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Abstract
Although laser irradiation with femtosecond pulses is known to generate crystallization and morphological changes, the contribution of optical parameters to material changes is still in discussion. Here, we compare two structures irradiated near Si–L2,3 edges by an extreme ultraviolet femtosecond pulse. Our result implies that, despite the femtosecond irradiation regime, these values of the optical attenuation length between the wavelengths of 10.3-nm and 13.5-nm differ by one order of magnitude. From the structural comparison, the original crystalline state was maintained upon irradiation at 13.5-nm, on the other hand, transition to an amorphous state occurred at 10.3-nm. The difference in optical attenuation length directly influence to the decision of material crystallization or morphological changes, even if the irradiation condition is under the femtosecond regime and same pulse duration. Our result reveals the contribution of optical attenuation length in ultrafast laser-induced structural change.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since the advent of femtosecond lasers [1], intense pulses with various central wavelengths have been used across a wide range of scientific [2–7] and industrial [8–13] applications. The mechanisms underlying the nonlinear responses of materials induced by femtosecond lasers are still under active discussion, especially in research fields of relatively high intensity laser, such as laser fusion [14], warm dense matter [15], and laser ion acceleration [16]. In lower fluence regime research areas, such as micromachining [17], laser welding [18], laser-induced forward transfer [19], and pulsed laser deposition [20], such investigations are also ongoing. Furthermore, recent advances in new materials, light sources, and measurement technologies have made it possible to provide new aspects of experiments related to microfabrication research.
From the perspective of industrial applications at relatively low fluences (<10 J/cm2) and intensities (<1014 W/cm2), it is important for the final material state to be predictable using an intuitive selection of optimum optical parameters. Several previous studies have reported observable evidence of pressure and temperature conditions, thereby providing important clues that indicate the link from optical parameters to pressure-temperature, and from pressure-temperature to crystalline state and morphology [21,22]. The optical parameters are fluence, intensity, pulse energy, energy density, pulse duration, wavelength, spot size, and repetition rate, and there are additional parameters such as surface reflectivity and optical attenuation length, which are dependent on the material properties. Each parameter sometimes dominates the other parameters; for example, since the values of fluence and pulse duration determine intensity, there is generally no need to independently consider the relationship. Meanwhile, a previous research [23] revealed the relationship between spot size and point defects in the material; it succeeded in independently considering pulse energy and spot size contained in a fluence. With respect to the contribution of beam size, wavelength, and pulse duration, it has been well discussed in the regime of pico- and nano-second time scales [24]. The value of time-resolved reflectance is measurable, and it has exhibited a causal relationship with other optical parameters and the final material state [25]. Furthermore, time- and space-dependent electron diffusion is visualized, and the correlation between spot size and temperature gradient is indirectly understood [26]. Recent experimental and theoretical comparative studies have shown good agreement in terms of the reproduction of crystallinity and morphology [27,28]. The handling of optical parameters has been based on the time-dependent (extended) Drude model, and the time-dependent optical attenuation length is generally within a range of 100 nm [29]. These facts indicate that these contributions of optical attenuation length and other optical parameters are non-discriminated under ultrafast laser-induced structural changes.
As extreme short-wavelength core-electron absorption is insensitive to changes in opto-electronic properties, it is possible that core-electron absorption edges could be used to adjust optical attenuation length. In the extreme ultraviolet (XUV) range, attenuation lengths range from 10 to 1000 nm, which is comparable to those achieved using visible femtosecond laser pulses [30]. In view of the relationship between optical and plasma frequencies, XUV wavelengths are approximately 100 times shorter than NIR wavelengths, resulting in XUV plasma cut-off densities higher than those of the NIR [31]. Further, as the electron inelastic mean free path is typically ≤1/10 of the optical attenuation length in the XUV regime, the spatial extent of the initial photoexcitation determines the scale of the entire structural change [32–34].
In this study, we identify the contribution of optical attenuation length via the Si–L2,3 absorption edges excited by XUV femtosecond pulses. The L absorption edges of Si are located at 99.2 and 99.8 eV, and the 10.3 nm (120 eV) and 13.5 nm (92 eV) wavelengths used in this study straddle these absorption edges [35,36], and the difference of these optical attenuation lengths are greater than one order of magnitude. The method is elegant in that non-cutoff shielding and core–edge adjustment is applied by the XUV wavelength excitation. Although, in the NIR regime, the crystal state was strongly dependent on the pulse duration compared with other optical parameters [37], in the XUV regime, it is possible to compare irradiated materials that have changed only in terms of the optical attenuation length under a fixed pulse duration.
2. Experimental procedure
2.1 Irradiation experiment and sample details
As shown schematically in Fig. 1(a), we conducted irradiation experiments at XUV beam line BL1 at the SPring-8 Angstrom Compact Free Electron Laser (SACLA) [38] facility in Japan. At this facility, XUV pulses are provided via a self-amplified spontaneous emission free-electron laser (SASE-FEL). Two different wavelengths, at 10.3 nm and 13.5 nm, were employed for irradiations. Sixteen different energy-attenuating filters were available to allow for pulse energy attenuation rates of between 0.0044% and 100%. The energetic fluctuation of the pulses was typically <20%, and hence, for accuracy, individual pulse energies were obtained using a gas monitor detector (GMD). The GMD was placed closer to the light source than the attenuation filter. The sample target was irradiated using a Kirkpatrick–Baez (K–B) mirror. The repetition rate was 60 Hz, and all of the experiments reported herein were conducted in the single shot regime. The target surface was set to a normal incidence angle and the pulse duration at 12.4 nm was measured by a correlation monitor to be 70 fs (full-width at half maximum, FWHM) [39,40]. Because the free electron laser (FEL) radiates a broad spectrum corresponding to part of the electron bunch, the durations of the pulses used in this study corresponded to the entire range of the radiation spectrum. More details of the setup were described by Owada et al. [38]. Note that, in order to secure sufficient data number of the experiment with synchrotron radiation facility, these experiments results shown in this paper were conducted with two beam times: 2018A and 2018B.
Fig. 1. Schematics of experimental procedures. (a) Irradiation setup, in which a pulse is focused onto the target surface using a K–B mirror and calibrated X-ray filters are employed for power adjustment. Irradiation is conducted under vacuum (<10−4 Pa). (b) Experimental procedure from laser irradiation to TEM. A single sample from each specific fluence regime was subjected to each step, and the consistent procedures were applied to 2018A and 2018B beam times.
The irradiated sample was a <110>-oriented crystalline silicon, which the size was 10 mm×10 mm×0.6 mm. The static optical attenuation lengths of silicon at 10.3 nm and 13.5 nm are 42.2 nm and 588 nm, respectively [35]. Several previous studies have indicated that silicon is one of the best samples for understanding transitions to and from crystalline states induced by femtosecond laser [41–47]. Although focal spot sizes are typically established via knife-edge scans or Liu’s plots [48], we employed an imprint method [49], which have developed to determine for non-Gaussian mode pulses. The contour for determine the beam shape was used the outline of the amorphous or crater, and this was measured via laser scanning microscope (LSM). Values of effective area were obtained and applied independently to the experimental data acquired during each of the two beam times, 2018A (91.4 μm2) and 2018B (57.9 μm2). The experimental procedure from irradiation to analysis is shown in Fig. 1(b).
2.2 Sample handling and analysis
After irradiation, the sample was stored in air. LSM, micro-Raman spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM) were employed for analysis. For the LSM, the laser diode had a central wavelength of 408 nm, and this method allowed discrimination of crystalline and amorphous structures at a high spatial resolution; a 20× objective lens was used, and the laser diode output energy was <1 mW. Micro-Raman spectroscopy was also used to identify signals from crystalline or amorphous materials. It was performed using an NRS-5500 (JASCO Corp., Japan), with a 532.1-nm laser. The laser was focused on the surface of the sample via an objective lens (NA = 0.9, Olympus MPlanFL N 100×). Raman scattering signal was detected as a line on a charge-coupled device (CCD) via an 1800-mm grating; the maximum laser power was 0.7 mW. During measurements, before X–Y mapping, the Z-position was adjusted to maximize the signal intensity of Raman-shift at the wavenumber of 520.5 cm-1. Using the measured signal intensity, Iirr, at 520.5 cm-1, corresponding to a wavelength of 547.3 nm, and the intensity for the non-irradiated sample, I0, the thickness of the amorphous layer was determined as
Here, da is the thickness of the amorphous layer and α532 and α547 are the optical attenuation lengths at the wavelength of 532.1 and 547.3 nm, respectively [50]. Note that, in this experiment, conventional Raman spectroscopy was employed. On the other hand, the novel nano-spectroscopy provides higher resolution at the crystal interface [51]. Also, the preprocessing of nano-spectroscopy is much less than that of transmission electron microscope and is expected to gain an enough sampling. Microscopic shapes were characterized by AFM and TEM. Initially, measurements were performed in both contact and tapping modes, and from these we confirmed that there were no observed structural differences dependent on the mode used. Entire measurements were then performed in contact mode. The AFM conditions included a scanning speed ≥2 s/line, a scanning range ≤49.74 × 49.74 μm, and an image resolution ≤0.19 μm in both the X- and Y-directions (256 × 256 pixels). This measurement result is shown in the Fig. S1 in Supplement 1. The irradiated sample was prepared for analysis via the procedure shown in Fig. 1(b).
Protective surface coating and focused ion beam (FIB) slicing pre-treatments were performed before TEM observation. TEM was performed in two modes: imaging and diffraction. The magnification of the imaging mode was <106. In imaging mode, high-resolution images were stitched to obtain a maximum field of view of 20 μm. To enhance image contrast, each of the raw diffraction images was adjusted to remove a constant level of base line and electron-beam-derived Gaussian noise, which had an arbitrary intensity.
3. Results
3.1 Crystal structure analysis using LSM, TEM and Raman spectroscopy
There are the differences of reflectivity between single crystals and amorphous silicon at wavelengths in the range of 400–1000 nm. The image is shown in Fig. 2 using LSM, which is equipped with the laser diode at 408 nm, and the brighter area of the damage represents silicon amorphization. Assuming that the sample surface is flat, from the result of Ref. [50], it is known that the combined use of TEM and LSM identifies amorphous thicknesses up to approximately 5 nm. The corresponding fluence is shown above each image. The amorphous layer is present in Fig. 2(a); meanwhile, it is absent in Fig. 2(b), across the entire fluence range.
Fig. 2. Comparison of irradiated silicon morphologies. Representative LSM images of silicon irradiated at wavelengths of (a) 10.3 nm and (b) 13.5 nm. Samples were obtained during the 2018A beam time. The irradiation points of 0.58 J/cm2 in (a) and 0.51 J/cm2 in (b) underwent TEM and correspond to the images shown in Fig. 3(c) and Fig. 3(k), respectively. The dashed lines indicate the crystal face orientation <112>, at which the samples were sliced by a FIB for TEM.
In Figs. 3(a)–(c), the cross-sections of samples irradiated at 10.3 nm with fluences of 0.083 J/cm2, 0.36 J/cm2, and 0.58 J/cm2, respectively, are shown. In Fig. 3(a), an amorphous layer formed without a recrystallized layer is also apparent. When the fluence is further increased and as lattice temperatures rises, the molten part breaks the amorphous layer, starting from the center of the crater and moving radially until it forms a small rim (Fig. 3(b)). Polycrystallinity or partial crystallization is promoted along the rim due to changes in the quenching time caused by increases in melting volume and the amorphous layer is excluded from the rim. At an even higher fluence, a recrystallized layer is formed under the crater (Fig. 3(c)). Recrystallization at high fluence occurs with ablation, and relatively large amounts of recrystallization occur beyond the ablation layer because of increased residual heat. Figures 3(d), 3(e), 3(h), and 3(i) show more detailed views of Figs. 3(a)–3(c) to highlight fractures and some of the finer laser-induced structure.
Fig. 3. TEM analysis of silicon irradiated at 10.3-nm (a–i). These Cross-sections of example craters irradiated with fluences of (a) 0.083 J/cm2, (b) 0.36 J/cm2, and (c) 0.58 J/cm2. Magnified images of the views shown in (d) Fig. 3(a), (e,h) Fig. 3(b), (i) Fig. 3(c); diffraction patterns from each point are shown as insets on panels (f, g). The TEM image data shown in (a,b) and (c) were obtained during the 2018B and 2018A beam times, respectively.
The formation of crystals and amorphous structures can also be observed via diffraction (Fig. 3(f), 3(g), 4(f) and 4(h)). This was corroborated by an experiment in which crystals and amorphous structures were first distinguished by LSM (Fig. 2). As the fluence increases, we first observe no change (∼0.02 J/cm2), then the formation of only an amorphous layer (∼0.2 J/cm2), and finally the formation of both amorphous and recrystallized layers (∼0.5 J/cm2).
Fig. 4. TEM analysis of silicon irradiated at13.5-nm (a–h). These Cross-sections of example craters irradiated with fluences of (a) 0.78 J/cm2 and (b) 0.51 J/cm2 by 13.5-nm. Magnified images of the views shown in (c,d) Fig. 4(a), (e) Fig. 4(c), (f) Fig. 4(e), (g) Fig. 4(d), (h) Fig. 4(g); diffraction patterns from each point are shown as insets on panels (f, h). The TEM image data shown in (a) and (b) were obtained during the 2018B and 2018A beam times, respectively.
In Figs. 4(a) and 4(b), the cross-sections of samples irradiated at 13.5 nm with fluences of 0.78 J/cm2 and 0.51 J/cm2, respectively, are shown. In Fig. 4(a), the crater was formed when thermal fluid diffusion without ablation was dominant. This thermal fluid diffusion behavior resembles a “drop–splash” effect [52,53]. Under these conditions, strong thermal stress occurred inside the crater, leading to cracking. Notably, in the single-shot regime, structural changes by thermal stress occurred at an even lower fluence than the ablation threshold, and the melted region was almost monocrystalline after re-solidification. It seems that the static Lambert-Beer law is established from the relationship between the shape of the cracked area and the fluence. As shown by comparing Figs. 4(a) and 4(b), though there were differences in the forms of the craters generated at the two fluence values, there was no difference in crystallinity. Figure 4(c) shows an enlarged section of the crack. In Figs. 4(c), 4(e), and 4(f), although the lattice structure of the regular stripe from the <112> orientation seems to disappear due to the distortion induced by thermal stress, it is actually an artificial problem originating from the observation angle with the TEM; the structure has crystallinity. Figure 4(d) shows an enlarged cross-section of the part of the crater that is raised from the original surface, while Fig. 4(g) shows an enlarged cross-section of Fig. 4(d); the crystallinity of the region shown in Figs. 4(d) and 4(g) is confirmed in Fig. 4(h). The details of Figs. 4(a)–4(h) reveal that the disappearance of crystallinity upon irradiation results entirely from thermally induced strain within crystal lattices. The crystalline stripe structure is also confirmed by the model structure.
A comparison of Figs. 3(a)–3(i) and 4(a)–4h reveals the differences in the laser-induced structures generated using different wavelengths. For example, Figs. 3(c) and 4(b) were acquired from samples irradiated with similar fluence levels. Thus, structures produced at these wavelengths, which straddle the L2,3 edge of Si, illustrate the principle of adjusting optical penetration via wavelength selection. The depth generated at the 10.3-nm wavelength differed by one order of magnitude with respect to that created using irradiation at 13.5 nm, and the structures induced at 10.3 nm were amorphous and polycrystalline while those induced at 13.5 nm were monocrystalline. Under 10.3-nm irradiation conditions, as the thickness of the laser-induced amorphous layer differed depending upon laser fluence, it was necessary to consider fluence dependency.
To fully establish that amorphization had occurred on the surface, the superposition of signals obtained via Raman spectral mapping at 520.5 cm-1 was selected to visualize the samples with reasonable resolution [50]. Figure 5(a) shows spectrums acquired at a point that had been irradiated at a wavelength of 10.3 nm and a non-irradiated, and these spectrum signals are distinguished amorphous from crystalline, that is, crystalline has a sharp peak at 520.5 cm-1 and amorphization has a broad peak from 400 to 500 cm-1. Figure 5(b) shows a thermal diffusion simulation result for a fluence of 0.022 J/cm2 and a wavelength of 10.3 nm, which was used to verify the amorphization threshold. Figure 5(c) illustrates the results of amorphous thickness mapping, as obtained from Eq. (1). Two 2D patterns are depicted as a representative irradiation point of non-ablated-amorphization (left) and ablated-amorphization (right), and it is visually understood that the amorphization threshold fluence is lower than the ablation threshold fluence.
Fig. 5. Raman spectroscopic analyses of point irradiated by 10.3-nm XUV light. (a) Raman spectra of a point previously irradiated at a fluence of 0.083 J/cm2 and a non-irradiated point. (b) Thermal diffusion simulation result at a fluence of 0.022 J/cm2 and a wavelength of 10.3 nm. The color corresponds to the temperature in Kelvin. (c) 2D mapping of the amorphous layer thickness from the Raman spectrum (upper plots) and the relationship between fluence and amorphization thickness (lower plot). The upper left and upper right images correspond to samples irradiated using fluences of 0.083 J/cm2 and 0.36 J/cm2, respectively; these samples are also shown in Fig. 3(a) and (c), respectively. Amorphous layer thicknesses were obtained from the maximum signal intensity point inside the crater. The data in (a) and (c) were obtained during the 2018B beam time.
From the results of fitting based on the Beer–Lambert law (Fig. 5(c), lower plot), an optical attenuation length of 37 nm and a maximum thickness for the amorphous layer of 88 nm were obtained. When the retrieved thickness became negative, the thickness of the amorphous layer was set to zero. This attenuation length value was consistent with the static value [35], thus we concluded that in this case, linear absorption was dominant. The amorphous layer thickness from TEM images was used as an absolute value for calibration. Before calibration, the amorphous thickness values acquired from the Raman and TEM measurements were in agreement (90% confidence level). From Fig. 5(b) and Ref. [54], the time from the maximum temperature to recrystallization was found to be ∼8 ns.
3.2 Comparison of experiment and simulation
The cooling time was estimated using a one-dimensional (1D) thermal diffusion toy model that included the experimentally obtained optical attenuation length. The 1D heat conduction equation from the enthalpy method, which includes the solid and liquid phases as well as the interface, was obtained as follows:
In Eq. (2) and (3), h(z,t) is the enthalpy, ρ is the density of silicon, k is the Boltzmann constant, S(z,t) is the laser source, F is the fluence, and τ is pulse duration, αeff is the optical attenuation length. This equation was solved using finite-difference methods. Since the beam size is well larger than the optical attenuation length, this simulation can be approximated in 1-dimensional. Calculations were performed using the experimentally obtained optical attenuation length. As described above, based on the Beer–Lambert law, the threshold fluence for amorphization was experimentally obtained as Famor ∼ 0.02 J/cm2. According to our simulation results, the fluence value of 0.02 J/cm2 corresponds to the temperature at which the silicon reaches its melting point at the surface. The amorphization threshold at the XUV wavelength is 10 times lower than the previously reported values using an NIR femtosecond laser by Bonse et al. (0.26–0.27 J/cm2) [50,55,56] and Izawa et al. (0.18–0.21 J/cm2) [37]. It is difficult to conduct a simple comparison of the amorphization thresholds of the NIR femtosecond and XUV femtosecond regimes, because the threshold in the NIR femtosecond regime drastically changes depending on the heating time, time-dependent reflectivity and time-dependent attenuation length. However, our result is in accordance with the conventional view of amorphization occurring via rapid cooling. Amorphization is closely related to the cooling time, and conventionally, by adjusting the pulse duration, we can determine whether it appears. However, heated volume is also closely related to the cooling time. The cooling time scale at the amorphization threshold including the experimentally determined optical attenuation length shown in Fig. 5(b) is located at the same order of magnitude as the previously reported values using an NIR femtosecond laser [37]. Also, the damage/ablation threshold value of silicon with nanosecond pulse at the wavelength of 13.5 nm have been reported by Barkusky et al. [57], and the range of the threshold is located at 4.1–5.0 J/cm2 and this threshold is approximately 10 times higher than the ablation threshold of 13.5 nm in the femtosecond regime. Also, Ref. [37] showed that when the pulse duration equals 8 ps or more, amorphization is untriggered and crystallization occurs. That is to say, the fact that nanosecond lasers do not cause amorphization is consistent with the fact that we have experimentally verified non-amorphization with femtosecond lasers at a wavelength of 13.5 nm.
4. Conclusion
In this study, we conducted the comparison of the crystal state of silicon by changing only the optical attenuation length under the pinned pulse duration. Since fluence was examined in a wide range, it intrinsically allows comparisons with a various fluence, intensity, and pulse duration. It is peculiar to the XUV wavelength that the optical attenuation length is changed selectively. Thus, from our results, materials are irradiated by XUV femtosecond pulses, which has a deeper optical attenuation length (here, approximately 500 nm), the crystal state of irradiated materials becomes like those state induced by an infrared nanosecond pulses. Also, when the optical attenuation length is shorter (here, approximately 40 nm), we observed the same effect as that reported for other investigations based on femtosecond pulses, though, here the crystal size was slightly larger than those reported elsewhere [46–48]. Eventually, we conclude that, in case of the fixed pulse duration of femtosecond regime, the crystal state between single crystal and amorphous depends only on optical attenuation length.
Very recently, a 300-nm spot size in the XUV regime was realized by an attractive method based on a high-numerical-aperture (NA) X-ray focusing elliptical mirror, and further developments in high-NA lens fabrication [58] and that of laser processing [59] are expected in the future. For the XUV, selection of the appropriate optical attenuation length makes it possible to form smooth surfaces and high aspect ratios via ablated holes [60,61]. The candidate of compact light sources includes plasma-based X-ray lasers, and high-harmonic generation light sources.
Funding
New Energy and Industrial Technology Development Organization (TACMI project); Ministry of Education, Culture, Sports, Science and Technology (JPMXS011806246, JPMXS0118070187, MEXT Q-LEAP).
Acknowledgments
The XFEL experiments were performed in the BL1 laboratory at the SACLA facility with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2018A8024, 2018B8024 and 2019B8033). The near-infrared mapping spectrometry (NIMS) microstructural characterization platform was part of a the “Nanotechnology Platform” of MEXT, Japan. The authors are grateful to H. Hara, Y. Ono, T. Tamura, T. Gisuji, T. Ogura, N. Shinozaki, S. Yamauchi, Y. Shimada, K. Maeda, R. Kageyama, H. Kawasaki, M. Shoji, H. Mori (Utsunomiya University), Y. Koshiba, Y. Tadenuma, E. Terasawa, and S. Hanai (Waseda University) for their technical support.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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