# Physico-Chemical characterization of D-LIPSS formation by femtosecond laser beam on N-doped (100) silicon under controlled atmospheres

Lynda Lounis, Barthélémy Aspe, Pierre Birnal, Loic Gimenez, Audrey Sauldubois, Hervé Rabat, A-L Thomann, N Semmar

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Lynda Lounis, Barthélémy Aspe, Pierre Birnal, Loic Gimenez, Audrey Sauldubois, et al., Physico-Chemical characterization of D-LIPSS formation by femtosecond laser beam on N-doped (100) silicon under controlled atmospheres. Applied Surface Sciences Advances, 2025, 30, pp.100860. 10.1016/j.apsadv.2025.100860. hal-05283412

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Full Length Article

# Physico-Chemical characterization of D-LIPSS formation by femtosecond laser beam on N-doped (100) silicon under controlled atmospheres

L. Lounis, B. Aspe, P. Birnal, L. Gimenez, A. Sauldubois, H. Rabat, A-L Thomann, N. Semmar*

GREMI UMR 7344 CNRS/Université d'Orléans, Orléans, 45067, France

ARTICLE INFO

Keywords:

Laser induced periodic surface structures

Silicon

Controlled atmosphere

Chemical composition

ABSTRACT

In this work, the formation of deep laser-induced periodic surface structures (D-LIPSS) on N-type silicon (100) was investigated using femtosecond laser pulses (pulse duration = 350 fs; wavelength = 1030 nm; repetition rate = 100 kHz) with linear polarization. Experiments were performed at atmospheric pressure and at 1 mbar under various atmospheres, including oxygen, nitrogen, argon, and ambient air, in both static mode (accumulation of successive pulses) and scanning mode (sample displacement under the beam). The study aimed to better understand the mechanisms involved in ultrashort laser-induced surface structuring by examining the influence of atmospheric pressure and gas composition on the morphology, topography, chemical composition, and wettability of the fabricated structures. Morphological characterization using SEM, AFM, and TEM revealed the formation of D-LIPSS with amplitudes around 650 ± 50 nm, significantly exceeding typical values reported in the literature. Material redeposition was markedly reduced under low-pressure conditions compared to atmospheric pressure, leading to cleaner and more well-defined surface structures.

1. Introduction

Laser structuring using ultrashort laser pulses has emerged as a manufacturing technique for the fabrication of nano- and microstructures, with applications spanning various fields, including data storage, optoelectronics, photonics, photovoltaics, and biomedicine. This technique has been applied to a diverse array of materials, including semiconductors [1–3], metals [4,5], polymers [6], and glasses [7,8]. Ultrashort laser treatment presents significant advantages in material processing, including reduced heat-affected zones and rapid energy deposition, in comparison to the effects produced by longer pulsed lasers [9].

Direct structuring of surfaces with ultrashort laser pulses allows the generation of diverse surface morphologies that are influenced by processing conditions such as ripples, grooves, and spikes. Ripples are formed at fluences near the material's ablation threshold, while higher fluences and an increased number of pulses lead to the formation of grooves and spike-like structures. The formation of the latter structures is commonly attributed to the interaction between surface plasmon excitation and the hydrodynamics of the molten material [10]. A particularly effective laser structuring technique involves the generation

of Laser-Induced Periodic Surface Structures (LIPSS) through the application of ultrafast laser pulses. LIPSS have been extensively studied since their discovery in the 1960s by Birnbaum [11]. They have attracted significant scientific interest due to their potential applications in surface functionalization, including wettability modifications through the creation of superhydrophobic surfaces [12,13], tribological property modulation [14,15], metal colorization [16], and enhancement of optical absorption [17]. The formation of these structures is influenced by laser parameters such as laser fluence, wavelength, beam polarization, number of pulses, surface roughness or ambient gas. Based on the spatial period (λ) and the polarization of the laser beam, LIPSS are classified into two distinct categories. Low-Spatial-Frequency LIPSS (LSFL) exhibit a period ranging from λ/2 to λ, where λ is the laser wavelength. Their orientation can be either parallel or perpendicular to the laser polarization. On the other hand, High-Spatial-Frequency LIPSS (HSFL) are characterized by a period smaller than λ/2, with orientations that can also be parallel or perpendicular to the laser polarization in the case of semi-conductors.

The mechanisms of formation of LIPSS are still a matter of debate. Several theories and models have been proposed so far to explain the formation of these nanostructures, including inhomogeneous energy

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: nadjib.semmar@univ-orleans.fr (N. Semmar).</footnote>
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** (a) Schematic of the experimental setup used for femtosecond (Yb:YAG) laser processing, and (b) the corresponding laser scanning pattern on the sample surface.</figcaption></figure>

<caption>**Table 1**</caption>

Values of the ablation threshold for a single shot, $F_{ih}(1)$, the ablation threshold for an infinite number of shots, $F_{ih}(\infty)$, and the incubation coefficient (S) obtained under four irradiation conditions (air, $N_2$, $O_2$, and Ar) at two pressure values (1 bar and 1 mbar).

<table><thead><tr><th></th><th></th><th>ω (μm)</th><th>F<sub>ih</sub>(1) (mJ/cm<sup>2</sup>)</th><th>F<sub>ih</sub>(∞) (mJ/cm<sup>2</sup>)</th><th>S</th></tr></thead><tbody><tr><td rowspan="2">1 bar</td><td>Air</td><td>24 ± 2</td><td>220 ± 10</td><td>7 ± 10</td><td>0.81 ± 0.04</td></tr><tr><td>$N_2$</td><td>35 ± 2</td><td>160 ± 10</td><td>-</td><td>0.92 ± 0.01</td></tr><tr><td rowspan="3">1 mbar</td><td>$N_2$</td><td>35 ± 2</td><td>170 ± 10</td><td>-</td><td>0.95 ± 0.01</td></tr><tr><td>Ar</td><td>35 ± 2</td><td>170 ± 10</td><td>-</td><td>0.94 ± 0.01</td></tr><tr><td>$O_2$</td><td>35 ± 2</td><td>170 ± 10</td><td>-</td><td>0.94 ± 0.01</td></tr></tbody></table>

deposition, Sipe's theory, models based on the interference of surface plasmons (SPs) and incident lasers, self-organization of surface instabilities, and hydrodynamic theories [10,18–20]. LIPSS formation typically occurs within a laser fluence range slightly below the material's ablation threshold [21]. In recent years, studies have focused on the role of oxidation to explain the formation of HSFL, suggesting that it is a necessary condition for their development. Öktem et al. [22] hypothesized a feedback mechanism between the creation of these structures and the presence of oxides. However, Dominic et al. [23] demonstrated in recent studies that oxygen does not play a role in the formation of HSFLs on tungsten. Instead, they emphasize the importance of ambient pressure on the resulting periodicity of the HSFLs. Regardless of these findings, it is obvious that the presence of oxides formed during laser irradiation has an impact on the physical and chemical properties of laser-treated surfaces.

The formation of LIPSS on silicon, one of the most extensively studied materials, has been thoroughly explored in the literature due to its unique response to ultrashort laser pulse irradiation. Various studies have demonstrated the fabrication of LIPSS on silicon surfaces, investigating the effects of different laser processing parameters, such as laser wavelength, fluence, pulse duration, repetition rate, pulse number, and polarization [24–28]. While the morphological and topographical modifications induced by ultrafast laser irradiation on silicon have been widely studied in recent years, comparatively little research has focused on the accompanying chemical alterations, often overlooked in favor of other types of modifications. For instance, Nivas et al. [29–31] examined the formation of LIPSS on silicon using ultrashort laser irradiation under vacuum and at various ambient pressures. Their findings highlight the significant role of ambient pressure in the development of surface structures, attributing the observed effects to the presence of target-material nanoparticles on the sample surface during processing in

air. Following this, Nivas et al. [5] further investigated LIPSS formation on copper in different environments, including air, vacuum, and nitrogen. Their study revealed notable differences in surface composition across these environments, suggesting that specific chemical effects may influence the final morphology of the sample during processing.

In this study, the morphological and chemical properties of the structures formed on the surface of silicon upon irradiation with femtosecond laser pulses are investigated. Various atmospheres, including oxygen, nitrogen, argon, and ambient air, were employed during the experiments. This research provides valuable insights into the mechanisms underlying direct ultrashort laser surface structuring and offers a deeper understanding of the role of oxidation in the formation of LIPSS. Furthermore, the wettability properties of the laser-induced nanostructures on silicon surfaces under different environmental conditions are examined.

## 2. Materials and methods

A double-sided polished N-type (100) silicon wafer with a resistivity of 20 to 30 Ω-cm, a thickness of 300 µm, and a band gap of 1.12 eV was used in this investigation. The silicon samples were irradiated with multiple femtosecond laser pulses from 100 to 100000 pulses and under various atmospheres and pressures: ambient air (1 bar), oxygen (1 mbar), argon (1 mbar) and nitrogen (1 bar and 1 mbar). To modify the chemistry of the atmospheres, the wafer was placed under vacuum at 10⁻² mbar and a continuous flow was maintained throughout the experiments. Prior to laser treatment, the silicon samples were ultrasonically cleaned in isopropanol. Fig. 1(a) shows a schematic of the experimental setup for fs-laser processing on silicon. The laser source is a Yb:YAG system (Satsuma HP-10 model from Amplitude Systems) that delivers linearly polarized pulses with a Gaussian spatial profile with a wavelength of $λ = 1030$ nm. The pulse duration is $τ = 350$ fs, with a maximum pulse energy $E_p$ of approximately 20 µJ. To calculate the laser fluence (F), given by $F = 2E_p/πω_0^2$, the laser spot size at the focal point ($ω_0$) was first determined using Liu's method (see below). The radius value obtained and used in this work is shown in Table 1. The repetition rate was set to $f = 100$ kHz. A scanning galvanometer was used to ensure that the laser beam covered the entire surface of the samples. The laser beam was focused on the target surface by an F-Theta lens with a focal length of about 134 mm. The laser device is coupled with a vacuum chamber where samples are placed to study the effect of the controlled atmosphere. In this work, laser irradiation was performed in both static and scanning mode.
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Evolution of the ablation threshold fluence $F_{th}(N)$ of silicon irradiated by a 350 fs, 1030 nm laser at 100 kHz, as a function of the number of pulses N. (a) In air and $N_2$ at a pressure of 1 bar. (b) in $N_2$, Ar, and $O_2$ environments at a pressure of 1 mbar.</figcaption></figure>

To irradiate the entire silicon surface, samples with dimensions of 2 cm × 2 cm were used, and the scanning speed was set to $v_x = 70$ mm/s, corresponding to a total of approximately 100 effective pulses. The scanning direction was oriented perpendicular to the laser polarization direction, with a translation step ($\Delta$Y) of 10 µm, as illustrated in Fig. 1(b). This choice was made to ensure the creation of homogeneously structured areas.

Laser-material interaction was investigated using several characterization techniques. The morphology of the LIPSS was examined by scanning electron microscopy (SEM) using a Zeiss Supra 40 and atomic force microscopy (AFM) with an ICON Bruker system. Two-dimensional fast Fourier transform (2D-FFT) analyses were performed on SEM and AFM images to evaluate the regularity and spatial periods of the formed structures. Crater sizes were measured using a Keyence VHX-5000 digital optical microscope. Transmission electron microscopy (TEM) was conducted on cross-sectional samples of laser-processed silicon to investigate the microstructure and subsurface features. For cross section observations a conventional preparation protocol was used. First, the sample is cut into thin slices with thickness of the order of 1 mm. A sandwich structure is prepared by bonding two slices, prepared with the deposited layers facing each other. The structure is then cut into slices mounted on a tripod and the specimen is mechanically polished. At the end, this specimen is glued to a 3 mm diameter copper support and a broad argon ion beam system is used to polish the specimen so that the

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** SEM images of craters formed on a silicon surface after femtosecond laser irradiation (350 fs pulses at 1030 nm, 100 kHz repetition rate) in static mode. Experiments were performed in ambient air (left column) and nitrogen atmosphere (right column) at 1 bar, with a fluence of 400 ± 9 mJ/cm² in air and 190 ± 7 mJ/cm² under $N_2$. Craters are shown after 100 pulses (panels (a) and (d)), 500 pulses (panels (b) and (e)), and 1000 pulses (panels (c) and (f)). The white double-headed arrow indicates the laser polarization direction. Insets in each panel show the two-dimensional Fourier transform (2D-FFT) of the corresponding SEM images.</figcaption></figure>

thinnest portions of the wedge are at the center for HRTEM imaging and analytical techniques.

Chemical composition and surface chemistry changes induced by laser processing were analyzed by X-ray photoelectron spectroscopy (XPS) using a Thermofisher Escalab Xi+. The performance of the X-ray monochromatized source (Kα Al =1487 eV) and the detection setup allow to achieve a photoelectron energy resolution of 0.1 eV. Position of the C1s spectrum at 285 eV was taken as reference.

In the aim of evidencing the modification of a surface property after laser structuring, wettability was assessed by contact angle measurements on silicon samples before and after femtosecond laser irradiation, using the DigiDrop system.

## 3. Results and discussion

### 3.1. Static mode

The ablation threshold was first determined at 100 kHz using Liu's method [32]. The silicon samples were irradiated under different atmospheres using 100, 500, 1000, 5000, 10,000, and 100,000 laser pulses, with varying energies ranging from 3.7 ± 0.1 to 9.4 ± 0.1 µJ. This corresponds to laser fluences ranging from 400 ± 9 mJ/cm² to 1000 ± 10 mJ/cm² in air, and from 190 ± 7 to 500 ± 10 mJ/cm² under controlled atmospheres. The difference in laser fluence between the two conditions is due to the variation in the laser spot size, as explained below. Fig. 2 displays the variation of the threshold fluence as a function of the number of pulses ($F_{th}(N)$) at 1 bar in air and nitrogen (Fig. 2(a)), and at 1 mbar for nitrogen, argon, and oxygen (Fig. 2(b)), as a function
<figure><img src="image_5.png" /><figcaption>Fig. 4. SEM images of craters formed on the silicon surface irradiated by a femtosecond laser with F = 190 ± 7 mJ/cm² and N = 100 for the left column, N = 500 for the center column, and N = 1000 for the right column, in different atmospheres at 1 mbar: (a, b, and c) in N₂, (d, e, and f) in Ar, and (g, h, and i) in O₂. The white double-headed arrow indicates the direction of laser polarization. The images within the yellow boxes show magnified views of the surface structures formed at the edges. The spectra in panels (a–i) show the two-dimensional Fourier transforms (2D-FFT) generated from the corresponding SEM images.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5. SEM image of LIPSS covered with redeposited material formed in ambient air. (a) SEM top view of surface morphology, (b) magnified view highlighting the details of the surface, and (c) SEM cross-section. The double-headed arrow indicates the laser polarization direction.</figcaption></figure>

of the number of shots. These curves show the decrease in threshold fluence with an increasing number of laser pulses, caused by the accumulation of defects from repeated shots at the same spot on the irradiated sample surface. This accumulation reduces the energy density threshold required to evaporate the material. This phenomenon is known as the "incubation effect". In this work, it was found that, under air, the process is described by a generalized incubation model introduced by Lenzner for dielectrics, given by the following formula [33]:

$$F_{th}(N) = F_{th}(1)N^{S-1} + F_{th}(\infty) \quad (3.1)$$

where $F_{th}(1)$ is the ablation threshold for a single shot, S is the incubation coefficient and $F_{th}(\infty)$ is the threshold fluence for an infinite number of shots. $F_{th}(\infty)$ represents the minimum fluence value reached after a large number of laser shots. Beyond this threshold, the diameter of the laser-ablated pattern stabilizes and remains constant, even with further increases in fluence or number of pulses. Consequently, the Si material is progressively ejected to greater depths, creating pits on the surface. This trend is shown in Fig. 2(a) under air, where for $N > 10^4$, $F_{th}$ remains nearly constant.

In contrast, a linear behavior is observed in $N_2$ at 1 bar, as well as in $N_2$, $O_2$, and Ar at 1 mbar. This behavior can be described using the incubation model proposed by Jee et al. [34]. The ablation threshold and the number of laser pulses are related by the following equation:

$$F_{th}(N) = F_{th}(1)N^{S-1} \quad (3.2)$$

In $N_2$ at 1 bar and 1 mbar (results shown in Fig. 2(b)), the threshold fluence decreases monotonically as the number of pulses increases. Under air, a dielectric-like behavior is observed at the surface due to the thicker oxide layer formed. However, at 1 bar in nitrogen and at 1 mbar in all three environmental conditions, the behavior becomes more similar to a metallic one. Table 1 summarizes the results for laser spot size $\omega$, single-pulse ablation threshold, ablation threshold for an infinite
<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Morphological features of the irradiated surface generated by scanning the target surface in a direction perpendicular to the laser polarization, and fluence of 190 ± 7 mJ/cm². (a, e, f) SEM images in top view and their corresponding 2D-FFT in the corner. (b, f, and j) SEM images in cross-section. (c, g, and k) AFM images, and (d, h, and l) the corresponding spatial profiles obtained from AFM images. The first row corresponds to the surface-N₂, the second to the surface-Ar, and the third to the surface-O₂. The double-headed arrow indicates the direction of laser polarization.</figcaption></figure>

<caption>**Table 2**</caption>

Values of LIPSS spatial periods obtained from the SEM and AFM analyses and their corresponding 2D-FFT.

<table><thead><tr><th rowspan="2"></th><th colspan="4">Λ (nm)</th></tr><tr><th>SEM</th><th>SEM-2DFFT</th><th>AFM</th><th>AFM-2DFFT</th></tr></thead><tbody><tr><td>Surface-N<sub>2</sub></td><td>850 ± 20</td><td>830 ± 20</td><td>870 ± 20</td><td>850 ± 20</td></tr><tr><td>Surface-Ar</td><td>830 ± 20</td><td>840 ± 20</td><td>850 ± 20</td><td>850 ± 20</td></tr><tr><td>Surface-O<sub>2</sub></td><td>830 ± 20</td><td>850 ± 20</td><td>840 ± 20</td><td>840 ± 20</td></tr></tbody></table>

number of shots and incubation coefficient for each environmental condition and for both pressure levels. The single-shot threshold fluence $F_{th}(1)$ of 220 ± 10 mJ/cm² and the incubation coefficient of 0.81 ± 0.04, obtained under air, are in good agreement with the results found in the literature [1,35]. The laser spot size obtained in air is smaller than that measured under other environmental conditions. This difference may be due to the significant redeposition observed in air (see Fig. 3 and Fig. 4). These effects may lead to a more localized energy deposition, resulting in a smaller effective spot size. Moreover, as shown in Table 1, $F_{th}(1)$ is higher in air than in other gaseous environments, further supporting the role of ambient atmosphere in modulating the ablation process.

Fig. 3 presents SEM images of the silicon surface following femtosecond laser irradiation, conducted under ambient air conditions (Fig. 3 (a)–(c)) and in a nitrogen atmosphere at 1 bar pressure (Fig. 3(d)–(f)) under different numbers of laser pulses (N = 100, 500, 1000) with a similar laser beam energy for both condition leading to a fluence of 400 ± 9 mJ/cm² under air and 190 ± 7 mJ/cm² under N₂. The SEM images in Fig. 3 show the formation of a slightly circular crater under both environmental conditions. The induced morphologies observed on the

SEM images at 100 pulses (Panels (a) and (d)) reveal the formation of LSFL, aligned parallel to the laser polarization. The orientation of the LSFL parallel to the polarization is due to the formation mechanism and similar to LSFL orientation reported for insulating or semi-conductor materials [19]. These structures are observed to cover almost the entire crater surface. In air, the average spatial period is $Λ = 770 ± 10$ nm, while in nitrogen, it is $Λ = 850 ± 10$ nm. At the centre of the crater, where the fluence is higher, supra-wavelength surface structures are observed. They are formed by the thermal coalescence of multiple ripples under high laser fluence [30]. After 500 pulses, in air (panel (b)), the LIPSS begin to disappear, whereas in nitrogen (panel (e)), the LSFL structures persist and are accompanied by supra-wavelength structures. This is confirmed by the 2D-FFT spectra presented in the top right corner of panels (a-f), generated from the corresponding SEM images. These spectra provide further insight into the surface features, showing that the periodicity and organization of these structures persist in N₂ after 500 pulses, whereas in air, LIPSS disappear after the same number of pulses. When the number of pulses is increased up to 1000, the LIPSS are almost entirely suppressed under air (panel (c)), resulting in the appearance of grooves and more irregular structures at the center. In contrast, in nitrogen (panel (f)), supra-wavelength structures are observed to cover the entire central region of the crater, while LSFL remain visible along its edges. Under both irradiation conditions, redeposited material is also visible following laser ablation. However, treatment in a nitrogen environment shows a surface less contaminated by redeposition compared to air.

Similarly, samples were prepared under controlled atmospheres (N₂, Ar, and O₂) at a pressure of 1 mbar. Fig. 4 illustrates the morphological features of the irradiated surface observed in different conditions for three pulse numbers: N = 100 (panels (a), (d), and (g)), N = 500 (panels
<figure><img src="image_8.png" /><figcaption>Fig. 7. Evolution of LIPSS amplitude as a function of scanning speed and laser fluence. These data were obtained under N₂ at 1 mbar.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. Cross-sectional images of Surface-O₂: (a) TEM and (c) STEM images; (b) Magnified view of the yellow box in (a)focusing on a single LIPSS (protrusion) and revealing different crystalline phases.</figcaption></figure>

(b), (e), and (h)), and N = 1000 (panels (c), (f), and (i)). The double-headed red arrow indicates the laser polarization direction. As shown in Fig. 4, in all three environmental conditions and for all pulse numbers, the observed craters exhibit a slightly elliptical shape, with average diameters of approximately 30 µm for N = 100, 32 µm for N = 500, and 34 µm for N = 1000. The formation of elliptically shaped structured craters on silicon has also been observed in the literature when the target is processed under reduced pressure [30,31,36]. The morphological features of these structures, as shown in Fig. 4, reveal the presence of bifurcation phenomena (i.e., the splitting or branching of periodic structures, leading to the formation of multiple, smaller features from a primary ripple). These bifurcations were also slightly observed in nitrogen at 1 bar (see Fig. 3(d)–(f)). The formation of such bifurcations on silicon has also been reported by Hu et al., who attributed the ripple bifurcation to the inhomogeneous distribution of absorbed laser energy across the target surface, as well as to the material’s intrinsic properties [37]. At 100 pulses, in all environmental conditions, LSFL oriented parallel to the laser polarization, with a spatial period of approximately 800 ± 10 nm, cover the entire crater surface. No significant differences of the morphology or periodicity were observed between the three atmospheres for a pulse number of 100. At N = 500 (panels (b) and (e) of Fig. 4), in N₂ and Ar atmospheres, High Spatial Frequency Laser Induced Structures (HSFL), oriented perpendicular to the laser polarization with a spatial period of approximately 200 ± 10 nm, are observed at the edges of the craters. These regions, characterized by low fluence, are highlighted in the yellow boxes in both N₂ and Ar environments. As the

fluence gradually increases from the edges toward the center, a transition from HSFL to less pronounced and discontinuous LSFL is observed. The LSFL are oriented parallel to the laser polarization. This behavior is consistent with findings reported in the literature, where it has been demonstrated that HSFL typically form in low-fluence regions near the damage threshold of the irradiated material [19,27]. Furthermore, the transition from HSFL to LSFL with increasing fluence has also been previously documented [38]. The formation of HSFL in this context supports the hypothesis of a hydrodynamic origin, though it may also be associated with thermal mechanisms inherent to the material’s response to laser irradiation. At the center of the crater, where the fluence is higher, supra-wavelength surface structures are formed. In oxygen, HSFL are also observed at the edge, but they are less visible and less pronounced than in nitrogen and argon. Furthermore, the crater is almost completely covered by LSFL parallel to the polarization, whereas in the other two atmospheres, the LSFL cover only the intermediate zone between the edge and the center. At 1000 pulses, in argon and nitrogen (panels (c) and (f)), the same morphological characteristics were observed as at 500 pulses. In O₂ atmosphere, however, the formation of HSFLs is absent. This absence may be explained by the fact that, in N₂ or Ar atmospheres, where inert gases prevail, the thermal effects of the laser dominate, promoting HSFL formation. In contrast, in an O₂ atmosphere, the reactivity of oxygen leads to significant oxidation of the material surface. According to the model proposed by Oktem et al. [22] that is based on a feedback mechanism between oxide and nanostructure formation, oxidation is required to initiate HSFL formation. However,
<figure><img src="image_10.png" /><figcaption>Fig. 9. XPS analysis of Si 2p spectra for (a) non-textured silicon, and laser-treated silicon in (b) nitrogen, (c) argon, and (d) oxygen atmospheres.</figcaption></figure>

once the oxide layer becomes too thick, it inhibits the development of these surface structures. In an $o_2$-rich environment, laser structuring induces the formation of a thick oxide layer that alters the surface properties and disrupts the thermal mechanisms necessary for HSFL formation, thus preventing their appearance.

## 3.2. Fabrication of Large Area Si LIPSS

To better understand the morphological and chemical modifications induced by laser processing under various atmospheres, large-scale surfaces were structured under different environmental conditions. For this purpose, a scanning speed of 70 mm/s was employed, corresponding to an effective number of 100 pulses, with a translation step ($\Delta$Y) of 10 µm and a laser fluence of $190 \pm 7$ mJ/cm². Initially, nano-structuring was performed in ambient air. The SEM analyses obtained show significant material redeposition on the surface (see the SEM images in Fig. 5), that masks part of the morphological details. These effects are particularly critical for applications where surface purity is essential.

To minimize these undesirable deposits, structuring was carried out at low pressure (1 mbar). Additionally, the experiments were conducted under a controlled atmosphere (N₂, Ar, and O₂). For simplicity, the surfaces processed under these conditions will be referred to as Surface-N₂, Surface-Ar, and Surface-O₂, respectively. Fig. 6 presents top view SEM images along with their corresponding 2D-FFT (Fig. 6(a), (d), and (g)), SEM cross-sectional images (Fig. 6(b), (e), and (h)) and AFM images (Fig. 6(c), (f), and (i)). These images illustrate the LIPSS formed on Surface-N₂, Surface-Ar, and Surface-O₂ after laser irradiation. The SEM analyses also show redeposition on the surface, but it is less significant than that observed in air. The presence of LSFL, aligned parallel to the laser polarization, can be observed in the SEM images and AFM under all three gases. The spatial periods, determined from SEM images, AFM, and their corresponding 2D-FFT analyses, are summarized in Table 2. All three techniques yielded consistent values, confirming good agreement. The spatial period is approximately the same under all environmental conditions.

Regarding the amplitude, AFM measurements revealed an average LIPSS height of approximately $550 \pm 50$ nm under all three gases. In contrast, cross-sectional SEM images showed slightly higher amplitudes, around $650 \pm 50$ nm. This ~100 nm difference is likely due to the limited ability of the AFM tip to probe narrow or recessed regions. The amplitudes obtained in this work are significantly higher than those commonly reported in the literature, typically ranging between 50 and 300 nm [31,39,40].

To gain a better understanding of the observed high amplitude and to investigate its evolution with laser dose, a series of experiments was conducted by varying the laser fluence and scanning speed under nitrogen at a pressure of 1 mbar. The results, presented in Fig. 7, indicate that no surface structuring was observed at scanning speeds exceeding 800 mm/s for a fluence of $190 \pm 7$ mJ/cm², and above 500 mm/s for a fluence of $160 \pm 5$ mJ/cm², suggesting the presence of a threshold effect. At a speed of 500 mm/s and a fluence of $160 \pm 5$ mJ/cm², the formation of LIPSS, resembling those reported in the literature, was observed. Within the speed ranges of 800 to 70 mm/s ($190 \pm 7$ mJ/cm²) and 500 to 70 mm/s ($160 \pm 5$ mJ/cm²), D-LIPSS were formed, with their amplitude increasing progressively as the laser dose was raised. These results suggest that heat accumulation plays a significant role in the formation of these pronounced structures. Greater heat accumulation leads to enhanced material ablation [41].

To gain further insight, TEM analysis was performed on Surface-O₂. The results, presented in Fig. 8, reveal the crystalline phases formed after laser treatment. A ~230 nm-thick amorphous layer is observed, which is attributed to material redeposition (see Fig 8(c)). Energy-dispersive X-ray (EDX) analysis of the TEM images indicated that this layer is composed of SiOx, resulting from oxidation during irradiation. It should be noted that this oxide layer was not taken into account in the amplitude measurement. Within a protrusion, a disordered crystalline phase is discernible, along with a cavity approximately 100 nm in diameter. These cavities appeared regularly and were also observed in the cross-sectional SEM images (see Fig. 6). Such cross-sectional SEM images are rarely reported in the literature, making it difficult to assess
<figure><img src="image_11.png" /><figcaption>**Fig. 10.** Images of 0.6 µL water droplets deposited on silicon before laser treatment (a), and on laser-textured surfaces one month after irradiation. Fig. (b), (d), and (f) correspond to measurements taken along the LIPSS formation direction (0°), while Fig. (c), (e), and (g) correspond to measurements taken perpendicular to this direction (90°).</figcaption></figure>

whether similar features are characteristic of LSFL structures on silicon. However, Dominic *et al* [41] reported subsurface voids in tungsten, with dimensions on the order of a few nanometers, under various processing conditions (ambient air, air at 10 mbar, argon at 10 mbar, and vacuum at 10⁻⁷ mbar), in association with HSFL. They explained these voids by hydrodynamic cavitation, triggered by a rarefaction wave generated during ultrafast laser-matter interaction, and then frozen in place by sub ps solidification of the molten metal. In our study, cavity formation was observed on LSFL structures in silicon, whereas in their work, cavities appeared on HSFL structures in tungsten. Furthermore, the cavity sizes do not appear to be of the same order of magnitude. These differences probably indicate that different physical phenomena are involved in each case.

In this work, one hypothesis for the formation of these cavities may be attributed to the following mechanism: during laser structuring, the treated region is assumed to enter a quasi-liquid state where atoms exhibit high mobility, followed by a fast solidification into polycrystalline structures. Given that the density of the solid phase is higher than that of the liquid phase, crystallization is accompanied by volumetric contraction. This contraction is likely to induce nanometer-scale cavities (voids) when the material flow is insufficient to compensate for the resulting volume decrease.

Although the resulting structures exhibit amplitude characteristics typical of Direct Laser Interference Patterning (DLIP), the patterns observed in this study cannot be attributed to the DLIP mechanism. DLIP

requires the simultaneous superposition of at least two coherent laser beams focused onto the sample surface. Furthermore, structures generated by DLIP typically have spatial periods larger than the laser wavelength, which is inconsistent with the features observed here [42,43].

Thanks to the TEM analysis, it is possible to make specific assumptions to expand the observation of Fig. 7. Indeed, in addition to the high repetition rate (100 kHz) of the femtosecond laser, the bandgap energy of the doped silicon used (1.1 eV) is slightly lower than the energy corresponding to the laser wavelength (1030 nm, 1.2 eV). This double effect (bandgap and heat accumulation) is leading to photon absorption enhancement simultaneously to the ablation threshold decrease. Moreover, at the top of a protrusion the oxide layer is thick compared to the lower region. As the theoretical bandgap energy of SiO₂ is of about 8.9 eV, the interaction with the laser is dominated by multiphoton absorption and may limits the ablation on the top compared to the thinner oxide layer allowing more effectively ablation in the bottom. Finally, as mentioned above, the disordered crystalline Si phase observed in Fig. 8 and the presence of a nano-void inside the protrusion that possibly indicate a quasi-liquid state of the matter may suggest that fluid mechanics phenomena are involved.

XPS analyses were performed to evaluate the chemical composition of Surface-N₂, Surface-Ar, and Surface-O₂ samples. The raw surface of the silicon substrate before laser structuring was also analyzed as a reference. Fig. 9(a) shows the XPS Si 2p spectrum for reference silicon, the Surface-N₂, Surface-Ar, and Surface-O₂. On the reference silicon, two
<figure><img src="image_12.png" /><figcaption>**Fig. 11.** Images of 0.6 µL water droplets deposited on silicon before laser treatment (a), and on laser-textured surfaces one month after irradiation. Fig. (b), (d), and (f) correspond to measurements taken along the LIPSS formation direction (0°), while Fig. (c), (e), and (g) correspond to measurements taken perpendicular to this direction (90°).</figcaption></figure>

<table><caption>**Table 3**
Contact angle measurements after laser irradiation in different environmental conditions.</caption><thead><tr><th rowspan="3"></th><th colspan="4">CA(°)</th></tr><tr><th colspan="2">One month</th><th colspan="2">Three months</th></tr><tr><th>0°</th><th>90°</th><th>0°</th><th>90°</th></tr></thead><tbody><tr><th>Surface-N<sub>2</sub></th><td>26 ± 5</td><td>23 ± 5</td><td>21 ± 5</td><td>20 ± 5</td></tr><tr><th>Surface-Ar</th><td>41 ± 5</td><td>49 ± 5</td><td>27 ± 5</td><td>30 ± 5</td></tr><tr><th>Surface-O<sub>2</sub></th><td>39 ± 5</td><td>36 ± 5</td><td>31 ± 5</td><td>32 ± 5</td></tr></tbody></table>

peaks can be observed. The first peak located at 103 eV correspond to an oxide superficial layer as the signal is attributed to the two responses characteristic of the Si-O bonds with the 2p$_{3/2}$ component at 103.0 eV and 2p$_{1/2}$ at 103.5 eV. The second peak at 99.4 eV corresponding to the Si-Si metallic responses remains dominant (2p$_{3/2}$ peak at 99.1 eV and 2p$_{1/2}$ peak at 99.5 eV), representing about 75 % of the total intensity. As with the estimated XPS analysis depth of approximately 10 nm, the 25 % intensity of the Si-O bonds indicates the presence of a very thin oxide layer consistent with the 1.5-nm-native-oxide-layer of silicone well reported in the literature [44]. For Surface-N$_{2}$, as shown in Fig. 9(b), a significant increase in the Si-O component is observed, making it the majority (87 %) compared to the metallic component (13 %). Similar results were obtained for Surface-Ar. These results show that residual oxygen gas is still present in the chamber for the 1 mbar pressure of Ar or N$_{2}$ gas used in this work. To avoid residual oxygen, lower vacuum in the order of at least 10$^{-4}$ mbar are usually required. Although oxidation is enhanced compared to the native oxide layer, the presence of the metallic component suggests that oxidation remains limited to a thin layer, less than 10 nm thick. Also, even in the case of Surface-N$_{2}$, XPS measurements do not show any evidence of Si-N response nor nitrogen. N$_{2}$ is considered a stable gas, especially compared to O$_{2}$, and the current process conditions do not enable for Si-N bond to be formed. Surface-O$_{2}$

(Fig. 9(d)), a more pronounced oxidation is observed. The Si-metal component at 99.4 eV is almost completely absent, suggesting that the oxide layer exceeds 10 nm. This is further supported by the significantly higher relative intensity of the Si-O bonds compared to the Si-Si ones, indicating deeper and/or more uniform oxidation. These results indicate that laser structuring performed in oxygen leads to a higher oxygen content on the surface compared to treatments carried out in other environments. This observation is consistent with the explanation presented in Section 3.1, where the absence of HSFL after 500 pulses in O$_{2}$ is attributed to the formation of a thicker oxide layer within this environment.

### 3.3. Wetting properties of LIPSS structure substrates

The wettability of silicon surfaces before and after laser irradiation was evaluated through contact angle (CA) measurements. This angle represents the interface between a liquid droplet and the solid surface. Since wettability is strongly influenced by surface topography and roughness [40], the effect of LIPSS on Surface-N$_{2}$, Surface-Ar, and Surface-O$_{2}$ was investigated. To perform these measurements, a 0.6 µL water droplet was first deposited using the syringe of the DigiDrop system. The droplet profile was then recorded with the system's integrated camera. Finally, the acquired images were processed using ImageJ software with the LB-ADSA plugin to determine the contact angle values. To assess its temporal evolution, multiple measurements were conducted one month and three months after laser structuring, allowing for the observation of potential changes in wettability over time. The samples were stored under air and at room temperature to enable a natural evolution of the surfaces. The CA measurements are presented in Fig. 10 and Fig. 11, with the corresponding numerical values summarized in Table 3. The measurements were made at two orientations: parallel (0°) and perpendicular (90°) to the direction of LIPSS formation. Fig. 8(a) shows the untreated silicon surface as a
reference, with a contact angle of approximately 70° ± 5°.

As shown in Fig. 10 and 11, and in Table 3, all the surfaces became more hydrophilic after laser structuring. The irradiation-induced oxidation process leads to the formation of an oxide layer on the material surface following laser treatment. This oxide layer promotes the adsorption of hydroxyl (-OH) groups when exposed to ambient humidity or water, thereby contributing to the development of superhydrophilic properties [45]. Furthermore, the enhanced hydrophilicity of the structured silicon substrates can also be attributed to an intensified capillary effect within the laser-modified regions [46]. These findings align with previous studies showing that water droplets typically exhibit hydrophilic behavior on laser-textured surfaces [40,47].

The results indicate that the contact angle measured in the direction of LIPSS formation is similar to that obtained in the perpendicular direction, suggesting that the contact angle is not significantly dependent on the orientation of the structures formed on the surface.

The comparative analysis between the two measurement times shows an overall decrease in the contact angle for all conditions. Due to the uncertainty of the contact angle technique, the effect of the different atmospheres is difficult to estimate accurately. Overall, the formation of LIPSS tends to increase hydrophilicity, an effect that is maintained over time.

## 4. Conclusion

In summary, the influence of the controlled atmosphere on the formation of LIPSS on silicon using a femtosecond laser with a 1030 nm wavelength was investigated. Experiments were carried out at atmospheric pressure and 1 mbar in various atmospheres, namely oxygen, nitrogen, argon, and ambient air, using both static and scanning modes. In static mode, the spatial period of the ripples was reduced under air compared to N₂ at 1 bar. At 1 mbar, under all environmental conditions (N₂, Ar, and O₂), LSFL oriented parallel to the laser polarization were obtained, with a spatial period of approximately 800 ± 10 nm. Additionally, HSFL with a spatial period of 200 ± 10 nm were formed at the edges of the irradiated areas in N₂ and Ar.

In scanning mode, the use of a low-pressure environment (1 mbar) reduces material redeposition compared to processing in ambient air, resulting in cleaner surface structures. The LIPSS formed under nitrogen, argon, and oxygen atmospheres show a similar morphology, with a slightly higher contact angle observed for samples processed in oxygen, likely due to increased surface oxidation. Deep-LSFL (D-LSFL) structures with amplitudes of about 650 ± 50 nm were produced under all environmental conditions, which is significantly larger than values commonly reported in the literature. Our results show that this higher depth could highlight the role played by in the formation process. Additionally, cross-sectional microscopy revealed regular nano-voids beneath the surface.

### CRediT authorship contribution statement

**L. Lounis:** Writing – original draft, Software, Investigation, Data curation. **B. Aspe:** Writing – review & editing, Writing – original draft, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation. **P. Birnal:** Validation, Software, Investigation, Data curation. **L. Gimenez:** Software, Resources, Investigation. **A. Sauldu-bois:** Resources, Formal analysis, Data curation. **H. Rabat:** Validation, Software, Methodology, Investigation, Data curation. **A-L Thomann:** Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition, Formal analysis. **N. Semmar:** Writing – review & editing, Validation, Supervision, Project administration, Methodology, Investigation, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial

interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

The authors would like to thank the Conseil Régional Centre-Val de Loire for supporting this work through Grant ARD-MATEX No. 2021-00145829 and the Grant CAMIL No. 2023-00155644. This project has benefited from the expertise and the facilities of the Platform MACLE - CVL which was co-funded by the European Union and Centre-Val de Loire Region (FEDER).

### Data availability

Data will be made available on request.

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