Laser induced periodic surface structuring on Si by ... - OSA Publishing

Laser induced periodic surface structuring on Si by temporal shaped femtosecond pulses G. F. B. Almeida, R. J. Martins, A. J. G. Otuka, J. P. Siqueira, and C. R. Mendonca* Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, + 55 (16) 33738085, Brazil * [email protected]

Abstract: We investigated the effect of temporal shaped femtosecond pulses on silicon laser micromachining. By using sinusoidal spectral phases, pulse trains composed of sub-pulses with distinct temporal separations were generated and applied to the silicon surface to produce Laser Induced Periodic Surface Structures (LIPSS). The LIPSS obtained with different sub-pulse separation were analyzed by comparing the intensity of the twodimensional fast Fourier Transform (2D-FFT) of the AFM images of the ripples (LIPSS). It was observed that LIPSS amplitude is more emphasized for the pulse train with sub-pulses separation of 128 fs, even when compared with the Fourier transform limited pulse. By estimating the carrier density achieved at the end of each pulse train, we have been able to interpret our results with the Sipe-Drude model, that predicts that LIPSS efficacy is higher for a specific induced carrier density. Hence, our results indicate that temporal shaping of the excitation pulse, performed by spectral phase modulation, can be explored in fs-laser microstructuring. ©2015 Optical Society of America OCIS codes: (160.6000) Semiconductor materials; (190.4180) Multiphoton processes; (320.5540) Pulse shaping; (220.4000) Microstructure fabrication.

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Received 27 Aug 2015; revised 5 Oct 2015; accepted 6 Oct 2015; published 12 Oct 2015 19 Oct 2015 | Vol. 23, No. 21 | DOI:10.1364/OE.23.027597 | OPTICS EXPRESS 27597

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1. Introduction Irradiation of crystalline silicon surface with ultrashort laser pulses has been a very active area of research in the last decade, which has led to interesting results in materials sciences with potential application for devices in optics and photonics [1–3]. It has been observed, for instance, the production of Si samples with enhanced absorption in the near infrared region upon fs-laser irradiation. Moreover, micro/nanostructures have been achieved on Si surface after ultrashort laser irradiation such as, for example, sharp nano-spikes when the irradiation takes place in atmosphere containing a halogen gas [4–7]. In fact, such laser induced periodic surface structures (LIPSS) have been observed on metal [8], dielectric [9–11] and semiconductor [12, 13] surfaces that are exposed to ultrashort laser pulses at or below the ablation threshold. The LIPSS spatial features, specifically its orientation, height and periodicity depends on the materials properties, as well as on the laser wavelength, polarization and angle of incidence [14]. In general, LIPSS produced with linearly polarized beam at normal incidence exhibits periodicity close to the light wavelength, height in the order of a few hundreds of nanometers and are oriented orthogonally to the laser polarization [15, 16]. In the last few years, additional control of the light matter interaction has been achieved by suitable shaping of the temporal/spectral profile of ultrashort pulses [17–21]. Such concept has also been applied to material laser processing [22–24], mostly using trains of fs pulses, studying the influence of duration [25–27], number of pulses in the train [28, 29] and separation between the sub-pulses [28, 30–33]. In principle, the use of fs pulse trains allows changing fundamental aspects of the light-matter interaction mechanism, as compared to single pulse, by altering, for instance, the transient electronic dynamics in the target material. In this work, 40-fs pulse-trains with distinct sub-pulse temporal separations, obtained by spectral phase modulation, were used to produce LIPSS on Si wafers. The ripples produced with each distinct pulse-train were analyzed by two-dimensional fast Fourier Transform (2DFFT) of the corresponding atomic force microscopy (AFM) images. We observed that LIPSS are more pronounced for the pulse train with sub-pulses separation of 128 fs, even when compared to the Fourier Transform limited pulse. We show that this behavior is related to the carrier density achieved at the end of the pulse train, in agreement with the Sipe-Drude [34] theory that predicts a range of excited carrier density that favors LIPSS formation. Therefore, such results indicate new venues to explore temporal pulse shaping in order to gain further control on laser micromachining. 2. Experimental The experiments were carried out in p-type Si (100) wafers with resistivity smaller than (ρ = 8-12 Ωcm), which were cleaned using the standard RCA method to remove surface

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contamination. The sample is positioned in the axial z direction using a translation stage and irradiated with 40-fs pulses from a Ti:sapphire multi-pass amplifier, centered at 770 nm (25 nm – FWHM) and operating at 1 kHz repetition rate. The laser pulses were focused onto the sample surface with a 20 cm focal length lens, yielding a nearly Gaussian spot with a beam waist of approximately 60 μm. The laser beam is scanned across the sample in the xy plane using a galvanometric mirror. The experimental setup is illustrated in Fig. 1. Each spot of the sample surface was exposed to 2 laser pulses, whose fluence was 0.64 J/cm2. The temporal profile of the excitation pulse was tailored by spectral phase modulation, using a 640-elements liquid crystal spatial light modulator (SLM) at the Fourier plane of a folded 4f zero-dispersion compressor, comprised of a 30 cm cylindrical mirror and a 600 lines/mm diffraction grating. We applied a sinusoidal spectral phase modulation ϕ (ω ) = α sin (ωΔτ + δ ) , where α is the modulation depth, Δτ is the frequency of modulation and δ is the position of the phase mask relative to the center of the pulse spectrum. Such phase modulation allows producing a train of fs-pulses, whose separation is given by Δτ. The pulses obtained with the pulse shaper were characterized using SH frequency-resolved optical gating (FROG) technique. The surface structures obtained are reproducible and were analyzed by atomic force microscopy (AFM).

Fig. 1. Illustration of the experimental setup. The graphs illustrate the FROG and autocorrelation traces for the FTL pulse.

3. Results and discussion The pulse train effect on the LIPSS produced on Si wafers was investigated by changing the sub-pulse separation. Such pulse trains were obtained by applying sinusoidal spectral phase masks with distinct values of Δτ to the Fourier transform limited (FTL) pulse, whose FROG trace is shown in Fig. 1 (40 fs and 25 nm FWHM bandwidth). Figure 2 shows the temporal profile of the excitation pulses (pulse trains) with sub-pulses separation of 64 fs, 85 fs, 128 fs and 170 fs, retrieved from the corresponding FROG traces displayed in the insets of the figure. It is worthwhile mentioning that the sub-pulses separation determined by FROG

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measurements, presented in Fig. 2, match the ones designed using the phase masks. For simplicity, each pulse train will be named by its corresponding sub-pulse separation.

Fig. 2. Temporal profile of pulse trains with sub-pulses separation of (a) 64 fs, (b) 85 fs, (c) 128 fs and (d) 170 fs, retrieved from the corresponding FROG traces (shown in the inset).

Figure 3 shows AFM micrographs (5 × 5 μm2) of LIPSS in Si, obtained from the central region of the laser spot, irradiated with pulse trains of 128 (Fig. 3(a)), 64 fs (Fig. 3(b)) and FTL pulse (Fig. 3(c)). In Fig. 3(d), as an illustration, we present the profile of the AFM image obtained along the dashed line in Fig. 3(c), which reveals a LIPSS period of 760 nm. The observed ripples present periodicity close to the light wavelength, orientation orthogonal to the laser polarization and height on the order of hundreds of nanometers, as reported in the literature [12, 14, 35, 36]. Similar structures have been observed for the other pulse trains. For all distinct pulse trains, the sample was exposed to two laser pulses with fluence of 0.64 J/cm2. The condition of few pulses and energy just above the ablation threshold was used to produce gentle structures, to evidence the effect of the fs-pulse train on the produced microstructures.

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Fig. 3. AFM images of the LIPSS obtained with a pulse train of (a) 128 fs, (b) 64 fs and (c) FTL pulse. (d) Profile of the AFM image, obtained along the dashed line in (c), displaying a LIPSS period of 760 nm.

In order to analyze the influence of the sub-pulse separation on the LIPSS, 2D-FFT (twodimensional fast Fourier Transform) was applied to the AFM topography data (matrix h(x,y) – height matrix). The 2D-FFT allows for an objective and quantitative analysis that provides information not only on the ripples amplitude, but also on its periodicity. It is important to mention that such analysis was carried out in the topography data that corresponds to an area of (5 × 5 μm2) for all samples, thus containing approximately the same number of LIPSS. The resulting 2D-FFT data (matrix H(u,v), in which u and v are the spatial frequencies) presents a peak at a spatial frequency that corresponds to the periodicity of the ripples. The amplitude of such peak (2D-FFT amplitude) was evaluated for each sample, and plotted as a function of the sub-pulse separation in Fig. 4 (circles, left and bottom axis). The solid line along the points in this figure was only draw to guide the eye. As it can be seen in Fig. 4 (circles), the highest 2D-FFT amplitude is obtained for a sub pulse separation Δτ = 128 fs. Interestingly, the 2D-FFT signal for the LIPSS produced with the FTL pulse is smaller than the ones obtained for the pulses with Δτ of 64 and 128 fs. Since the amplitude of the 2D-FFT reflects the LIPSS amplitude, the result in Fig. 4 indicates that the LIPSS amplitude is increased, particularly for the pulse train with separation of 128 fs. The error bars in this figure refers to the deviation encountered during repeated experiments that consistently revealed the same behavior. The primary mechanism of fs-laser structuring is related to the laser energy deposition in the materials electronic system, followed by a time dependent release of energy to the lattice [22, 37, 38]. If the laser pulse energy is enough, the surface melts according to the energy deposition profile, generating a specific surface pattern upon resolidification. The currently accepted mechanism to explain the LIPSS is the interference between the incident laser beam and surface plasmon polaritons (SPPs), being the later attributed to the coupling of the incident laser light to the surface roughness (Sipe’s theory). Hence, the resulting inhomogeneous absorption of the laser light below the material rough surface leads to the ripples (LIPSS) formation [14]. One of the merits of Sipe’s theory is to be able to predict the inhomogeneous energy deposition on the sample, in the spatial frequency domain, which is further quantified by a scalar function called efficacy factor η. By combining Sipe’s theory with a Drude model (Sipe-Drude theory), Bonse et al. [34] were able to take into account transient changes in the optical properties of the material, due to the excitation of a dense electron-hole plasma, on the LIPSS formation. With this model, a #248153 © 2015 OSA


strong dependence of the LIPSS efficacy factor, η, with the laser induced carrier density is observed [34]. To analyze our results (Fig. 4) in the framework of the Sipe-Drude theory, we calculated the carrier density in the conduction band, achieved at the end of the pulse train. Such task was accomplished by estimating the carrier density generated by each sub-pulse, considering one- and two-photon absorption, using N e = Φ 0 (1 − R ) {α + Φ 0 (1 − R ) β 2τ } hν [39], in

which Φ0 is the fluence of the sub-pulse, R = 0.328 is the surface reflectivity of silicon at normal incidence [40], α = 1.1 × 10−5 m−1 is the linear absorption coefficient of silicon [41], β = 6.8 × 10−11 m/W is the two-photon absorption coefficient of silicon [39], hν is the photon energy and τ is the pulse duration. Until the arrival of the next sub-pulse, during the interval Δτ, relaxation of the carrier density occurs, for highly excited semiconductors and thus 2 considering screening, with a characteristic time given by τ c = τ 0 1 + ( N e N c )  , where τ0 is   the relaxation time at low carrier densities and Nc is the critical carrier density [42, 43]. From transient gratings measurement in silicon, it has been determined that τ0 = 240 fs [44]. The critical density, Nc, was calculated to be 4.3 × 1027 m3, using the optical effective mass (m*opt = 0.18me) and Drude damping time τD = 1.1 fs according to [41]. Using such approach, we are able to determine residual carrier density before the next sub-pulse arrives, which is then added to the carrier density generated by this sub-pulse. The process is repeated for all subpulse in the pulse train, taking in consideration its specific Δτ and energy profile, according to Fig. 2. The top axis in Fig. 4 represents the carrier density achieved by each pulse train, that presents distinct sub-pulse separation (displayed in the bottom axis). The dashed line in Fig. 4 shows the LIPSS efficacy factor, η, obtained from the Sipe-Drude theory, extracted from [34]. As it can be seen, our result exhibits a 2D-FFT maximum for a sub-pulse separation of Δτ = 128 fs, which corresponds to a carrier density at the end of the pulse train of approximately 6.0 × 1027 m3. Such result in agreement with the position where the efficacy factor (dashed line) presents a maximum, according to the Sipe-Drude theory. In fact, as predicted by the theory, the carrier density in which the efficacy reaches a maximum is a little higher than Nc, also in agreement with our data. The Sipe-Drude theory predicts a decrease of the LIPSS efficacy for carrier densities significantly bigger than Nc, as observed for pulse trains with Δτ of 64 and 85 fs, as well as for carrier densities much smaller than Nc, as in the case of the pulse trains with Δτ = 170 fs.

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Fig. 4. Amplitude of the 2D-FFT (circles) for LIPSS obtained with pulse trains with sub-pulse separations of Δτ (left and bottom axis). The solid is a guide to the eye. The dashed line represents the efficacy factor (right axis) obtained from the Sipe-Drude theory, extracted from [34], as a function of the carrier density Ne.

Therefore, our results show that the use of pulse trains on the LIPSS formation is basically determined by the carrier density generated at the end of the train of pulses, in good agreement with strong dependence of the LIPSS efficacy with the carrier density observed in the Sipe-Drude theory. Since significant relaxation of the carrier density may take place between sub-pulses in the pulse train via carrier diffusion, Auger recombination and carrierphonon coupling [42, 44–47], knowledge about the relaxation time is fundamental to understand the LIPSS, which occurs at high carrier densities where the electron-phonon coupling screening plays an important role. Other works regarding LIPSS production in silicon under multiple double-pulses irradiation with varying pulse delay can be found in the literature [45–48]. In such works, theoretical modelling involving silicon electronic dynamics and energy relaxation were used to explain the dependence of ripples on experimental parameters and its periodicity [45–48], as observed in optical and scanning electron microscopies. In this work, however, we focus on the LIPSS efficacy factor, resulting from multiple pulses excitation, by using 2D-FFT to analyse the AFM topographies. Although our results present a reasonable agreement with the Sipe-Drude model, a complete understanding of the process depends of a complex interplay between energy deposition and dissipation, as well as fundamentals aspects of electron dynamics that affects silicon dielectric permittivity, which could explain the deviation between the experimental data and the theory in Fig. 4, what is out of the scope of this work. 4. Conclusions

In this work we have studied femtosecond pulse shaping applied to silicon laser micromachining. Sinusoidal phase masks were used to produce pulse trains with specific subpulse temporal separations, in order to explore its influence on laser induced periodic #248153 © 2015 OSA


structures. Experimental results revealed that LIPSS are more emphasized for the pulse train with separation of 128 fs, as determined by 2D-FFT of AFM topography data of the ripples. This behavior was assigned to the higher efficacy of the LIPSS for the carrier density achieved at the end of the pulse train, in agreement with the Sipe-Drude model that predicts an induced carrier density range in which the efficacy factor for LIPSS is higher, favoring the ripple formation. Such result indicates that the proper engineering of the pulse temporal profile, achieved by spectral phase modulation, can be advantageously used to further control the fs-laser micromachining in silicon and potentially in other materials. Acknowledgments

This work has been supported by São Paulo Research Foundation (FAPESP) Grant # 2011/12399-0, 2011/23587-1, 2012/00702-2 and 2012/03513-6, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).

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