanical protection, as well as in the optical industry. The formation of randomly distributed nanoparticles attached to surfaces by the irradiating of extremely thin metal films on thermally insulating substrates with nanosecond-pulsed laser has been demonstrated by several groups [16–18]. At low-laser fluences, the mechanism of metal formation comprises melting of the film, dewetting of the molten metal film due to hydrodynamic instabilities, and resolidification after redistribution of the molten metal, developing nanodroplet distributions [16,19]. The formation of random or specific organized droplet distributions in different thin metal films like Co, Cu, Ag, Fe, Ni, Pt, Zn, Ti, V, and Mn (layer thickness 1–25 nm) on fused silica applying Nd: YAG laser (pulse duration 9 ns, wavelength 266 nm) was studied by Favazza, et al. [19]. Laser-induced droplet formation by thin metal film dewetting can be described by combining heat equation, phase transitions, and Navier–Stokes equation, which describe laser heating, melting, and mass transport in liquids, respectively [20]. Such metal nanodroplet distributions can be utilized for nanopattern fabrication in bulk material through laser-enhanced pattern transfer of such attached nanodroplets into the substrate, e.g., fused silica, as reported by Lorenz, et al. [21]. Laser-induced periodic surface structures (LIPSS) had been first observed in the 1960s by irradiating crystalline materials with a pulsed ruby laser [22]; this phenomenon was observed later for different ma-
Corresponding author. E-mail address: [email protected] (I. Zagoranskiy).
https://doi.org/10.1016/j.optlaseng.2018.10.005 Received 18 May 2018; Received in revised form 20 August 2018; Accepted 7 October 2018 0143-8166/© 2018 Elsevier Ltd. All rights reserved.
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terials exposed to nanosecond-pulsed lasers [23,24]. Currently, the formation of LIPSS — this term is now related to ripples and other types of laser-induced, partially ordered nanopatterns — is often observed at ultrashort, pulsed laser irradiation of surfaces [25–29]. The LIPSS can be classified into LSFL (low-spatial frequency LIPSS) and HSFL (highspatial frequency LIPSS). The wavelength Λ of LSFL has a period close to 𝜆/n where 𝜆 and n are laser wavelength and refraction index, respectively. The LSFL can be classified into two types. In type 1 LSFL, the periodic structures are regularly oriented perpendicularly to the laser beam polarization. In the case of type 2, LSFL are oriented parallel to the laser beam polarization [28]. The formation of type 1 LSFL is normally considered to be related to the excitation of surface plasmon-polaritons (SPP) [28]. Due to multipulse irradiation, the formation processes of LSFL can be enhanced through feedback. HSFL that feature a period shorter than half of the laser wavelength [28] are primarily observed close to the modification threshold for high number of applied laser pulses. The mechanism of HSFL formation is still under discussion and may include a hydrodynamic approach [30–33]. LIPSS are found by irradiation of solids in gases and in liquids, whereas the period is influenced by the surrounding material— for instance by the refractive index. A higher refractive index of the confining material (e.g., water) may result in shorter LIPSS periods in comparison to those formed in air [34]. LIPSS in non-metallic films, including diamond-like carbon and ZnO [35], have been observed for ultrashort laser pulses irradiation, too; here, the strong excitation of the material may provide conditions for the temporary formation of SPP in different materials. In relation to the current work, Scorticati, et al. [36] and Lopez, et al. [37] reported ps- and fs-laser induced ripples generation in thin molybdenum layers. LIPSS formation was studied for bulk materials by Bonse, et al. [38] using different pulse durations, ranging from 150 fs to 7 ns. It was found that LIPSS appear close to the melting point. The guided formation of droplet distributions providing at least a partial order has been shown already [39–43]. However, the final achieved periodically distributed droplets can been fabricated only by laser irradiation, if very precise lithographic metal patterns [39,40], e.g., metal strips [41] as well as metal rings on dielectric surface [42,43] have been used. This limits the applicability, as challenging lithographic or high-resolving processes has to be applied. The driving force for this periodic forming process during temporal laser melting is surface tension, which is supported by the Plateau–Rayleigh instability. Also, the laser ablation with interfering laser beams, which can be achieved by phase mask projection in order to produce periodic metal film patterns, has been utilized to guide the droplet position. In this way, nanodot arrays and grids can be induced on different metal-covered substrates [44,45]. Here, we report on the guided self-organization by utilizing only one laser source for the fast fabrication of ordered metal nanodroplets on dielectric substrates. The LIPSS are induced firstly in the form of ripples and remolded in the next step to periodic ordered droplets. Both the ripples formation in molybdenum films by ns-laser irradiation, as well as the utilization of the formed molybdenum ripples for ordered nanodroplet fabrication, demonstrate the abilities of ns lasers irradiation for guiding the self-organizing process of nanopattern fabrication with low cost equipment and fabrication allowing high processing speed.
sample is washed in deionized water and dried in a nitrogen stream. The cleaned sample is coated with 15 nm molybdenum (Mo) film. Magnetron sputtering was applied with argon pressure of 10−3 mbar, plasma power of 150 W, resulting in a deposition rate ∼0.5 nm/min. The analysis of the layer thickness distribution that was performed by optical transmission measurement with a CW laser of about 100 μm spot size, showed that the variation of the metal layer thickness is less than 1%.
2.2. Laser treatment The experimental set-up for laser irradiation is schematically presented in Fig. 1a). A fibre laser (PyroFlex 25, EOLITE) is incorporated into a workstation (INNOLAS Systems). The fibre laser emits at a wavelength of 1064 nm, features a TEM00 spatial mode, and has a beam quality M2 of less than 1.3. The pulse duration Δtp can be varied from 1 to 600 ns with a precision of 1 ns. The repetition rate is fixed at f = 10 kHz. The sample is mounted with a spacing of 10 mm to the stage in order to reduce reflections from the stage surface. The XY and Z stages allow the sample positioning in relation to the scanner head with 1 μm precision. X and Y motions of the laser spot during the laser irradiation are accomplished by the scanner head. The laser beam is focussed with an F-Theta lens (focal length of 160 mm) that is attached to the scanner. A particular Gaussian radius can be chosen by varying the distance between scanner head and the sample surface; the size needs to be evaluated for each defocusing position. Liu-method is applied in order to determine the Gaussian radius in a 1 mm defocusing step [47]. From this dataset, the waist position and Gaussian radius in focus plane can be found. In the focus plane the Gaussian radius is calculated to be 𝜔 = (12.7 ± 0.5) μm. The laser irradiation is performed in two steps: (i) Mo ripple formation and (ii) transferring of the rippled pattern into Mo droplets. Different laser spot sizes that were realized by defocusing are used for the two irradiation steps. For ripple formation, a spot with a radius of 𝜔 = (115 ± 6) μm is applied so that a wider patterning (broader rippled area) can be achieved. For this spot size, the fluence ranges for ripple formation were determined for all chosen pulse durations. It is most suitable to choose such laser fluence that ripples appear near the centre of the Gaussian energy density distribution in order to realize a reliable fabrication of rippled lines, to maximize the ripples lines width, and to avoid locally strong variation of the ripple pattern near FWHM (Full Width at Half Maximum) of the laser spot. This condition can be achieved by selecting appropriate laser pulse energy. A suitable scanning speed is found to be 0.05 m/s and can be converted in terms of number of pulses, N = 46 ± 2. Lines of 6 mm length with ripples appear orthogonal to the line scan direction are fabricated. Step two is aimed to reorganize the ripples pattern into periodic distributed droplets. Selected laser spots were irradiated in a step-andrepeat process (dots in Fig. 1b) and d). Further, the number of pulses is set to N = 10, and a Gaussian beam radius 𝜔 of (37 ± 2) μm is applied in order to achieve higher laser fluences. The laser-treated samples are analyzed by optical and scanning electron microscopy (SEM) and the sample surface is covered by 10 nm gold for the SEM measurements. From the SEM images, the size and the distribution of the formed particles are evaluated by image processing.
2. Experimental set-up 2.1. Sample preparation
3. Results
Double-sided polished Al2 O3 (1–102) wafer (diameter 76.2 mm) is used. The surface roughness is measured to be 0.25 nm RMS (root mean square value). First, the sample is cleaned by RCA 1 solution (ammoniac solution 30%, hydrogen peroxide, and water in 1:1:5 combinations). Further cleaning is realized by RCA 2 solution (hydrochloric acid, hydrogen peroxide, and water in 1:1:5 combinations) [46]. Each treatment is applied for 30 minutes in an ultra-sonic bath at 333 K. Finally, the
The laser-based fabrication is performed in two separate steps: ripples fabrication and remelting of the formed ripples to produced ordered droplets patterns. The separation in a sequence of two distinct steps allows full control on the laser processing parameters for ripple and droplet fabrication. 56
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Fig. 1. Sketch of the experimental set-up, (a) and the procedure of laser irradiation, (b). Two steps are applied for the whole laser irradiation procedure: (i) scanning with a defocused spot (larger Gaussian radius) for inducing of ripples in form of extensive long lines of Mo and (ii) forming of Mo ripples pattern into droplets by fast laser melting at higher laser fluences (applying smaller Gaussian radius). c) Adjustment of Gaussian beam radiuses for two step processing. d) SEM image of pattern on sample. Both processes are visible: Ripple formation in some fluence range along a line treatment and forming process in the form of dots.
3.1. Ripple formation at laser irradiation
3.2. Remelting of rippled Mo film
At first, the appearance of ripples in the molybdenum film is studied regarding dependence on the pulse duration and the laser fluence. Ripples are found for pulse durations Δtp from 25 to 100 ns. The lower and upper limits of the laser fluence for ripples appearance can be determined from their distance y from the laser spot centre (from laser scan center to the rippled lines) of the Gaussian distribution of the laser intensity. The lower fluence limit is defined as the transitions from a smooth to a capillary wave-like surface. The upper fluence limit is defined as the transition from a capillary wave-like surface to hole-like patterns. A typical SEM image of the irradiated area is shown in Fig. 2a), where ripples appear. The calculated local fluence of the Gaussian laser intensity distribution is presented on the right side. In addition, the determined thresholds of ripples appearance in dependence on pulse duration, including the inaccuracy of determination, are shown schematically. The measurement uncertainties of the laser pulse energy and the Gaussian beam radius have been considered for the estimation of the overall inaccuracy. SEM images show that the ripples appear orthogonally to the polarization direction of laser. The period of the ripples is calculated from SEM images to be Λ = (1086 ± 30) nm. The SEM image of Fig. 2b) shows a reference ripple structure that is utilized for further laser irradiation to form droplets. In Fig. 2c), measured limits of the laser fluence for the appearance of ripples are shown. In general, the fluence for ripple formation increases with the increasing of the laser pulse duration.
Thereafter, additional laser pulses with a pulse duration of 100 ns are applied to the rippled line pattern (Fig. 2b) in order to produce periodical distribution of droplets. In Fig. 3, different results of the remelting processes are presented. For calculating, the droplet size distribution image processing of SEM photographs is carried out in subsequent steps. Applying image-processing software, the particle size distribution is calculated. The circularity of particles was limited to the range of 0.8 to 1.0 in order to detect only droplets and to exclude strip-like structures. Data of droplet size distribution is statistically analyzed. The occurrence of particles was calculated with a data interval width of 25 nm. Fig. 3a shows incomplete separation of droplets at a fluence of F = (1.1 ±0.1) J/cm2 . In particular, metal bridges between droplets and non-circular shaped particles can be observed. With a slight increase of the laser fluence to F = (1.2 ± 0.1) J/cm2 , a complete separation of droplets can be found (Fig. 3b). They have primarily a circular shape. Additionally, deep holes can be found in the sapphire substrate at fluence F = (1.7 ± 0.1)J/cm2 (see Fig. 3c). Below the SEM images, the related droplet size distributions are shown, wherein two peaks can be clearly distinguished. The total number of droplets in the intervals representing a particular droplet size range was fitted with Gaussian curves in order to find mean values of droplet size distribution. According to the SEM image in Fig. 3b) that was taken at a fluence F = (1.2 ± 0.1) J/cm2 , a periodicity in lateral distribution of the droplet is observed. For droplets with a mean size of 0.38 ± 0.09 μm, two different periods can be found: along and orthogonally to the previous formed
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Fig. 2. (a) and (b) SEM image of the irradiated 15 nm Mo/Al2 O3 : structured with N = 46 ± 2, 𝜔 = 115 μm and Δtp = 100 ns. Lower and upper limits of fluence for ripples are shown schematically. The y scale serves to represent distance from laser spot centre. At b), the reference structure is presented that will be applied for further forming procedures (applied fluence in the range of (1.1–1.2 ± 0.1) J/cm2 ). c) Laser fluencies found for ripples formation: upper and lower limits are presented in dependence on pulse duration.
Fig. 3. (a), (b) and (c) SEM images of the irradiated 15 nm Mo/Al2 O3 : Additional N = 10 pulses with 𝜔 = 37 μm, Δtp = 100 ns and a) F = (1.1 ± 0.1) J/cm2 , b) F = (1.2 ± 0.1) J/cm2 and c) F = (1.7 ± 0.1) J/cm2 were applied to reference ripples pattern (F = (1.1–1.2 ± 0.1) J/cm2 , N = 46) presented at Fig. 2b). SEM image graphs present droplet size distribution dependent upon applied fluence. Gaussian fits were applied. Here, the tolerance of peak values corresponds to the radius of the Gaussian droplet distribution.
ripples. Droplet periods of (0.965 ± 0.118) μm and (1.095 ± 0.069) μm were measured, respectively. The droplet period orthogonal to the ripples is similar to the period of original ripple structures. In the SEM image, it can also be observed that small droplets appear between big droplets along previous ripples hills.
of the sample. The surface plasmon-polariton (SPP) can be induced by laser irradiation on interface between thin the molybdenum layer and air (Fig. 4(1)). The SPP interacts with the laser light and cause interference effects [25–28]. Finally, the periodic modulated laser irradiation results in a periodical temperature profile (Fig. 4(2)). Localised melting, in addition to temperature profile induced alterations of the surface tension and viscosity may occur [48]. The mass movement occurs obviously in liquid state and follows the gradient of temperature (Marangoni-effect) [49]. Therefore, the mass movement of molten metal is periodically predefined (Fig. 4(3)). The local thinning of the metal film finally results in rupture of the film and in dewetting of the molten molybdenum from the sapphire. Multipulse laser irradiation allows the enhancement of metal ripple formation. After several pulses, the surface tension of molten metal contributes in the material separation from rippled surface to the strips and further from strips (Fig. 4(4)) to droplets (Fig. 4(5)).
4. Discussion 4.1. LIPSS formation Formally, the formation process of ripples can be presented as a stepwise process, as shown in Fig. 4. A short representation of the process is shown here. More detailed information will be given in the following two parts. The molybdenum layer surface has a roughness of 0.25 nm RMS. Laser interaction with surface material is affected by the roughness, which results in enhanced scattering of laser light in the plane 58
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Fig. 4. Schematic illustration of the formation process. (1) The SPP is induced to interface air/molybdenum (period of ripples close to excitation wavelength). SPP interferes with scattered light, which results in a periodic temperature modulation. At moment (2), the modulated temperature field is above the melting point of molybdenum. Coloring in shades of red serves to express higher temperature. The low thermal conductivity of Al2 O3 (metal layer is too thin and negligible) allows the ripple formation in some certain pulse durations. A final product is a rippled surface (3), strips (4). or a periodic droplet distribution (5). Three different types of droplets sizes can be formed: Type S1, S2, and B.
4.2. Rippling of Mo films, upper and lower limits In the experiment, it was found that ripples could be generated in an interval of fluences that depends on the pulse duration (Fig. 3c). Considering the thermal diffusion length LTD as √ a length measure for the energy dissipation, that is given by 𝐿TD = 2 𝐷𝑇 ℎ 𝑡 with t for a characteristic time (e.g., duration of laser pulse) characteristic lengths can be calculated for material specific thermal diffusivity Dth . with the DTh ∼0.8 ∗ 10−6 m2 /s for Al2 O3 and 24.6 ∗ 10−6 m2 /s for Mo at ∼2000 K reported by Touloukian, et al. [50]. Significant contributions of the metal film to the heat dissipation can be neglected due to the low thickness of the metal film in relation to the heated thickness of the substrate. With the observed ripples period Λ of ∼1.1 μm and considering of the symmetry of the heat dissipation from the hot sections of the laser-induced temperature distribution, the maximal pulse duration below which thermal equalization is prevented can be calculated with (LTD /2)2 /(4 ∗ DTh ). The calculated value of ∼125 ns is slightly higher than experimentally observed 100 ns but explains well the reason for the upper limit of the pulse length. Thermal simulation of the temperatures for melting and evaporation for the experimental studies parameter range according to the simulations in Lorenz, et al. [48] was performed. The results show that the absolute difference fluence thresholds for evaporation and melting is reduced with decreasing pulse duration (see Fig. 5). Also in the experiment, the upper and lower limits of ripple appearance come closer at decreased pulse duration. The lower limit of ripples appearance may be related, in addition to heating effects from the laser plume or the dynamics of the heating and melting processes that reduce significantly with shorter pulse durations. In particular, the lifetime of the molten molybdenum might be too short. Longer liquid lifetimes can be achieved with higher fluencies as the cooling time down to the melting point is longer, but the threshold for evaporation / laser ablation should not be exceeded. Therefore, we conclude that the dynamics of the heating, including the related phase transitions, control the short pulse limit of ripples appearance. In general, as expected, the same tendency as in the experiments of the need for lower fluence required for heating to a significant temperature (Tm ) has been found. The achieved ripples orientation is not influenced by scanning direction or sample orientation and orthogonal to the laser polarization direction. The period of ripples Λ is (1086 ± 30)nm and close to the ex-
Fig. 5. Laser fluence thresholds for evaporation and melting of 15 nm Mo on Al2 O3 dependent upon pulse duration are estimated by FEM simulation, based on [48]. The grey curve represents the absolute difference between two values.
citation wavelength (1064 nm) and to the wavelength of the SPP at the molybdenum air interface that has been calculated according to [51] to be 1052 nm. The found ripples belong to first type of LSFL. Other types of ripples were not observed. 4.3. Remelting of rippled Mo film The formed ripples [period Λ ∼ 1086 nm, line width of the formed broad stripes (line patterns of the ripples peaks) ∼190 nm and of the slim stripes (ripples valleys) ∼65 nm] were performed by a second step of multipulse irradiation. This forming process is probably a stepwise process comprising melting, mass transport in the liquid, and resolidification from each step. The experimental results (see Fig. 3) show a transfer from strips to droplets where two different kinds of droplets have been found (see Figs. 3 and 4): Type B: big droplets are localized at the former position of the broad stripes of the ripples. Type S belongs to small droplets. The small droplets can be also be distinguished in two kinds of droplets: Type S1 — small droplets are localized at the former position of the slim stripes (in the middle between two broad stripes) and Type S2 — small droplets are localized between two big droplets (Type B) at the former position of a broad stripe. 59
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References
The fitted droplet size distribution (see Fig. 3b) feature two maxima at 140 nm (Type S) and 380 nm (Type B), respectively. The period of type B droplets that are orthogonally organized to the ripples is, with ∼1.1 μm, similar to the ripples period. The formation of the periodic ordered droplets along the ripples can be explained by the PlateauRayleigh instability. The Plateau-Rayleigh instability describes the separation of a liquid cylinder into droplets due to inherent instabilities. The period of formed droplets can be estimated by equation ΛRP = 9.02∗ R0 , where the R0 is radius of a liquid cylinder, here representing the molten film metal. For simplicity, the interaction with the sapphire surface is not taken into account. Neglecting evaporation of molybdenum, the volume of the film volume should be constant within redistribution of the film during melting. Therefore, the estimated thickness of the wide Mo stripes is Δz = 86 nm. With the radius of an equivalent cylinder that is given with R0 = ((d ∗ Δz)/𝜋)0.5 , the droplet period of ΛRP ∼700 nm can be found. This estimated value is near the experimental found period of ∼644 nm. The SEM image in Fig. 2a) can be also helpful for understanding the process of type S2 droplets formation. During laser irradiation of rippled metal film, the droplets start to separate from each other. This effect is supported by Plateau-Rayleigh instability and was already simulated for nickel strips in [41]. However, filaments between unseparated droplets can be observed (Fig. 3a). Further irradiation of such bridgecoupled droplets is caused by filament separation from the droplets and subsequent remelting of the filaments to type S2 droplets. In [41], it is mentioned that the size of these droplets and period of breaking up are dependent on filament length and width. The type S1 droplets are formed on the former position of the slim stripes that have an average width of at least a factor of 3 smaller than the broad lines. Furthermore. the SEM images imply that the slim stripes have a lower thickness compared to the broad lines (see Fig. 2b). The distinct, smaller average cross-section of the slim stripes results in droplets of type S1 that can be explained by Plateau-Rayleigh instability.
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5. Conclusion All-laser fabrication of periodical nanodroplet distributions by guided self-organization processes without utilization high resolution laser techniques has been shown. The process comprises two steps, with each particular process parameters: first the generation of ripples and second the remelting of the ripples stripes to ordered droplets. Hydrodynamic effects provide the main driving forces for the ripples and the droplet formation, due to the needed movement of molten material. Ripples can be generated only in a limited pulse length range from 25 to 100 ns. The long pulse limit is determined by partial vanishing of the temperature modulation, due to heat dissipation in the substrate. Hence, the thermal diffusion length in substrate material must be smaller than the ripple period that is defined by the interference of the incidence and the surface scattered waves. The short pulse limit is probably given by the reduction of the molten time of the metal film, due to the shorter pulse duration. Further laser irradiation of rippled metal film leads to formation of periodic ordered droplets. Different mechanism determine the period of the droplets in the two orthogonal directions. Hence, the distance can be selected by different process parameters, in particular by selecting the wavelength that determine the droplet period perpendicular to the ripple direction and the film thickness that determine the droplet distance along the ripple direction.
Acknowledgement We appreciate the support by the Deutsche Forschungsgemeinschaft (DFG) under LO 1986/2-1. 60
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